Photophysical properties of bis(2,2'-bithiophene-5-yl)benzenes

Article abstract of DOI:10.1039/a802514b

The synthesis of three kinds ofbis(2,2'-bithiophene-5-yl)benzenes (1,2-, 1,3- and l,4-bis(2,2'-bithiophene-5-yl)benzene; o-PhBT2, m-PhBT2 and p-PhBT2) and analyses of their photophysical properties are reported.The electronic structures were also studied

Full text of DOI:10.1039/a802514b

View Article Online / Journal Homepage / Table of Contents for this issue
Photophysical properties of bis(2,2º-bithiophene-5-yl)benzenes
Tadatake Sato,a Kiyotaka Hori,a Mamoru Fujitsuka,b Akira Watanabe,b Osamu Itob and
Kazuyoshi Tanakaa*
a Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,
Sakyo-ku, Kyoto 606-8501, Japan
b Institute for Chemical Reaction Science, T ohoku University, Katahira, Aoba-ku, Sendai
980-8577, Japan
The synthesis of three kinds of bis(2,2@-bithiophene-5-yl)benzenes (1,2-, 1,3- and 1,4-bis(2,2@-bithiophene-5-yl)benzene; o-PhBT ,
2
m-PhBT and p-PhBT ) and analyses of their photophysical properties are reported. The electronic structures were also studied
2
2
by the molecular orbital (MO) method. It was found that the extension of p-conjugation varies with the substitution mode (o-, m-,
or p-), which in turn inÑuences the absorption, Ñuorescence and transient absorption spectra. The unique behaviour of o-PhBT
was interpreted in terms of its lowest unoccupied MO (LUMO) pattern, in which p-orbitals on the adjacent sulfur atoms in each
2
bithienyl moiety can have considerable overlap.
In recent years, oligomers possessing p-conjugation, especially
oligothiophene, have received much attention as model com-
pounds for related conjugated polymers. Since these oligomers
are well deÐned chemical systems, investigations of their size-
dependent properties aids understanding of the properties of
conjugated polymers and these have been extensively studied,
both theoretically and experimentally.1 In particular, studies
on their radical cations and dications have produced a better
understanding of the nature of the polarons and bipolarons in
polythiophenes, that are responsible for their electrical and
optical properties.1a,b Moreover, the photophysical properties
of their neutral state have been comprehensively studied by
several groups.2 These photophysical properties have a close
relationship to their electronic structures. In these studies,
state order is an important subject.
can be model oligomers for a copolymer composed of thieny-
lene and phenylene.9 The degree of extension of the p-
conjugation was tuned by the substitution in the phenylene
moieties. This idea originated from the fact that the phenylene
moieties are expected to induce a remarkable di†erence in the
electronic structures of these compounds. Their photophysical
properties were investigated in detail and compared with
those of the reference compounds and oligothiophenes. Their
electronic structures were also analysed theoretically.
