Excited states of bromine-substituted distyrylbenzenes: Models for conjugated polymer emission

Article abstract of DOI:10.1021/jp030041s

Model 1,4-bis(styryl)benzene derivatives 1a,b related to poly(2-bromo-5-methoxy-1,4-phenylenevinylene) were synthesized to probe the effect of bromination on excited-state behavior of conjugated phenylenevinylene analogues. They showed solution absorption maxima at about 385 nm, without evidence of excimer formation up to 100μM. Their fluorescence spectra showed overlapping bands at 435 and 460 nm, with quantum yields in tetrahydrofuran of 0.16 and 0.21 for 1a and 1b, respectively. Excited-state transients were monitored by time-resolved laser flash spectroscopy at room temperature. The triplet states were characterized by absorption maxima at about 520-530 nm with lifetimes of about 0.6 μs, much longer than would be observed for prompt fluorescence states. The quantum yields of singlet to triplet-state intersystem crossing were determined to be 0.45 and 0.28 for 1a and 1b, respectively. The acceleration of triplet transient decay rates under increased oxygen pressure according to the Stern-Volmer law further supports the triplet-state assignments. The relatively high yields of intersystem crossing and low yields of fluorescence are attributed to a bromine-modulated heavy atom effect that enhances intersystem crossing between excited singlet and triplet states.

Full text of DOI:10.1021/jp030041s

J. Phys. Chem. A 2003, 107, 6533-6537
6533
Excited States of Bromine-Substituted Distyrylbenzenes: Models for Conjugated Polymer
Emission
Ananda M. Sarker,† Yuji Kaneko,‡ Paul M. Lahti,*,§ and Frank E. Karasz†
Department of Polymer Science & Engineering, and Department of Chemistry, UniVersity of Massachusetts,
Amherst, Massachusetts 01003, and Photonics Research Institute, National Institute of AdVanced Industrial
Science and Technology, Tsukuba, 305 Japan
ReceiVed: January 13, 2003; In Final Form: April 28, 2003
Model 1,4-bis(styryl)benzene derivatives 1a,b related to poly(2-bromo-5-methoxy-1,4-phenylenevinylene) were
synthesized to probe the effect of bromination on excited-state behavior of conjugated phenylenevinylene
analogues. They showed solution absorption maxima at about 385 nm, without evidence of excimer formation
up to 100 µM. Their fluorescence spectra showed overlapping bands at 435 and 460 nm, with quantum yields
in tetrahydrofuran of 0.16 and 0.21 for 1a and 1b, respectively. Excited-state transients were monitored by
time-resolved laser flash spectroscopy at room temperature. The triplet states were characterized by absorption
maxima at about 520-530 nm with lifetimes of about 0.6 µs, much longer than would be observed for prompt
fluorescence states. The quantum yields of singlet to triplet-state intersystem crossing were determined to be
0.45 and 0.28 for 1a and 1b, respectively. The acceleration of triplet transient decay rates under increased
oxygen pressure according to the Stern-Volmer law further supports the triplet-state assignments. The relatively
high yields of intersystem crossing and low yields of fluorescence are attributed to a bromine-modulated
heavy atom effect that enhances intersystem crossing between excited singlet and triplet states.
Introduction
CHART 1
Poly(p-phenylenevinylene) (PPV) and its derivatives have
been intensely studied due to their potential use in technology
employing organic light-emitting diodes (OLEDs).1 Despite
much recent progress in the fabrication of OLEDs, the nature
of the photoexcited states and the efficiency of their generation
is still not entirely clear. Photoluminescence, especially prompt
fluorescence, is known to originate from the radiative decay of
singlet polaronic excitons, whereas photogenerated triplet ex-
citons in PPVs yield long-lived photoinduced optical absorp-
tions.2 Direct emission from triplet excitons (phosphorescence)
in PPVs and related model compounds has apparently not been
observed, although the presence of such excitons has been
deduced by use of transient absorption,2 photon photoelectron
spectroscopy,3 optically detected magnetic resonance,4 and
pulse-radiolysis experiments.5 The triplet states in PPV-based
OLEDs are particularly interesting, because statistically one
expects three triplet microstates per singlet exciton to be
generated by excitation.6 Thus, we aimed to investigate heavy
atom substitution effects on triplet-state spectroscopy of PPV
oligomers.
