Laser flash photolysis study on the photoinduccd reactions of 3,3′-bridged bithiophenes

Article abstract of DOI:10.1039/a806072j

Photophysical properties and photoinduced reactions of 3,3′-bridged-2,2′-bithiophenes {dithieno[3,2-b:2′,3′-d]-thiophene, 4H-cyclopenta[2,1-b:3,4-b′]dithiophene, and 4H-dithieno[3,2-b;2′,3′-d]pyrrole (DTP)} and 2,2′-bithiophene (BT) were investigated by observing the transient absorption spectra in the visible and near-IR regions using nanosecond laser flash photolysis. Fluorescence quantum yields for the bridged bithiophenes were low compared with that for BT. An especially low quantum yield for DTP in acetonitrile was attributed to an addition reaction with the solvent. The triplet energy of BT was the lowest amongst the examined bithiophenes, indicating some conformational change in the triplet state. Triplet-energy-transfer reactions at diffusion limited rates were confirmed between bithiophenes and triplet-energy donors or acceptors. Photochemical generation of the radical cations of the bithiophenes was confirmed by the transient absorption spectra, which show good correspondence with those observed in γ-ray radiolysis. It was found that the triplet quenching rates of the electron-transfer reactions were small when the Gibbs energy change for the reaction was ≥ -25 kJ mol^-1. The observed tendency agreed with the semi-empirical equation of Rehm-Weller. The generated radical cations decay according to a second-order function, indicating deactivation by a back electron-transfer reaction.

Full text of DOI:10.1039/a806072j

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Laser Ñash photolysis study on the photoinduced reactions of
,3º-bridged bithiophenes
3
Mamoru Fujitsuka,*a Tadatake Sato,b Fumiyasu Sezaki,b Kazuyoshi Tanaka,b Akira Watanabea
and Osamu Ito*a
a Institute for Chemical Reaction Science, T ohoku University, Katahira, Aoba-ku, Sendai,
9
80-8577, Japan
b Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,
Sakyo-ku, Kyoto, 606-8501, Japan
Recei¿ed 3rd August 1998, Accepted 17th September 1998
Photophysical properties and photoinduced reactions of 3,3@-bridged-2,2@-bithiophenes Mdithieno[3,2-b:2@,3@-d]-
thiophene, 4H-cyclopenta[2,1-b:3,4-b@]dithiophene, and 4H-dithieno[3,2-b;2@,3@-d]pyrrole (DTP)N and 2,2@-
bithiophene (BT) were investigated by observing the transient absorption spectra in the visible and near-IR
regions using nanosecond laser Ñash photolysis. Fluorescence quantum yields for the bridged bithiophenes
were low compared with that for BT. An especially low quantum yield for DTP in acetonitrile was attributed
to an addition reaction with the solvent. The triplet energy of BT was the lowest amongst the examined
bithiophenes, indicating some conformational change in the triplet state. Triplet-energy-transfer reactions at
di†usion limited rates were conÐrmed between bithiophenes and triplet-energy donors or acceptors.
Photochemical generation of the radical cations of the bithiophenes was conÐrmed by the transient absorption
spectra, which show good correspondence with those observed in c-ray radiolysis. It was found that the triplet
quenching rates of the electron-transfer reactions were small when the Gibbs energy change for the reaction
was P [25 kJ mol~1. The observed tendency agreed with the semi-empirical equation of RehmÈWeller. The
generated radical cations decay according to a second-order function, indicating deactivation by a back
electron-transfer reaction.
thiophene.20,22 In these dithienothiophenes, sulfur forces the
Introduction
two thiophene rings to lie in a plane, thus increasing the p-
conjugation. The well deÐned structure excludes cisÈtrans con-
formational change due to S-C(a)wC(a)-S twisting, which is
expected for a-linked oligothiophenes such as 2,2@-bithiop-
hene.23h25 Dithieno[3,2-b:2@,3@-d]thiophene has been investi-
gated as a donor for a conducting charge-transfer complex or
as a monomer for a conducting polymer.26h29 A thin Ðlm of
dithienothiophene dimer was reported to show high mobility
and a high on/o† ratio, comparable to that of amorphous
silicon, resulting from the closely packed face-to-face stacking
and wide HOMOÈLUMO gap of the material.28 Several
bithiophene derivatives of which the 3,3-positions are bridged
have already been reported and used as the starting monomer
in chemical or electrochemical polymerization.30 Properties of
the bridged bithiophenes, such as the oxidation potential,
largely depend on the electronic nature of the bridging
group. For application of these oligomers to opto- and
electro-functional materials, systematic studies on their ex-
cited states and charge generation process are indispensable.
