1

SYNTHESIS OF NOVEL PHOSPHONIUM

SALTS WITH DONOR ACCEPTOR

MOLECULAR ARCHITECTURE

Nasrulla Majid Khan

MASTER’S THESIS

Inorganic Chemistry

International Master’s Program for Research Chemists

657/2020

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Synthesis of novel phosphonium salts with Donor-Acceptor molecular

architecture

Nasrulla Majid Khan, 300278

University of Eastern Finland, Department of Chemistry

Supervisor: Professor Igor Koshevoy

Joensuu 16.05.2020

Abstract

In the present times the research related to fabricate luminophores based on the rational

design at molecular level is of central importance because of its applications in

bioimaging, OLEDs, medicine, fluorescent probes etc. To meet the growing demands

various strategies have been formulated to design the luminophores accordingly. Over

the last decade polyaromatic hydrocarbons (PAH) scaffolds have been utilized for

tuning the gap, the specific property of the molecule depends upon the shape and -

conjugation of the molecule. By doping PAH’s with nitrogen, phosphorus, boron,

sulfur, etc the structures, reactivity and optoelectronic properties can be diversified. The

presence of phosphorus in PAHs scaffolds provides a vital tool for molecular

engineering of this PAHs family.

Phosphorus based compounds is one of the class of compounds utilized to fabricate

luminophores because of its comparable electronegativity with carbon and simplistic

transformation of trivalent phosphorus compounds into tetra and pentavalent

compounds. The above-mentioned characteristics of phosphorus paves the way to

design compounds based on phosphorus to tune the HOMO-LUMO gap and thus these

compounds can be used to design luminophores with different photophysical properties.

The process of careful design and tailoring of the photophysical properties of

compounds based on phosphorus present a plethora of opportunities to the ever-growing

scope of luminescent materials. The phosphorus can be incorporated in cyclic aromatic

systems like phosphines phospholes or phosphine oxides. The trivalent phosphorus can

act as a donor in the −conjugation but its complexation or alkylation makes it an

important electron acceptor in the donor-acceptor systems, which are extensively known

for the intra-molecular charge transfer (ICT). In ICT, acceptor moiety is connected

through system to the electron donor moiety and is responsible for ICT which plays

an important role in energy conversion

These molecules with donor-acceptor systems are often referred to as push-pull.

Characteristic groups with positive mesomeric or inductive (+M/+I) effects (OR, NH2,

OH and NR2) are used as electron donors while the most well-known electron acceptors

include groups showing negative mesomeric (-M/-I) effects (CHO, NO2, CN and also +PR3) has been utilised as an electron acceptor. To meet the growing demand, there are

continuous efforts to develop library of luminophores with different applications. The

optoelectronic properties are greatly influenced by introducing the electron donor and

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acceptor moieties in luminophores. The donor and acceptor moieties are connected

by −systems depending upon the donor-acceptor moieties the energy levels can be

tailored accordingly. Moreover, depending upon the length of system the fluorophores

show different emission and absorption maxima. The emission from locally excited state

(ES) gives generally smaller stokes shift since there is no apparent variation in dipole

moment in ES while in ICT there is prominent difference in dipole moment with more

polar nature thus ES is stabilised by polar solvent molecules, hence it leads to larger

change in Stokes shift.

The research presented in thesis was to design phosphonium salts with Donor Acceptor

molecular architecture utilizing the R3P+– (Acceptor moiety) and NPh2 (Donor moiety).

The salts were synthesised by Nickel catalysed methods and characterised by 1H-NMR, 31P-NMR, elemental analysis and mass spectrometry. The photophysical studies were

also performed and it was found that the salts emit in between 522-586 nm in different

solvents. The salts displayed different behaviour in different solvents with unusual dual

emission in non-polar solvent like toluene attributed to conventional ICT emission and

counterion migration respectively.

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Contents

Abbreviations ............................................................................................... 5

1. Introduction ................................................................................................................. 6

1.1. Luminescence ............................................................................................................. 6

1.2. Delayed fluorescence ................................................................................................. 7

1.3. Triplet-triplet annihilation .......................................................................................... 7

1.4. Solvatochromism ....................................................................................................... 8

1.5. ICT and Dual emission............................................................................................... 9

1.6. Phosphorus ............................................................................................................... 10

1.7. Organophosphorus Compounds ............................................................................... 10

1.7.1. Applications in medicine....................................................................................... 10

1.7.2. Agricultural and industrial application.................................................................. 11

1.7.3. Applications in organic synthesis.......................................................................... 12

1.8. Cyclic organophosphorus chromophores ................................................................. 15

1.8.1. Five membered heterocycles based on phosphorus .............................................. 15

1.8.2. Six membered heterocycles based on phosphorus ................................................ 19

1.8.3. Three Linearly Fused Six-Membered Rings analogues of anthracene ................. 20

1.8.4. Four Linearly Fused Six-Membered Rings ........................................................... 21

1.8.5. Phosphaphenalenes based luminophores .............................................................. 22

1.8.6. Six linearly fused six membered rings .................................................................. 23

1.9. Acyclic organophosphorus chromophores ............................................................... 25

2. Aims of the study ....................................................................................................... 33

3. Experimental procedure ........................................................................................... 34

3.1. General information ................................................................................................. 34

3.2. General methods for synthesis of phosphines .......................................................... 34

3.3. General method for the synthesis of phoshonium salts (21-24[Br]) using NiBr2 as a

catalyst............................................................................................................................. 35

3.4. General procedure for preparation of the 21-24[OTf] salts....................................... 37

4. Results and discussions ............................................................................................. 39

4.1. Synthesis of phosphonium salts ............................................................................... 39

4.2. Characterisation by 1H-NMR and 31P-NMR .......................................................... 43

4.3. Characterisation by Mass spectrometry ................................................................... 52

4.4. Photophysical properties .......................................................................................... 53

Conclusion

Acknowledgement

References

5

Abbreviations

HOMO Highest Unoccupied Molecular Orbital

LUMO Lowest Unoccupied Molecular Orbital

ICT Intramolecular Charge Transfer

ISC Intersystem Crossing

RISC Reverse Intersystem Crossing

TTET Triplet Triplet Energy Transfer

UT UP-Conversion

ES/GS/TS/SS Excited State/Ground State/Triplet State/Singlet State

TADF Thermally Activated Delayed Fluorescence

OLED Organic Light Emitting Diode

D–A Donor-Acceptor

ESI Electrospray Ionization

LE Locally Excited

DMSO Dimethyl Sulphoxide

DNA Deoxy Ribonucleic Acid

RNA Ribonucleic Acid

TTA Triplet Triplet Annihilation

ATP Adenosine triphosphate

NMR Nuclear Magnetic Resonance

OPC,s Organophosphorus Compounds

PPh2Cl Chlorodiphenylphoine

MeCN Acetonitrile

THF Tetrahydrofuran

DMF Dimethylformamide

DCM Dichloromethane

CHCl3 Chloroform

n-BuLi n-Butyllithium

t-BuLi tertiary Butyllithium NBS N-bromosuccinimide

NMR Nuclear Magnetic Resonance

MS Mass Spectrometry

ESI Electrospray Ionization

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1. Introduction

1.1. Luminescence

Luminescence can be defined as emission of radiations by a material due to transitions

in electronic states. Depending upon the nature of the ES, luminescence can be

classified as phosphorescence and fluorescence. In fluorescence, electron in ES is of

opposite spin as compared to electron in ground state (GS) (Figure 1). The de-

excitation of electron in fluorescence is spin allowed with an emission rate of 10-8 s.1

The substance exhibiting fluorescence is called fluorophore. The fabrication of

fluorophores in the contemporary times is in great demand because of its great

significance in the development of solar cells,2 field effect transistors,3 non-linear

optical materials,4 fluorescent biosensors,5 medicine,6 and optoelectronic devices.7 The

fluorescent dyes emitting in the range of near-infrared regions (650-900 nm) find its use

in bioimaging because of its good penetration in tissues and less interference from

autofluorescence of biomolecules.8

Figure 1: Various transitions processes like Absorption, Fluorescence, Intersystem

Crossing, Internal Conversion, Phosphorescence etc.9

Phosphorescence is phenomenon of emission of radiation due to triplet ESs, where spin

orientation of electron for ES is parallel as compared to the electron in the GS. The

phosphorescence is a spin forbidden transition as the transition occurs from the triplet

state (TS) to the GS with emission rates of 103 to 100 s–1 and lifetimes varying from 10-6

to 100 s.1 Since this transition involves the ISC from singlet state (SS) to TS it has more

Stokes shift as compared to the fluorescence. The phosphorescence is rarely observed in

purely organic compounds because of the other accessible de-excitation processes.

Therefore, incorporation of heavy atoms like halogens and transition metals in the

molecules is used to observe phosphorescence in the molecule. The design of organic

luminophores is limited because the spin change in ISC is forbidden.10 The

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phosphorescence is mostly limited to inorganic and organometallic compounds because

of the presence of heavy metals atoms which facilitates the intersystem crossing (ISC)

because of spin orbit interactions.11

Occasionally, ISC is found in pure organic luminophores by the presence of some

functional groups like carbonyl or some heteroatoms. The presence of quenchers like

oxygen in the environment effectively quenches phosphorescence because of the

durability of triplet excitons. Recently the crystallization induced phosphorescence

observed in purely organic luminophores like benzophenone which usually does not

show phosphorescence in amorphous and solution forms. The crystallization locks the

conformations and thus minimizing the loss of non-radiative transitions and thus

enhancing the emission of phosphorescence.10 Room temperature phosphorescence has

produced widespread interest due to its fascinating applications and exceptional

photophysical properties.12

1.2. Delayed fluorescence

Thermally activated delayed fluorescence is type of emissive transition that results

from exciton transition from S1 state that leads to ISC, delayed emission and has

prolonged lifetime as compared to normal fluorescence.13 The motive behind TADF is

to harvest both triplet as well as singlet excitons in fluorescence. Reverse ISC (Figure

1) from T1 to S1 creates ultimate ES of excitons which is responsible for TADF

phenomena. Reverse inter-crossing in the TADF process is an endothermic because the

SS has greater energy as contrasted to TS as a result of which it becomes highly

emissive at higher temperatures. In 2011 Adachi et al found the TADF while working

with pure organic molecules and it was a revolution in the development of OLEDs on

TADF phenomenon.14 It was observed that the energy difference of SS and TS is

reducible by minimising overlapping of HOMO-LUMO orbitals. This resulted in

chromophore expansion with twisted electron donor–acceptor (D–A) architecture

initiated by steric hindrance of bulky groups and leads to proficient TADF.9

Luminophores based on TADF now are categorised as third generation emitters.

1.3. Triplet-triplet annihilation

The second method to harvest triplet excitons is by triplet-triplet annihilation (TTA)

which exploits a bimolecular phenomenon of two triplet excitons. In the pleasing

method, donor initially excites to SS that results in ISC to generate TS. This is followed

by TTET amongst donor and acceptor resulting in enrichment of acceptor in TS.

Sequentially, the acceptors in TS can simply interact via TTA. The interactions result in

the formation of one acceptor in ES S1 for emission and another in GS S0 (Figure 2).

Since Triplet excitons are longer lived, therefore, the two TSs can easily interact with

each other. The manifestation of a very small gap between TS and SSs in TADF

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fluorophores gives 100 % internal quantum efficiency because both triplets, as well as

SSs, are converted into photons under electrical excitation.

Figure 2: Mechanism of triplet-triplet annihilation showing Triplet Triplet Energy

Transfer (TTET), Up-Conversion (UC).13

1.4. Solvatochromism

The absorption of compounds is greatly influenced by the solvents because of different

interactions like hydrogen bonding dipolar-dipolar interactions between solute and

solvent molecules. The phenomenon is ubiquitous in the field of chemistry and is

commonly called as solvatochromism. The shift towards the blue region (hypsochromic

shift) on rising the polarity of solvent termed as negative solvatochromism while the

shift towards the red region (bathochromic shift) is termed as positive solvatochromism.

These effects are observed due to differential solvation of ground or ES.15

Solvatochromism probes are helpful in indicating the surrounding medium which

greatly affects the emission energy, quantum yield and emission lifetime. Generally, the

solvatochromic chromophore design depends on CT due to D-A interactions within the

fluorophore.16 During excitations, the charge transfer results in different electron

distribution from that of the GS as a result of this solvent molecules rearrange

themselves to stabilize the ES which is widely known as solvent relaxation.

The recent phenomenon of dual emission in nonpolar solvents by anion migration is

quite fascinating. During ICT in donor acceptor systems the CT from donor to acceptor

is stabilised by polar solvent molecules. While in nonpolar solvents the solvent

molecules are not able to stabilise, the charge developed at the donor moiety as a result

of which there is migration of counterion from acceptor side to donor side which paves

way to dual emission. The stokes shift in the case of nonpolar solvent is greater than in

polar solvents because of the destabilisation of GS by nonpolar solvent molecules. This

dual emission can be described as a molecular machine in which there is the relocation

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of cation and anion. Moreover, the emission can be significantly affected by extension

of conjugated system, viscosity of medium and nature of counterion.17

1.5. ICT and Dual emission

The intramolecular transfer of charge originating from electron abundant group (Donor)

to electron scarce group (Acceptor) is termed as the ICT process. This is usually

observed under photoexcitation when the electron is transferred from one part to another

part of the molecule and charge distribution is entirely different from that of the GS.

