PHOSPHORESCENCE OF 2-BROMOBENZOPHENONE FROM 1.6 K TO MELTING M. A. Strzhemechny, A. A. Avdeenko, O....

PHOSPHORESCENCE OF

2-BROMOBENZOPHENONE FROM 1.6 K TO

MELTING

M. A. Strzhemechny, A. A. Avdeenko , O. S. Pyshkin, & L. M. Buravtseva

Verkin Institute for Low Temperature Physics & EngineeringKharkov, Ukraine

MSS-62 OSU June 2007

Talk Layout• Benzophenones as proper materials to study triplet exciton

transport. Shape of 2Br-BP molecule as the key factor.• Technicalities• Phosphorescence in the crystal versus temperature• Origin of strikingly unusual behavior of 2Br-BP• Phosphorescence in the glass state• Conclusions


Benzophenones

Substitution of the hydrogens can substantially or even radically change the molecular shape both in the free molecule and the solid. Our aim was to study (combining structure and phosphorescence measurements with quantum-chemical calculations) how the substitution place influences molecular shape, crystal structure, and optics.

Benzophenone is an ideal model of conformation flexible compounds and a suitable test ground for studying excitation (triplet exciton) transport in crystals and glasses.

The balance between the conjugation energy and the repulsion of the hydrogens at 6 and 6΄ the phenyl rings are rotated out of plane making two twist angles with the ketone group plane (about 32° in free unsubstituted molecule). The twist angles are the key parameters that determine electronic configuration and energy of the free molecule and the crystal packing.


Technicalities


Material: compound 2Br-BP (R-grade, Shostka factory) was purified by (a) very slow recrystallization from solutions & (b) by zone melting

Sample preparation:Single crystals from ethanol solution. Crystals: colorless, slightly

opaque, plate-shaped.Vitreous samples by abrupt cooling to about 90-120 K.

Photoluminescence measurements:Excitation by N2 laser (337 nm), pulse duration 10 ns, pulse power 20 kW/cm2.Recording by cooled FEU-106 photo-multiplier (photon count mode).Luminescence monitored at right angle to excitation path with a double-grating, 0.8-m

scanning spectrometer (inverse dispersion 5.2 Å/mm).PL spectra corrected for spectral sensitivity of recording equipment.Time resolved measurements: strobe duration 1-10 mcs, interval 5-20 mcs.

Quantum-chemical calculations:

DFT B3LYP/cc-pVDZ MP2/cc-pVDZ

Phosphorescence in the crystal versus temperature

14000 16000 18000 20000 22000 24000 26000

1

2

3

4

5

1.6 K

13 K

inte

nsity

, ar

b.un

its

energy, cm-1

293 K

192 K

136 K

119 K

95 K

77 K

52 K

40 K


Unusual phosphorescence properties of 2Br-BP

At low T, the emission is typically monomeric (C=O stretch repetition series). But as T is raised, TWO monomeric series are present!

At around 100 K the PL spectrum strongly shifts to red, the monomeric emission disappearing completely, unlike in any other benzophenone

At lowest T, the spectrum is NOT structured (like in any other BP, which usually evidences triplet exciton transport to various traps)

414 417 420 423 4260,0

0,2

0,4

0,6

0,8

1,0

4Br-BP crystalslow T (1.6 K)

absorption (non-normalized)

PL

PL

inte

nsity

(arb

.uni

ts)

Wavelength, nm


Origin of strikingly unusual PL in 2Br-BP

Our structure studies (Baumer et al., Acta Cryst. E, 2005) and quantum-chemical calculations show that, unlike other BP, the 2Br-BP molecule is highly asymmetric, both in the free ground state and in the crystal.

Unsubstituted

32º 32º

O

Br

32º 24º

4Br-BP

O

22º 70º

2Br-BP

O

Br


But what is more IMPORTANT for luminescence, unlike other PB the 2Br-BP molecule changes its shape DRAMATICALLY upon electronic excitation (Avdeenko et al., Low Temp. Phys. 2006)

22º 70º

2Br-BP

O

Ground state

Br

58º 1º

2Br-BP

O

Excited triplet state

Br Which forecasts steric “difficulties” when an excited molecule is trying to comply with the crystal environment


16000 20000 240000.0

0.4

0.8

1.2

1.6

2.0

PL in

tens

ity (

arb.