Experimental
Materials
1,2-, 1,3- and 1,4-bis(2,2@-bithiophene-5-yl)benzene (o-PhBT ,
2
Most studies on oligothiophene were carried out in highly
dilute solution in order to suppress any intermolecular inter-
actions. However, in conjugated polymers, intermolecular pro-
cesses are important, in phenomena such as charge transport
and relaxation of the photo-excited state is much a†ected by
higher-order structure. By linking two oligothiophenes it will
become possible to observe clearly the intermolecular pro-
cesses between them. Moreover, Aviram has proposed the
idea of a molecular electronic device using the oligothiophene
dimer.3 The dimer system composed of oligothiophenes has
also been studied. We have already succeeded in the synthesis
of homogeneous and inhomogeneous oligothiophene dimers,
linked by trimethylene, in order to understand the intermolec-
ular processes in polythiophene.4 Kanemitsu et al. have
studied several kinds of oligothiophene dimers as a quasi-one-
dimensional system, separated by a potential-energy barrier.5
Orthogonally fused oligothiophenes have been synthesized by
Diaz et al.6 based on AviramÏs hypothetical structure.3
Kuroda et al. have synthesized oligothiophene dimers bridged
by anthracene.7
m-PhBT and p-PhBT ) were synthesized following the pro-
2
2
cedure described below. 2,2@-Bithiophene (BT), 2,2@:5@2A-ter-
thiophene (TT) and 2,2@-bithiophene-5-ylbenzene (PhBT) were
used as the reference compounds in the present study. BT was
purchased from Aldrich and recrystallized from methanol. TT
and PhBT were synthesized by the Ni-catalysed Grignard
coupling reaction10 and puriÐed by column chromatography
on SiO , followed by recrystallization. The other chemicals
2
In the present study, three kinds of phenylene-bridged oli-
gothiophene dimers (PhBT s) were synthesized in which two
2
2,2@-bithiophene-5-yl (bithienyl) groups are bridged by o-, m-
and p-phenylene groups, respectively. Their molecular struc-
tures are depicted in Fig. 1. The dimer system bridged by
phenylene groups has been studied with respect to the inter-
action between chromophores such as porphyrin.8 However,
the p-conjugation of the bithienyl group in the present com-
pounds should be somewhat extended owing to the connec-
tion with the phenylene group. In this sense, these compounds
Fig. 1 Molecular structures of the series of PhBT s
2
J. Chem. Soc., Faraday T rans., 1998, 94(16), 2355È2360
2355

View Article Online
used for the measurements were of the best commercial grade
available.
C4334-01). In the present study, all the samples were excited
at 368 nm.
The transient-absorption spectra in the visible and near-IR
region (350È1000 nm) were measured using FHG (266 nm) of
an Nd:YAG laser (Quanta-Ray, GCR-130, fwhm 6 ns) as an
excitation source. A pulsed xenon Ñash lamp (Tokyo Instru-
ments, XF80-60) was used as a monitor light. An Si-PIN
photodiode (Hamamatsu Photonics, S1722-02) detector
attached to a monochromator (Ritsu MC-10N) was employed.
The output signal from the detector was recorded with a digi-
tizing oscilloscope (Hewlett-Packard 55410B) and analysed by
a personal computer. All the measurements were carried out
in a 1 cm quartz cell after 15 min of argon bubbling.
m-PhBT . Under a N atmosphere, an anhydrous ether
2
2
solution (15 ml) of thienyl magnesium bromide prepared from
9.8 g of 2-bromothiophene and 1.5 g of magnesium was added
to a mixture of 1,3-dibromobenzene (5.7 g) and NiCl [dppp]
2
(0.32 g) suspended in anhydrous ether. After stirring for 24 h
under reÑux, water was added to the reaction mixture and the
crude product was extracted. The crude product was puriÐed
by SiO column chromatography followed by recrystallization
2
from methanol, giving 5.7 g of 1,3-dithienylbenzene (yield
81%).
6.0 g of 1,3-dithienylbenzene was dissolved in 148 ml of a
1 : 1 mixture of chloroform and acetic acid. 10 g of N-
bromosuccinimide was added and then stirred for 4 h at room
temperature.11 After removal of solvent, the crude product
was recrystallized from n-hexane to give 8.5 g of a-brominated
compound (yield 86%). This compound was applied to the
Grignard cross-coupling reaction with thienyl magnesium
bromide, following the same procedure as that for the synthe-
Results
Absorption and emission
The absorption spectra of the three types of PhBT s are
2
shown in Fig. 2, together with those of the reference com-
pounds. Structureless absorption bands assigned to the pÈp*
transition were observed in m- and p-PhBT . In the spectrum
sis of 1,3-dithienylbenzene. m-PhBT was obtained after puri-
2
of o-PhBT , the observed absorption band was accompanied
2
2
Ðcation by SiO column chromatography followed by
2
by a shoulder on the lower energy side. Absorption maxima
recrystallization from n-hexaneÈbenzene (yield: 9%);
and molar absorption coefficients for the PhBT s are listed in
1H-NMR (400 MHz, CDCl ) d 7.04 (2H, dd, J \ 5, 4 Hz), 7.17
2
3
Table 1.