The CT is possible when the Donor and Acceptor are connected through

conjugation.18. The charge shift through space is also reported but ICT literature is

mostly devoted to the molecules with Donor Acceptor system connected by conjugated

systems. The commonly used donors to synthesise ICT molecules are NR2, NH2, OH

etc. while CHO, NO2 and +PR3 groups have been utilized as acceptors. During ICT

phenomenon there is drastic change in the charge distribution resulting in the formation

of ICT species with high dipolar moment as compared to its GS. This is substantiated

by the fact that ICT proceeds faster in polar solvent media and at the same the ICT

species shows red emission with increasing solvent polarity. However, it can be noted

not all ICT species show red shift upon photoexcitation. Species with zwitter ionic GS

show blue shift because the ES is less polar. Some of the ICT species show dual

emission and these two bands are attributed to locally excited towards blue end of the

emission region and one due to ICT at the red end. The ICT based luminophores are

used as sensors to detect various metal ions like Hg, Cu, Zn, etc.19,20,21 They also find

use in recognition of protein with high sensitivity and in protein staining.22

The dual emission was first reported by the Lippert and co-workers in 4-N,N-

dimethylaminobenzonitrile (DMABN) (Figure 3) for the first time in 1962. The ICT

fluorescence is usually observed in polar solvents to stabilize the ES.23 To illustrate this

unusual behaviour various models were proposed but among those twisted and planar

ICT are widely accepted ones. The former was proposed by Grabowski in which the

initially ES twists its dialkylamine group from a planer to nearly perpendicular

alignment as compared to benzonitrile system to yield another energy minimum along

with ICT from donor to acceptor substituents in the ES.

Figure 3: Chemical structure of (DMABN).

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The emission bands for DMABN in nonpolar solvent like cyclohexane shows one band

was observed at 342 nm independent upon excitation is attributed to locally ES. While,

in polar solvents like acetonitrile shows two bands with max emission at wavelength of

353 and 469 nm attributed to locally ES and CT respectively. Moreover, the emission

bands depend upon the excitation frequencies thus indicating that these two bands

originate from two different electronic state.24 Our work was also devoted to synthesise

fluorophores based on ICT in which strong electron acceptor +PR3 acts as a acceptor

moiety while NPh2 acts as donor moiety connected by biphenyl system.

1.6. Phosphorus

Phosphorous is a p-block element belonging to the Pnictogen group (VA) of periodic

table. It is represented by atomic symbol, P, with atomic number 15 and exists as stable

isotope 31P having nuclear spin of ½ and thus is NMR active. Phosphorus has several

isotopic forms ranging from 24P to 47P but 31P is the only stable isotope. The atomic

weight of Phosphorous is 30.974 g/mol. The electronic configuration of the

Phosphorous atom can be written as 1s22s22p63s23p3, the electronic configuration of the

phosphorus shows that phosphorus is mostly trivalent or pentavalent.

1.7. Organophosphorus Compounds

The phosphorus is ubiquitous in all living organisms in the form of DNA, RNA,

phospholipids and energy carrier molecules like ATP. The applications of phosphorus

compounds can be enlisted below.

1.7.1. Applications in medicine

Organophosphorus compounds have been widely used in medicine and pharmacology

fields like bisphosphonates (olpadronic), N,N′,N′′-Triethylenethiophosphoramide

(thioTEPA) and combretastatin A-4phosphate (CA4P) (Figure 4). Olpadronic acid is

used as significant and efficient drug for the medication of bone disorders like

osteoporosis and Paget’s disease.6 The bisphosphonates are competent inhibitors against

tumour induced bone destruction, and considerably minimize risk for skeletal

deformation and difficulties in bone metastasis patients because of various type of

tumours.

Organophosphorus compound, thioTEPA has application as anticancer drug that

inhibits cell duplication of cancerous cells upon binding to DNA.25 It was designed in

mid of 20th century and has extensive range of antitumor activity. Moreover, it is

considered one of the most effective drugs in treating breast cancer, ovarian cancer,

lymphosarcoma and Hodgkin’s disease. CA4P is additional drug used in cancer

treatment fabricated to damage the blood vessels of cancer tumours and cause central

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necrosis.6 Triphenylphosphine cations are widely used to provide antioxidants, probes

and pharmacophores to mitochondria.26

Figure 4: Organophosphorus compounds used in medicine.

1.7.2. Agricultural and industrial application

Phosphorous based organic compounds have also extensive applications in agricultural

and industrial fields. Organo-phosphorous based pesticides are broadly used (38% of

total pesticide) to destroy pests in agriculture and residential society.27 In 1854, first

potential pesticide, tetraethyl pyrophosphate was prepared. Later, Germans synthesized

Organophosphate nerve agents including tabun, sarin and soman (Figure 5) which are

considered among the most toxic warfare agents.28

Figure 5: Organophosphate nerve agents.

Organophosphorus compounds (OPCs) with different functional groups like, esters,

phosphonates and phosphites, can be useful as additives, stabilizers, flame retardants (in

plastics, textiles, building materials), plasticizers, antifoaming and wetting agents.

Tris(2-butoxyethyl) phosphate (TBEP) finds its use for flame retardants and plasticizers

and Tris(2-chloroethyl) phosphite (CLP) as flame retardant, stabilizer and in hydraulic

fluids (Figure 6).29

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Figure 6: Structures of TBEP and CLP.

1.7.3. Applications in organic synthesis

Organophosphorus compounds are extensively utilized in production of organic and

organometallic compounds. Phosphorous ylide is a distinct class of zwitterions that

contain electron rich carbon ions (nucleophilic carbon), also called ylidic carbon which

is responsible for mechanistic properties and their use as Wittig reagents. This active

compound shows a broad range of reactions due to exceptional molecular and electronic

structure. As reagents, they have gotten immense importance for linking synthetic

building blocks by forming C=C double bonds, that provoked more attention for

synthesis and applications of Wittig reagents and their derivatives.30

High reactivity of tervalent phosphorus compounds (such as trialkyl phosphites or

triarylphosphines) is due to their ability to expand their valence shell, to form strong

bonds with C, N, O and their nucleophilic character. For that reason, tervalent

phosphorus compounds have gained great interest as reagent in various organic

synthesis. Some organic synthesis reactions are, desulphurization (Synthesis scheme 1),

deoxygenation and reaction with halogen-containing materials, all of which are very

central in organic chemistry.31

Synthesis scheme 1: Desulphurization by organophosphorus compounds.

The BINAP (Figure 7) metal complexes like Rh and Ru have been utilised in

asymmetric synthesis like asymmetric hydrogenation of alkenes, asymmetric

hydrogenation of ketones and enantioselective allylic hydrogen shift.32

13

Figure 7: Structure of BINAP used in asymmetric synthesis.

Nomenclature Organic compounds containing phosphorus represents organophosphorus compounds.

These compounds are holds very important place in Chemistry because of its very

diverse applications. Organophosphorus compounds are defined by coordination

number and by their valency (Figure ) The phosphorus usually exists in its

trivalent phosphorus with coordination number 3 represented by There are very

less phosphines reported in literature as emitters due to quenching of luminescence by

the presence of lone pair on phosphines which is partially involved in conjugation.33,34

The presence of lone pair in trivalent phosphorus can be used to tune the photophysical

properties by oxidation, alkylation or complexation.34,35 phosphorus with coordination

number 4 is represented by . The other bonding modes of phosphorus are

represented by (sp2) and (sp).36

Figure 8: Organophosphorus compounds with different valency.

The presence of lone pair in the trivalent phosphorus atoms acts as a typical donor in

organophosphorus compounds however oxidation to pentavalent phosphoryl drastically

changes its properties and reverses its properties to an electron acceptor. The

phosphoryl compounds are denoted by the double bond between phosphorus and E

(whereas E = O, NR2, S, Se, or CR2). The phosphoryl bond is strongly polarised as

shown by resonating structures below (Figure 9).37

14

The phosphoryl bond is strongly polarised. The presence of electron pair on E is

responsible for negative hyperconjugation in which the back bonding donates in the σ*

molecular orbital of phosphorus, this communication strongly affects the photophysical

behaviour of phosphoryl compounds.38 In the case of positive hyperconjugation, the

electrons are donated from σ orbital to empty orbital. The stabilization of the orbitals

is represented (Figure 10).39

The increase in the electronegativity causes an electron-withdrawing effect relative to

the trivalent phosphorus atom. The stabilization of orbitals has been used in fabricating

OLED’s. The high-energy inversion barrier of phosphorus even in aromatic rings as

compared to nitrogen facilitates negative hyperconjugation and unfolds pronounced

optoelectronic properties. The presence of lone pair on trivalent phosphorus is available

for additional functionalization which basically creates one σ* antibonding orbital for

other hyperconjugation interaction with the conjugated core thus increasing electron-

accepting properties.37

Figure 9: Showing the effect of polarisation.

Figure 10: Orbital interaction between sigma and orbitals.

15

Over the year’s different types of organophosphorus luminophores have been

engineered to meet the ever-growing demands because of their remarkable properties.

The organic luminophores have been classified into cyclic and acyclic compounds.

1.8. Cyclic organophosphorus chromophores

1.8.1. Five membered heterocycles based on phosphorus

The phosphole unlike other heterocycles shares some different properties based upon its

structure. The presence of phosphorus atom in 5 phosphole ring having pyramidal shape

with a very large energy barrier makes it very less aromatic. The presence of P-R bond

in phosphole results in the σ- hyperconjugation with the endocyclic diene system and

exocyclic P-R bond.40 The phosphorus atom can be further functionalized by the

presence of lone pair on phosphorus atom since it does not interact much with the

endocyclic system of phosphole, so it remains accessible for further modification like

oxidation (Se, S and O), alkylation (CH3) and coordination with a varied choice of

transition metals. These modifications give rise to different optoelectronic properties.

The pyramidal shape of phosphorus creates a steric hindrance preventing the −stacking

in the solid-state thus facilitates solid-state emission.

Phospholes were synthesised in 1959,41 and their material application importantly

became subject of study from last two decades. The chemically synthesised the 2,5-

diarylphosphole by Reau et al and probability of tailoring opto-electronic properties of

phospholes by modifying substituents on the phosphole ring or substituents directly

attached to phosphorus (Figure 11) was breakthrough in designing various devices

based on principle of luminescence.42

Figure 11: Compounds synthesised by Reau and co-workers.42

The oligomer compound 2 showed absorption maximum max = 427nm in UV/vis region

clearly longer wavelength than 2,5-bis(2-dipyridyl) phosphole max =390nm which

indicates the extended conjugation in compound 2. The fluorescence spectrum of this

oligomer showed emission maximum at 570 nm reflecting large stokes shift while the

16

compound 1 shows absorption maximum at 412 nm slightly shorter than 2 but

considerably longer than the electron rich pyrrole rings like compound 1 and showed

emission maximum at 501 nm. The Reau and co-workers studied the cationic

phospholes to study impact of electron density on opto-electronic behaviour of the

compounds. The group succeeded experimentally and concluded that the oxidation of

phosphorus atom causes bathochromic shift in the emission spectroscopy as reflected by

the UV/vis absorption of compound 3 (max = 442 nm) while emission maximum at 593

nm.

In 2004 T. Baumgartner and his group extended this work by incorporating phospholes

into polymer built on polystyrene and modified the dithianophospholes (Synthesis

scheme 2) by further functionalising the existing phospholes and their effect on

optoelectronic properties. The introduction of these novel compounds showed highly

luminescent properties. The compound 4 shows absorption maximum at max =338 nm

and emission spectrum at 415 nm. The compound 4 was further functionalised by

attaching the silyl group directly to the phosphorus atom. It was interesting to find that

dithianophosphole derivative monomer 6 showed red shift as compared to 4 (10–15 nm)

for the absorption and emission bands(ex =352, em =422 nm) contrasted to 4 (ex

=338, em =415 nm) and 5 (ex = 336 nm em 408 nm) thus displays that silyl group acts

as an electron acceptor.

The similar style is followed by the compounds bearing borane adducts and showing red

shift about 10 nm and phosphole oxides which are also electron acceptors give rise to

bathochromic shift nearly 30 nm. Most importantly these compounds showed high

quantum yield ranging from 55 to 90 percent which is unprecedented for phospholes.

The compounds synthesised exhibited intense photoluminescence particularly by the

boron and oxygen adducts.43

Synthesis scheme 2: Synthesis of dithianophospholes precursors and derivatives. a)

2nBuLi, R’PCl2, Et2O, -78 °C RT (4: R=H, R’=Ph, 5: R=H, R’=4-tBuC6H4); b) 2n-

BuLi, (4-vinylphenyl)PCl2, TMEDA, Et2O, -78oRT (6: R=Sit-BuMe2, R’=4-

vinylphenyl); c) BH3·SMe2 (1m in CH2Cl2), CH2Cl2, RTE=BH3 ; 7: R=H, R’=Ph,8:

R=H, R’=4-tBuC6H4); e) H2O2 (30% in H2O), pentane, RT (E=O;9: R=H, R’=Ph,10:

R=H, R’=4- tBuC6H4.43

17

In 2008 the same group modified the parent dithianophosphole and by extending

conjugation on both sides of thiophene rings with the help of benzene and studied its

changes on the photophysical behaviour. The Benzannelated phosphole 11 displayed

(Synthesis scheme 3) as bright blue luminescence due to its extended conjugation at

em = 440 nm in solution as compared to parent dithianophosphole 4 (415 nm) with high

quantum yield 63%. The phosphole 11 also shows peculiar florescence in solid state

with bright green emission (ex =450, em =512 nm).44

Synthesis scheme 3: Benzannelated phosphole.44

The oxidation of phosphorus atom by H2O2 and S results in the formation of compound

12 and 13 respectively. The absorption and emission bands of 13 (ex = 407 nm and em

= 461 nm) show In between 11 and 12. The parent phosphole 11 was further modified

by the complexation with [Au(tht)Cl] where tht is tetrahydrothiophene in DCM at RT

resulting in compound 14 with 56 % yield (Synthesis scheme 4). The fluorescence

emission spectra of the compound 14 shows red shift in absorption and emission spectra

(ex = 398 nm and em = 471 nm).