These are key processes of many useful and interesting
properties such as photoconductivity of organic semicon-
ductors.
Over the past decade, oligomers of conjugated polymers have
attracted considerable attention because of their importance
in understanding the interesting physical and chemical
properties of conjugated polymers.1 In particular, radical
cations and dications of oligomers have been investigated
widely, since they are responsible for electrical and optical
properties.2h11 The studies on oligomers have been carried
out not only to gain understanding of the electrical and
optical properties but also for their practical applications.
Sexithiophenes and larger oligomers have been examined as
materials for non-linear optics12 and Ðeld-e†ect tran-
sistors.13,14 Photochemical processes of small a-
oligothiophenes, such as bithiophene and terthiophene, have
been studied also in the context of biophotosensitizers15 and
singlet oxygen sensitizers.16,17
While large oligomers are stable in their oxidized forms,
oxidation of smaller oligomers often results in a coupling reac-
tion to form longer oligomers and polymers.18h20 Recently,
Guyard et al. reported investigations on the generation and
coupling reactions of radical cations and neutral radicals of
bipyrrole derivatives.21 They concluded from the kinetic data
that only the cation radicals, and not the neutral radicals,
produce higher oligomers via a coupling reaction followed by
a deprotonation process.
In the present report, we describe a study of the photo-
physical and photochemical properties of 2,2@-bithiophene
(BT) and three bridged bithiophenes (Scheme 1), dithieno[3,2-
The investigations on the oligothiophenes have been carried
out mainly with linear oligomers bonded at the a-positions of
the thiophene ring. Previously, we reported photochemical
generation of radical cations and polymerization of
dithieno[3,2-b:2@,3@-d]thiophene and dithieno[3,4-b:3@,4@-d]
b:2@,3@-d]thiophene
(DTT),
4H-cyclopenta[2,1-b:3,4-b@]
dithiophene (CPDT), and 4H-dithieno[3,2-b;2@,3@-d]pyrrole
(DTP) by a laser Ñash photolysis method. Their properties in
the excited states and the photoinduced reactions are dis-
cussed, based on the kinetic data.
J. Chem. Soc., Faraday T rans., 1998, 94, 3331È3337
3331

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Scheme 1
Experimental
Materials
DTT,31 CPDT,32 and DTP33 were synthesized according to
the literature procedures. BT (Aldrich) and p-dinitrobenzene
(
p-DNB, Wako Pure Chemicals) were puriÐed by several rec-
rystallizations from methanol and ethyl acetate, respectively.
Solvents for the measurements were of spectroscopic grade.
Other chemicals were of the best commercial grade available.
Fig. 1 Absorption and Ñuorescence spectra of bithiophenes in AN.
absorption coefficients of the bridged bithiophenes at the peak
at ca. 300 nm are larger than that of BT. The absorption spec-
trum of a mixed solution of bithiophenes and dinitrobenzenes
(DNB), which act as electron acceptors in the present electron-
transfer studies, was the same with the superposition of the
components. This Ðnding indicates that, in the ground state,
charge-transfer complex formation between them is negligible
under the experimental conditions (bithiophenes 1.0 ] 10~3
mol dm~3, DNB \ 5 ] 10~4 mol dm~3). In the laser Ñash
photolysis experiments on the electron-transfer reactions, the
concentration of DNB or triplet energy acceptors was adjust-
ed so that the excitation light (266 nm) was absorbed pre-
dominantly by the bithiophenes.
Measurements
The transient absorption spectra in the visible and near-IR
regions (350È1000 nm) were measured using an Si-PIN photo-
diode (Hamamatsu Photonics, S1722-02) as the detector and
fourth harmonic generation (FHG, 266 nm) or third harmonic
generation (THG, 355 nm) of a Nd:YAG laser (Quanta-Ray,
GCR-130, fwhm 6 ns) as the excitation source.34 Long-
timescale phenomena were observed by using a photomulti-
plier as the detector. All the samples were contained in a 1 cm
quartz cell and were deaerated by argon bubbling. In order to
avoid the e†ect of polymerization of the resultant radical
cations on the kinetic studies, the concentration of the
bithiophenes was adjusted to be O1 ] 10~3 mol dm~3 in
most of the experiments.