We synthesized model compounds 1a,b analogous to previ-
ously described8 polymer 2Br5OHexPPV, which has a push-
pull pattern of hexyloxy and bromine substituent on the aryl
moiety (Chart 1). Most oligomeric PPV models studied previ-
ously have only electron donor9 ring substitution or vinylic
substitution by electron-withdrawing cyano groups.10 The pres-
ence of the bromine atom could, in principle, increase the triplet
populations in photoexcited 1a,b by intersystem crossing due
to a heavy atom effect. We hoped, therefore, to see effects of
the bromine substitution upon the optical spectroscopy. Here
we describe excited-state photophysical studies of these two
model compounds.
Short chain model compounds are suitable to benchmark the
PPV photophysical process in solution. The quenching of
excitons in trapping sites is limited in model systems due to
their finite size, easily achieved chemical purity, and the low
degree of structural disorder. Furthermore, excimer formation
by interchain interactions, which is common in conjugated
systems,7 should be less efficient in model oligo(phenylenevi-
nylenes) in solution than in higher polymers.
Experimental Section
Synthesis. The syntheses of 1a,b are shown in Scheme 1;
synthetic details for 2-4 are given in the Supporting Informa-
tion. Horner-Emmons condensation between phosphonate ester
4 and aldehydes 2a,b in the high-boiling solvent dimethoxy-
ethane gave trans configured 1a and 1b. The trans configuration
was confirmed by the coupling constants of the vinylic protons,
1
J ) >16 Hz in the H NMR. No evidence for the cis isomer
† Department of Polymer Science & Engineering, University of Mas-
sachusetts.
1
was observed by H NMR after recrystallization.
‡ National Institute of Advanced Industrial Science and Technology.
§ Department of Chemistry, University of Massachusetts.
Spectroscopy. Absorption spectra were recorded using
JASCO, Ubest-55, or Shimazu UV-1600 spectrophotometers.
10.1021/jp030041s CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/01/2003

6534 J. Phys. Chem. A, Vol. 107, No. 34, 2003
Sarker et al.
SCHEME 1
Fluorescence spectra were recorded with the correction of the
wavelength-dependent instrument responses using a Hitachi
F-4000 spectrofluorometer. The Φf values were measured
relative to 9,10-diphenylanthracene in cyclohexane as external
standard,11 with correction for the refractive indices of the
solvents. For all the fluorescence measurements, the optical
density of the solutions were kept at OD ∼ 0.2 at the excitation
wavelength.
All transient absorption spectra were obtained using a kinetic
spectrophotometric detection system previously described.12 For
these measurements, either 308 nm excitation pulses from an
XeCl excimer laser (Lambda Physik LPX-100) or 425 nm pulses
from a dye laser (Lambda Physik FL-3002; fwhm 10 ns)
pumped by the 308 nm excimer laser were used. Laser energies
employed were less than 10 mJ. The monitoring light was
oriented perpendicular to the exciting laser beam, passed through
a grating monochromator (JASCO CT-25C), and detected with
a photomultiplier (Hamamatsu Photonix R928) and a storage
oscilloscope (Iwatsu TS-8123). The system was computer
controlled and the data analyzed using a NEC PC-9801vm Intel-
chip personal computer.
Figure 1. UV-vis absorption and fluorescence spectra of 1a in THF
(1 × 10-5 M).