Synthesis scheme 4: Compounds synthesised by T. Baumgartner and his group.44

In 2017 the T. Baumgartner and Z. Wang further extended the research on

dithienophospholes and modified the existing scaffold by reaction with aniline and

different hydroxypyridines to construct phospholes with different optoelectronic

properties. The pyridines can be further modified by complexation with metals or

18

methylated to enhance electron acceptor character of central framework. Reactions

between 2- and 3-hydroxypyridines with p-chloro dithienophosphole oxide gave good

results while the 4-hydroxypyridine didn’t reacted well because of the 4-

hydroxypyridine exist as tautomeric pair in the solution. The triethylamine was used in

to scavenge the produced HCl and shift the reaction towards the products 22. The

reaction scheme for the compounds mentioned is depicted (Synthesis scheme 5).

Synthesis scheme 5: Dithienophospholes derivatives based on pyridines.45

The quantum yields of 18a and 18b are very high (up to 92% in DCM solution). To date

these are most efficient fluorescence quantum yields for dithienophospholes. The

photophysical behaviour of anilinyl-phosphole 17 are different then the

alkylaminophospholes reported in literature.46 The most aminophospholes reported are

extremely emissive in solution while 17 gives minimal emission (L=) However, 17

in solid form shows limited fluorescence with higher quantum yields (L = 5%) as

compared to the solution (Figure 12). This effect is called aggregation induced

enhanced emission (AIEE) contrary to the reported aggregation caused quenching

(ACQ) for aminophospholes.

19

Figure 12: Emission spectra of compounds 18a, 18b and 17.45

1.8.2. Six membered heterocycles based on phosphorus

It is now widely known that the introduction of electron donors in the system leads to

electron repulsions and thus increasing the HOMO energy thus increases the p-type

character while the incorporation of electron-accepting groups to the scaffolds decreases

the LUMO energy and thus increases the n-type character. The intriguing properties of

phosphorus are remarkable it not only alters the properties of the system but can be

further modified because of the presence of lone pair. The six-membered heterocycle

phosphinine an analogue of benzene was first time reported in late 1960s and its fusion

with the system thus came after it. 2,4,6-triphenylphosphinine (Figure 13), was first

time synthesized by Markl (1966) that epitomizes breakthrough in organophosphorus

field. This phosphorus aromatic heterocycle in low coordination state exhibits

considerably variable electronic characteristics with respect to nitrogen-containing

compounds (pyridine). Whereas, unsubstituted phosphinine (Figure 13), was prepared

later in 1971 by Ashe.47

Figure 13: 2,4,6-triphenylphosphinine and phosphinine.

20

1.8.3. Three Linearly Fused Six-Membered Rings analogues of anthracene

Interestingly these phosphines were reported before phosphonapthalenes, the first

compound in phosphoanthracenes 19 (Figure 14) was reported first time in 1967.48 A

year later Bickelhaupt and de Koe substituted this at position 10 with phenyl ring.49 The

research on such systems was further carried by modifying the existing derivatives.

Recently Ito, Mikami et al incorporated CF3 groups at 4 and 5 positions of

phosphoantharacene (20a ,20b and 20c) (Figure 14), enhancing the electron

withdrawing nature and thus enhancing the n character of such systems of phosphorus

with endo system.50 The compound 20a and 20b showed identical emission (L =

0.13) while 20c shows negligible emission (L = 0.001). Additionally the compound 20c

shows red emission shift (em = 583 nm ) as compared to 20a (em = 508 nm) and 20b

(em = 511 nm).

Figure 14: Showing substituted phosphonapthalene and compounds synthesised by

Mikami.51

The phosphoanthracene molecules with substitutions at 3,6 positions have been

extensively studied for their optoelectronic properties. Wang et al in 2015 studied the

rhodamine scaffold by replacing oxygen with phosphorus atom.52 These

phosphoanthracenes with diethylamino at 3,6 positions and −endo and − exo

phosphorus centre the compounds are called phospharhodamines (Figure 15). The

introduction of phosphorus in rhodamines exhibits red shift 140-40 nm contrasted to

oxygen and sulfur analogue rhodamines. The presence of P=O with high inductive

effect and the hyperconjugation by methyl group lowers the LUMO in 21a and 21b. 52

In 2018 Chang et al. studied phospharhodol 22 (Figure 15), related photophysical

behaviour along with silicon and carbon analogue. The thioether fragment attached with

the central ring system is used to detect copper ions. The compound exhibited the

bathochromic shift in absorption (ex = 569 nm) and in emission (em = 679 nm) in

comparison with carbon (ex = 516 nm, ex = 608 nm) and silicon analogue (ex = 568

nm, em = 638 nm).53 while the photophysical properties change on complexation with

copper ions. The 22 when complexed with copper showed absorption at (ex = 654 nm)

while it displayed emission band at (em = 680 nm) there by indicating a significant

bathochromic shift in absorption bands as compared to 22 without complexation.

21

Figure 15: Compounds synthesised by wang et al (21a and 21b) 52 and Chang et al

(22).53

1.8.4. Four Linearly Fused Six-Membered Rings

In 2017 Aiko Fukazawa further modified the xanthene-based dyes with phosphorus

oxide groups like phospha-fluoresceins (POF’s). The presence of P=O group in the

flouresceins decreases the PKa value (5.7) as compared to parent fluorescein (6.2). The

POF-Me 23 based analogue seminaphtho-phospha-fluorescein (SNAP- Me2) 24 was

synthesised (Figure 16) and the photophysical properties were studied. The

photophysical characteristics of 23 and 24 were studied in the neutral solution with 1%

DMSO acting as co-solvent. Both 23 and 24 exhibited similar PH dependent

fluorescence. The absorption spectrum of 23 showed (ex = 488 nm) reduced while

raising PH for solution. The PH of the solution was raised from highly acidic with

PH=3 to basic solution with PH=11 new emission maxima appeared at (ex = 628 nm),

with quantum yield (L=) This bathochromic shift was assigned to anionic form of

POF-Me2. The 24 exhibited the absorption band (ex = 654 nm) showing bathochromic

shift as compared to 23 due to extended conjugation, with quantum yields (L=0.03).54

22

Figure 16: Compounds synthesised by Aiko Fukazawa.

1.8.5. Phosphaphenalenes based luminophores

In 2015 Romero-Nieto and his group merged phosphorus heterocycles to develop

innovative phosphorus systems having extraordinary stability and extended conjugation

having , −endo −exo configuration.55 The novel phosphaphenalenes were

synthesised by treatment of lithiated derivatives with dichlorophenylphosphane via

intermolecular aromatic substitution. The trivalent phosphorus compounds were further

oxidised by hydrogen peroxide to yield pentavalent phosphorus oxide with

, −endo −exo configuration. The synthetic procedure is given (Synthesis

scheme 6).

Synthesis scheme 6: Reaction scheme by Romero-Nieto to yield fused heterocycles

with phosphorus.

All phosphaphenalenes (Figure 17, 18) absorb in UV region while the emission bands

were found in violet-blue region. The fusion of the aryl moieties with the main

phosphaphenalene framework greatly affects emission and absorption bands at the same

time quantum yield. The absorption, emission and quantum yield for 25 were found to

be 344 and 402 nm and 0.04 respectively. While for 26 it was found to be 335 and 399

nm and 0.12. For compound 27 it was found to be 357 and 420 nm and 0.2 respectively.

23

Figure 17: The compounds reported by Romero-Nieto and his group.55

The fusion of electron rich rings with the phenalenes exhibits red shift in absorption

maxima and emission maxima. The order follows as 27 > 28 > 29 >30. It is important

to mention here that pyrrole containing phenalene 30 exhibits the highest quantum

yields (0.8). on the other side the opposite results were found when electron deficient

rings like pyridine were fused with phenalene, 31 displayed blue shift with the emission

band at 383 nm. The optoelectronic properties can be further modified by

functionalisation of phosphorus atom. The quantum yield of 32 decreased to 0.001 as

compared to all the phosphaphenalenes synthesised.

Figure 18: The compounds reported by Romero-Nieto and his group.55

1.8.6. Six linearly fused six membered rings

Successful synthesis of phosphaphenalenes and their optoelectronic properties

motivated them to develop more complex molecules. In 2017 C. Romero-Nieto and his

group developed more complex chromophores having six linearly merged rings with

phosphorus heterocycles. The idea was to study it effect on the optoelectronic

properties. They envisaged to develop new diphosphahexaarenes developed from 6

linearly merged with phosphorus heterocycle centres. The diphosphahexaarenes were

developed by careful Stille coupling to form C-C bonds by reacting triflate analogues

dibromotriflate A and B with Sn functionalised bromonapthalene. The tetrabromo

derivatives were lithiated and subsequently reacted with dichlorophenylphosphane

24

which was followed by oxidation of phosphorus by treatment with H2O2 to give syn 33

and anti 34 derivatives. The synthesis pathway is given below (Synthesis scheme 7).56

Synthesis scheme 7: Compounds synthesised by C. Romero-Nieto a) CsF, Pd(PPh3)4,

CuI, 50 °C/(8-bromonaphthalen-1-yl)tributylstannane; b) 1. tBuLi, –78 °C, 2. PhPCl2,

3. H2O2, 0 °C.

The chromophores reported were observed as air stable and moisture stable equally in

solution and solid state. The new compounds synthesised having configuration

, −endo −exo were observed soluble in different organic solvents including

tetrahydrofuran, dichloromethane, acetonitrile along with chloroform. The compounds

33a and 33b exhibits two absorption bands at 348 and 405 nm. The compounds 34a and

34b displayed absorption bands at 372 nm along a shoulder peak at 388 nm. The 33

exhibits emission maxima at (415, 438 and 463 nm) while 34 exhibits dual emission

bands (420 and 442 nm). The quantum yields for 33a and 33b were found to be 0.80

and 0.85 while for 34a and 34b it was found to be 0.84 and 0.81 respectively. These

novel six membered fused rings with outclass quantum yields and their stability

intrigued to design new novel molecules with such extended conjugation.

25

1.9. Acyclic organophosphorus chromophores

In 1997 David W. Allen synthesised dyes based on phosphonium betaine (Synthesis

scheme 8) all of which showed negative solvatochromism, the absorption band shifted

to shorter wavelength on moving from nonpolar to polar solvents.57 The compounds

were synthesised by using NiBr2 as a catalyst and ethanol acting as solvent.

Synthesis scheme 8: Synthesis of phosphonium betaine.

The compound 35 ( R= Ph, X =Ph) existed as green (toluene, max = 686 nm) blue

(acetone, max = 604 nm) purple in ( DCM, max = 596 nm) violet (acetonitrile, max =

562 nm) and red (MeOH, max = 498 nm). The compound 36 (R = Ph, X = But) showed

(max = 662 nm) in tetrahydrofuran, (max = 616 nm) in dichloromethane (max =

598 nm) in acetonitrile and (max = 532 nm) in methanol. The hypsochromic shift is

attributed to the stabilisation of GSs by the polar solvents. The 35 and 36 exists in high

polar GS as compared to less dipolar ESs (Figure 19).57

Figure 19: Compounds reported by David W. Allen.57

Christoph Lambert in 1998 reported compounds 37, 38 and 39 (Figure 20) based on

substituted azobenzene chromophores and their optical properties with a conventional

dialkylamino group as a donor moiety and triorganophosphonium as acceptor moiety.

Three and four azobenzene subchromophores were connected with phosphorus centre in

37 and 38 respectively. The 37 exhibits absorbance at (max = 494 nm) in MeCN while

in CHCl3 (max = 506 nm). The compound 38 absorbance at (max = 499 nm) in MeCN

26

and (max = 511 nm) in CHCl3. The compound 39 displayed absorbance at (max = 483

nm) in MeCN and (max = 491 nm).58 All the compounds displayed moderate negative

solvatochromism. This phenomenon can be attribute to stabilisation of GS and

destabilisation of ES with increase in polarity of solvent.

Figure 20: Compounds reported by Christoph Lambert.58

In 2000 David W. Allen synthesised compounds based on dipolar aryl-phosphonium

systems and studied their solvatochromism. The compounds were synthesised by

Horner Reaction in which suitable bromoarene was heated with triphenylphosphine to

yield compounds 40, 41 and 42 (Figure 21).59

The 41 was found as pale-yellow powder while 40 was obtained as bright red and were

emissive in dichloromethane. The compounds exhibited negative solvatochromism on

increasing polarity of the solvent. The 40 displayed absorbance at (max = 422 nm) in

MeOH and (max = 442 nm) in DCM. The thienyl based phosphonium salt 41 displayed

smaller solvatochromic effects with absorbance (max = 352 nm) in MEOH and (max =

358 nm) in DCM. The 42 was isolated was orange red and exhibits absorbance having

max-406 nm in MeOH and 422 nm in DCM thus indicating negative solvatochromism.

