Photophysics of transition metal complexes

Francesco Nastasi, Maria Letizia Di Pietro, Emanuela Trovato andFausto Puntoriero*DOI: 10.1039/9781849737722-00156

This chapter deals with studies on the photophysical properties of selected metal

complexes and their polynuclear supramolecular assemblies reported in literature in

the period January 2011 to December 2012. The transition metal species considered

here belong to families of complexes featuring largely studied optical properties. The

complexes are from the following metal centres: d6 Ru(II), Os(II), Re(I), Ir(III) and

Rh(III), d8 Pt(II) and Pd(II), d10 Cu(I) and Au(I), d3 Cr(III); nally some example is

given of lanthanide (Ln) complexes or supramolecular arrays.

1 Introduction

The study of the photophysical and photochemical properties of co-ordination and organometallic compounds is at the center of a large andgrowing interest. Several reasons are at the basis of such an interest,including the use of coordination and organometallic compounds asphoto-active components in processes aimed to perform articial photo-synthesis (e.g., photochemical water splitting and CO2 photoreduction),the design of new dye-sensitized solar cells, the development of newluminescent sensors for environment and biological systems, the devel-opment of new systems for illumination, the preparation of nanostructurescapable of exhibiting new optical properties and luminescence imaging tobe used in diagnostic and medicine. Whereas research in these abovementioned elds is relatively new (or has been strongly revitalized byrecent results) and has literally exploded in the last decade, the photo-chemical and photophysical properties of metal compounds continue to beinvestigated also for fundamental reasons, such as for detailed studiesaimed to increase the knowledge on photoinduced energy and electrontransfer in supramolecular systems.The result of the impressive work focused on the photophysics of

metal compounds in the last years makes impossible to write a reallycomprehensive report on this eld.1 The area is a broad one and in orderto adopt criteria for introducing the choice from the available material,we have made eorts to provide an extended layout of quoted papers;this should allow the interested reader to be aware of the main part ofthe literature available. For instance, in recent years the use of neutralluminescent species in the fabrication of OLEDs, particularly of Ir(III)complexes, has become an important eld of activity for the photo-physical characterization of the luminophores; consideration of this aspecttherefore has provided a criterium for grouping contributions. Among the

Dipartimento di Scienze Chimiche - Universita` degli Studi di Messina, Viale F. StagnodAlcontres, 31, I-98166 Messina Italy and SolarChem - Centro di Ricerca Interuniversitarioper la Conversione Chimica dellEnergia Solare. E-mail: fpuntoriero@unime.it

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c The Royal Society of Chemistry 2013

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transition metal complexes, also the various families of lanthanidederivatives constitute both a distinct and enormeous and expanding area,and for these reasons a summary on this topic is reported at the end of thisreview.In 2011 an highlight paper has been focused on various luminescent

complexes with dierent transition metal centres of d6, d8 and d10 electroniccongurations. Through the systematic study on the variation of ligands,structural and bonding modes of dierent metal centres, the structure-property relationships of the various classes of luminescent transition metalcomplexes have been reported.2

It is essentially clear that an approach like function by design couldbe employed to explore and exploit the potential applications of suchluminescent transition metal complexes. Future development of luminescenttransition metal complexes with improved and optimized functions inthe widely diverse elds of immunoassay, chemosensing, OLEDs, OPVsand DSSCs, multimodal imaging, diagnostics, therapy, photo-catalysis, and photosensitizers for clean and renewable energy is clearlyenvisaged.313

Several review articles on the use of transition metal complexes aschemosensors have been published too.14 In the context of luminescentsensing, transition metal complexes have unique advantages that make themsuitable for chemosensing or biosensing applications: (i) high luminescencequantum yield, (ii) long excited state lifetime, (iii) large Stokes shift,(iv) sensitivity of their photophysical properties to changes in the localenvironment. In light of these advantages ruthenium(II), platinum(II),iridium(III), osmium(II), gold(I) and rhenium(I) compounds have beenwidely studied for luminescent sensing applications.15

2 Ruthenium and osmium

The long-lasting trend for studying complexes based on Ru(II) and Os(II)centres is continuously going up. In the time interval considered, a lot ofnew papers have appeared which report the synthesys and properties of suchspecies. Relevant lines of activity employed Ru(II) complexes as activeprobes towards various substrates, or as active centres for light/energyinterconversion devices.1620

Ru(II) complexes decorated with a bithienyl amide pendant have beenshown to exhibit a very long-lived charge separated state (in the range of 3 to7ms ligand centered-CT) storing at least 2 eV of energy.21 Excited statetuning of a series of supramolecular building blocks based on 2,20:4,400:40,4000-quaterpyridine (qtpy) as ligand and Ru(II) as metal centre has beeninvestigated. The excited states were shown to be qtpy centered exceptwhere the ancillary ligands are dppz.22 Reviews on supramolecular dyadsin which perylene-3,4:9,10-bis(dicarboximide) (DPI) is combined with abpyRuCl2(CN

tBu)2 fragment have been reported and the new studied speciesshow an exoergonic (DGCS of about 1 eV) PET from Ru to DPI. The TAspectroscopy reveals that the charge recombination process happens in 63ps.23

A [Ru(bpy)2(dppz)]2 complex with a dppz ligand decorated by benzimidazolyl-

coumarin group exhibits a non-emissive 3IL state but is able to

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sensitize triplettriplet annihilation up-conversion with DPA.24 Thepresence of two acetyl carbazole moyeties appended to a phen ligandallows an homoleptic Ru(II) complex to show NIR two-photonabsorption.25

The species [Ru(bpy)2(dppz)]2 is non luminescent in aqueous solution or

in the presence of amyloid-beta peptide monomers but exhibits long-livedluminescence upon peptide aggregation, this property allows to use it intime-resolved imaging microscopy.26

It is quite well known that the pure D and L enantiomers of[Ru(phen)2(dppz)]

2 and [Ru(bpy)2(dppz)]2exhibit biexponential excited-

state emission decays when bound to mixed-sequence DNA. By a combinedanalysis of calorimetric and photophysical data for binding of the D and Lenantiomers, the two emission lifetimes have been assigned to two distinctintercalation geometries.One strategy to prolong excited-state lifetimes for bis(tridentate) ruthe-

nium(II) complexes is to bestow a more idealized octahedral ligand envir-onment. Within this eld a series of heteroleptic bis(tridentate)ruthenium(II) complexes, each bearing a substituted 2,2 0:60,200-terpyridine(terpy) ligand, has been reported to be characterized by room temperaturemicrosecond excited-state lifetimes. These results are the consequence of thestrongly s-donating and weakly p-accepting tridentate carbene ligand, 20,60-bis(1-mesityl-3-methyl-1,2,3-triazol-4-yl-5-idene)pyridine (CLNLC), adja-cent to the terpy maintaining a large separation between the ligand eld andmetal-to-ligand charge transfer (MLCT) states while also preserving a large3MLCT energy.27