Fluorescence spectra of bithiophenes
Steady-state Ñuorescence spectra of the samples were mea-
sured on a Shimadzu RF-5300PC Ñuorospectrometer. Fluo-
rescence lifetimes were measured by a single-photon counting
method using an argon ion laser (Spectra-Physics, BeamLok
Steady-state Ñuorescence spectra, Ñuorescence quantum yields
(
U ), and lifetimes (q ) are estimated as shown in Fig. 1 and
F
F
Table 1. It has been reported that U for the a-linked oligo-
thiophenes tends to increase with the number of repeating
F
2060-10-SA), a pumped Ti:sapphire laser (Spectra-Physics,
units and becomes constant for larger oligomers than the pen-
Tsunami 3950-L2S, fwhm 1.5 ps) with a pulse selector
tamer: U for BT was estimated to be 0.013, while that for
(
(
Spectra-Physics, Model 3980), a third harmonic generator
GWU-23PS), and a streakscope (Hamamatsu Photonics,
F
sexithiophene is reported to be 0.40.1 q also increases with
F
the size of the oligomer. Small U can be attributed to e†ective
F
C4334-01). In the present study, samples were excited at
spinÈorbit coupling caused by the internal heavy atom e†ect
282 nm.
of sulfur. For BT, the intersystem crossing process is reported
to be the dominant deactivation process of the singlet excited
Phosphorescence spectra were recorded with a Hitachi 850
Ñuorescence spectrometer equipped with a phosphorescence
unit.
state; the quantum yield for the triplet generation (U ) is
T
reported to be ca. 0.9.1 U for the bridged bithiophenes is very
F
Steady-state absorption spectra in the UV, visible, and
near-IR regions were measured with a JASCO V-570DS spec-
trophotometer. Radical cations of bithiophenes at 77 K were
generated by c-ray irradiation of butyl chloride solutions con-
taining 5 ] 10~3 mol dm~3 of bithiophenes at 2.4 ] 104
R h~1 for ca. 6 or 22 h. Butyl chloride was puriÐed according
to a published procedure.35
The products of the photochemical reactions were analysed
with a gas chromatograph mass spectrometer (GCÈMS, Shi-
madzu GCMS-QP5000).
small compared with that for BT (Table 1). In particular, that
for DTP is very small: it was difficult to estimate an accurate
Ñuorescence lifetime by the photon counting method because
of the weak Ñuorescence. It is noteworthy that the U for DTP
F
Table 1 Peak positions (j ), lifetimes (q ), and quantum yields (U ) of
F
F
F
Ñuorescence of bithiophene derivatives
j /nm
q /ns
UF (in AN)a
F
F
BT
362
365
333
337
0.04
0.03
Èb
1.3 ] 10~2
3.7 ] 10~3
1.0 ] 10~3
7.7 ] 10~5
CPDT
DTT
DTP
Results and discussion
Èc
Ground-state absorption spectra of bithiophenes
a U values were estimated by using quinine sulfate in 1 mol dm~3
F
H SO aq. as a reference. b q depends on the concentration of DTT
2
4
F
Fig. 1 shows the absorption spectra of bithiophene derivatives.
While their peaks appeared at similar positions, the molar
(
ref. 22). c q was difficult to estimate because of weak Ñuorescence.
F
3332
J. Chem. Soc., Faraday T rans., 1998, 94, 3331È3337

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depends largely on the solvent; 7.7 ] 10~5 in acetonitrile
(
AN) compared with an estimated value of 2.7 ] 10~4 in
cyclohexane. The large di†erence between them indicates the
existence of some photochemical reaction, which is discussed
in the next section.
Triplet excited state of bithiophenes
Phosphorescence spectra of the bithiophenes were measured
in MeOH/EtOH glass at 77 K (Fig. 2). While DTT, CPDT,
and DTP show phosphorescence with clear vibrational struc-
tures, BT does not. This Ðnding can be attributed to the rigid
structure caused by the bridging group at the 3,3@-positions.
The triplet energy of BT will be consumed by a non-radiative
deactivation process, due to its Ñexible molecular structure.
Triplet energies estimated from the 0È0 bands of the phos-
phorescence spectra are summarized in Table 2.
Fig. 3 Transient absorption spectrum of 1.0 ] 10~3 mol dm~3
CPDT in AN at 100 ns after FHG laser irradiation. Insert:
AbsorptionÈtime proÐle at 380 nm.