Figure 21: Compounds reported by David W. Allen.59

27

The coumarin based phosphonium salt 43 (Figure 22) was prepared by Moussa Ali in

2015 which exhibited absorbance at (max = 441 nm) with quantum yield (f = 91%).60

The 44 phosphonium cation derivative Mitochondrial membrane potential (MMP) was

designed on lipophilic Boron-dipyrromethene (BODIPY) backbone (Figure 22) to

probe the mitochondrial function. (BODIPY) based derivatives are widely used

biomedical imaging due to tuneable photophysical properties and biological media

compatibility. 61

Figure 22: Phosphonium modified chromophores based on coumarin and BODIPY.

Gaocan Li synthesised phosphonium salts based on pyrene by quaternization 1-

(bromomethyl) pyrene with trisubstituted phosphine (Synthesis scheme 9) and

consequent anion exchange. The photophysical properties showed that absorption

maxima of 45[Br] (max = 328 nm), while 45[NTf2] displayed (max = 332 nm), 45[PF6]

showed (max = 331 nm) and 45[BF4] showed c) in dichloromethane. The emission

spectra of pristine 44[Br] exhibited at (max = 400 nm), 45[NTf2] exhibited at (max =

464 nm), 45[NTf2] exhibited at (max = 399 nm), 45[BF4] exhibited at (max = 398 nm).

These compounds also exhibited mechanochromic luminescence.62 The 45[Br]

underwent a redshift of emission of 85 nm upon grinding using spatula. After grinding

45[PF6], 45[BF4] displayed similar mechanochromic results as 45[Br] with wavelength

changes of 87 nm and 85 nm, respectively.

Synthesis scheme 9: Compounds reported by Gaocan Li.

28

Gaocan Li in 2016 developed a specific dual-mode switchable mechanochromic

luminophore which unlike previous fluorophores exhibit both red and blue shift of

luminescence. The idea was to develop a fluorophore to enrich the mechanochromic

luminophores and to study the structure-property relationship. The luminophores were

based on the cation anion interaction which prompts an important factor in designing

mechanochromic luminophores. The luminophore designed

tributyl(perylen‐3‐ylmethyl) phosphonium (TPMP) fluorophore, which displays a dual

mode switchable mechanochromic emission at ambient temperature.62

The photophysical behaviour of pure sample of 46 was recorded with different

counterions like Br−, BF4−, PF6− and NTf2− (Figure 23) all exhibited marvellous

luminescent properties in solid state. The absorption maxima of 46[Br], 46[BF4],

46[PF6], and 46[NTf2] in dichloromethane are 446 nm, 447 nm, 447nm and 448nm

respectively. While the emission spectra of pristine samples of 46[Br], 46[BF4], 46[PF6],

and 46[NTf2] are (525 nm, 547 nm, 598 nm and 622 nm, respectively).

Figure 23: Molecular structure of mechanochromic luminophore 46.62

In 2016 Yoshinari Koyanagi synthesised a series of compounds based on thienyl

phospholes and studied the effect of, phosphino Boryl and phosphonio substituents

(Figure 24) on photophysical nature of phospholes. The compound 47 exhibited (max =

434 nm) and (max = 518 nm) in methanol while 48 exhibited absorbance at (max = 434

nm) and emission at (max = 519 nm). The compound 48 exhibited absorbance at (max =

407 nm) and emission at (max = 511 nm) while compound 50 exhibited absorbance at

(max = 416 nm) and emission at (max = 510 nm) in methanol.

The fluorescence exhibited by Ph2MeP+ substituted derivatives were greatly affected by

the counter ion, concentration and solvent. In dichloromethane at lower concentrations

47 is moderately fluorescent and weakly fluorescent at high concentration, while in

methanol it is highly fluorescent. The 48 exhibits high fluorescence at all concentrations

in both solvents.63 The 49 and 50 exhibited moderate fluorescence in both solvents and

showed very less concentration dependences. The lifetime of ESs was supported by

contact ion pairs or solvent dissociated ion pairs in nonpolar solvents in the former case

the lifetime will be manipulated by characteristic quality of anion i.e. the heavy atom

effect.

29

Figure 24: Compounds reported by Yoshinari Koyanagi.63

The photophysical properties of 47, 48, 49 and 50 can be explained by taking two or

three emitting species into consideration. In the case of diphosphonium salts 47 and 48

the equilibria exist between three emitting species with A as contact pair and B as

partially solvent separated and C as fully solvent separated as shown (Figure 25).

Whereas in the case of phosphonium salts 49 and 50 the equilibria exist between

electron pairs D as contact ion pair and E as solvent separated ion pair. Thus, in the case

of A, B and D the iodide ion is tightly paired leading to internal heavy effects on S1

states of phosphonium salts. While in the case of C and E the heavy atom effects are

supressed because the ion pairs are fully separated.

Figure 25: Plausible equilibria of 47 48 49 and 50 .63

30

In 2018 J. N. Gayton et al reported the influence of counterion on Photophysical

Properties of Emissive Indolizine-Cyanine Dyes (Figure 26) in Solution and Solid

State. The Cl ͞ NO3¯, ClO4

¯ were selected as smaller-sized ions. (TFSI), (BARF), (TPB)

were selected as the largest anions to study to the photophysical properties of indolizine

cyanine dye. The sizes were compared by volume computationally, and it was observed

that NO3¯ ≈ Cl < ClO4< PF6 < TFSI << TPB << BARF. while studying molar

absorptivity no significant change was observed in MeCN with different counterions

absorbing at around 810 nm. In less polar solvent DCM, the change was found to be

remarkable ranging from 120000 to 238000 M-1 Cm-1. The dye with larger ions (TFSI),

(BARF), (TPB) displayed higher molar absorptivities as compared to smaller counterion

as shown (Table 1). The different behaviour of counterions in DCM and MeCN can be

explained as the less polar solvent giving stronger ion contact pair while in the case of

highly polar solvent the ions are mostly dissociated so in the former case the tighter ion

pair changes the photophysical behaviour of the fluorophore.64

Table 1: Photophysical data of Indolizine-Cyanine Dyes.64

Figure 26: Indolizine-Cyanine Dyes reported by Gayton et al.

31

In 2019 A. Balyaev developed a series of chromophores in which -R3P+ groups act as an

electron acceptor and diphnylamino -NPh2 group acts as a donor separated by -systems

(Figure 27). The chromophores were synthesised by Nickel catalysed methods.65 The

yields were found to be decreased by increasing the length of oligophenylene and with

extended conjugation probably due to steric hindrance and electronic factors which

might supress oxidative addition to nickel. The series of compounds synthesised have

been described below (Synthesis scheme 10).

Synthesis of nickel catalysed bond formation.

Synthesis scheme 10: Nickle catalysed synthesis of phosphonium salts.66

Figure 27: Compounds reported by Andrei. Belyaev.

The photophysical properties of compounds were recorded in highly polar solvents as

well as in weakly polar solvents. In highly polar solvents the absorption and emission

were recorded in acetonitrile, dichloromethane, methanol, and water. The absorption

maximum was red shifted with increase in −spacer. The 51 exhibited absorbance at

333 nm while 52 at 387 nm, 55 at 407 nm and 56 showed absorbance at 519 nm. It was

observed with further elongation from 52 to 54 no red shift was observed.

In weakly nonpolar solvents the fluorophores displayed dual emission while as the

emission exhibited by fluorophores in polar solvents was exclusively by ICT emission.

The dual emission has been attributed to two bands F1 and F2, high and low energy

bands respectively. Moreover, it was also observed the exchange of bromide ion by a

bulkier triflate ion OTf ¯ considerably quenches the F1 band and enhances the emission

of F2 band. The emission shown by compounds in polar and nonpolar solvents can be

summed up (Figure 28).

32

Figure 28: Emission mechanism in polar and nonpolar solvents.17

33

2. Aims of the study The presented literature review exemplifies diverse applications of organophosphorus

compounds for fabricating photoluminescent materials. The phosphorus moiety

connected to aromatic systems offers optical tuning to design luminophores according

to demand. The trivalent phosphorus can be modified to pentavalent phosphorus thus

tailoring the photophysical properties of organophosphorus compounds. The cyclic

system of organophosphorus luminophores were predominant in the literature as

compared to acyclic systems. Amongst the cyclic 5 membered phosphole derivative

chromophores were found mostly with pendent –PR2=E where E = S, O etc. Whereas

acyclic chromophores were mostly found in the literature with terminal groups like –

R2P=E (E = O, S, and Se) and the strong electron acceptor R3P+ were casually used in

the phosphorus chromophores. Moreover, the nature of the counterion and the nature of

the solvent also affects the photophysical behaviour of chromophores as presented in

the literature.

Figure 29: Phosphonium based fluorophores based on push pull system

The main of the work was to design the donor acceptor system with the NPh2 group

acting as an electron donor with strongly –+PR3 as an electron acceptor. The idea was to

add more chromophores with NPh2 groups connected by biphenyl to phosphonium

centre to investigate the effect on photophysical behaviour, studying the ICT process

depending upon the counterion and the nature of solvent.

34

3. Experimental procedure 3.1. General information

The following reagents Ag(OTf), HCl (38 % in water), n-BuLi (1.6 M in hexane),

B(OMe)3, NiBr2, PPh3, PPh2Cl, PPhCl2, PCl3 and Pd(dba)2 and solvents

dichloromethane (DCM), diethyl ether (Et2O), n-hexane and methanol (MeOH) were

used as received. The tetrahydrofurane (THF) and toluene were dried using Na-

benzophenone ketyl under inert atmosphere of nitrogen. 4-bromo-N,N-

diphenylaniline,67 4-(N,N-diphenylamino)-benzeneboronic acid68 , 4-bromo-N,N-

diphenyl-[1,1-biphenyl]-4-amine68, and 4'-(diphenylphosphino)-N,N-diphenyl-[1,1'-

biphenyl]-4-amine69 (L1) were synthesised according to published literature.70 The

solution 1H, 31P{1H}, 13C{1H} and 1H–1H COSY NMR spectra were recorded on Bruker

Avance 400 and AMX-400 spectrometers. The UV-visible absorption was recorded

using a Hitachi spectrophotometer (U-3310). The reagents used in the synthesis were

dried under vacuum and all the reactions were performed under the nitrogen atmosphere

prior to use. Microanalysis were performed at the analytical laboratory of the University

of Eastern Finland.

3.2. General methods for synthesis of phosphines

4'-bromo-N,N-diphenyl-[1,1'-biphenyl]-4-amine (1.00 eq.) was dried in Schlenk flask

and was dissolved in freshly distilled THF (80 mL), cooled to -80 °C and 1.6 M solution

of n-BuLi in hexanes (1.05 eq.) was added dropwise in 10 minutes to the reaction

mixture. The resulting solution was stirred for one and a half hour at -50 °C and then

was cooled down to -80 °C again and treated with PCl3 (0.33 eq) or PPhCl2 (0.5 eq) and

left for stirring for additional 1hour. The reaction mixture was quenched by methanol (2

mL) and volatiles were evaporated using Schlenk line using warm water bath. The

residue was washed with methanol 3× 20 mL and dried. The crude product was

dissolved in dicholoromethane (10 mL) diluted with hexanes 10 mL and passed through

Sillica gel with eluent DCM: hexane (1:1). Solvents were evaporated in vacuo to give

white foamish precipitate.

Synthesis of phosphine PPh[[(C6H4)2]NPh2]2 (L2)

4'-bromo-N,N-diphenyl-[1,1'-biphenyl]-4-amine (2.17 g, 5.42 mmol, 2.00 eq.), 1.6 M

solution of n-BuLi in hexanes (3.56 mL, 5.69 mmol, 2.05 eq.), the reaction mixture was

treated with neat PPhCl2 (0.48 g, 2.71 mmol, 1.00 eq.) to give white powder L2. Yield

78 %.