By exploiting the complexes-as-metals/complexes-as-ligands syntheticstrategy, six new dinuclear achiral complexes of RuII, containing the tri-podal ligand tris(1-pyrazolyl)methane, a chelating ligand such as 2,20-bipyridine, 1,10-phenanthroline or 2,20-biquinoline, and either pyrazine or4,40-bipyridine as bridging ligands, have been prepared and characterized.Spectroelectrochemical experiments have conrmed that in mixed-valence(II/III) species pz is a good electron coupling mediator between the metalcenters. On the contrary, 4,40-bpy does not help the intermetalliccommunication.28

In 2011 a number of reports have appeared dealing with thephotophysical properties of complexes based on 1,2,3-triazole ligands.The mixed valence complex [(tpy)-Ru(ttapyr)Ru(tpy)]3 (ttapyr=1,3,6,8-tetrakis(1-butyl-1,2,3-triazol-4-yl)pyrene) exhibits intervalence chargetransfer transitions.29 Dyads based on [Ru(bpy)3]

2 and naphthalene-bisdiimide-acceptor fragment connected by a 1,2,3-triazole linker showecient electron transfer through the triazole moiety.30

Proton-coupled electron transfer (PCET) from tyrosine (TyrOH) to acovalently linked [Ru(bpy)3]

2 photosensitizer in aqueous media has beenreinvestigated by laser ash-quench kinetics as a model system for PCET inradical enzymes and in photochemical energy conversion. The resultsshowed the mechanistic sensitivity and complexity of PCET reactions withwater as proton acceptor and provide experimental model systems forseveral dierent PCET mechanisms.31

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ADye-sensitised solar cells (DSSCs) remain an active area for the applicationof ruthenium complexes. Several reports have appeared for the developmentof ecient cyclometallated thiocyanate-free complexes as new ecient dyesfor DSSC applications. Complexes of the type A32 and B33 for example, showeciencies of ca. 8%. Remote substituent eects on DSSC performance havebeen investigated in complexes of the form [Ru(dcb)(4,40-(p-C6H4X)2-2,20-bpy)(NCS)2]. An eciency of 8.3% was obtained in devices based on a dyewhere X=OMe.34 [Ru(dcb)(4-{pyrid-2-yl}-1,2,3-triazole)-(NCS)2] basedcomplexes result in DSSC devices with high eciency (7.8%), despite reducedoptical absorption compared to established dyes such as N3.35 In 2012 a newambidentate dicarboxylic acid ligand, dfm, that provides a continuous con-jugation pathway from 2,20-bipyridine (bpy) to a metal oxide surface wassynthesized and coordinated to [Ru(bpy)2]- fragment for sensitization ofTiO2. An ecient rinj = 0.70 0.05 interfacial electron transfer to TiO2 wasobserved even though the [Ru(bpy)2(dfm)]

2MLCT excited state lifetime waso10 ns, providing a new alternative to the commonly utilized 4,40-(CO2H)2-2,20-bipyridine (dcb) ligand for the coordination of transition metal com-pounds to metal oxide surfaces.36

B

A new family complexes containing Ru(II) as the central metal atom and 5-diethylamino-1,10-phenanthroline (5-Et2Nphen) as an environmentally sensitiveligand has been synthesized and their potential as pH reporters examinated.37

The photophysical properties of osmium compounds are to a large extentvery similar to those of ruthenium ones. Indeed, most of the luminescent Os

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compounds are Os(II) polypyridine complexes, analogous to the rutheniumones.The photophysics of Os(II) imine complexes and the light induced energy

and electron transfer reactions involving them have been extensively reviewed.A series of [Os(N6N)2(P

6P)] complexes (N6N=5-(1-isoquinolyl)-1,2,4-triazolate) with relevance to OLEDs containing chelating diphosphineligands P6P has been investigated by DFT and TDDFT methods. Resultsindicate that they are better electron than hole transporters.38

Osmium(II) complexes containing N-heterocyclic carbene (NHC)-basedpincer ligand 1,3-bis(1-methylimidazolin-2-ylidene)phenyl anion (C16C6C1)or 1,3-bis(3-methylbenzimidazolin-2-ylidene)phenyl anion (C26C6C2) andaromatic diimine (2,20-bipyridine (bpy), 1,10-phenanthroline (phen), or 4,40-diphenyl-2,20-bipyridine (Ph2bpy)) in the form of [Os(C

6C6C)(N6N)(CO)]

have been prepared and investigated from a theoretical and experimentalpoint of view. The results indicate that these complexes are emissive with longlifetimes, and the emissive excited-states are shown to be derived fromHOMO-LUMO transitions. Moreover the authors demonstrated that theseligands can aect the photophysical properties via the formation of the hybrid[OsC6C6C] frontier orbitals.39A series of newly synthesized Os(II) complexes bearing isoquinoline-

triazolate/pyrazolate ligands showed a remarkable ratiometric intensitychanges in phosphorescence versus uorescence that are excitation wave-length dependent.40

The rst example of an [Os(tpy)2]-like species able to intercalate intoDNA has been reported in 2012. This compound is completely quenched inaqueous solution and becomes luminescent in the near-IR spectral region inthe presence of a small amount of biopolymer. Thus, this osmium speciesbehaves as a real OFF-ON DNA light switch.41

3 Rhenium

The photochemistry of rhenium compounds occupies a prominent positionin the photochemistry of transition-metal compounds. As pointed out inrecent books and reviews,42 the current photochemistry of rhenium com-pounds is extremely rich, spanning eight oxidation states, from formal Re(0)to formal Re(VII).In 2012 the rst Re(I)-dipyrrinato complexes have been reported. Com-

plexes with the general formulae fac-[ReL(CO)3Cl], fac-[ReL(CO)3PR3],and [ReL(CO)2(PR3)(PR

03)], where L is one of a series of meso-aryl

dipyrrinato ligands and R or R0 are phenyl or butyl, have been preparedand studied. The results showed that the new species are luminescent froman excited state centred on dipyrrinato ligand.43

The introduction of a uorous moiety on the structure of polypyridineRe(I) complexes has oered unique properties for rendering themnovel biological probes. The new reported species are three luminescentrhenium(I) polypyridine uorous complexes [Re(Me2bpy)(CO)3(L)]

(Me2bpy=4,40-dimethyl-2,20-bipyridine; L=3-amino-5-(N-((3-peruorooctyl)-

propyl)aminocarbonyl)pyridine, 3-ethylthioureidyl-5-(N-((3-peruorooctyl)propyl)-aminocarbonyl)pyridine, that oer interesting luminescence behavior to the