Determination of Bimolecular Rate Constants for Reac-
tion With Oxygen. Compounds 1a and 1b were excited at 308
nm at room temperature and their absorption decay curves
monitored at 530 nm. Excited-state lifetimes were determined
at various oxygen concentrations (calculated from the measured
pressure). The eq 1/τ ) 1/τ0 + kq[Q] was used to calculate the
rate constants, where τ is the observed lifetime, τ0 is the lifetime
in the absence of oxygen (argon degassed), kq is a bimolecular
rate constant, and [Q] is the concentration of oxygen. The rate
of decay was plotted against oxygen concentration at room
temperature to yield the oxygen-quenching rate from the slope.
Figure 2. Triplet-triplet absorption spectra of 1a in THF (1.1 ×
10-5 M).
gous spectra for 1b in THF are very similar to those of 1a under
the same conditions. Therefore, we assign these fluorescence
bands to the lowest excited singlet states of isolated 1a and 1b,
corresponding to about 2.95 eV, or 285 kJ/mol above the ground
state. The fluorescence quantum yields Φf for 1a and 1b were
found to be 0.16 and 0.21, respectively. These were independent
of excitation wavelength throughout the absorption bands,
consistent with the presumption of only one emitting conforma-
tion in solution.
Results and Discussion
Steady-State Spectroscopy. The absorption spectrum of 1a
in THF (10 µM) shows intense absorption with a maximum at
Triplet-State Solution Absorption Spectroscopy. The 308
nm laser excitation of 1a in oxygen-free THF (1.10 × 10-5 M)
at room temperature led to the formation of a very strong
transient absorption spectra with a maximum at 530 nm (Figure
2). The transient lifetime is 0.64 µs under our experimental
conditions with decayed by first-order kinetics (kT ) 1.4 × 106
s-1). Transient absorption spectra of 12.0 µM 1b in THF under
the same conditions also shows a band maximum at 530 nm
(Figure 3). The transient spectrum decayed within a few
nanoseconds in oxygen-saturated solutions (Figure 4). The
behavior of the transient generated from 1b is quite similar under
the same conditions, with kT ) 1.2 × 106 s-1 (Figure 5). The
clean exponential decay observed in the oxygen-free experiments
suggests that triplet-triplet bimolecular annihilation13 is neg-
ligible under these conditions. The transients were assigned to
triplet excited states of 1a and 1b, based on their long lifetimes
385 nm [ꢀ ) 46 000 dm3/(mol cm)] assigned to the π f π*
transition (Figure 1). The shape and width of this band remains
constant over a wide range of concentrations (from 10 to 100
µM). The absence of broadening at high concentrations suggests
a minimal contribution from ground-state aggregates. The lack
of additional peaks at higher concentrations shows that excimers
are not formed to a significant extent.
1
1
A typical fluorescence spectrum of 1a excited at 355 nm in
room-temperature tetrahydrofuran (THF, Figure 1) shows well-
resolved peaks at 435 and 460 nm, the relative shapes of which
remain constant over a range of concentration with no increase
in low-energy emission. Fluorescence excitation spectra moni-
tored at 435 and 460 nm appeared the same, and overlapped
the absorption spectrum (see supporting material). The analo-

Excited States of Bromine-Substituted Distyrylbenzenes
J. Phys. Chem. A, Vol. 107, No. 34, 2003 6535
Figure 3. Triplet-triplet absorption spectra of 1b in THF (1.2 ×
10-5 M).
Figure 6. Buildup profiles for triplet absorption from irradiation of
1a and 1b in THF in the presence of benzil. See also Supporting
Information for further details.
Figure 4. Kinetic profiles for triplet spectral absorption of 1a at 530
nm in THF at room temperature.
quenching rates previously determined for p-phenylenevinylene
oligomers5a and polymers.14,15 The kinetic results of the quench-
ing experiments are included in Table 1, together with sum-
maries of the photophysical studies described above. Overall,
all of these transient absorption spectra were assigned to triplet-
state transitions of 1a,b, based on their long lifetimes and
quenching behavior with oxygen.