35

Synthesis of phosphine P[[(C6H4)2]NPh2]3 (L3)70

4'-bromo-N,N-diphenyl-[1,1'-biphenyl]-4-amine(2.02 g, 5.00 mmol, 1.00 eq.),1.6 M

solution of n-BuLi in hexanes (3.28 mL,5.69 mmol, 1.05 eq.), the reaction mixture was

treated with neat PCl3 (0.20 g, 1.60 mmol, 0.33 eq) to give nearly colorless L3. Yield 71

%. 1H-NMR (400 MHz, CD2Cl2, 298 K; δ): 7.61 (dd, JHH 8.3, 1.3 Hz, 6H, P-C6H4-

C6H4-N), 7.53 (dm, JHH 8.7 Hz, 6H, P-C6H4-C6H4-N), 7.46 (dd, JHH 8.3 Hz, JHP 7.6 Hz,

6H, P-C6H4-C6H4-N), 7.30 (dd, JHH 8.5, 7.3 Hz,12H, meta-H Ph), 7.14(dd, JHH 8.5, 1.2

Hz, 12H, ortho-H Ph), 7.13 (d, JHH 8.7 Hz, 6H, PC6H4-C6H4-N), 7.08 (t, JHH 7.3 Hz, 6H,

para-H Ph). NMR data identical to previously published.70

3.3. General method for the synthesis of phoshonium salts (21-24[Br]) using NiBr2 as a catalyst66

4-bromo-N,N-diphenyl-[1,1-biphenyl]-4-amine ( 1.00 eq.), phosphine (L1/L2/L3) (

1.20 eq.) and nickel bromide (0.20 eq.) were placed in tube. To this mixture ethylene

glycol (3 ml) and magnetic stir bar was added in the tube, the mixture was degassed

with nitrogen for 15 minutes, then the tube was sealed with Teflon cap. The sealed tube

was heated to180 oC in an oil bath for two days. The reaction mixture was diluted with

DCM and washed with water (3× 80 mL). The organic layer was dried under anhydrous

sodium sulphate for 30 min and dried in vacuo. The residue was purified by column

chromatography (Silica gel 70-230 mesh, ⌀3×20 cm, eluent dichloromethane-methanol,

99:1→94:6 v/v mixture) to afford compounds 21-24 [Br]

Synthesis of 4-(diphenyamino)-[1,1-biphenyl]-4-yl)triphenylphosphonium

bromide 21[Br].

4-bromo-N,N-diphenyl-[1,1-biphenyl]-4-amine (0.30 g, 0.75 mmol, 1.00 eq.), triphenyl

phosphine (PPh3) (0.20 g, 0.90 mmol, 1.20 eq.) and nickel bromide (0.07 g, 0.32 mmol,

0.2 eq.) were used as a starting material to afford 21[Br]. Greenish precipitate yield

65%. 1H-NMR (400 MHz, CDCl3, 298 K; δ): 7.91–7.95(m, para-H PPh3+ and -biph-,

5H), 7.78 (td, JHH = 7.9, 3.5 Hz, meta-H PPh3+, 6H), 7.63–7.71(m, ortho-H PPh3

+ and -

biph-, 8H), 7.54 (d, JHH = 8.6 Hz, -biph-, 2H), 7.30 (dd, JHH = 7.7 Hz, meta-H NPh2,

4H), 7.11–7.16 (m, ortho-H NPh2 and -biph-, 6H), 7.09 (t, JHH = 7.7 Hz, para-HNPh2,

2H).31P-NMR (162 MHz, CHCl3 298 K) 23.5 (s, 1P, PPh3+). NMR and ESI+ data

similar to previously reported data.17

36

Synthesis of 4-(diphenyamino)-[1,1-biphenyl]-4-yl)diphenylphosphonium

bromide22[Br]

4-(diphenylphosphino)-N,N-diphenyl-[1,1-biphenyl]-4-amine (0.40 g, 0.79 mmol, 1.10

eq.), 4-bromo-N,N-diphenyl-[1,1 biphenyl]-4- amine (0.30 g, 0.72 mmol, 1.00 eq.) and

nickel bromide (0.06 g, 0.46 mmol) were used as starting material to afford 22[Br]. The

yield of the product was 50%. 1H-NMR (400 MHz, CH2Cl2, 298 K) δ 8.05 – 7.92 (m,

para-H PPh2+ and -biph-,6H), 7.83 (d, J = 7.9, Hz, ortho-H PPh2

+ 4H), 7.78 – 7.66 (-

biph-, 8H), 7.66 – 7.55 (d, J = 8.7 Hz, PPh2+ -, 4H), 7.34 (dd, J = 9.7, 5.8, 2.2 Hz, meta-

H NPh2 8H), 7.24 – 7.08 (m, para , ortho-H NPh2 and biph 16H). 13C-NMR (101 MHz,

CH2Cl2, 298 K) δ 149.88 , 148.26 , 147.63 , 136.20 , 135.50 (d, J = 10.8 Hz), 135.00 (d,

J = 10.4 Hz), 132.87 – 130.81 (m), 130.06 , 128.61 (d, J = 15.0 Hz), 125.82 , 124.50 ,

122.97. ). 31P-NMR (162 MHz, CH2Cl2-d2) δ 23.13. Anal. Calcd for C60H46BrN2P: C,

79.55; H, 5.12; N, 3.09. Found: C, 79.28; H, 4.99; N, 2.87. ESI-MS (m/z): [M]+

825.3406 (calcd 825.3399).

Synthesis of tris(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)(phenylphosphoni-

um bromide 23[Br]

L2 (0.23 g, 0.31 mmol,1.0 eq.), 4'-bromo-N,N-diphenyl-[1,1'-biphenyl]-4-amine(0.12 g,

0.31 mmol, 1.0 eq.), and nickel bromide (0.05 g, 0.06 mmol, 0.20 eq.) were used as

starting material to afford 23[Br]. The yield of the compound was 54%. 1H-NMR (400

MHz, CH2Cl2 298 K) δ 8.05 – 7.90 (m, para-H PPh+ and -biph 7H), 7.84 (d, J = 7.8,

3.6 Hz, ortho-H PPh+ 2H), 7.82 – 7.68 (m, biph and meta-H PPh+ 8H), 7.68 – 7.52 (m -

biph 6H), 7.44 – 7.28 (m, meta-H NPh2 12H), 7.26 – 7.07 (m ortho/para NPh2 and -

biph 24H). 13C-NMR (101 MHz, CH2Cl2 298 K) δ 149.87 , 148.22 , 147.64 , 135.49 (d,

J = 10.7 Hz), 134.98 (d, J = 10.4 Hz), 131.18 (d, J = 12.8 Hz), 130.07 , 128.61 (d, J =

16.5 Hz), 125.82 , 124.51 , 122.99 , 115.38 (d, J = 92.7 Hz). 31P-NMR (162 MHz,

CH2Cl2 298 K) δ 22.93. Anal. Calcd for C78H59BrN3P: C, 81.52; H, 5.17; N, 3.66.

Found: C, 81.75; H, 5.00; N, 3.49. ESI-MS (m/z): [M]+ 1068.4422 (calcd 1068.4447).

Synthesis of tetrakis(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl) phosphonium

bromide 24[Br].

4'-bromo-N,N-diphenyl-[1,1'-biphenyl]-4-amine (0.87 g, 0.87 mmol, 1.00 eq.) L3 (0.35

g, 0.87 mmol, 1.00 eq.) and nickel bromide (0.05 g, 0.17 mmol, 0.20 eq.) were used as

starting material to afford 24[Br]. The yield was 20%.1H-NMR (400 MHz, CH2Cl2 298

K) δ 7.98 (d, J = 8.4 Hz, biph- 8H), 7.76 (d, J = 8.3 Hz, biph-8H), 7.67 – 7.53 (d, 8.8 Hz

biph- 8H), 7.43 – 7.28 (m, meta-NPH2 16H), 7.22 – 7.03 (m, para, ortho HNPh2 and

biph-32H). 13C-NMR (101 MHz, CH2Cl2 298 K) δ 149.86, 147.64, 135.46 (d, J = 10.8

Hz), 131.28, 130.07, 129.23 – 128.10 (m), 125.82, 124.51, 123.00, 116.15. 31P-NMR

(162 MHz CH2Cl2 298 K) δ 22.61. Anal. Calcd for C96H72BrN4P: C, 82.80; H, 5.21; N,

37

4.02. Found: C, 82.97; H, 5.01; N, 4.11. ESI-MS (m/z): [M]+ 1312.5511 (calcd

1312.5527).

Synthesi of bis(4-(diphenylamino)-[1,1-biphenyl]-4-yl)diphenylphosphoni-

um bromide 22[Br] with palladium catalyst.65

4-(diphenylphosphino)-N,N-diphenyl-[1,1-biphenyl]-4-amine (0.20 g, 0.39 mmol, 1.10

eq.), 4-bromo-N,N-diphenyl-[1,1-biphenyl]-4-amine (0.40g, 0.36 mmol, 1.00 eq.)

[Bis(dibenzylideneacetone)palladium] (0.01 g, 0.05 eq.) were taken in tube and

dissolved in toluene (1mL). The tube was sealed by Teflon cap and heated at 115 oC in

an oil bath for two days. After two days, the reaction mixture was cooled to room

temperature and washed with toluene 5 ml. This was followed by washing with diethyl

ether (3×5mL). Last, the reaction mixture was centrifuged, vacuum dried and the

desired compound was purified by column chromatography (Sillica gel 70-230 mesh,

⌀3×20 cm, eluent dichloromethane-methanol, 97:3→94:6 v/v mixture) to afford

compound 22Br. The yield was 30 %. The rotary evaporator was used to evaporate the

solvent. The reaction was again performed with the same set of reagents at different

temperatures 130 and 140 oC.

3.4. General procedure for preparation of the 21-24[OTf] salts

The phosphonium salts 21-24[Br] were dissolved in DCM (10 mL) and AgOTf in

acetone (1mL) was added in one portion. Mixture was left for stirring in a round bottom

flask 30 min. The solvents were evaporated in vacuo. The phosphonium salts were

extracted with dichloromethane 3×5 mL and passed through Celite pad to separate

AgBr and evaporated by rotary vapour to give greenish residue. The compounds were

finally purified by column chromatography (Sillica gel 70-230 mesh, ⌀3×10 cm, eluent

dichloromethane/methanol, 95:5 v/v mixture) to afford 21-24[OTf] as a greenish

precipitate.

21[OTf]

The 21[Br] (0.10 g, 0.15 mmol, 1.0 eq), AgOTf (0.04 g 0.15 mmol, 1.05 eq.) were used

as staring material to convert 21[Br] into 21[OTf]. The yield was 88%.1H-NMR (400

MHz, CHCl3 298 K) δ 8.02 – 7.86 (m, para-H PPh3+ and -biph-, 5H), 7.79 (td, J = 7.8,

3.6Hz Hz, meta-H PPh3+,6H), 7.73 – 7.59 (m, ortho-H PPh3

+ and -biph-, 8H), 7.56 –

7.49 (d, J = 8.7 Hz -biph-, 2H), 7.35 – 7.22 (m, meta-H NPh2, 4H), 7.22 – 7.00 (m,

ortho-H NPh2 and -biph-,para-H NPh2, 8H) 31P-NMR (162 MHz, CD2Cl2 298 K) δ 23.5

(s, 1P, PPh3+). NMR and ESI+ data similar to previously reported data.17

38

22[OTf]

The 22[Br] (0.1 g 0.11 mmol 1.0 eq.), AgOTf (0.03 g, 0.11 mmol, 1.05 eq.) were used

as starting material to convert 22[Br] into 22[OTf]. The yield of product was 68%. 1H-

NMR (400 MHz, , CH2Cl2, 298 K) δ 8.05 – 7.89 (m, para-H PPh2+ and -biph 6H), 7.87

– 7.77 (m, meta-H PPh2+ 4H), 7.77 – 7.64 (m, ortho-H PPh2

+ and -biph-, 8H), 7.63 –

7.53 (m, -biph-, 4H), 7.43 – 7.29 (m, meta-H NPh2 8H), 7.24 – 7.04 (m, para and

ortho-H 16H). 13C-NMR (101 MHz, CH2Cl2, 298 K) δ 149.90, 147.98 (d, J = 71.3 Hz),

137.86 – 133.80 (m), 131.95 – 130.54 (m), 130.06, 129.21 – 127.61 (m), 125.84,

124.52, 122.95, 119.20. 31P-NMR (162 MHz, CH2Cl2, 298 K) δ 23.27. Anal. Calcd for

C61H46F3N2O3PS: C, 75.14; H, 4.76; N, 2.87. Found: C, 74.97; H, 4.71; N, 2.90. ESI-

MS (m/z): [M]+ 825.3386 (calcd 825.3399).

23[OTf].

The 23[Br] (0.1 g, 0.09 mmol, 1.0 eq.), AgOTf (0.02 g, 0.09 mmol, 1.05 eq.), were used

as starting material to convert 23[Br] into 23[OTf]. Percentage yield was found to be

85%. 1H-NMR (400 MHz, CH2Cl2) δ 7.97 (dd, J = 8.6, 3.1 Hz, para-H PPh+ and -biph

7H), 7.82 (ddd, J = 13.0, 8.6, 4.6 Hz, meta-H PPh+ 2H), 7.81 – 7.66 (para-H NPh2 and

biph NPh2 8H), 7.71 – 7.51 (m ortho-H PPh+ and -biph 6H), 7.46 – 7.26 (m, ortho-

NPh2 and -biph 12H), 7.29 – 6.92 (m, para-H NPh2 24H). 13C-NMR (101 MHz,

CH2Cl2, 298 K) δ 149.87, 148.83 – 146.85 (m), 137.16 – 133.47 (m), 131.27, 130.07,

129.22 – 127.47 (m), 125.83, 124.51, 122.99, 115.39 (d, J = 92.7 Hz). δ 31P-NMR (162

MHz, CH2Cl2, 298 K) δ 22.96 . Anal. Calcd for C79H59F3N3O3PS: C, 77.88; H, 4.88; N,

3.45. Found: C, 78.02; H, 4.91; N, 3.33. ESI-MS (m/z): [M]+ 1068.4416 (calcd

1068.4447).