(2H, d, J \ 4 Hz), 7.22 (2H, dd, J \ 4, 1 Hz), 7.24 (2H, dd,
The absorption maxima of the PhBT s were observed at
J \ 5, 1 Hz), 7.28 (2H, d, J \ 4 Hz), 7.37È7.41 (1H, m), 7.50È
7.52 (2H, m), 7.80È7.81 (1H, m), EIMS m/z; 406 (M`).
2
lower energy than for BT; the energy of absorption maximum
is a measure of the spatial extent of p-conjugation. The degree
o-PhBT . o-PhBT was synthesized from 1,2-dibromoben-
of the redshift from BT was in the order p-PhBT [ m-
2
2
2
zene and 2-bromothiophene following the same procedure as
PhBT [ o-PhBT . It has been reported, for oligothiophenes,
2
2
for the synthesis of m-PhBT . 1H-NMR (400 MHz, CDCl ) d
that the energies of the absorption maxima change inversely
proportional to the conjugation length.2 These results have
been interpreted by the spatial extension of the singlet excited
state over the molecule.1e Supposing that the conjugation
length can be represented by the number of double bonds (n),
the peak energy (E) of the unsubstituted oligothiophenes in
the literature1a,2 can be described by
2
3
6.82 (2H, d, J \ 4 Hz), 6.99 (2H, dd, J \ 4, 5 Hz), 7.04 (2H, d,
J \ 4 Hz), 7.13 (2H, dd, J \ 4, 1 Hz), 7.19 (2H, dd, J \ 1, 5
Hz), 7.37 (2H, m), 7.52 (2H, m). EIMS m/z; 406 (M`).
p-PhBT . p-PhBT was synthesized by the Suzuki reac-
2
2
tion.12 Under an N atmosphere, 0.492 g of p-phenylene
2
bis(2,2@-dimethytrimelthylene)boronate was synthesized fol-
7.32
lowing the literature,13 0.980 g of 5-bromo-2,2@-bithiophene
and 0.228 g of tetrakis(triphenylphosphine)-palladium were
added to the 40 ml of an equivolume mixture of 1,2-dime-
thoxyethane and 2 mol l~1 Na CO aqueous solution. After
E \
] 2.26
(1)
n
which gives n values of 4.7, 5.8 and 8.0 for o-PhBT , m-PhBT
extent of p-conjugation in those molecules. Incidentally,
n \ 5.3 was estimated for PhBT.
2
2
2
3
stirring for 87 h under reÑux, the crude product was extracted
with benzene. The crude product was puriÐed by SiO column
2
chromatography followed by recrystallization from chloro-
form and n-hexaneÈbenzene, giving 0.047 g of p-PhBT (yield:
2
6%). 1H-NMR (400 MHz, CDCl ) d 7.04 (2H, dd, J \ 5, 4
3
Hz), 7.16 (2H, d, J \ 4 Hz), 7.22 (2H, dd, J \ 4, 1 Hz), 7.23
(2H, dd, J \ 5, 1 Hz), 7.26 (2H, d, J \ 4 Hz), 7.61 (4H, s),
EIMS m/z; 406 (M`).
Optical measurements
The absorption spectra were measured with a Shimadzu
UV-2200 spectrophotometer. The Ñuorescence and Ñuores-
cence excitation spectra were measured with a Shimadzu
RF-503A spectroÑuorometer. These spectra were obtained at
20 ¡C for CH Cl solution (10~5È10~4 mol l~1). The Ñuores-
2
2
cence quantum yields were estimated using quinine sulfate dis-
solved in 0.5 mol l~1 sulfuric acid aqueous solution as
standard. For Ñuorescence measurements at 77 K, a solution
in methanolÈethanol \ 1/1(v/v) was used (ca. 10~5 mol l~1).
Before the solutions were cooled to 77 K, Ñuorescence mea-
surements were carried out under identical conditions at room
temperature.