Fig. 3 shows a transient absorption spectrum of CPDT in
AN obtained by the FHG laser irradiation. The absorption
band at 380 nm can be attributed to the triplet excited state of
CPDT (3CPDT*), because the transient absodrption band
was quenched in the presence of triplet energy acceptors such
the addition reaction between DTP and AN, which occurred
in the singlet excited state of DTP.
In order to observe an absorption band of 3DTP*, an
energy-transfer reaction from a triplet energy donor to DTP
was attempted. Here, benzophenone (BP) was adopted as the
triplet energy donor because it has a sufficiently high triplet
as oxygen and b-carotene. The peak position and lifetime (q )
T
of 3CPDT* are similar to those of BT and DTT (Table 2).22
energy (2.97 eV) and U (1.0).37 Fig. 5 shows the transient
In the case of DTP in AN, the solution was quite sensitive
to the laser irradiation. The colourless solution became yellow
after a few shots of the FHG laser pulse. Transient absorption
was obtained as in Fig. 4. The absorption band at 440 nm
appeared within the laser duration and did not decay. The
solution after the photolysis experiment showed the same
steady-state absorption as the transient spectrum (insert of
Fig. 4), indicating generation of a photoproduct. From the
GCÈMS analysis of the coloured solution, the generation of
an adduct of DTP with AN was conÐrmed.36 As mentioned
T
absorption spectra of solutions containing DTP and BP in
AN, obtained by the THG laser excitation which excites BP
selectively. With decrease in the absorption band at 510 nm of
3
BP*, a new absorption band appears at 400 nm with an
increasing absorptionÈtime proÐle (Insert of Fig. 5). The new
absorption band can be attributed to 3DTP* generated by an
energy-transfer reaction [eqn. (1)], while an absorption band
at 520 nm, at 1 ls after the laser excitation, can be attributed
to the BP ketyl radical.
above, the U for DTP is much smaller in AN than that in
F
BP Èh l (È3 5 È5 n Èm )
Õ
1BP* È iÈs c È
Õ 3BP*
cyclohexane. The large di†erence in UF
can be attributed to
(1)
3
BP* ] DTP Èk e nÈt (B ÈP )
Õ
BP ] 3DTP*
The bimolecular rate constant of this reaction [k (BP)] was
ent
estimated to be 1.5 ] 1010 dm3 mol~1 s~1, from the relation
Fig. 4 Transient absorption spectrum of 1.0 ] 10~3 mol dm~3 DTP
in AN at 100 ns after FHG laser irradiation. Insert: Absorption
spectra of DTP in AN before (É É É) and after (È) laser irradiation.
Fig. 2 Phosphorescence spectra of bithiophenes in MeOH/EtOH at
7
7 K.
Table 2 Properties of TÈT absorption bands and reaction rates of bithiophene derivatives in triplet excited state
Triplet
energya/eV
k
(bc)d
/dm3 mol~1 s~1
k
(BP)b
/dm3 mol~1 s~1
k
b
ent
ent
TT
j b/nm
e b,c/dm3 mol~1 cm~1
q b/ls
/dm3 mol~1 s~1
T
T
T
BT
2.1e
2.23
2.57
2.58
374
375
384
400
2.1 ] 104
2.3 ] 104
2.2 ] 104
1.4 ] 104
26
27
45
È f
9.5 ] 109
7.4 ] 109
5.5 ] 109
6.4 ] 109
1.2 ] 1010
1.2 ] 1010
1.3 ] 1010
1.5 ] 1010
1.2 ] 1010
1.5 ] 1010
2.7 ] 1010
È f
CPDT
DTT
DTP
a Estimated from 0È0 band of phosphorescence. b In acetonitrile. c Assuming e of TÈT absorption band of benzophenone to be 6.5 ] 103 dm3
mol~1 cm~1 at 520 nm (ref. 38). d In cyclohexane. e Estimated from energy-transfer reactions (Table 3). f The value was not obtained because of
the photoinduced chemical reaction with the solvent (see text).
J. Chem. Soc., Faraday T rans., 1998, 94, 3331È3337
3333

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Table 3 Energy transfer rates from 3BT* to energy acceptors
of energy
E
T
Energy acceptor
acceptora/eV
k
b/dm3 mol~1 s~1
ent
b-Carotene
0.91
1.83
2.04
2.10
2.19
2.97
9.5 ] 109
5.1 ] 109
7.3 ] 109
3.9 ] 109
n.r.c
1
,4-Diphenylbutadiene
Benz[a]anthracene
Pyrene
9
-Fluorenone
Benzophenoned
1.2 ] 1010
a From ref. 37. b In cyclohexane. c No reaction. d Energy transfer from
BP* to BT in AN.