24[OTf]

The 24[Br] (0.10 g, 0.07 mmol, 1.05 eq.), AgOTf (0.02 g, 0.08 mmol,1.05 eq.) were

used as starting material to convert 24[Br] into 24[OTf]. Percentage yield was found to

be 90%. 1H-NMR (400 MHz, CH2Cl2, 298 K) δ 8.12 – 7.91 (m, , biph- 8H), 7.89 – 7.69

(m, biph- 8H), 7.69 – 7.50 (m, biph- 8H), 7.45 – 7.27 (m, meta-NPH2 16H), 7.23 – 7.04

(m, para, ortho HNPh2 and biph- 32H). 13C-NMR (101 MHz, CH2Cl2, 298 K) δ 149.86

, 147.91 (d, J = 53.7 Hz), 135.43 (d, J = 10.7 Hz), 131.31 , 130.07 , 128.58 (d, J = 18.1

Hz), 125.82 , 124.51 , 123.01 , 115.71 (d, J = 93.0 Hz). 31P-NMR (162 MHz, CH2Cl2,

298 K) δ 22.64. Anal. Calcd for C97H72F3N4O3PS: C, 79.71; H, 4.97; N, 3.83. Found: C,

79.61; H, 5.11; N, 3.78. ESI-MS (m/z): [M]+ 1312.5491 (calcd 1312.5527).

39

4. Results and discussions 4.1. Synthesis of phosphonium salts

Figure 30: Phosphonium salts 21-24[X] reported in our work, where X = Br or CF3SO3.

Synthesis of 1a, 1b and 1c

In order to synthesise biphenyl based phosphonium salts several precursors were

synthesised (1a, 1b, 1c, L2, L3). The Synthesis of 1a was carried out according to

published literature under a nitrogen atmosphere by addition of triphenylamine in

CHCl3 and N-bromosuccinimide. The mixture followed by recrystallisation of residue

by methanol to afford white solid crystalline product. The percentage yield was found to

be 84 % in good agreement with the published in literature (91%).67

Then, 1a was later converted into 1b under inert atmosphere of nitrogen followed by the

lithiation protocol of 1a by n-BuLi and by addition of trimethylborate. Then, reaction

mixture was left for stirring, HCl solution was charged into it and neutralised by sodium

bicarbonate to yield greenish white solid. The percentage yield was found 67 % as

compared to 55 % in published literature.67

40

Lastly, 1b was used as precursor for the synthesis of 1c by Suzuki cross coupling

reaction with 1-bromo-4-iodobenze and catalysed by Pd(PPh3)3Cl2 in presence of

K2CO3 in toluene. The compound was synthesised according to published literature and

the percentage yield were found to be 71 % as compared to 91 % in the published

literature.68 Reaction sequence for 1a, 1b, 1c is illustrated (Synthesis scheme 11) and

these precursors were characterised by 1H-NMR.

Synthesis Scheme 11: Synthesis of 1c. a = NBS CHCl3 , b = THF, n-BuLi ,B(OMe)3, -

80 °C, B(OMe)3, -80 °C, HCl – 40 °C to RT, c = 1-bromo-4-iodobenzene, K2CO3,

Pd(PPh3)Cl2, Methanol, Toluene 40 °C.

Synthesis of L2 and L3

L2 and L3 were synthesised from same precursor 1c by lithiation protocol and

subsequently was coupled with stoichiometric amount of PPhCl2 to afford white powder

L2 with 75 % yield and coupled with stoichiometric amount of PCl3 to afford nearly

colourless L3 with 70% respectively. The reaction procedure and molecular structure is

given (Synthesis scheme 12).70

Synthesis Scheme 12: Synthesis Scheme of L2, a = THF, n-BuLi, -78 °C, ½ PPhCl2. b

= THF, n-BuLi, -78 °C, 1/3 PCl3.

41

Preparation of phosphonium salts

The salts were designed with D--A system in which the NPh2 acts as typical donating

moiety and PR3+ acts as an electron accepting moiety. The 22 was synthesised by

palladium catalysed reactions at different temperatures 115, 130 and 140 oC.65 The 31P-

NMR of showed mixture of phosphonium salts along with the desired phosphonium

salt 22. Nevertheless, it was observed that at 115 °C the reaction works better in terms of

purity as compared to 130 and 140 °C as depicted (Figure 31). Due to its constraints

this synthetic procedure was discarded.

Figure 31: 31P-NMR spectra of 22[Br] at different temperatures with palladium as a

catalyst.

The synthesis of compounds 21-24[Br] were carried out according to D. Marcoux Nickel

catalysed synthetic procedure for phosphonium salts.66 The phosphonium salts 21-

24[Br] were synthesised from a common precursor 1c by Nickel catalysed coupling

with different phosphines (PPh3/ L1/ L2/ L3) with ethylene glycol acting as a solvent.

The reaction mixture turned into homogenous solution after 1 hour and the colour

turned to greenish as the reaction proceeded because of the displacement of Br⁻ with

solvent molecules. The three solvent molecules form octahedral complex with the Ni

ion. These solvent molecules are replaced by phosphine ligands followed by oxidative

addition of aryl halide. Finally, reductive elimination results in the formation of

phosphonium salts.

42

The reaction was stirred for two days at 180 °C. Salts were extracted with DCM and

were dried over anhydrous sulphate. The 21 was synthesised from 1c coupled with PPh3

to give greenish yellow precipitate with a yield of 65%. The 22 was synthesised from 1c

and coupled with PPh2[(C6H4)2NPh2] to give greenish precipitate with a yield of 50 %.

While as compound 23 and 24 were synthesised by coupling 1c with L2 and L3 with a

yield of 54% and 20 % respectively. It was observed that there was decrease in the yield

from 21-24 probably due to increase in the steric effects. The compounds were

characterised by 1H-NMR and 31P-NMR. The 1H-NMR and 31P-NMR was in good

agreement with the similar phosphonium salts reported in the literature.17 The molecular

structure and reaction are given (Synthesis scheme 13).

Synthesis Scheme 13: Synthesis of phosphonium salts 21-24. a) PPh3. b) L1 c) L2 d)

L3, with ethylene glycol as solvent and NiBr2 as catalyst 180 °C.

43

Counterion exchange and synthesis of OTf salts

The bromide counterion of the salts 21-24[Br] were exchanged by triflate ions by

dissolving the synthesised compounds in DCM with AgOTf in acetone after stirring

results in greenish precipitate with OTf as counterion. The yields ranged from 68% to

90%.

4.2. Characterisation by 1H-NMR and 31P-NMR

The synthesised salts were confirmed by 1H-NMR, 13C-NMR, and 31P-NMR

spectroscopy which are defined below.

Figure 32: Stacked spectra of phosphonium salts.

1H-NMR of phosphonium salt 21[OTf] is depicted in the (Figure 33). The multiplet

signal for para H of phenyl ring represented by 1 and the meta H of biphenyl ring

represented by 2 are presented towards the low field region at 7.91 ppm because of

positive charge on phosphorus atom. The triplet of doublets signal at 7.79 ppm with J =

7.8, 3.6 Hz is assigned to the meta protons of phenyl ring. The multiplet signal at 7.65

ppm have been assigned to ortho H of phenyl and biphenyl ring represented by 4 and 5

respectively. The doublet signal at 7.53 ppm with J = 8.7 Hz have been assigned to

biphenyl protons represented by 6. The meta protons of diphenylamine shows signal at

7.27 ppm and is denoted by 7. The multiplet signals 7.16 to 7.10 ppm have been

44

assigned to ortho/para protons of phenyl rings of diphenyl amine and ortho protons of

biphenyl ring represented by 8,9 and 10 respectively. The signals are towards high field

and can be justified by the presence of the lone pair present on nitrogen which enhances

the electron density on these positions.

Figure 33: 400 MHz 1H-NMR (CHCl3, 298 K; ppm) of 21[OTf]

The presence of single peak in phosphorus spectrum (Figure 34) confirms that there is

single kind of organophosphorus compounds in the synthesised compound moreover the

single peak at 23.5 ppm confirms the presence of PR3+ group in the compound. It has

been reported that phosphonium salts having similar structure show simillar chemical

shift around 23 ppm.17

45

Figure 34: 400 MHz 31P-NMR (CHCl3, 298 K; ppm) of 21[OTf]

The presence of single peak in the 31P-NMR (Figure 35) confirms the presence of

single phosphorus compound. The single peak at 23.1 ppm which is slightly towards

high field as compared to 21[OTf] because of the presence of one more electron donor

in the 22[Br].

46

Figure 35: 400 MHz 31P-NMR (CH2Cl2, 298 K; ppm) of 22[Br]

1H-NMR of phosphonium salt 22[Br] is depicted (Figure 36). The multiplet signal

assigned at 7.97 ppm is attributed to para H of phenyl rings attached to phosphorus

centre and ortho H of biphenyl rings represented by 1 and 2. The doublet at 7.83 ppm

with J = 7.6 Hz is assigned to ortho protons of phenyl ring attached to phosphorus

represented by 3. The shifting of signals towards low field is because of the presence of

Phosphonium cation attached directly to rings. The multiplet signal at 7.73 ppm is

assigned to biphenyl protons signified by 4. The doublet signal at 7.60 ppm with J = 8.7

Hz has been assigned to meta H of phenyl rings because the phosphonium cation does

not affect much for meta Hs. The dd signal at 7.34 ppm with J = 8.5, 7.2 Hz is assigned

to meta Hs of phenyl rings attached to nitrogen represented by 6. The signals 7.19-7.16

ppm denoted by 7, 8 and 9 towards the high field are because of the shielding by the

resonance of lone pair of electrons on nitrogen.

47

Figure 36: 400 MHz 1H-NMR (CH2Cl2, 298 K; ppm) of 22[Br]

1H-NMR of phosphonium salt 23[Br] is depicted (Figure 37). The signals at 7.97 ppm

are assigned to para H of phenyl ring attached to phosphorus and ortho H protons of

biphenyl rings attached to phosphorus represented by 1 and 2 respectively. The doublet

at 7.84 ppm is assigned to ortho protons of phenyl ring of PPh+ with J = 7.8 Hz

represented by 3. The 4 and 5 signals are assigned to meta protons of PPh+ and meta

protons of biphenyl ring. The doublet at 7.63 ppm with J = 8.8 Hz represented by 6 is

assigned to H of biphenyl rings. The multiplet at 7.35 ppm is assigned to meta H of

phenyl rings attached to nitrogen. The high field signals at 7.19 to7.17 ppm are assigned

to 8, 9 and 10 positions because of the shielding by the lone pair on nitrogen.

48

Figure 37: 400 MHz 1H-NMR (CH2Cl2, 298 K; ppm) of 23[Br]

The presence of single peak in the 31P-NMR (Figure 38) confirms the presence of

single phosphorus compound. The single peak at 22.8 ppm which is slightly towards

high field as compared to 22[Br] because of the presence of three electron donor in the

23[Br].

49

Figure 38: 400 MHz 31P-NMR (CH2Cl2, 298 K; ppm) of 23[Br]

1H-NMR of phosphonium salt 24[Br] is depicted (Figure 39). This compound is the

most symmetrical among all the synthesised compound thus showing smaller number of

signals. The signals at 7.98 ppm towards the low field are assigned to ortho H of

biphenyl rings attached to P+ with J = 8.4 Hz and is represented by 2. The signals at 7.76

were allotted for meta H of biphenyl, represented by 2 with J = 8.3 Hz. The doublet

signal at 7.64 ppm is assigned to H of biphenyl rings shown by 3 with J = 8.8 Hz. The

multiplet at 7.35 is assigned to meta H of phenyl rings attached to nitrogen. The

multiplet signal at 7.19 to 7.16 ppm are assigned to ortho/para H of phenyl rings

attached to nitrogen and ortho H of biphenyl ring and are represented by 5, 6 and 7.

50

.

Figure 39: 400 MHz 1H-NMR (CHCl3, 298 K; ppm) of 24[Br]

The presence of single peak in the 31P-NMR (Figure 40) confirms the presence of

single phosphorus compound. The single peak at 22.6 ppm which is slightly towards

high field as compared to 23[Br] because of the presence of four electron donors in the

24[Br].

51

Figure 40: 400 MHz 31P-NMR (CHCl3, 298 K; ppm) of 24[Br]

52

4.3. Characterisation by Mass spectrometry

The ESI mass spectra of compounds 22[Br], 23Br], 24[Br], 22[OTf], 23[OTf] and

24[OTf] along with corresponding calculated isotopic distribution were found to be in

good agreement with each other as shown (Figure 41).

Figure 41: ESI+ MS of compounds 22Br-24[X] (X = Br or OTf) and the calculated

isotopic distributions for molecular cations.

53

4.4. Photophysical properties

The photoluminescence spectra were measured by the collaborative team at National

University of Taiwan (group of Prof. P.T. Chou).

54

s

55

Figure 42: Absorption and normalized emission spectra of 22Br-24[Br] in DCM,

toluene and acetonitrile at 298 K.

56

Figure 43: Molar absorptivity of 22Br-24[OTf] in DCM, toluene and acetonitrile at 298

K.

The absorption and emission spectra for compounds 22-24[X] (X =Br or OTf) were

recorded in dichloromethane, toluene and acetonitrile. Since these chromophores are

designed based on Donor-Acceptor system the absorption bands observed towards red

region is distinctive band of ICT. The novel compounds reported in our work were

similar to the work reported. Since the photophysical properties of 21Br (abs 268, 387

nm and em 528 nm, DCM) is already reported17. Our work was to study the influence

of chromophores with electron donating group NPh2 connected by biphenyl to the

Phosphonium cation. It was observed that the absorption bands wavelengths (abs) are

almost similar with very small bathochromic shift on introducing electron donating

groups which destabilize the HOMO orbitals thus tailoring the photophysical properties.