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labeled molecules, which would allow new assay design and the possibilityof studying the cellular uptake of the labeled molecules by confocalmicroscopy.44

The species of general formula [Re(N6N)(CO)3L]n(N6N=bpy, phen,

4,7-dimethyl-phen; L=Cl, n=0; L=imidazole, n=1) have been studied byultrafast spectroscopy. Between them [Re(bpy)(CO)3(L)]

complexes (L is amonodentate 1,2,3-ligand) have been reported to exhibit long luminescentlifetimes (in the ms time-scale) in air equilibrated solutions at RT.45 Theultrafast spectroscopy experiments demonstrated that the latter complexundergoes to rapid ISC (in about 150 fs) populating two long-lived tripletstates.46

In 2011 the cluster [Re6S8Cl5(4-ppy)]3 (4-ppy=4-phenylpyridine) was

prepared and the photophysical studies show that this species emits in thered in CH3CN. In detail the transient absorption spectroscopy revealed thatthe emissive state is a CT that involves Re6-core as donor and ppy asacceptor.47 The emission properties of a new family of Re(I) diiminecarbonyl complexes containing isocyano ligands was demonstrated to be netuned.48 [Re(N6N)(CO)3(5-aryltetrazolate)] complexes have been preparedand the photophysical studies show excited state lifetimes that range from 100to 900 ns with quantum yields from 1% to 10%.49 New cyclometalatedcomplexes with general formula [Re(CO)3(N

6C)X](N6C=3-butyl-1-(2-pyridyl)benzimidazolin-2-ylidene, X=Cl, Br) exhibit emission from apartially mixed 3MLCT/3LLCT state.50

Re(I) tricarbonyl complexes with pyrazolylpyridyl based chelate ligandsare luminescent with quantum yields up to 0.05 and exhibit electro-chemiluminescence properties.51

4 Iridium

No doubt, iridium is the metal whose study of the photophysical propertiesof the coordination and organometallic compounds has experienced thelargest increase, in percentage, in the last few years.The studies on complexes of the octahedral d6 Ir(III) centre, for the most

part coordinated by cyclometalated ligands, of which the deprotonatedform of Hppy (2-phenylpyridine) can be considered a sort of archetype,have suddenly expanded in recent years, either as mononuclear complexes,or as a part of larger assemblies, or upon incorporation in polymers andsolid-state devices. Cyclometalated complexes of Ir(III) are by far the mostutilized class of ionic transition-metal complexes (iTMCs) light-emittingelectrochemical cells (LECs).5255

Emission wavelength tuning has been demonstrated through post-cyclometallation modication of [Ir(mppy)3] (mppy=2-(4-methoxyphenyl)-pyridine) at the aryl 5-position.56 Tuning has also been achieved by using asubstituted 2-phenylbenzothiazole,57,58 phenylimidazole59 and substitution in[Ir(tolylpyridine)3]-based complexes.

60 Complexes of the type [Ir(C6C6N)-(C6N6N)] (C6C6N=pyridylbiphenylene, C6N6N=6-phenyl-2,20-bpy)have been prepared and showed a red-shifted emission spectra with respect tothe one of the previously reported bicyclometallated diphenylpyridinecomplexes.61

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The inuence of halogen atoms on a homologous series of bis-cyclome-talated Ir(III) complexes of general formula Ir(2,4-di-X-phenyl-pyr-idine)2(picolinate) (X=H, F, Cl, Br), have been studied showing that thesubstituents have a strong impact on the optical properties of the complexeswith both electronic and geometric eects.62

Triplet state photophysics have been generated in two distinct diketo-pyrrolopyrrole (DPP) chromophores terminated with either phenyl orthienyl spacers, when sandwiched between two Ir(III) complexes usingbipyridyl linkers. Low energy excitation of these metal complexes producedstrongly quenched singlet uorescence, generated quite intense, long-lived(t=3 ms) absorption transients in the red and sensitized 1O2 photo-luminescence in aerated solutions.63

Recent studies have revealed that many Ir(III) complexes possess highstructural diversity and rich photophysical properties that can be exploitedin the development of probes to determine biological structures and tounderstand molecular recognition and cellular processes. In particular, ithas been established that water solubility, lipophilicity, cytotoxicity, cellularuptake, and intracellular localization can all be tuned by using variouscyclometalating and polypyridine ligands.64

The eect of the position of aryl substituents in [Ir(Arpz)2(bpy)] (ArpzH=

1-arylpyrazole) has been investigated from a photophysical point ofview. The para position to the metal has an increased eect on emissionenergy than that in the meta position.65 Enhanced luminescent emissionis observed in complexes of the formula [Ir(atz)2(tBu2bpy)] (atzH=4-aryl-1,2,3-triazole, tBu2bpy=4,4

0-bis-tert-butyl- 2,20-bipyridyl) with quantumyields that reach 80%.66

[Ir(bt)2(acac)] (bt=2-phenylbenzothiazole) acts as a ratiometric lumi-nescent sensor for Hg(II) ions resulting in blue-shifted absorption andemission.67 A carbazate complex [Ir(ppy)2(NH2NHCO2)] has been shownto exhibit long luminescent lifetime and high quantum yield (t=1.8 ms,f=0.45).68 Biscyclometalated iridium complexes with pyrene and peryleneappended acetylacetonate ligands undergo quenched 3MLCT emissionthrough triplettriplet energy transfer processes leading to long-lived 3ILstates.69 A complex containing two [Ir(ppy)2] moieties bridged by a 2,2

0:60,200-terpyridyl-6,600-dicarboxylate ligand shows intense luminescent emis-sion (quantum yield of 18%, only 11% less than the mononuclear picolinateanalogue).70 Iridium complexes with two cyclometallated 1-aryl-3-methyli-midazol-2-ylidene ligands and a pyrazolylpyridine-based ancillary ligand-have been investigated as complexes for OLED applications.71 Introductionof phenyl substituents into the 6- and 60-positions of the bpy ligandin [Ir(ppy)2(bpy)]

complexes allows the thermal population of the non-radiative 3MC states.72 [Ir(ppy)2(Me2SO)2]PF6 has been shown to be anucleus-specic turn-on luminescent staining agent for living cells.73 ComplexC exhibits extremely long-lived coumarin-centred 3IL states (75.5ms) andsensitizes DPA-based triplettriplet annihilation up-conversion.74 An Ir(III)complex bearing a spiro-ring as pendant on a bpy ligand, undergoesselective rhodamine spiro-ring opening in the presence of Hg(II) ionsresulting in modulation of energy transfer to iridium and increased emissionintensity.75