Fluorescence lifetimes were measured by the single-photon
counting method using an argon ion laser (Spectra-Physics,
BeamLok 2060-10-SA), a pumped Ti:sapphire laser (Spectra
Physics, Tsunami 3950-L2S, 1.5 ps fwhm) with a pulse selector
(Spectra Physics, Model 3980), a third harmonic generator
(GWU-23PS) and a streakscope (Hamamatsu Photonics,
Fig. 2 Top: absorption spectra of (a) o-PhBT , (b) m-PhBT and (c)
2
2
p-PhBT . Bottom; those of (a) BT, (b) TT and (c) PhBT. All spectra
2
were recorded in CH Cl at room temperature.
2
2
2356
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

View Article Online
Table 1 Photophysical data for PhBT s in CH Cl at room tem-
The Ñuorescence and Ñuorescence excitation spectra in an
2
2 2
perature
ethanolÈmethanol glass matrix were observed in PhBT s and
2
PhBT at 77 K and room temperature, and are shown in Fig.
absorbance
Ñuorescence
4. A considerable increase in the Ñuorescence intensity at 77 K
was observed in m-PhBT , as well as in PhBT. Since the
vibrational resolution in both the Ñuorescence and excitation
spectra generally increases in comparison with that at room
temperature, these results indicate that suppression of the
inter-ring rotation a†ected the quantum efficiencies in m-
compound
j
/nm
e
/l mol~1 cm~1
j
/nma
U
max
max
max
482
F
2
o-PhBT
324.6
350.8
390.8
3.27 ] 104
5.02 ] 104
5.06 ] 104
0.14
0.13
0.69
2
m-PhBT
402, 422
448, 474
2
p-PhBT
2
PhBT and PhBT. On the other hand, the Ñuorescence inten-
a The italicized wavelength correspond to the band maximum, the
other designates another distinct band (see Fig. 2).
2
sity of p-PhBT did not much increase, although considerable
2
increase in vibrational resolution was seen. The Ñuorescence
excitation bands of p-PhBT shifted to lower energy. In con-
trast, no remarkable di†erence was observed between the Ñuo-
2
The Ñuorescence spectra of the PhBT s are shown in Fig. 3,
together with those of their reference compounds. In contrast
to the absorption spectra, structured Ñuorescence bands were
2
rescence spectra of o-PhBT at 77 K and room temperature,
2
apart from an increase in the Ñuorescence intensity. The Ñuo-
observed in the spectra of m- and p-PhBT s, although that of
rescence excitation spectrum at room temperature showed a
shoulder peak, being more clearly resolved at 77 K.
2
o-PhBT was structureless and observed at the lowest energy
2
region in contrast with the highest energy absorption band.
The Ñuorescence decay curves of the PhBT s in CH Cl are
2
2 2
The shape of the Ñuorescence band, particularly its vibronic
structure, changes in oligothiophenes with the number of
thiophene rings. The Ñuorescence bands observed for o-, m-
given in Fig. 5. The decay curves of m- and p-PhBT s could be
2
Ðt by a single exponential, implying a typical monomolecular
decay. In contrast, that of o-PhBT was Ðt by a double expo-
2
and p-PhBT bore resemblance to those of BT, TT and penta-
nential. The transient Ñuorescence was also measured for the
2
thiophene, respectively.2b The Ñuorescence quantum yields
methanolÈethanol solutions of PhBT s at both room tem-
2
(U ) estimated for the PhBT s are listed in Table 1. These
perature and 77 K. The lifetimes obtained are listed in Table
F
2
values are higher than those of BT (0.013È0.026) and TT
(0.054È0.07);2b the quantum yield increases with the number
of thiophene rings in oligothiophenes. Note that a remarkable
2, where similar behaviour is seen. The Ñuorescence decay
kinetics of o-PhBT did not depend on the concentration
2
(10~4È10~5 mol l~1). Moreover, similar kinetics was also
increase in the yield was observed in p-PhBT . This yield is
observed in the glass matrix at 77 K, thus ruling out a
bimolecular process. In general, the double-exponential model
has been used to describe the existence of two emissive
2
rather higher than those of pentathiophene and hexa-
thiophene (0.28È0.44).2
species.14 Hence, in o-PhBT , two emissive species should
2
exist, although the detail of these is currently not known.