The salts displayed absorption bands (22Br, 380 nm), (23Br 382 nm), and (24Br, 383

nm) in toluene, while in DCM the salts showed the absorption bands (22Br, 390 nm),

57

(23Br 392 nm), and (24Br, 393 nm), thereby indicating bathochromic shift as compared

to toluene. The bathochromic shift can be defined by slight stabilization of ES by DCM

solvent molecules. The lowest lying absorptions of salts exhibit blue shift of 1026 cm -1

for 22Br 1080 cm-1 for 23Br and 1074 cm -1 for 24Br respectively in a polar solvent

ACN as compared to DCM thus these salts demonstrate negative solvatochromism

which is in agreement with the push pull phosphonium chromophores reported in

literature.58

The emission maxima for salts 21Br-24Br also varied as a function of the solvent nature

and biphenyl chromophores connected to Phosphorus center. The salts displayed

luminescence ranging from 522 to 527 nm in DCM with lifetimes extending from 4.08

to 4.40 ns with quantum yield increasing from 73 % to 79 % in DCM. The emission

bands displayed regular trend on increasing electron donating groups it was observed

that 21Br, 528 nm >22Br, 525 nm > 23Br, 523 nm > 24Br, 522 nm in DCM thereby

showing small hypsochromic shift. The high stokes shift of 8485 cm -1 for 22Br, 8310

cm -1 for 23Br and 8928 cm-1 for 24Br shift in highly polar solvent like ACN can be

assigned to stabilization of GS by polar-polar interaction between polar ACN solvent

molecules and polar GS. However, in polar solvent ACN it resulted in fluorescence

quenching and the quantum yield decreased to 30 % and this can be accredited to static

interactions of salts with polar solvent molecules like ACN. The large stokes shift,

significant solvatochromism and quenching with increase in solvent polarity can be

assigned to ICT.23

The photophysical studies for the salts synthesized were also investigated in nonpolar

solvent toluene. The salts demonstrated anomalous dual emission with two bands F1 and

F2 attributed to high and low energy bands respectively. The ratio of the intensities

increased in 21Br-24Br from 0.57/1 to 0.67/1 for F1 and F2 bands respectively. The

quantum yield was around 15 %. Moreover, no significant change was observed on

ICT bands and highest energy bands with increase in chromophores from 21Br-24Br as

displayed in the Table 2.

58

Table 2: Photophysical properties of 22-24[X] where X = Br or OTf in different solvents

at 298 K

solvent λabs, nm 103 M-1 cm-1 λem, nm Δν, cm-1 τobs, ns Φem F1/F2

DCM 268, 390 525 6593 4.40 0.73

22Br toluene 380 468, 586 9251

0.15 0.57/1

ACN 375 550 8485 3.24 0.39

DCM 268, 392 37.35 527 6534 4.41 0.76

22Otf toluene 385 38.23 468, 586 8909

0.38 0.29/1

ACN 373 32.01 551 8660 3.27 0.34

DCM 272, 392 523 6390 4.17 0.76

23Br toluene 382 468, 585 9084

0.15 0.60/1

ACN 377 549 8310 2.86 0.34

DCM 269, 393 62.37 528 6506 4.20 0.81

23OTf toluene 386 38.58 468, 584 8783

0.34 0.30/1

ACN 377 83.00 549 8310 2.90 0.29

DCM 273, 393 522 6288 4.08 0.79

24Br toluene 383 468, 582 8928

0.10 0.69/1

ACN 378 547 8174 2.71 0.30

DCM 273, 394 102.83 523 6260 4.08 0.77

24OTf toluene 386 39.29 468, 580 8665

0.30 0.35/1

ACN 378 88.47 549 8240 2.73 0.25

59

Furthermore, the change of counterion from Br⁻ to bulkier OTf ⁻ ion has a significant

contribution in the photophysical properties. It was observed bulkier triflate increased

the overall quantum yield and at the same time intensity of F2 band also increased in all

salts. The counterion also affects ICT absorption maxima for 22OTf, 23OTf, and 24OTf

were recorded as 392, 393 and 394 nm respectively in DCM. The molar absorptivity for

triflate salts displayed a dramatic increase in molar absorptivity as compared to 21Br

(27200 M-1 cm-1)17. Molar extinction coefficient of 22OTf, 23OTf and 24OTf was

observed at 37335 M-1 cm-1, 62366 M-1 cm-1 and 102825 M-1 cm-1 respectively in DCM,

this increase was attributed to increase in chromophores from 21Br-24Br respectively.

However, in nonpolar solvent no significant change in molar absorptivity was observed.

The photophysical behavior of chromophores designed were found reliant on the

polarity of solvent, nature of counterion. The spectacular dual emission of salts is

rationalized by mechanism put forward in the literature17. In non-polar solvents the

counterion of the salt is having stronger electrostatic force. During excitation to S1 state

followed by ICT there is complete transfer of charge from +PPh3 to NPh2 as a result of

which the PPh3 becomes neutral and NPh2 becomes positively charged. During this

charge redistribution the counterion migrates towards the positively charged NPh2 and

stabilizes this ES at the same time the destabilization of GS after counterion migration

leads to emission with higher stokes shift and is observed as F2 in emission spectra of

toluene. While in the case of polar solvents upon charge redistribution the positive

charged at NPh2 is stabilized by the polar solvent molecules this doesn’t lead to

counterion migration as a result we get only single emission in the case of polar solvents

like ACN.

60

Conclusion

The work presented in this thesis is mainly directed on the synthesis characterisation

and photophysical study of novel phosphonium salts. The following conclusions can be

summarized as.

➢ The work was dedicated in synthesizing novel acyclic luminophores based on

Donor Acceptor system with λ4σ4 phosphonium group +PR3 group acting as an

acceptor and Nph2 as donor moieties connected by biphenyl systems.

➢ The phosphonium salts were confirmed by 1H-NMR, 31P-NMR and elemental

analysis. Complete assignment of protons and phosphorus signals was assigned

to reported compounds.

➢ The ESI mass spectra of the compounds was done and was found in complete

agreement with calculated isotopic distribution.

➢ The photophysical properties of reported compounds showed emission in the

range from 522 nm to 586 nm in different solvents with Φem up to 81 %. The

emission can be defined in terms of conventional ICT excited state.

➢ The salts showed entirely different behaviour in different solvents while in

polar solvent like ACN only one emission band was observed. Intriguingly in

non-polar solvent like toluene anomalous dual emission was attained. The dual

emission was attributed to intramolecular charge transfer driven counterion

migration resulting in different excited state.

61

Acknowledgment

This work was carried out at the Department of the Chemistry University of Eastern

Finland. Foremost I would like to express my sense of gratification to my supervisor

Prof. Igor Koshevoy for his immense support, motivation and enthusiasm. It was really

an honour to work under the guidance of such a renowned Chemist.

I am also grateful to my mentors Wani Ayaz and Mudasir Shah for their continuous

guidance support and wonderful contribution in my student carrier.

I also extend my heartfelt appreciation to my co-supervisor Andrei Beliaev who was

always there for his insightful thoughts and helpful in developing my research skills.

I want to extend my special gratitude to my friends Asma Bashir, Zahid Rasool and

Younis Amin for continuous assistance and encouragement. I am beholden to them for

always their presence through thick and thin.

Finally, I am extremely thankful for my wonderful parents for their continuous efforts,

love and blessings Abdul Majid and Fameeda and my loving siblings Suhail, Muskan

and Zainab for their love and moral support.

62

References

1. Lakowicz, Joseph, R. Principles of Fluorescence Spectroscopy, Third Ed.; Springer

Science, 2006.

2. Coughlin, J. E.; Henson, Z. B.; Welch, G. C.; Bazan, G. C. Design and Synthesis of

Molecular Donors for Solution-Processed High-Efficiency Organic Solar Cells.

Accounts of Chemical Research 2013, 47 (1), 257–270.

3. Komori, T.; Nakanotani, H.; Yasuda, T.; Adachi, C. Light-Emitting Organic Field-

Effect Transistors Based on Highly Luminescent Single Crystals of

Thiophene/Phenylene Co-Oligomers. Journal of Materials Chemistry C 2014, 2

(25), 4918.

4. Li, Y.; Liu, T.; Liu, H.; Tian, M.-Z.; Li, Y. Self-Assembly of Intramolecular

Charge-Transfer Compounds into Functional Molecular Systems. Accounts of

Chemical Research 2014, 47 (4), 1186–1198.

5. Vanengelenburg, S. B.; Palmer, A. E. Fluorescent Biosensors of Protein Function.

Current Opinion in Chemical Biology 2008, 12 (1), 60–65

6. Demkowicz, S.; Rachon, J.; Daśko, M.; Kozak, W. Selected Organophosphorus

Compounds with Biological Activity. Applications in Medicine. RSC Advances

2016, 6 (9), 7101–7112.

7. Guan, X.; Zhang, K.; Huang, F.; Bazan, G. C.; Cao, Y. Amino N-Oxide

Functionalized Conjugated Polymers and Their Amino-Functionalized Precursors:

New Cathode Interlayers for High-Performance Optoelectronic Devices. Advanced

Functional Materials 2012, 22 (13), 2846–2854.

8. Becker, A.; Hessenius, C.; Licha, K.; Ebert, B.; Sukowski, U.; Semmler, W.;

Wiedenmann, B.; Grötzinger, C. Receptor-Targeted Optical Imaging of Tumors

with near-Infrared Fluorescent Ligands. Nature Biotechnology 2001, 19 (4), 327–

331.

9. Yang, Z.; Mao, Z.; Xie, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Xu, J.; Chi, Z.; Aldred, M.

P. Recent Advances in Organic Thermally Activated Delayed Fluorescence

Materials. Chemical Society Reviews 2017, 46 (3), 915–1016.

10. Eeckhout, K. V. D.; Smet, P. F.; Poelman, D. Persistent Luminescence in Eu2 -

Doped Compounds: A Review. Materials 2010, 3 (4), 2536–2566.

11. Yuan, W. Z.; Shen, X. Y.; Zhao, H.; Lam, J. W. Y.; Tang, L.; Lu, P.; Wang, C.; Liu,

Y.; Wang, Z.; Zheng, Q.; Sun, J. Z.; Ma, Y.; Tang, B. Z. Crystallization-Induced

Phosphorescence of Pure Organic Luminogens at Room Temperature. The Journal

of Physical Chemistry C 2010, 114 (13), 6090–6099.

12. Gong, Y.; Zhao, L.; Peng, Q.; Fan, D.; Yuan, W. Z.; Zhang, Y.; Tang, B. Z.

Crystallization-Induced Dual Emission from Metal- and Heavy Atom-Free

Aromatic Acids and Esters. Chemical Science 2015, 6 (8), 4438–4444.

13. Wei, D.; Ni, F.; Zhu, Z.; Zou, Y.; Yang, C. A Red Thermally Activated Delayed

Fluorescence Material as a Triplet Sensitizer for Triplet–Triplet Annihilation up-

Conversion with High Efficiency and Low Energy Loss. Journal of Materials

Chemistry C 2017, 5 (48), 12674–12677.

63

14. Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C.

Efficient up-Conversion of Triplet Excitons into a Singlet State and Its Application

for Organic Light Emitting Diodes. Applied Physics Letters 2011, 98 (8), 083302.

15. Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chemical

Reviews 1994, 94 (8), 2319–2358.

16. Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural Changes Accompanying

Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer

States and Structures. ChemInform 2003, 34 (52).

17. Belyaev, A.; Cheng, Y. H.; Liu, Z. Y.; Karttunen, A. J.; Chou, P. T.; Koshevoy, I.

O. Angewandte Chemie 2019, 131 (38), 13590.

18. Valeur, B.; Berberan-Santos, Mário, N. A Brief History of Fluorescence and

Phosphorescence before the Emergence of Quantum Theory. J. Chem. Educ. 2011,

88 (6), 731–738.

19. Hu, J.; Hu, Z.; Liu, S.; Zhang, Q.; Gao, H.-W.; Uvdal, K. Sensors and Actuators B:

Chemical 2016, 230, 639.

20. Xu, Z.; Xiao, Y.; Qian, X.; Cui, J.; Cui, D. Ratiometric and Selective Fluorescent

Sensor for CuIIBased on Internal Charge Transfer (ICT). Organic Letters 2005, 7

(5), 889–892.

21. Sutariya, P. G.; Modi, N. R.; Pandya, A.; Joshi, B. K.; Joshi, K. V.; Menon, S. K.

An ICT Based “Turn on/off” Quinoline Armed Calix[4]Arene Fluoroionophore: Its

Sensing Efficiency towards Fluoride from Waste Water and Zn2 from Blood Serum.

The Analyst 2012, 137 (23), 5491.

22. Suzuki, Y.; Yokoyama, K. Design and Synthesis of ICT-Based Fluorescent Probe

for High-Sensitivity Protein Detection and Application to Rapid Protein Staining for

SDS-PAGE. Proteomics 2008, 8 (14), 2785–2790.