3
Fig. 5 Transient absorption spectra of AN solution containing
] 10~4 mol dm~3 DTP and 1.0 ] 10~3 mol dm~3 BP at 100 ns
2
sesses a biradical nature, has energy minima when the thio-
phene rings are in a perpendicular arrangement as well as when
they are in a planar arrangement.24 The Ñexible structure of
BT will be favourable for lowering its triplet energy, while the
other bridged bithiophenes cannot take more stable arrange-
ments because of the rigid structures due to the bridging
group.
The absorptionÈtime proÐles of the TÈT absorption band of
oligothiophenes deviated somewhat from a Ðrst-order decay
function when the excitation laser power became high (Fig. 7
and insert). This deviation can be attributed to a TÈT annihi-
lation process [eqn. (2)].
(
…) and 1.0 ls (L) after THG laser irradiation. Insert: AbsorptionÈ
time proÐles at 400 and 510 nm.
between a pseudo-Ðrst order decay rate constant and the con-
centration of DTP. The population of the triplet energy of the
other bithiophenes by the energy-transfer reaction from 3BP*
[eqn. (1)] have been conÐrmed similarly. The estimated
bimolecular rate constants are summarized in Table 2. The
rate constants were quite similar to that of DTP, reÑecting the
di†usion-limited rate of the solvent, AN. The molar absorp-
tion coefficients (e ) of bithiophenes at the peak positions (j )
T
T
were estimated by using the e of BP (6.5 ] 103 dm3 mol~1
T
kTT
cm~1 at 510 nm)38 as a reference (Table 2). The estimated e
T
3Th* ] 3Th* ÈÈÈ
Õ
1Th* ] Th
(2)
are ca. 2 ] 104 dm3 mol~1 cm~1, which are similar to the
where Th represents a bithiophene. Assuming that absorption
of 3Th* regenerated via 1Th* in eqn. (2) is negligible, then the
absorptionÈtime proÐle of the TÈT absorption can be
analysed by the following equation:
reported values for oligothiophenes.9
The TÈT absorption bands of bithiophenes were quenched
in the presence of triplet energy acceptors such as oxygen and
b-carotene. Fig. 6 shows the transient absorption spectra
observed in the reaction between 3CPDT* and b-carotene. In
the case of the triplet energy transfer reaction from CPDT to
b-carotene, generation of an absorption band of 3(b-carotene)*
at 520 nm was conÐrmed, with decrease in 3CPDT* at 380
nm (Insert of Fig. 6). The rate constants of the present energy-
GA
1
k
B
k
H
~1
*A \
]
TT exp (k t) [ TT
e k e k
T T T T
(3)
A
T
0
where A is the initial absorbance of the TÈT absorption, k
0
T
and k are the Ðrst- and second-order decay rate constants,
TT
respectively. The rate constants for the TÈT annihilation
transfer reactions [k (bc)] were also estimated as listed in
ent
process indicate that the present deactivation process is di†u-
sion controlled (Table 2).
Table 2. The di†usion-limited rate in cyclohexane is calculated
to be 7 ] 109 dm3 mol~1 s~1, which shows good correspon-
dence with the estimated energy transfer rates.
As mentioned above, phosphorescence of BT was not
observed. Here, in order to evaluate the triplet energy of BT,
energy transfer from 3BT* to triplet energy acceptors was
examined in cyclohexane. Table 3 summarizes the triplet ener-
Absorption spectra of radical cations
The generation of radical cations by c-ray irradiation in a
frozen matrix is a well established process whose reaction
mechanism is shown below.35
gies of the acceptors and the observed reaction rates (k ).
ent
RX ] c ] RX` ] e~
From the results in the Table, the triplet energy is considered
(
4)
to be 2.1 eV.
RX` ] Th ] RX ] Th`
The triplet energy of BT is the smallest amongst the
bithiophenes examined in this study. Recently, Di Ce sare et al.
reported, in their theoretical study based on a semi-empirical
MO method, that the triplet excited state of BT, which pos-
where RX represents butyl chloride, used as a solvent in this
study. In the previous report we applied this method to DTT
and dithieno[3,4-b:3@,4@-d]thiophene and conÐrmed the good
correspondence with the absorption of radical cations
observed in the photoinduced electron-transfer reaction.22
Fig. 6 Transient absorption spectra of cyclohexane solution contain-
ing 1.0 ] 10~3 mol dm~3 CPDT and 3.0 ] 10~5 mol dm~3 b-
carotene at 250 ns (…) and 2.5 ls (L) after FHG laser irradiation.