Rotational conformers around the inter-ring bond, for
instance, can be considered to rationalize the kinetics, because
large steric hindrance is expected in o-PhBT , as discussed
2
later. The lifetimes obtained for o- and p-PhBT s did not
2
depend on temperature but that for m-PhBT increased on
2
lowering the temperature. This corresponds to an increase in
quantum yield. The lifetimes of the m- and p-PhBT s did not
2
change much with the solvent, whereas that of o-PhBT in
2
methanolÈethanol solutions decreased.
The experimental values of the radiative rate constants
(k
) in Table 2 were estimated from Ñuorescence lifetime (q)
R, exp
and quantum yield (U ):
F
U
F
k
\
(2)
R, exp
q
On the other hand, k
Berg equation (3a)15 from the absorption and Ñuorescence
spectra
can be estimated from the SticklerÈ
R, calc
8 ] 2303ncn2
P
e(l)
l
k
\
Sl~3T~1
dl
(3a)
(3b)
R, calc
AV
N
Fig. 3 Top: Ñuorescence spectra of (a) o-PhBT , (b) m-PhBT and
2
2
/ I(l) dl
(c) p-PhBT . Bottom: those of (a) BT, (b) TT and (c) PhBT. All
Sl~3T~1 \
2
AV
spectra were recorded in CH Cl at room temperature.
/ l~3I(l) dl
2
2
Fig. 4 Fluorescence and Ñuorescence excitation spectra of (a) o-PhBT , (b) m-PhBT , (c) p-PhBT and (d) PhBT at room temperature (É É É É É É É)
2
2
2
and 77 K (ÈÈÈ) in methanolÈethanol
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
2357

View Article Online
Fig. 5 Fluorescence decay curves of (a) o-PhBT , (b) m-PhBT and (c) p-PhBT recorded in CH Cl at room temperature
2
2
2
2 2
where Sl~3T~1 was estimated from the Ñuorescence spectra
assigned to the tripletÈtriplet (T ^ T ) absorption. It has been
AV
n
1
shown in Fig. 3 and / [e(l)/l]dl from the lowest pÈp* absorp-
reported for oligothiophenes that the tripletÈtriplet absorption
maxima shifted to lower energy, inversely proportional to n,
the number of double bonds.2b Beljonne et al. have reported
tion bands shown in Fig. 2, the bands being extracted by
means of curve Ðtting with Gaussian functions. This equation
can be applied when the absorption is sufficiently intense and
the absorption and Ñuorescence spectra are the mirror image
of each other. Since the latter condition is not completely met
that the lowest triplet excited state (T ) of oligothiophenes is
1
strongly conÐned and the higher triplet state is delocalized
(T ), then the shift in tripletÈtriplet absorption is attributed to
n
for PhBT s, some deviation should be contained in the calcu-
the delocalization of the T state.1e The transient absorption
2
n
lated values. However, rather good agreement between the
band of p-PhBT was composed of a single absorption band
2
experimental and calculated values have been reported for oli-
gothiophenes, in spite of similar absorption and Ñuorescence
spectra.2b,16
around 620 nm, closely resembling that of pentathiophene.2b
The m-PhBT transient absorption band was observed
2
around 450 nm and was accompanied by a shoulder peak
For o-PhBT , two emissive species could exist, as discussed
around 570 nm. The spectra were quite similar to those of
TT2b and PhBT, indicating a similar spatial extent of the
triplet excited states (T and T ) of m-PhBT , TT and PhBT.
2
earlier. In such case, the estimation of k
based on the
R, calc
StricklerÈBerg equation was not acceptable in the strict sense.