23. Misra, R.; Bhattacharyya, S. P. Intramolecular charge transfer: theory and

applications; Wiley-VCH Verlag.: Weinheim, Germany, 2018.

24. Atsbeha, T.; Mohammed, A. M.; Redi-Abshiro, M. Excitation Wavelength

Dependence of Dual Fluorescence of DMABN in Polar Solvents. Journal of

Fluorescence 2010, 20 (6), 1241–1248.

25. Maanen, M.; Smeets, C.; Beijnen, J. Chemistry, Pharmacology and

Pharmacokinetics of N,N′,N′′ -Triethylenethiophosphoramide (ThioTEPA). Cancer

Treatment Reviews 2000, 26 (4), 257–268.

26. Zielonka, J.; Joseph, J.; Sikora, A.; Hardy, M.; Ouari, O.; Vasquez-Vivar, J.; Cheng,

G.; Lopez, M.; Kalyanaraman, B. Mitochondria-Targeted Triphenylphosphonium-

Based Compounds: Syntheses, Mechanisms of Action, and Therapeutic and

Diagnostic Applications. Chemical Reviews 2017, 117 (15), 10043–10120.

27. Singh, B. K.; Walker, A. Microbial Degradation of Organophosphorus Compounds.

FEMS Microbiology Reviews 2006, 30 (3), 428–471

28. Barthold, C. L.; Schier, J. G. Organic Phosphorus Compounds—Nerve Agents.

Critical Care Clinics 2005, 21 (4), 673–689.

29. Marklund, A.; Andersson, B.; Haglund, P. Screening of Organophosphorus

Compounds and Their Distribution in Various Indoor Environments. Chemosphere

2003, 53 (9), 1137–1146.

64

30. Ramazani, A.; Kazemizadeh, A. R. Preparation of Stabilized Phosphorus Ylides via

Multicomponent Reactions and Their Synthetic Applications. Current Organic

Chemistry 2011, 15 (23), 3986–4020.

31. Cadogan, J. I. G.; Mackie, R. K. Tervalent Phosphorus Compounds in Organic

Synthesis. Chemical Society Reviews 1974, 3 (1), 87.

32. Noyori, R.; Takaya, H. BINAP: an Efficient Chiral Element for Asymmetric

Catalysis. Accounts of Chemical Research 1990, 23 (10), 345–350.

33. Giordan, J. C.; Moore, J. H.; Tossell, J. A.; Kaim, W. Interaction of Frontier

Orbitals of Group 15 and Group 16 Methides with the Frontier Orbitals of Benzene.

Journal of the American Chemical Society 1985, 107 (20), 5600–5604.

34. Organophosphorus derivatives for electronic devices. J. Mater. Chem. C 4, 3686–

3698 (2016).

35. Bouit, P.-A.; Escande, A.; Szűcs, R.; Szieberth, D.; Lescop, C.; Nyulászi, L.;

Hissler, M.; Réau, R. Dibenzophosphapentaphenes: Exploiting P Chemistry for Gap

Fine-Tuning and Coordination-Driven Assembly of Planar Polycyclic Aromatic

Hydrocarbons. Journal of the American Chemical Society 2012, 134 (15), 6524–

6527.

36. Floch, P. Phosphaalkene, Phospholyl and Phosphinine Ligands: New Tools in

Coordination Chemistry and Catalysis. Coordination Chemistry Reviews 2006, 250

(5-6), 627–681.

37. Baumgartner, T. ChemInform Abstract: Insights on the Design and Electron-

Acceptor Properties of Conjugated Organophosphorus Materials. ChemInform

2014, 45 (31).

38. Leyssens, T.; Peeters, D. Negative Hyperconjugation in Phosphorus Stabilized

Carbanions. The Journal of Organic Chemistry 2008, 73 (7), 2725–2730.

39. Larrañaga, O.; Romero‐Nieto, C.; De Cózar, A. Dismantling the Hyperconjugation

of π‐Conjugated Phosphorus Heterocycles. Chemistry – A European Journal 2019,

25 (38), 9035–9044.

40. Baumgartner, T.; Réau, R. Organophosphorus π-Conjugated Materials. Chemical

Reviews 2006, 106 (11), 4681–4727.

41. Dauth, A.; Love, J. A. Synthesis and Reactivity of 2-Azametallacyclobutanes.

Dalton Transactions 2012, 41 (26), 7782.

42. Hay, C.; Fischmeister, C.; Hissler, M.; Toupet, L.; Réau, R. Electropolymerization

of π-Conjugated Oligomers Containing Phosphole Cores and Terminal Thienyl

Moieties: Optical and Electronic Properties. Angewandte Chemie 2000, 112 (10),

1882–1885.

43. Baumgartner, T.; Neumann, T.; Wirges, B. The Dithieno[3,2-b:2?,3?-d]Phosphole

System: A Novel Building Block for Highly Luminescent ?-Conjugated Materials.

Angewandte Chemie International Edition 2004, 43 (45), 6197–6201.

44. Dienes, Y.; Eggenstein, M.; Kárpáti, T.; Sutherland, T. C.; Nyulászi, L.;

Baumgartner, T. Phosphorus-Based Heteropentacenes: Efficiently Tunable

Materials for Organic n-Type Semiconductors. Chemistry - A European Journal

2008, 14 (32), 9878–9889.

65

45. Wang, Z.; Spasyuk, D.; Baumgartner, T. On the Reactivity of P-Chloro

Dithieno[3,2-b:2′,3′-d]Phosphole Oxide. Canadian Journal of Chemistry 2018, 96

(6), 555–560.

46. Wang, Z.; Gelfand, B. S.; Baumgartner, T. Dithienophosphole-Based

Phosphinamides with Intriguing Self-Assembly Behavior. Angewandte Chemie

2016, 128 (10), 3542–3546.

47. Ashe, A. J. Phosphabenzene and Arsabenzene. Journal of the American Chemical

Society 1971, 93 (13), 3293–3295.

48. Koe, P. D.; Bickelhaupt, F. Dibenzo[b,e]Phosphorin. Angewandte Chemie

International Edition in English 1967, 6 (6), 567–568.

49. Koe, P. D.; Bickelhaupt, F. 10-Phenyldibenzo[b,e]Phosphorin. Angewandte Chemie

1968, 80 (21), 912–913.

50. Ito, S.; Koshino, K.; Mikami, K. CF3-Inspired Synthesis of Air-Tolerant 9-

Phosphaanthracenes That Feature Fluorescence and Crystalline

Polymorphs. Chemistry - An Asian Journal 2018, 13 (7), 830–837.

51. Regulska, E.; Hindenberg, P.; Romero-Nieto, C. From Phosphaphenalenes to

Diphosphahexaarenes: An Overview of Linearly Fused Six-Membered Phosphorus

Heterocycles. European Journal of Inorganic Chemistry 2019, 2019 (11-12), 1519–

1528.

52. Chai, X.; Cui, X.; Wang, B.; Yang, F.; Cai, Y.; Wu, Q.; Wang, T. Near-Infrared

Phosphorus-Substituted Rhodamine with Emission Wavelength above 700 Nm for

Bioimaging. Chemistry - A European Journal 2015, 21 (47), 16754–16758.

53. Jia, S.; Ramos-Torres, K. M.; Kolemen, S.; Ackerman, C. M.; Chang, C. J. Tuning

the Color Palette of Fluorescent Copper Sensors through Systematic Heteroatom

Substitution at Rhodol Cores. ACS Chemical Biology 2017, 13 (7), 1844–1852.

54. Fukazawa, A.; Usuba, J.; Adler, R. A.; Yamaguchi, S. Synthesis of Seminaphtho-

Phospha-Fluorescein Dyes Based on the Consecutive Arylation of

Aryldichlorophosphines. Chemical Communications 2017, 53 (61), 8565–8568.

55. Romero-Nieto, C.; López-Andarias, A.; Egler-Lucas, C.; Gebert, F.; Neus, J.-P.;

Pilgram, O. Paving the Way to Novel Phosphorus-Based Architectures: A

Noncatalyzed Protocol to Access Six-Membered Heterocycles. Angewandte Chemie

International Edition 2015, 54 (52), 15872–15875.

56. Hindenberg, P.; Rominger, F.; Romero‐Nieto, C. Phosphorus Post‐Functionalization

of Diphosphahexaarenes. Chemistry – A European Journal 2019, 25 (57), 13146–

13151.

57. Allen, D. W.; Li, X. Solvatochromic and Halochromic Properties of Some

Phosphonioarylimino- and Phosphonioarylazo-Phenolate Betaine Dyes. Journal of

the Chemical Society, Perkin Transactions 2 1997, No. 6, 1099–1104.

58. Lambert, C.; Schmälzlin, E.; Meerholz, K.; Bräuchle, C. Synthesis and Nonlinear

Optical Properties of Three-Dimensional Phosphonium Ion Chromophores.

Chemistry - A European Journal 1998, 4 (3), 512–521.

59. Allen, D. W.; Mifflin, J. P.; Skabara, P. J. Synthesis and Solvatochromism of Some

Dipolar Aryl-Phosphonium and -Phosphine Oxide Systems. Journal of

Organometallic Chemistry 2000, 601 (2), 293–298.

66

60. Ali, M.; Dondaine, L.; Adolle, A.; Sampaio, C.; Chotard, F.; Richard, P.; Denat, F.;

Bettaieb, A.; Gendre, P. L.; Laurens, V.; Goze, C.; Paul, C.; Bodio, E. Anticancer

Agents: Does a Phosphonium Behave Like a Gold(I) Phosphine Complex? Let a

“Smart” Probe Answer! Journal of Medicinal Chemistry 2015, 58 (11), 4521–4528.

61. Nigam, S.; Burke, B. P.; Davies, L. H.; Domarkas, J.; Wallis, J. F.; Waddell, P. G.;

Waby, J. S.; Benoit, D. M.; Seymour, A.-M.; Cawthorne, C.; Higham, L. J.;

Archibald, S. J. Structurally Optimised BODIPY Derivatives for Imaging of

Mitochondrial Dysfunction in Cancer and Heart Cells. Chemical Communications

2016, 52 (44), 7114–7117.

62. Li, G.; Xu, Y.; Zhuang, W.; Wang, Y. Preparation of Organic Mechanochromic

Fluorophores with Simple Structures and Promising Mechanochromic

Luminescence Properties. RSC Advances 2016, 6 (88), 84787–84793.

63. Koyanagi, Y.; Kimura, Y.; Matano, Y. Effects of Boryl, Phosphino, and Phosphonio

Substituents on Optical, Electrochemical, and Photophysical Properties of 2,5-

Dithienylphospholes and 2-Phenyl-5-Thienylphospholes. Dalton Transactions 2016,

45 (5), 2190–2200.

64. Gayton, J.; Autry, S.; Fortenberry, R.; Hammer, N.; Delcamp, J. Counter Anion

Effect on the Photophysical Properties of Emissive Indolizine-Cyanine Dyes in

Solution and Solid State. Molecules 2018, 23 (12), 3051.

65. Marcoux, D.; Charette, A. B. Palladium-Catalyzed Synthesis of Functionalized

Tetraarylphosphonium Salts. The Journal of Organic Chemistry 2008, 73 (2), 590–

593.

66. Marcoux, D.; Charette, A. B. Nickel-Catalyzed Synthesis of Phosphonium Salts

from Aryl Halides and Triphenylphosphine. Advanced Synthesis & Catalysis 2008,

350 (18), 2967–2974.

67. Yang, X.; Zhao, Y.; Zhang, X.; Li, R.; Dang, J.; Li, Y.; Zhou, G.; Wu, Z.; Ma, D.;

Wong, W.-Y.; Zhao, X.; Ren, A.; Wang, L.; Hou, X. Thiazole-Based

Metallophosphors of Iridium with Balanced Carrier Injection/Transporting Features

and Their Two-Colour WOLEDs Fabricated by Both Vacuum Deposition and

Solution Processing-Vacuum Deposition Hybrid Strategy. Journal of Materials

Chemistry 2012, 22 (15), 7136.

68. Klikar, M.; Poul, P. L.; Růžička, A.; Pytela, O.; Barsella, A.; Dorkenoo, K. D.;

Guen, F. R.-L.; Bureš, F.; Achelle, S. Dipolar NLO Chromophores Bearing Diazine

Rings as π-Conjugated Linkers. The Journal of Organic Chemistry 2017, 82 (18),

9435–9451.

69. Yang, X.; Yan, X.; Guo, H.; Liu, B.; Zhao, J.; Zhou, G.; Wu, Y.; Wu, Z.; Wong,

W.-Y. Charged Dinuclear Cu(I) Complexes for Solution-Processed Single-Emitter

Warm White Organic Light-Emitting Devices. Dyes and Pigments 2017, 143, 151–

164.

70. Kondrasenko, I.; Tsai, Z.-H.; Chung, K.-Y.; Chen, Y.-T.; Ershova, Y. Y.;

Doménech-Carbó, A.; Hung, W.-Y.; Chou, P.-T.; Karttunen, A. J.; Koshevoy, I. O.

Ambipolar Phosphine Derivatives to Attain True Blue OLEDs with 6.5% EQE.

ACS Applied Materials & Interfaces 2016, 8 (17), 10968–10976.

71. Misra R., Bhattacharyya S. P. John Wiley & Sons. Intramolecular Charge Transfer:

Theory and Applications; Viley-VCH, 2018.

67