Photosensitized Triplet Formation in Solution. Triplet-
triplet energy transfer is one route to generate triplet states of
organic molecules. We found that we could produce the triplet
states of 1 both directly and by photosensitization. Upon
excitation at 308 nm, an argon-saturated 0.01-0.02 M solution
of benzil in THF gave a triplet-triplet absorption at 480 nm.16
When either 1a or 1b was added to a concentration of 0.01
mM, the benzil triplet absorption band at 480 nm decayed with
concomitant formation of a new absorption band at about 530
nm. Figure 6 shows the buildup curve at 530 nm of the triplet
absorption of 1a,b in an argon saturated benzil solution (THF).
These findings demonstrate triplet-triplet energy transfer from
the excited triplet state of benzil to ground state 1a,b, followed
by formation of excited triplet state 1a,b. Benzil thus is an
effective triplet photosensitizer for 1a,b.
The observed rate of triplet buildup was linearly dependent
on the 1a,b concentrations. With increasing concentration of
1a,b, a corresponding increase in the amount of excited triplet
state 1a,b was observed. The energy transfer rate constants (ket)
were calculated by a Stern-Volmer plot in the same manner
as described for the quenching experiments and found to be
1.3 × 1010 and 1.1 × 1010 M-1 s-1 for 1a and 1b, respectively.
The molar extinction coefficients (ꢀT) for the triplet spectra were
estimated relative to the reported ꢀT value of triplet benzil as a
standard.17 The ꢀT values for 1a and 1b at 530 nm in THF were
8200 and 11 000 M-1 cm-1, respectively. Given that the
transients in Figures 4-6 show different rates of decay upon
sensitization than upon direct excitation, it appears that the net
observed behavior involves some equilibration between the
triplet states of benzil and 1a,b.
Figure 5. Kinetic profiles for triplet spectral absorption of 1b at 530
nm in THF at room temperature.
TABLE 1: Photophysical Properties of 1a,b in Excited
Triplet Statesa
properties
1a
1b
λmaxT (nm)
ꢀT (M-1 cm-1
τT (µs)
530
8200
0.64
530
11000
0.50
)
kT (s-1
)
1.4 × 106
1.5 × 109
1.3 × 1010
0.45
1.2 × 106
1.3 × 109
1.1 × 1010
0.28
k
O2 (M-1 s-1
)
ket (M-1 s-1
)
Φisc
a λmax ) absorption maximum, ꢀ ) molar absorptivity estimated by
the method of ref 17, “et” ) energy transfer to benzil, “isc” )
intersystem crossing.
and quenching by oxygen. After laser excitation, the solution
of 1a, displayed a good UV-vis spectral match with the original
solution. Apparently, little or no chemical change occurs upon
laser photoexcitation.
The rate of triplet quenching by oxygen was estimated by a
Stern-Volmer plot of observed rates of transient spectral decay
as a function of oxygen pressure over THF at room temperature.
The transient decay rates were linearly dependent on the oxygen
concentration, consistent with bimolecular quenching. The
quenching rate constants, kO , were found to be 1.5 × 109 and
2
1.3 × 109 M-1 s-1 for 1a and 1b, respectively. Quenching rates
Quantum Yield of Intersystem Crossing. The quantum
yields of intersystem crossing (Φisc) for 1a,b were determined
on the order of 109 M-1 s-1 are consistent with triplet-state

6536 J. Phys. Chem. A, Vol. 107, No. 34, 2003
Sarker et al.
by the triplet-triplet energy transfer method using benzil as
photosensitizer. By comparing the spectral intensity of 1a or
1b triplets formed in each experiment, it is possible to determine
the triplet quantum yields relative to anthracene as a standard
(Φisc ) 0.66) by the following17 equation
so our difficulties in observing 77 K phosphorescence for 1a
are not unique.21
The relative energies of the singlet and triplet excited states
in 1a,b will affect their intersystem crossing rates.22 Because
the S1-T1 gap depends on the oligomer chain length,23 results
for 1a cannot readily be inferred from organic conjugated high
molecular weight polymers. For example, the S1-T1 gap for a
ladder type poly(p-phenylene) and an analogous oligomer was
found to be 0.62 eV from phosphorescence emission,18 and an
estimate of 0.6-0.7 eV for an infinite PPV chain has also been
made.24 Often, nonradiative decay processes in conjugated
oligomers and polymers dominate over phosphorescence due
to the long radiative lifetime of the latter process. The various
decay processes not only are affected by the S1-T1 energy gap
but may also be affected by the substituents on the chro-
mophores.25 Our results are consistent with this, in showing a
strong heavy atom effect. The yields of fluorescent radiative
decay in 1a,b are low, indicating the importance of nonradiative
deactivation pathways. Overall, the present studies demonstrate
singlet excited-state deactivation in bromine-substituted lumi-
nescent molecules, and enhanced crossover between triplet and
singlet excited-state manifolds by substitution on the emitting
chromophore.