1
n
2
However, the observed absorption and Ñuorescence bands of
On the other hand, di†erent behaviour was observed for o-
o-PhBT that should be the superimposed spectra of two
species were not so broad in comparison with those of the m-
PhBT : the absorption maximum was observed around 460
2
2
nm, as in m-PhBT , but an additional peak was observed at
2
and p-PhBT s. The spectra of two emissive species should
longer wavelength. Incidentally, in the PhBT s, phosphor-
2
2
thus bear close resemblance to each other. Therefore, the k
escence could not be observed, similarly to the oligothiop-
R
values estimated from the observed spectra should be close to
henes.
those of two species. The calculated rate constant values are
Theoretical analysis
also listed in Table 2. The calculated values for PhBT s are in
2
reasonable agreement with the experimental results, in which
As described earlier, there should be various rotational con-
the smaller rate constant of o-PhBT corresponds to the cal-
formers in PhBT s. In order to examine the geometries of
2
2
culated value. These results indicate the one-to-one correspon-
PhBT s, molecular orbital (MO) calculations were carried out
2
dence of the ground state to the lowest singlet excited state.
in the semi-empirical level (AM1 approximation17) using the
MOPAC program.18 The potential barrier for rotation
around the inter-ring bond has been studied extensively for
BT and its derivatives.19
Triplet excited state
The transient absorption spectra of the PhBT s and PhBT are
The potential-energy curves plotted for the rotation around
thiopheneÈthiophene (ThÈTh) and thiopheneÈphenylene
2
shown in Fig. 6. These transient absorption bands were
Table 2 Fluorescence lifetimes of the PhBT s
2
CH Cl solution at room temperature
methanolÈethanol solution
2
2
compound
q/ns
k
/109 s~1
k
/109 s~1
q/ns (rt)
q/ns (77 K)
R, exp
0.19, 0.94
R, calc
o-PhBT
0.71, 1.49
0.23
0.97
0.22
0.46
0.35
0.46, 1.22
0.24
0.93
0.50, 1.22
0.41
0.93
2
m-PhBT
0.57
0.71
2
p-PhBT
2
Fig. 6 Transient absorption spectra of (a) o-PhBT , (b) m-PhBT , (c) p-PhBT and (d) PhBT observed at 250 ns (…) and 2.5 ls (L) after
2
2
2
excitation
2358
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

View Article Online
approximation-invariant manner, as described in what
follows: The highest occupied (HO) and lowest unoccupied
(LU) MO patterns of PhBT s were all of pseudo p-type for
2
each ring, as illustrated in Fig. 9. These patterns of p-PhBT
2
are of a p-type, delocalized over the whole molecule. More-
over, the aromatic and quinoid characters are induced by in-
phase overlapping at the HOMO and the LUMO,
respectively.21 On the other hand, for m-PhBT , the nodal
2
plane cuts the p-conjugation in the centre of the phenylene
rings in both the HOMO and the LUMO. Hence p electrons
of m-PhBT are delocalized within the bithienyl unit and a
2
vinyl part in the m-phenylene moiety, and aromatic and
quinoid characters exist within this spatial extent.
In the HOMO of o-PhBT , p-lobes exist on the bithienyl
and the carbon atoms bonded to bithienyl groups. Therefore,
Fig. 7 Potential-energy curves for the rotation around PhÈTh (…)
and ThÈTh (=) rings
2
the relationship between the electronic properties of o-PhBT
and a bithiophene dimer bridged by a vinylene group would
2
(ThÈPh) rings obtained in the present calculation are shown in
Fig. 7. Other structural parameters than h were all energeti-
cally optimized. It is seen that the twisted anti-conformer was
most stable for ThÈTh rotation and that the barrier for
ThÈPh rotation is larger than that for ThÈTh. It has been
reported that the anti-conformer is the more stable, based on
the NMR spectroscopy20 and gas-phase electron di†rac-
tion.19d The potential barrier for the rotation around ThÈTh
rings was calculated to be ca. 0.5 kcal mol~1. This value is
similar to the reported value,19b but rather smaller than those
determined by NMR20 and by ab initio calculation.19c
be of interest. However, the absorption and the Ñuorescence
spectra of the latter are notably di†erent from those of the
former5 and such di†erence could be attributed to the Ðxation
of the two bithienyl groups in the neighbourhood of o-PhBT .
