Transcript MF14-Strzhemechny-2007.ppt

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 & Engineering
Kharkov, Ukraine
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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
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Benzophenones
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.
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.
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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
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Phosphorescence in the crystal
versus temperature
Unusual phosphorescence properties of 2Br-BP
5
293 K
At low T, the emission is typically monomeric
(C=O stretch repetition series). But as T is raised,
TWO monomeric series are present!
192 K
intensity, arb.units
4
136 K
3
119 K
At around 100 K the PL spectrum strongly shifts
to red, the monomeric emission disappearing
completely, unlike in any other benzophenone
95 K
2
77 K
1
40 K
13 K
1.6 K
14000 16000 18000 20000 22000 24000 26000
energy, cm
-1
At lowest T, the spectrum is
NOT structured (like in any
other BP, which usually
evidences triplet exciton
transport to various traps)
PL intensity (arb.units)
52 K
1,0
4Br-BP crystals
low T (1.6 K)
absorption
(non-normalized)
PL
0,8
0,6
0,4
0,2
0,0
414
417
420
423
Wavelength, nm
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426
Origin of strikingly unusual PL
in 2Br-BP
Our structure studies (Baumer et al., Acta Cryst. E, 2005) and quantumchemical calculations show that, unlike other BP, the 2Br-BP molecule
is highly asymmetric, both in the free ground state and in the crystal.
O
O
O
Br
Br
32º 32º
32º 24º
Unsubstituted
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22º 70º
2Br-BP
4Br-BP
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)
O
O
Br
Br
22º 70º
2Br-BP
Ground
state
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58º
1º
2Br-BP
Excited
triplet
state
Which forecasts steric
“difficulties” when an excited
molecule is trying to comply
with the crystal environment
100
90
metastable
80
E
Energy, arb. units
70
"stable"
60
50
40
30
20
CO stretch
vibron levels
10
0
-10
0
2
4
6
8
10
12
14
16
18
20
22
24
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 crystalrelated barrier, which results in two
repetition band series
twist angle
metastable
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]
PL intensity (arb. units)
2.0
stable
1.6
1.2
0.8
0.4
0.0
16000
20000
24000
-1
energy (cm )
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PL intensity (arb.units)
5.4
4.8
The emission at higher temperatures is from
a triplet excimer (Strzhemechny et al.,
Chem.Phys.Lett, 2007). The arguments:
293 K
4.2
192 K
136 K
3.6
1. The red energy shift (about 6000 cm-1) is reasonable for the
excimer binding energy.
119 K
3.0
15000
18000
21000
24000
27000
-1
energy (cm )
1.0
no delay
delay 300 mcs
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.
intensity, arb.units
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
phosphorescence intensity (a.u.)
wavelength, nm
3. Photoluminescence in ethanol solutions has similar features
only at large enough concentrations of 2Br-BP.
1.0
longwave
0.8
0.6
shortwave
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
excitation intensity (a.u.)
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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
glass
4.2 K
phosphorescence intensity
1.0
0.8
glass
111 K
crystal
4.2 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.
0.6
0.4
0.2
0.0
350
400
450
500
550
600
Even at low T the excimer emission contributes
substantially; monomer emission disappears
close to 80-90 K.
650
wave length, nm
1.0
2BrBP glass
room T
excimer emission
PL intensity
0.8
0.6
The excimer band is a sum of two Gaussian
bands broader than in the crystal but
centered similarly
0.4
0.2
0.0
1.4
1.6
1.8
2.0
2.2
2.4
2
wavenumber, 10 nm
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2.6
2.8
6000
Strobe duration 1 mcs
Time interval 5 mcs
T = 294 K
5000
Intensity
4000
3000
2000
1000
0
420
440
460
480
500
520
540
560
580
600
Wavelength, nm
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.
620
quantum yield
0.20
2BrBP glass
Quantum yield gives us information
concerning the temperature-related changes in
sample morphology
0.15
1
0.10
2
0.05
0.00
100
150
temperature, K
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200
250
Conclusions
A 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).
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