OD × ODstd(s) × Astd
ODstd × OD(s) × A
Φics
)
Φics(std)
where OD(s) and OD are the respective maximal intensity of
triplet absorption of 1a or 1b (at 530 nm) with and without
sensitization by benzil. Similarly, ODstd(s) and ODstd are the
maximal optical intensity of anthracene triplet absorption (at
430 nm) with and without benzil, respectively. A and Astd are
the ground-state absorption intensities at 308 nm of 1a or 1b
and anthracene, respectively. The transient absorption spectra
of 1a or 1b and anthracene in THF (at 10 µM) were measured
with excitation at 308 nm. The same measurements using benzil
as sensitizer were recorded with excitation at 425 nm. Only
benzil solutions of absorption intensity ∼1.0 at 425 nm were
used throughout the experiments. Using the equation above, the
yields of triplet formation via intersystem crossing (Φisc) were
determined to be 0.45 and 0.28 for 1a and 1b, respectively.
Overall Considerations. The quantum yield of singlet-state
radiative deactivation for 1a and 1b is low by comparison to
analogues without bromine atom substitution.18 The presence
of the three bromine atoms strongly promotes radiative deac-
tivation of singlets by intersystem crossing (ISC) into triplet
manifolds. Indeed, the quantum yields of ISC for 1a,b are high
enough (Table 1) to be competitive with fluorescence. Efficient
ISC results in higher population of the triplet states, and
increased intensity of triplet manifold electronic spectroscopy.
The 640 ns lifetime of triplet 1a (Figure 2) is much longer than
would be expected for singlet-state transients. Excited singlet
states of less substituted phenylenevinylene derivatives exhibit
prompt fluorescence with very short lifetimes (τ > 2 ns), by
comparison to typical triplet-state lifetimes on the order of a
hundred nanoseconds to several microseconds.19 The triplet-
state absorption maxima of 1a,b at 520-530 nm are similar to
bands observed by laser flash photolysis for other distyrylben-
zene derivatives in dilute solution.19 In addition, the putative
triplet bands of 1a-b are efficiently quenched with molecular
oxygen, reflecting the expected bimolecular energy transfer from
triplet states to triplet oxygen. Excited-state phenylenevinylenes
are known to generate singlet oxygen as a result of bimolecular
quenching by triplet molecular oxygen.14 Recently, the genera-
tion of singlet oxygen was observed from distyrylbenzene with
symmetrical bromine substitution on the central aromatic ring.20
Thus, available data very strongly support the generation of
triplet states in 1a,b.
Acknowledgment. We are grateful to Prof. T. Arai, Depart-
ment of Chemistry, University of Tsukuba, Japan, for use of
laser flash photolysis apparatus, spectrophotometer, and spec-
trofluorometer in carrying out this work.
Supporting Information Available: Synthetic procedures
and characterization data for 1a,b and intermediates in Scheme
1, transient decay curves at 530 nm for calculation of energy
transfer from the triplet state of benzil to 1a or 1b, fluorescence
excitation spectra of 1a at room temperature, 77 K emission
spectrum from irradiation of 1a in EPA glass. This information
is freely available for download from the Internet at http://
pubs.acs.org.
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Excited States of Bromine-Substituted Distyrylbenzenes
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