2
In fact, in o-PhBT , since the distance between adjacent sulfur
2
atoms is rather small (2.444 Ó), the LUMO on the two sulfur
atoms can be overlapped, as depicted in Fig. 10. This result
indicates that the interaction between two bithienyl groups
cannot be neglected in its excited state. This interaction of two
bithienyl groups can be considered as one of the reasons for
the structureless absorption and emission.
A large potential barrier for the rotation around PhÈTh
rings should be expected for o-PhBT because of steric hin-
drance. In order to examine that potential barrier, MO calcu-
lations for o-dithienylbenzene were also carried out. After
2
Discussion
Structureless absorption and structured Ñuorescence spectra
were observed for m- and p-PhBT . In a class of non-rigid
aromatic molecules, the di†use absorption band and the struc-
total energetical optimization, two kinds of dihedral angle (h
L
and h ) were made to change. Other structural parameters
R
2
than h were all energetically optimized. In Fig. 8, the potential
tured emission band are observed and interpreted as showing
that the structure in the ground state is non-planar and non-
rigid and that in the excited state is more planar and rigid.22
Such absorption and Ñuorescence spectra have also been
observed for various oligothiophenes.2 These spectra have
been interpreted as follows: The HOMO and the LUMO of
oligothiophenes have aromatic and quinoid character,
respectively16 and, hence, the inter-ring bonds gain double
bond character by HOMOÈLUMO electronic transition
resulting in a more planar and rigid molecular structure. Simi-
larly, the redshift and increase in the vibrational resolution of
proÐles vs. h (dihedral angle between the right-hand Th and
R
the Ph rings) for a couple of Ðxed values of h (that between
L
the left-hand Th and the Ph rings) are given. It is seen that the
conÐguration in which the sulfur atoms on the two thiophene
rings become in close proximity is the most stable and that
the potential barrier to any other conformation is rather large
(ca. 5 kcal mol~1) compared with that of the ThÈPh rotation
in Fig. 7 (ca. 0.9 kcal mol~1).
With respect to the PhBT s, all the structural parameters
2
were energetically optimized except for the assumption of C
s
symmetry in o-PhBT . The dihedral angles between the thio-
2
phene rings obtained by this optimization were 152¡, 156¡ and
153¡ for o-, m- and p-PhBT , respectively. The dihedral angles
2
between ThÈPh rings were 44¡, 28¡ and 20¡ for o-, m- and
p-PhBT , respectively.
2
Although the present MO calculations are only qualitative,
correlation between the photophysical properties of the
PhBT s and their electronic structures can be found by an
2
Fig. 8 Potential-energy curves for rotation around PhÈTh rings of
o-dithienylbenzene. The dihedral angle h (see text) is Ðxed at [45¡
Fig. 9 Frontier MO patterns of (a) o-PhBT , (b) m-PhBT and (c)
L
2
2
(>), [90¡ (L) and [135¡ (…).
p-PhBT
2
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
2359

View Article Online
physical properties di†ered considerably depending on the
substitution mode in the phenylene moieties. It is shown from
the MO patterns of these compounds that such a change is
due to a remarkable di†erence in the extent of the p-
conjugation. In particular, the result for o-PhBT was quite
2
di†erent from those of m- and p-PhBT . The electronic struc-
2
ture of o-PhBT is rather unique and requires more attention.
The LUMO pattern of o-PhBT indicated that its excited
state can be delocalized by interaction between the excited
2
2
and the unexcited bithienyl moieties. The structureless Ñuores-
cence and decrease in lifetime with solvent polarity suggested
a contribution from the charge-transfer state to the singlet
excited state.