Insert: AbsorptionÈtime proÐles at 380 and 525 nm.
Fig. 7 AbsorptionÈtime proÐles at 380 nm of an AN solution con-
taining 1.0 ] 10~3 mol dm~3 CPDT. Insert: First-order plot.
3334
J. Chem. Soc., Faraday T rans., 1998, 94, 3331È3337

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Fig. 9 Transient absorption spectra of AN solution containing
1
.0 ] 10~3 mol dm~3 CPDT and 1 ] 10~4 mol dm~3 p-DNB at
50 ns (…) and 2.5 ls (L) after FHG laser irradiation. Insert:
2
AbsorptionÈtime proÐles at 930, 400 and 380 nm.
absorption bands at 930, 550, and 400 nm was conÐrmed with
a decrease in the band at 380 nm due to 3CPDT*. The
absorption band at 930 nm can be attributed to the radical
anion of p-DNB (p-DNB~~).35 The bands at 550 and 400 nm
are assigned to the radical cation of CPDT (CPDT~`), which
show good correspondence with those observed in the c-ray
radiolysis experiment (Fig. 8). AbsorptionÈtime proÐles at 930,
400, and 380 nm are shown in the insert of Fig. 9. The time
proÐle at 930 nm does not show fast radical generation within
the laser duration, indicating that any contribution of
1CPDT* to the electron-transfer reaction will be negligible
under the present experimental conditions, p-DNB
Fig. 8 Absorption spectra of c-irradiated butyl chloride solutions
containing bithiophenes at 77 K.
The absorption spectra of radical cations of the bridged
bithiophenes are shown in Fig. 8. The absorption spectra of
these radical cations are quite similar to each other with a
weak broad band at 550È600 nm and a sharp well structured
band at 380È430 nm.
\
5 ] 10~4 mol dm~3. The rising absorptionÈtime proÐle
at 930 nm shows good correspondence with the decay of the
CPDT*. These Ðndings indicate the generation of radical
Electron-transfer reaction of bridged bithiophenes
3
Photo-illumination of the solution containing thiophene deri-
vatives and an appropriate electron acceptor results in an
electron-transfer reaction which generates the radical cation of
the thiophene derivatives.9h11,18h20,22 In the present report,
o-, m-, and p-DNB have been used as electron acceptors.
Based on their reduction potentials ([0.81, [0.90, [0.69 V
vs. SCE for o-, m-, p-DNB, respectively),39 Gibbs energy
changes for the electron-transfer reactions (*G) were estimated
using the RehmÈWeller equation [eqn. (5), Table 4]:40
ions by an electron-transfer reaction via 3CPDT*, i.e.
kT
q
3
CPDT* ] p-DNB ÈÈÈ
Õ
CPDT~` ] p-DNB~~
(6)
The absorptionÈtime proÐle at 930 nm can be analysed by a
Ðrst-order rise function [eqn. (7)],
*
A \ A [1 [ exp ([k t)]
(7)
0
obs
where it is assumed that the generation rate of radical ions
) is much larger than the decay rate of the generated
(k
*G(kJ mol~1) \ 96.49[E(D~`/D)
obs
radical ions. From the dependence of k on the concentra-
obs
[
E(A/A~~) [ *E [ e2/ae]
(5)
tion of p-DNB, the bimolecular rate constant for the quen-
ching of 3CPDT* (kT) was estimated to be 2.0 ] 1010 dm3
mol~1 s~1. The quenching rate constant at the di†usion limit
of the solvent is reasonable for a reaction where the Gibbs
energy change is estimated from eqn. (5) to be [59.8 kJ
mol~1. The molar absorption coefficient for CPDT~` at 400
nm was estimated to be 1.2 ] 104 dm3 mol~1 cm~1, from
comparison with the absorbance of pDNB~~ at 930 nm. The
estimated molar absorption coefficient of CPDT~` was similar
to those of BT~` and DTT~` (Table 5).
0
,0
0
q
where E(D~`/D) is the oxidation potential of the bithiophene
derivative,41 E(A/A~~) is the reduction potential of the accep-
tor, *E
is the excitation energy of the bithiophene, and
e2/ae is the Coulombic term (0.06 eV for AN).40 A wide
0,0
0
variety of negative *G were expected by combining dinitro-
benzenes and bithiophenes, for which E(D~`/D) depends on
the electronic nature of a bridging group.