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CThe complex [Ir(tBu2bpy)(L)(NCS)], where L is a tripodal bis-cyclome-talated triphenylphosphite ligand, is luminescent and when integrated inOLED devices allows eciencies up to 14.1%.76 Ir(III) and Pt(II) cyclo-metalated complexes of oligothiophenylpyridines show a changeover from3LC/3MLCT phosphorescence to thiophene-based uorescence on goingfrom thiophenylpyridine to the bi- and trithiophenyl analogues.77 The tet-ranuclear complex in which two [Ir(ppy)2(CN)2]

anions are bridged by two[Ir(ppy)2]

cations via the cyanide ligands retains the luminescent propertiesof the mononuclear components with a phosphorescent quantum yield ofabout 65%.78 An Ir(III)-based soft salt made by [Ir(dfppy)2(NCS)2]

anionsand a dinuclear cation with two [Ir(dfppy)2] moieties bridged by a bis-(imidazophenanthroline)carbozole ligand yields OLED devices with anexternal eciency of 0.44.79 A complex with an [Ir(ppy)2(acac)] core andphenylethynyl substituents in the pyridine 4-positions exhibits two-photonabsorption (TPA) at 800 nm with a TPA cross-section of 44GM andemission at 570 nm in toluene.80 A novel dinuclear Ir(II)Ir(II) dinitrogencomplex has been prepared and exhibits luminescent emission at 582 nm inCH2Cl2 (f=2.5%).

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5 Platinum and gold

Platinum and gold coordination and organometallic compounds have alarge variety of photochemical properties, even larger than the probablymost investigated d6 metal complexes. Luminescent platinum compoundsare usually Pt(II) species, whereas the oxidation states of gold which cangive rise to luminescent species are Au(I), Au(II), and Au(III). Pt(II), Au(III), and Au(I) share the property to be capable of forming luminescentcompounds exhibiting strong metal-metal interaction, a feature which is notusually showed by d6 metal complexes.[Pt(bpy)(1,2-benzenedithiolate)] complexes with a BODIPY chromo-

phore attached to either the bpy or dithiolate ligand show neither BODIPY-based uorescence or Pt-based phosphorescent emission. Transientabsorption spectroscopy shows that BODIPY excitation results in singletenergy transfer to the 1MLLCT state which then undergoes ISC to yield the3MLLCT. Population of the BODIPY 3LC then occurs through energytransfer on the ps time scale and by a Dexter mechanism.82 A Pt(II) benzo-porphyrin complex with four pendant BODIPY chromophores exhibitsfast singlet and triplet bidirectional energy transfer processes betweenBODIPY and porphyrin units showing red phosphorescent at 722 nm, with aquantum yield of 0.26.83 Cyclometalated dipyridylbenzene Pt(II) chloride

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complexes with CF3 pyridyl substituents are shown to result in red-shiftedexcimer emission that reach the NIR region.84

The synthesis and the photophysical behavior of a Pt(II) terpyridyl per-ylenediimide (PDI) acetylide charge-transfer complex have been reported.These studies revealed the spectroscopic characteristics of its associatedligand-localized triplet state in both transient absorption and, for the rsttime, time-resolved step-scan FT-IR.85

Pt(II)acac complexes with benzothiazolylcoumarin,in which the coumarinmoiety is directly cycloplatinated, give triplet-triplet annihilation quantumyields up to 15.4% in the presence of DPA.86 A Pt(II) chloride complex with acyclometalated 2,6-bis(N-alkylbenzimidazol-2-yl)benzene ligand exhibitsbright green luminescence (500587nm, t=5.3ms, f=0.19) and OLEDdevices with an eciency of 11.5%.87 [(tBu3tpy)Pt(CRCtpy)PtCRCt-Bu](OTf)2 exhibits unusually higher emission quantum yield in CH2Cl2(f=0.43) than in MeCN (less than 10%) attributed to the formation of anexcited state contact ion pair in CH2Cl2 solutions.

88 A [Pt(ppy)(acac)]complex with dimesitylboron electron-transporting group attached topyridine and naphthanlenylphenylamine hole-transporting group appendedto phenyl yields bright orange OLED devices with eciencies up to 10.6%.89

Cyclometalated Pt(II) 2,6-bis(oxazol)phenyl complexes are moderatelyemissive with easy tuning of emission wavelength available throughthe oxazole moieties.90 Binuclear complexes [{Pt(Me)(L)}2(m-dppm)] (L=ppyor benzo[h]quinoline, dppm=(diphenylphosphine)methane) show intenseorange-red emission stemming from 3MMLCT excited states.91 1,3-bis-(2-pyridylimino)isoindoline Pt(II) complexes with monodentate N-donorssuch as pyridine, phthalazine or phenanthridine show orange-red emissionfrom 3IL states mixed with some 3MLCT character.92

The DFT calculations performed and the luminescence measurements ona series of tetranuclear gold(I) uorinated amidinate complexes of generalformula [Au(ArN)2C(H)]4 (Ar=4-FC6H4; 3,5-F2C6H3; 2,4,6-F3C6H2;2,3,5,6-F4C6H), elucidated the eect of the uorine number on the prop-erties of this kind of species. In particular, it has been showed that the lessuorinated amidinate complexes display structured emission.This beha-viour is due to metal-perturbed intraligand transitions with a low inuencefrom the Au(I) centers. However, the higher metal contribution to thefrontier orbitals of the more uorinated complexes leads to highly metal-perturbed unstructured LMCT emissions.93

Binuclear complexes with two [Pt(PEt3)2(CRCPh)] moieties bridged by apentacenyl-6,13-diacetylide unit show ligand uorescence in the NIRspectral region (710730 nm).94 The complex [Pt(acac)(C6N)] (C6N is acyclometalated 4-pyrazolylnaphthalimide ligand) yields a triplettripletannihilation up-conversion eciency of 14.1% with DPA.95 The complex[Pt(N6C6N)Cl] (N6C6N=N-phenyl-N-(3-(pyridin-2-yl)phenyl)pyridin-2-amine) has improved square-planar geometry compared to the dipyr-idylbenzene analogue and is intensely luminescent (f=0.65).96 Increasedelectronic insulation between metal and photochromic dithienylethenemoieties in a series of Pt(II) tpy complexes with dithienylethene-functio-nalised acetylide ligands has been shown to reduce the rate of MLCT sen-sitized ring closure and promotes Pt(tpy)-based phosphorescent emission.97