uni

ts)

energy (cm-1)

stable

metastable

0 2 4 6 8 10 12 14 16 18 20 22 24-10

0

10

20

30

40

50

60

70

80

90

100E

nerg

y, a

rb. u

nits

twist angle

metastable

"stable"

CO stretchvibron levels

E

Thus, upon excitation the molecule at low temperatures the molecule can find itself in a metastable (due to the interaction with crystal environment) state. Extra energy is required to overcome the crystal-related barrier, which results in two repetition band series

Very crude estimations from the PL intensities Ims from the metastable excited state versus T yields ΔE = 40 ± 10 K, assuming that Ims exp[- ΔE/kT]


0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

phos

phor

esce

nce

inte

nsity

(a.u

.)

excitation intensity (a.u.)

shortwave

longwave

15000 18000 21000 24000 27000

3.0

3.6

4.2

4.8

5.4

119 K

136 K

192 K

293 K

PL in

tens

ity (

arb.

units

)

energy (cm-1)

The emission at higher temperatures is from a triplet excimer (Strzhemechny et al., Chem.Phys.Lett, 2007). The arguments:

1. The red energy shift (about 6000 cm-1) is reasonable for the excimer binding energy.

2. Time-resolved PL data shows that this emission is VERY long-lived (up to a tenth of second and longer), which is typical of excimer emission.

400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

inte

nsity

, arb

.uni

ts

wavelength, nm

no delay delay 300 mcs

3. Photoluminescence in ethanol solutions has similar features only at large enough concentrations of 2Br-BP.

4. Photoluminescence is a linear function of the excitation power, which means that the excimer is single-excitation and bimolecular.

5. The distance between CO groups in the crystal is short enough (3.4 Å) to produce an excimer and tends to become shorter upon excitation.

Phosphorescence in the glass state

350 400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0 glass111 K

phos

phor

esce

nce

inte

nsity

wave length, nm

glass4.2 K

crystal4.2 K Only one set of monomer C=O stretch bands is

present, namely, the one from the global minimum in excited state. No or very small energy barrier.

Even at low T the excimer emission contributes substantially; monomer emission disappears close to 80-90 K.

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.80.0

0.2

0.4

0.6

0.8

1.0

PL

inte

nsity

wavenumber, 102 nm

2BrBP glassroom Texcimer emission

The excimer band is a sum of two Gaussian bands broader than in the crystal but centered similarly


Time resolved phosphorescence in unsubstituted vitreous BP was used to quantitatively study energy-space dispersive relaxation (Richert & Bässler, Chem. Phys. Lett., 1985): the 0-0 band must shift by up to 10 cm-1 with increasing delay time. No such effect was observed in 2Br-BP which corroborate our inference from the data for the crystal that triplet exciton transport in 2Br-BP is strongly suppressed. Molecules emit where excited.

420 440 460 480 500 520 540 560 580 600 6200

1000

2000

3000

4000

5000

6000

Inte

nsity

Wavelength, nm

Strobe duration 1 mcsTime interval 5 mcs T = 294 K

100 150 200 2500.00

0.05

0.10

0.15

0.20

quan

tum

yie

ld

temperature, K

2BrBP glass1

2

Quantum yield gives us information concerning the temperature-related changes in sample morphology



ConclusionsA combined investigation of 2-bromobenzophenone crystal involving single-crystal structure and photoluminescence measurements as well as quantum-chemical calculations led to following conclusions:

1. The title molecule is strongly asymmetric in the ground, changing its shape radically upon excitation. This entails two minima in the excited state in the crystal environment, which explains two sets of CO stretch vibron bands in the low-T phosphorescence .

2. At higher temperatures the emission is mainly from a long-lived single-photon bi-molecular excimer. This is a rare occasion when a robust excimer exists in a molecular crystal.

3. Absence of fine structure in the low-T phosphorescence suggests a suppressed (tunneling) transport of triplet excitons in 2Br-BP. This conclusion is confirmed by time-resolved phosphorescence data for the vitreous state. Reason: the deformation of the molecular shape upon excitation is too large to be transported via a tunneling mechanism (quasi-polaron coherent band narrowing).

PHOSPHORESCENCE OF 2-BROMOBENZOPHENONE FROM 1.6 K TO MELTING M. A. Strzhemechny, A. A. Avdeenko, O....

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