In the transient absorption spectra of an AN solution con-
taining CPDT and p-DNB (Fig. 9), the generation of new
Upon 266 nm laser irradiation of a cyclohexane solution
containig CPDT and p-DNB, the absorption bands which
indicate electron-transfer reactions were not observed, while
the decay of 3CPDT* was accelerated. After decay of
Table 4 Oxidation potentials of bithiophene derivatives and Gibbs
energy changes for the electron-transfer reactions with dinitro-
benzenes
3CPDT*, an absorption band due to 3(p-DNB)* was observed
at a shorter wavelength than 350 nm. Thus, the decay of
3CPDT* can be attributed to triplet-energy transfer to
p-DNB; radical ion formation is not preferential in a non-
polar solvent. Similar energy transfer was observed in a cyclo-
hexane solution containing DTT and p-DNB.22 The
bimolecular rate constant for the present energy transfer reac-
tion was estimated to be 8.6 ] 109 dm3 mol~1 s~1, indicating
a di†usion-controlled process.
*
Gb/kJ mol~1
E(D~`/D)a
V vs. SCE
/
p-DNB
o-DNB
m-DNB
BT
1.09
0.98
1.07
0.75
[39.7
[59.8
[83.9
[115.8
[25.1
[48.3
[72.4
[104.2
[16.4
[39.6
[63.7
[95.5
CPDT
DTT
DTP
When an AN solution containing DTP and p-DNB was
irradiated with the FHG laser, generation of p-DNB~~ was
observed in the transient absorption spectra, in addition to an
a Onset potential of electropolymerizations. b 0.06 eV of Coulombic
term was employed.
J. Chem. Soc., Faraday T rans., 1998, 94, 3331È3337
3335

View Article Online
Table 5 Peak positions (j ) and molar absorption coefficients (e) of cation radicals of bithiophene derivatives and reaction rates (kT and k
bet)
max
q
with o-, m-, and p-DNB
kT/dm3 mol~1 s~1
k /dm3 mol~1 s~1
q
bet
jmax/nm
417, 600
395, 560
406, 586
[ c
ea/dm3 mol~1 cm~1
p-DNB
o-DNB
m-DNB
p-DNB
o-DNB
m-DNB
Èb
3.2 ] 1010
8.5 ] 109
[ c
BT
8.8 ] 103
1.2 ] 104
6.5 ] 103
[ c
1.5 ] 1010
2.0 ] 1010
2.4 ] 1010
ca. 1010 d
2.6 ] 109
1.6 ] 1010
9.2 ] 109
[ c
2.5 ] 109
1.5 ] 1010
9.3 ] 109
[ c
8.4 ] 109
1.3 ] 1010
1.1 ] 1010
[ c
3.8 ] 1010
2.4 ] 1010
3.3 ] 1010
[ c
CPDT
DDT
DTP
a e values at peaks around 400 nm were estimated from e of p-DNB~~ (5.9 ] 103 dm3 mol~1 s~1 at 930 nm, see ref. 22). b k was difficult to
bet
estimate because of small *A. c Radical cation and reaction rates were not obtained from transient spectra because of large *A of photoreaction
product of DTP and AN. d The rate was estimated from the absorption band of p-DNB~~ at 930 nm.
addition reaction between excited DTP and AN, which shows
an absorption band at 450 nm (Fig. 10). The rate for the quen-
ching of 3DTP* by p-DNB can be estimated to be ca. 1010
dm3 mol~1 s~1 from the absorptionÈtime proÐle at 930 nm
eqn. (10); the upper limit of k (CRÈS) will be ca. 106 dm3
dim
mol~1 s~1. Since the absorption band which can be attributed
to the products of radical coupling reaction was not observed
in either the transient or steady-state absorption spectra after
laser experiments, the contribution of eqn. (9) will also be
(
insert of Fig. 10).