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Arylacetylide complexes of the form [Pt(C6C)(CCAr)2], where C6Cis the chelating carbene 1,10-dipentyl-3,30-methylene-diimidazol-2,20-diy-lidene, display emission from 3IL/3MLCT mixed states.98 [Pt(tpy)Cl] com-plexes with 9,9-di(2-ethylhexyl)-7-diphenylaminouoren-2-yl substituentsin the tpy 4-position exhibit TPA (Two Photon Absorption) Z-scan-derived cross sections of 6002000 GM.99 The complex [{Pt(C6N6N)}2(m-P-Fc-P)]2 (C6N6N=4-tolyl-6-phenyl-2,20-bipyridine, P-Fc-P=1,10-bis-(diphenylphosphino)ferrocene) undergoes PET from ferrocene to Pt with aconstant rate of 2 106 s1 yielding a charge separated state living 9.4 ms.100

D

Bisacetylide complexes (D) with electron donating substituents (in theplace of R in D) have triplet excitons localised away from the Pt centre,resulting in red-shifted spectra and prolongated triplet state lifetime withthe opposite true with inclusion of donating groups. The rate of ISC and theS1-T1 energy gap show for the Marcus relation a reorganisation energy of0.83 eV.101

The emission quantum yields and the lifetimes of a series of alkynyl goldderivatives with general formulae [(diphos)(AuCCpy)2] (diphosphane=2,20-bis(diphenylphosphanyl)propane) have shown a correlation with theAu(I) Au(I) distance. The analysis of the experimental results demon-strated the existence of a correlation between Au(I) . . .Au(I) distance and theradiative rate constant for the deactivation of the emissive triplet states.102

A series of gold(III) complexes containing various tridentate cyclometa-lating ligands derived from 6-phenyl-2,20-bipyridine and alkynyl ligands hasbeen reported to be luminescent at 77 K from an intraligand excited state withsome charge transfer character from the aryl to the bipyridine moiety.103

A cluster with the formula [Au6(C)Cu2(dppy)6]4 (dppy= 2-(diphenyl-

phosphino)-pyridine) with an unprecedented trigonal prismatic Au6 corecontaining a hypervalent C-atom has been reported. The cluster exhibitsbright red luminescent emission.104

A reported gold(III) complex of a tris(pentauorophenyl)corroleligand brominated at each b-pyrrole position exhibits emission insolution at RT (l=769 nm, t=195ps, f=0.003).105 A series of highlyluminescent octanuclear Au(I)/Cu(I) clusters with the formula[Au6Cu2(CRCR)6(P

6P)2]2 (P6P=1,4-bis(diphenylphosphino)benzene) has

been reported. When R=1-cyclohexanolyl emission at 488 nm (quantumyield=94%) is observed.106 Gold(I) complexes containing oligo(o- or m-phenyleneethynylene) ligands exhibit emission attributed to a short-livedprompt emission and a long-lived delayed emission.107 A series of Cu/Ausystems of the form {[Au(C6X5)2][Cu(NCR)(m2-C4H4N2)]}

n (X=F, Cl;R=Me, Ph, CHCHPh) is brightly luminescent when R is Me or Ph

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stemming from a pyrimidine-localised MLCT state, whereas the styrenylsubstituent results in non-radiative decay from a nitrile ligand based MLCTstate.108 Cyclometalated gold(III) complexes of the type [(C6N)Au(CCR)2]have been reported (C6N=ppy, dfppy, 2-(thien-2-yl)pyridine, benzoqui-noline, phenylisoquinoline; R = aryl, silyl groups). Emission occurs frommetal perturbed C6N 3IL states allowing emission tuning on the basis of theidentity of C6N.109,110 Organodiselone Au(I)-Tl(I) complexes are lumi-nescent in the solid state and at 77K with lifetimes in the ns range. Theemissive state is an admixture of 3MMLCT and 3LC states.111 Tetra- andoctanuclear dendritic-like arylthiolate gold(I) complexes around an oligo-phosphino core display mainly 3LMCT based emission with F-, MeO- andMe-4-aryl substituents and 3LC emission in the presence of NO2substituents.112 An 8-hydroxyquinoline-funtionalized rhodamine-basedreceptor acts as a reversible uorescent sensor for gold(III) ions with highselectivity and has been used for imaging of gold(III) in living cells.113

6 Copper

Copper in solution has two common oxidation states: 1 and 2. Due totheir intrinsically superior photochemical and photophysical properties,herein our attention is focused on Cu(I) complexes. Anionic Cu(I) com-plexes do not exhibit attractive photophysical properties (e.g., lumines-cence), unlike cluster and cationic complexes which show a very richphotophysical behaviour. Among the latter, the most extensively investi-gated are NN-type (where NN indicates a chelating polypyridine-likeligand, typically 1,10-phenanthroline) or PP-type (where PP indicates abisphosphine ligand). Both homoleptic [Cu(NN)2]

and heteroleptic[Cu(NN)(PP)] compounds have been investigated.The emissive states are assigned as mixtures of 3ILCT and 3MLCT or

3LLCT and3MLCT.114 The complex [Ir(ppy)2(L)] (L is a dipicolylamino-methyl-appended 2-(benzothienyl)pyridine ligand) acts as a ratiometricsensor for Cu2 under physiological conditions.115 DSSC devices preparedusing Cu(I) complexes with bpy ligands functionalized at the 6-positionwith furyl, thienyl, methylpyrrolyl, phenyl and methyl substituents exhibiteciencies approaching 1.5% (versus 4.5% for N719).116 Neutral Cu(I)complexes such as [Cu(Hqbm)(PPh3)2] (Hbqm=2-(20-quinolyl)- benzimi-dazole), exhibit blue-shifted phosphorescent emission that is longer-livedthan their cationic counterparts, such as [Cu(qbm)(PPh3)2]

+, attributed toadditional LC transitions besides MLCT transitions.117 [Cu(N6N)(P6P)]

complexes (N6N=bpy or phen, P6P=pop or dppb) yield light emittingelectrochemical cells with eciencies approaching those based on Ru(II)and Ir(III) complexes.118 The species E has been reported as a selective two-photon uorescent probe for Cu(I) ions and has been used to image Cu(I) inlive cells using two-photon uorescence microscopy.119 A Cu(II) complexwith a tridentate dansyl-functionalized ligand acts as a turn-on uorescentsensor for nitric oxide.120 A sensor comprising a tricarbocyanine receptorand a 2,20-azanediyl bis(N-hydroxyacetamide) NIR uorophore gives a10-fold enhancement in NIR emission in the presence of Cu2 ions and hasbeen applied in live cell imaging.121

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ERecently the prototype [Cu(CN-xylyl)2(dmp)]tfpb (dmp=2,9-dimethyl-1,10-phenanthroline; CN-xylyl=2,6-dimethylphenylisocyanide; tfpb=tetrakis-(bis-3,5-triuoromethylphenylborate)), having a millisecond emissionlifetime, has been reported to be at least 40 times more sensitive to dioxygen,with respect to [Ru(phen)3]tfpb2 (phen=1,10-phenanthroline).

122 Poly-pyridyl Cu(I) complexes have also being considered as a replacement forruthenium complex sensitizers in dye-sensitized solar cells, owing to theirstriking similarities in absorption spectra and photophysics. Photoinducedcharge-transfer dynamics from a Cu(I) diimine complex to TiO2 nano-particles were investigated by combining multiple time-resolved spectro-scopic methods. An ecient and ultrafast electron transfer process from thesinglet MLCT state was discovered as a result of structural control owing tothe attening of the tetrahedral geometry in the complex and the bulkygroups in the ligands.123

7 Lanthanides

The lanthanides 3 centeres (Ln(III)) exhibit the most external electronicconguration with fn orbitals, n=1 to 14. The Ln(III) centeres becomeluminescent only upon inclusion in Ln-antenna complexes, obtained bysaturation of 89 positions around the metal ion. Several reviews areavailable on the photophysics of this quite large family of complexes.The mechanism of non-overlapping FRET (nFRET) (where energy

transfer is observed between a spectrally non-overlapping donor emitter andan acceptor absorber) has been investigated using an europium chelateemitter and an organic uorophore separated by an oligonucleotide. Fromdistance and temperature dependent data the mechanism is proposed toproceed through thermal excitation from the lowest Eu emission state to ahigher ionic or LMCT state.124

The sensitisation of lanthanide-based emission and energy transfer inmixed transition metal/lanthanide assemblies remains an active area with anumber of reports appearing in the literature.125130 Tb(III) complexeswith octadentate bis(diaminethane) capped ligands containing four 2-hydroxyisonaphthalamide chelate groups have been investigated as newluminescent lanthanide standards and show high quantum yields and longlifetimes in water (ftot more than 50%, t>2.45ms).

131 A watersoluble Yb(III) rhodamine-appended porphyrin complex capped with a[CpCo{(MeO)2P=O}3]

anion exhibits TPA and impressive NIR emission(1060 nm, f=2.5%).132 The complex [Tb(deppa)3] (deppa=6-(diethox-yphosphoryl)picolinate) exhibits emission at 545 nm (f=0.4) with a lifetimeof 2.4ms.133 An Eu(III)/ferrocene dyad exhibits reversible switching ofEu(III) luminescent emission.134

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Luminescent [Ln(Tf2N)3]complexes in a [1-butyl-3-methylpyrrolidi-nium][Tf2N] ionic liquid (Tf2N =bis(triuoromethyl-sulfonyl)amide) dis-play remarkably long lifetimes compared to values quoted for Ln(III)complexes in solution due to weak vibrational quenching.135 Europium andterbium tris(hfac) (hfac=1,1,1,5,5,5-hexauoropentane-2,4-dione) com-plexes with chiral bis(oxazole)pyridine (pybox) ligands display circularlypolarised luminescent (CPL) emission. Despite having the same chir-ality,phenyl-substituted pybox ligands result in opposite CPL spectra tothose of Me- and iPr-substituted complexes.136 Dimeric lanthanide (Eu, Gd,Tb) complexes with bridging biphenyl or diphenylmethane bridges giveenhanced luminescent emission for imaging in the presence of human serumalbumin.137

Arylphosphonate-decorated complexes of Tb(III) are emissive withquantum yields between 0.18 and 0.24 and lifetimes of about 43 ms.138

Nd(III), Yb(III) and Eu(III) complexes of ligands decorated with 8-aminoquinoline and anthraquinone uorophores display dual emissionproperties that are tuned in the presence of dierent metal ions.139

F

Complex F is brightly luminescent in water (f=0.13, t=1.8ms) butdisplays poor two photon absorption coecients. Introduction of 4-aryla-cetylene substituents on the pendant pyridyl rings reduces the luminescentintensity but yield a TPA cross-section of 45 GM at 720 nm.140 Eu(III)complexes with uorenyl-acac ligands with phenyl, 2-naphthyl or 4-biphe-nyl substituents allow tuning of the excitation wavelength. Replacement ofphenyl by naphthyl or biphenyl signicantly shifts excitation into the visible(ca. 500 nm) with quantum yields between 19 and 43% inthe solid state,amongst the highest reported for a visible wavelength sensitised Eu(III)complex.141

Lanthanide complexes of a 2,6-bis(pyrazolyl)pyridine ligand functiona-lized with two iminobismethylenephosphate groups are stable in biologicalmedia and are luminescent with very long lifetimes (t=1.50 and 3.28ms forEu and Tb respectively). Lanthanide complexes based on bis(amide)-derivatives of diethylenetriaminepentaacetic acid with thiol functionalitiesallow access to heterobimetallic complexes through disulphide bridge for-mation. The Eu/Tb complex exhibits dual colour ff emission from bothEu and Tb centres but with some energy transfer from Tb(III) to Eu(III).142

An 8-hydroxyquinoline ligand substituted with a BODIPY group at the5-position has been used for the enhancing of NIR emission from Yb(III),Er(III) and Nd(III). Nd(III) and Er(III) complexes emit weakly at 1060 and

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1382 nm respectively whereas the Yb(III) complex emits strongly at 976 and1003 nm.143

Tris(thienoyltriuoroacetonato) Eu(III) complexes with 4,7-disubstitutedphen ligands have been investigated by DFT/TDDFT methods. Calcula-tions reveal that the luminescent 5D0 state can be populated for H, Me,OMe, Cl and Br substituents but not for p-conjugating CO2Et andC4H3OMe groups through tuning of the energy of the excited tripletstate.144 A terpyridinetetraacetate Eu(III) complex connected to an ampi-cillin moiety is able to bind b-lactamase. The long lifetime of the emissiveEu(III) sensor (41.25ms) has been exploited for time-gated multicolourbioimaging applications.145 A related complex with a pendant 3,4-diaminophenoxy group acts as a light-switch-on time-gated imaging probefor NO which reacts with the diaminophenyl group to form a benzotriazolemoiety.146

In 2012 the photophysical properties of a series of lanthanoid cryptates(Eu and Tb) with an increasing number of 2,20-bipyridine units have beeninvestigated in aqueous solution. The results showed that increasing thenumber of bipyridine units in the cryptates ions both an increase in theenergy of the rst excited singlet state S1 and a lowering of the T1 tripletenergy levels is observed.147,148

G

With the goal to prepare new sensitizers for lanthanoids, a newmultidentate ligand (G) has been reported to sensitize both visible andnear-infrared (NIR) emitters by using the same excitation wavelength, withsignicantly high quantum yields. In particular a new holmium luminescentcomplex was described to emit in both the visible and NIR region.149