When o-DNB or m-DNB was used as an acceptor for the
electron-transfer reaction, generation of radical cations of
bithiophenes was similarly conÐrmed in the transient absorp-
tion spectra (Fig. 11). The rate constants for reaction between
bithiophenes in the triplet state and an electron acceptor are
summarized in Table 5. While kT for CPDT and DTT were
q
close to the di†usion-limited rate, those of BT were ca. (2È3)
]
109 dm3 mol~1 s~1 (Table 5). In the case of m-DNB, the
quantum yield for the generation of BT~` was ca. 1/20 of that
for p-DNB. Therefore, the rate constant for the electron-
transfer reaction (k ) between 3BT* and m-DNB will be ca.
et
1
3
08 dm3 mol~1 s~1. Similarly, k for the reaction between
et
BT* and o-DNB was ca. 1 ] 109 dm3 mol~1 s~1. The small
kT and k can be attributed to the small Gibbs energy changes
q
et
for the reactions (Table 4). The observed *G dependence of k
et
is in accord with the semi-empirical equation of RehmÈ
Weller.40
The absorption bands of the generated radical ions decay
after reaching a maximum (Insert of Fig. 12). For the decay
mechanism of the radical cation, a back electron-transfer reac-
tion [eqn. (8)], a coupling reaction between cation radicals
[CRÈCR, eqn. (9)], and that of cation radical and neutral sub-
strate [CRÈS, eqn. (10)] are expected.
kbet
CPDT~` ] p-DNB~~ ÈÈÈÈÕ CPDT ] p-DNB (8)
kdim(CRhCR)
CPDT~` ] CPDT~` ÈÈÈÈ
Õ
(CPDT) 2`
(9)
2
kdim(CRhS)
CPDT~` ] CPDT ÈÈÈÈ
Õ
(CPDT) ~`
(10)
2
The Ðrst two processes obey a second-order decay function
while the third is of Ðrst order. As shown in Fig. 12, the time
proÐle of CPDT~` obeys a second-order decay function over
the examined timescale and does not depend on the concen-
tration of CPDT, indicating a negligible contribution from
Fig. 11 Transient absorption spectra of AN solutions containing BT
and (a) p-DNB, (b) o-DNB, and (c) m-DNB at 250 ns (…) and 2.5 ls
(
L) after FHG laser irradiation. Concentrations of BT and DNB are
1
.0 ] 10~3 and 1.0 ] 10~4 mol dm~3, respectively.
Fig. 10 Transient absorption spectra of AN solution containing
Fig. 12 Second-order plot of an absorptionÈtime proÐle at 410 nm
of AN solution containing 1.0 ] 10~3 mol dm~3 CPDT and
1.0 ] 10~4 mol dm~3 p-DNB. Insert: AbsorptionÈtime proÐle at
410 nm.
1
.0 ] 10~3 mol dm~3 DTP and 2 ] 10~4 mol dm~3 p-DNB at
00 ns (…) and 1.0 ls (L) after FHG laser irradiation. Insert:
1
AbsorptionÈtime proÐles at 920 and 460 nm.
3336
J. Chem. Soc., Faraday T rans., 1998, 94, 3331È3337

View Article Online
1
1
1
0
1
2
V. Wintgens, P. Valat and F. Garnier, J. Phys. Chem., 1994, 98,
minor under the present experimental conditions. Thus, the
generated radical ions are considered to decay mainly accord-
ing to a back electron-transfer reaction. From the slope of the
second-order plot of the absorptionÈtime proÐle at 400 nm,
2
28.
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/e was estimated to be 1.1 ] 106 cm s~1. Employing the
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bet
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ions return to the neutral form at the di†usion limited rate.
14
15
16
17
18
19
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Conclusion
M. Fujitsuka, T. Sato, H. Segawa and T. Shimidzu, Synth. Met.,
The photophysical properties and photoinduced reactions of
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tions proceed via triplet states of bithiophenes under the
present experimental conditions (bithiophenes 1.0 ] 10~3 mol
dm~3, acceptors \5 ] 10~4 mol dm~3). The reaction rates
depend on the Gibbs energy changes estimated by the RehmÈ
Weller equation. The di†erent reactivities of the bithiophenes
reÑect the electron-donor ability at each, which is an impor-
tant characteristic in their application in conducting charge-
transfer complexes and as monomers for conducting polymers.
The generated radical ions decay mainly by back electron-
transfer reactions.
1
995, 69, 309.
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ü
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2
In the present work, c-ray radiolysis experiments were carried
out at Tohoku University Cobalt 60 c-ray Irradiation
Facility. We are grateful to Prof. T. Shimizu and Dr. K. Yana-
gita, Tohoku University, for GCÈMS analysis of photo-
irradiated bithiophenes. The present work is partly supported
by the Grant-in Aid on Priority-Area-Research on ““Carbon
AlloysÏÏ (No. 10137203) from the Ministry of Education,
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Paper 8/06072J
J. Chem. Soc., Faraday T rans., 1998, 94, 3331È3337
3337