A divalent Eu(II) complex exhibits luminescent emission at 528 nmwhich can be attributed to a 4f-5d transition and is a rare example of thephotophysical study of a Eu(II) complex.150 Dinuclear complexes contain-ing two Ln(hfac)3 moieties bridged by two m-phenolic hydroxyquinolineligands display uoride ion induced luminescent enhancement throughreplacement of H2O ligands or formation of OH-F hydrogen bonds thatsuppress non-radiative O-H oscillators.151 Lanthanide nitrate complexes ofthe oligoimidazole ligands bis- and tris{[2-{(imidazol-4-yl)methylidene}-amino]ethyl}amine show enhancement of Ln(III) phosphorescence due to

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high ligand triplet state energies.152 Remarkable Nd(III) and Yb(III) con-taining clusters where the Ln(III) ions are surrounded by a complex array ofthree metallacrown structures comprising 16 Zn(II) ions and 16 picoline-hydroxamic acid ligands have been reported. These new structures exhibitNIR emission.153

An Eu(III) cyclen-based complex with a 4,7-diphenyl-1,10-phenanthro-line-disulfonate (BPS) antenna ligand acts as a turn-o luminescent sensorfor transition metal ions through BPS displacement. A detection limit ofemission quenching by Fe(II) ions of about 10 pM was observed.154,155

Eu(III) and Sm(III) trigonal dodecahedral complexes with chelatingbis(phoshineoxide) ligands such as 4,5-bis(diphenylphosphoryl)-9,9-dime-thylxanthene and hfac ligands display markedly high emission quantumyield (Eu, f=5572%; Sm, f=2.45.0% in d6-acetone) due to enhance-ment of the electric dipole transition and suppression of vibrationalrelaxation.156

8 Miscellanea

A conjugate of a N,N-bis(2-phenylthioethyl)amine binding domain and a1,2-dihydroxyanthraquinone uorophore is a selective and sensitive sensorfor Al(III) ions through switching o of uorescence-quenching PET pro-cesses on Al3 binding.157 Al(III) complexes of a tetraiodinated corroleligand exhibit deep red emission (612 to 700 nm) from a long-lived tripletexcited state (t=92 ms in degassed toluene).158 The eects of the presence ofboth electron-donating and electron-withdrawing 8-hydroxyquinolate (q)ligand substituents on the photophysical properties of their Alq3 complexeshave been investigated. High OLED eciencies of up to 4.6% wereobserved using these uorophores.159 The tris-homoleptic Al(III) complexof the fused-ring hydroxyquinolate ligand 11H-indolo[3,2-c]quinolin-4-olexhibits deep blue luminescent emission and three times the uorescentquantum yield compared to the parent Alq3.

160 A series of stableindium bacteriochlorins has been prepared and shows NIR absorption(740780 nm).161 Fluorescent indium and gallium bis(thiosemicarbazonate)complexes have been investigated as confocal luminescent imagingagents.162 A series of pyrrolyldipyrrinato tin(IV) complexes shows uor-escent quantum yields between 0.28 and 0.61.163 Cyclen-functionalisedperylenediimides have been reported to act as selective and highly sensitiveuorescent probes for lead(II) ions and can be used for imaging of Pb(II) inliving cells.164

Complexes of the type trans-[M(cyclam)(CCCF3)2]OTf (whereM=Cr3, Co3, and Rh3; OTf=triuoromethanesulfonate) were pre-pared and in the case of the rhodium complex the triuoropropynyl ligandenhances the MC luminescence to a quantum yield greater than 0.1.165

The rhodamine B based ligand precursor N-(30,60-Bis(diethylamino)-3-oxospiro-[isoindoline-1,9 0-xanthene]-2-yl)picolinamide is a light switch-onsensor for Fe(III) that could be used as intracellular uorescent imagingagent.166 An organic assembly based on BODIPY, cyanine uorophoresand a terpyridyl metal binding domain acts as a sensor for Fe(II) ions.On iron binding, MLCT state quenches cyanine emission whilst

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leaves BODIPY emission unchanged.167 Clusters with the formula[Mo6X8(n-C3F7CO2)6]

2 where X is Br or I show extraordinarily brightlong-lived red luminescence with f=0.23 and 0.36 respectively and t=370and 303 ms respectively.168

The hexauorophosphate salt of the cationic dyad 5-{4-[Re(CO)3(pico-line)-(4-methyl-2,20-bipyridyl-40-carboxamidyl)]phenyl}-10,15,20-triphenyl-porphyrinatopalladium(II)photoreduces CO2 to CO. Time-resolvedstudies show a multistep energy transfer from Pd to porphyrin and nallyto Re as energy trap.169 Tetranuclear assemblies of the form[{(phen)2M(dpp)}2Ru{(dpp)PtCl2}]

6 (M=Ru, Os, dpp=dipyridylpyr-azine) exhibit terminal metal to dpp 3MLCT transition. Emission fromthese states undergoes quenching by population of triplet charge transferstates centred on the dpp ligand bridging to Pt.170

Temperature-dependent excited state equilibria have been directlyobserved between 3MLCT states and 3LC states in dinuclear complexescontaining [Ru(tpy)2] units spaced by conjugated electron-poor bridges.

171

Dyads of the form [(bpy)2-Ru(dpdpz)Pt(CRCAr)2]2 (dpdpz=2,3-di(pyrid-

2-yl)-5,6-diphenylpyrazine), in which the Pt(II) centre is cyclometalated todpdpz, exhibit intense visible and NIR mixed Ru- and Pt-based MLCTabsorption and NIR emission.172 Ru(bpy)2/Ru(bpy)2 and Ru(bpy)2/Os(bpy)2 bimetallic complexes bridged by 4,5-(benzimidazol-2-yl)imidazo-late show MLCT-based phosphorescence; moreover energy transfer fromRu to Os is observed in the heterobimetallic complex.173 An Ir(III)/Pt(II)triad displays red-shifted luminescent emission compared to both themononuclear Ir(III) bis-cyclometalated and Pt(II) mono-cyclometalatedcomponent analogues such that the metallic units do not retain their indi-vidual photophysical properties.174 Multinuclear platinum(II) acetylidecomplexes containing triethynylbenzene or 1,4-bis(3,5-diethynylphe-nyl)buta-1,3-diyne cores show TPA induced luminescence and TPA crosssections from 6 to 191 GM upon excitation at 720 nm.175,176

The synthesis, characterization, photophysics, and time-dependent den-sityfunctional theory (TD-DFT) calculations of spirobiuorene-bipyridinebased Ir(III), Os(II), and mixed Ir/Os complexes have been presented. Theresults clearly show that the ligand uorescence is almost quantitativelyquenched in all the mono- and bimetallic systems at both temperatures andecient energy transfer toward the appended metal chromophores isoperative.177

The synthesis of p-bonded ruthenium, rhodium, and iridium o-benzo-quinones of the general formula [M(L)2(OM

0-linker)][X]n (n=0, 1, 2) whereM=Ru, L=bpy and M0=Rh, Ir, L=ppy. In these binuclear compounds,the luminophore brick adopts a distorted octahedral geometry due to thetwo polypyridine ligands and to cis O6O coordination of the OM-linkers.The emission properties recorded at low temperature show a red-shiftedemission compared to those of the parent [M(L-L)3] (M=Ru, Rh, Ir)compounds with polypyridine or phenylpyridine ligands.178

A hexakis(zinc(II)porphyrin) nano-ring with butadiynyl inter-porphyrinlinkers has been prepared. Light absorption generates an excited statedelocalized over the whole p-system with a lifetime of 0.5 ps.179 A zincporphyrin dye with a cobalt-based electrolyte has been used to assembly

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DSSC devices obtaining the greatest eciencies to date (up to 13.1%).180

Tetra-meso-arylporphyrinatozinc polyimide containing dendrons built on adimethylxanthenebis(M(II)porphyrin) core (M=Cu, Zn) have been inves-tigated. The dendrons can act as singlet and triplet energy acceptors ordonors depending on the individual dendrimeric system.181

Cyclic bis-zincporphyrin forms host-guest inclusion complexes with C60and C70 fullerenes. These exhibit an excited state involving electron transferto fullerene which decays to the ground state over several hundred pico-seconds.182 Picosecond TA spectroscopy reveals that excitation in the Soretband of tetraphenylporphyrinzinc(II) (ZnTPP) in the presence of excess C60results in the formation of a relatively long-lived charge separated state.183

A diethylpyrrole-bridge dizincporphyrin is able to interact with pyrene.Fluorescence studies show photoinduced singlet-singlet energy transferfrom excited pyrene to Zn.184 Assemblies containing two zinc phthalocia-nines (ZnPc) bridged by an imidazole-functionalized perylenediimide unitdisplay CS states with lifetimes between 3 and 9.8 ns.185 Photoinducedenergy transfer between noncovalent complexes of ZnPc and C60 pyrroli-dine tris-acid ethyl ester (PyC60) occurs predominantly by energy transferfrom a PyC60 triplet excited state to ZnPc.

186 A distorted fused porphyrin-phthalocyanine conjugate ZnPZnPc has been reported. In the presence ofpyridylfullerene, excitation results in energy transfer from ZnP to ZnPcfollowed by electron transfer to fullerene to generate a CS state, whichdeactivates to the ground state in about 1.5 ns.187 A conjugate, two C60fullerenes appended to a ZnP moiety by 1,2,3-triazole linkers, exhibits PETand a long-lived charge-separated state (t=0.48 ms) in benzonitrile. Intoluene, intramolecular energy transfer occurs resulting in C60 triplet stategeneration on ZnP excitation.188

A crown-BODIPY-ZnP assembly acts as a receptor for alkyl ammoniumfunctionalized C60. Excitation of the BODIPY moiety results in singletenergy transfer to ZnP occurring in 7 ps. Following electron transfer pro-cesses result in the formation of the charge separated state which has alifetime approaching 100 ms, the longest reported for a system of thistype.189 A supramolecular array comprising a tetrakispyridine substitutedperylenebisdiimide acceptor coordinated to four Ru(II) carbonyl phthalo-cyanine (RuPc) donor moieties has been reported. Excitation of either thedonor or acceptor groups results in formation of a radical ion pair state witha lifetime of several hundred ps in toluene.190 RuPc and Ru naphthalo-cyanines with dendronised axial pyridine ligands show optical power lim-iting properties.191 Supramolecular arrays based on Sn(IV) porphyrincoordinated to a Ru(II) carbonyl porphyrin with two expanded thiapor-phyrins coordinated axially to Sn have been reported. Photophysical studiesshow little coupling between components showing that they retain theirindividual features.Sn-based emission is signicantly reduced, however, due to non-radiative

decay pathways operating in these systems.192 In contrast to thenon-luminescent Ni(II) complex, Pd(II) and Pt(II) complexes of1,4,8,11,15,18,22,25-octabutoxyphthalocyanine exhibit deep red uorescentand NIR phosphorescent emission from singlet and triplet excited statesrespectively.193 Water-soluble Pt and Pd phthalocyanine complexes have

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been shown to sensitize singlet-oxygen formation with a quantum yield of0.24.194 A multinuclear porphyrin (P) array based on an Au(III)P coresurrounded by four free-base (fb) P units and 12 ZnP units at the peripheryhas been obtained through click chemistry. Excitation of ZnP results inenergy transfer to fbP and in turn charge transfer to Au(III)P. The resultantcharge-shift state lives for microseconds at room temperature.195

Mixed porphyrin/phthalocyanine double-decker rare-earth comp-lexes of Y(III), Sm(III), Eu(III), Tb(III), Dy(III), and Lu(III) have beendeveloped as broad band light absorbers and electron donors inbulk heterojunction solar cells. Eciencies of up to 0.82% have beenachieved.196

9 Abbreviations

BL Bridging LigandCS Charge Separation/SeparatedCT Charge TransferDFT Density Functional TheoryDSSC Dye Sensitized Solar CellsGM two-photon absorption cross section units (Goeppert-

Mayer unit)HOMO Highest Occupied Molecular OrbitalIL IntraligandILCT Intraligand Charge TransferISC intersystem crossingLC Ligand Centred (pp*)LEC Light Emitting CellsLLCT Ligand-to-Ligand Charge TransferLMCT Ligand to metal charge transferLMMCT Ligand to metal to metal charge transferLUMO Lowest Unoccupied Molecular OrbitalMLCT Metal to ligand charge transferMLLCT Metal to ligand to ligand charge transferMMLCT Metal to metal to ligand charge transferMC Metal Centred (dd)MLCT Metal-to-Ligand Charge TransferNIR Near InfraredOLED Organic Light Emitting DiodeQD Quantum DotTDDFT Time Dependent Density Functional TheoryAcac monoanion of acetyl acetonebpy 2,20-bipyridinetpy or terpy 2,20;60,20-terpyridinedppz dipyrido[3,2-a:20,30-c:]phenazinephen 1,10-phenathrolinepic monoanion of 2-picolinic acidppy (anion of) 2-phenylpyridineppz (anion of) phenylpyrazoletppz tetrakis(2-pyridyl)pyrazine

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