Fig. 10 The SÈS overlapping (pointed by an arrow) in the LUMO of
o-PhBT : (a) Pattern seen from the same direction as Fig. 9 and (b)
2
from the right-hand side
This work is a part of the project of Institute for Fundamental
Chemistry, supported by Japan Society for the Promotion of
ScienceÈResearch for the Future Program (JSPS-
RFTF96P00206).
m- and p-PhBT s in a glass matrix at 77 K can be explained
2
by the molecular structure becoming more planar in the
ground state. The change in the Ñuorescence spectra of m- and
References
p-PhBT s is similar to those of TT and pentathiophene.2b In
2
1
(a) G. Horowitz, A. Yassar and H. J. von Bardeleben, Synth. Met.,
1994, 62, 245; (b) J. Guay, P. Kasai, A. Diaz, R. Wu, J. M. Tour
and L. H. Dao, Chem. Mater., 1992, 4, 1097; (c) D. Fichou, G.
Horowitz and F. Garnier, Synth. Met., 1990, 39, 125; (d) D.
Fichou, G. Horowitz, B. Xu and F. Garnier, Synth. Met., 1990,
39, 243; (e) D. Beljonne, J. Cornil, J. L. Bredas and R. H. Friend,
Synth. Met., 1996, 76, 61; ( f ) J. Cornil, D. Beljonne and J. L.
Bredas, J. Chem. Phys., 1995, 103, 842.
m- and p-PhBT s, the Ñuorescence excitation spectra at 77 K
2
and Ñuorescence spectra at room temperature form a good
mirror image, as seen in Fig. 4. These results indicate that
these molecules in the lowest singlet excited state at room
temperature and the ground state at 77 K have similar plan-
arity. However, the observed behaviour of o-PhBT was quite
2
di†erent. The shape of the Ñuorescence band resembles that of
2
(a) D. Grebner, M. Helbig and S. Rentsch, J. Phys. Chem., 1995,
99, 16991; (b) R. S. Becker, J. Seixas de Melo, A. L. Macanita and
F. Elisei, J. Phys. Chem., 1996, 100, 18683.
BT at room temperature, as seen in Fig. 3. However, it has
been reported that BT showed a well resolved spectrum at 77
K.2b This indicates that the di†used absorption and emission
3
4
A. Aviram, J. Am. Chem. Soc., 1988, 110, 5687.
bands observed for o-PhBT do not come from the inter-ring
T. Sato, H. Ino, M. Fujitsuka and K. Tanaka, T etrahedron L ett.,
1997, 38, 6039.
2
rotation of bithienyl moieties.
5
6
7
Y. Kanemitsu, K. Suzuki, Y. Masumoto, Y. Tomiuchi, Y. Shi-
raishi and M. Kuroda, Phys. Rev. B, 1994, 50, 2301.
The absorption, Ñuorescence and transient absorption of m-
and p-PhBT s can be interpreted in terms of the extent of
2
J. Guay, A. Diaz, R. Wu and J. M. Tour, J. Am. Chem. Soc., 1993,
115, 1869.
p-conjugation, as discussed in the theoretical analysis. Inci-
dentally, “cuttingÏ of the p-conjugation observed in m-PhBT
has also been seen in the polymer prepared by its electro-
(a) M. Kuroda, J. Nakayama, M. Hoshino and N. Furusho,
T etrahedron L ett., 1992, 33, 7553; (b) M. Kuroda, J. Nakayama,
M. Hoshino, N. Furusho and O. Shigeru, T etrahedron L ett.,
1994, 35, 3957.
2
chemical polymerization.9c For o-PhBT , unusual behaviour
2
was observed in the absorption, Ñuorescence and transient
8
9
(a) K. Maruyama and A. Osuka, Pure Appl. Chem., 1990, 62,
1511; (b) A. Osuka, S. Nakajima, T. Nagata, K. Maruyama and
K. Toriumi, Angew. Chem. Int. Ed. Engl., 1991, 30, 582.
(a) R. Danieli, P. Ostoja, M. Tiecco, R. Zamboni and C. Taliani,
J. Chem. Soc., Chem. Commun., 1986, 1473; (b) J. Birgerson, K.
Kaeriyama and P. Barta, Adv. Mater., 1996, 8, 982; (c) T. Sato, K.
Hori and K. Tanaka, J. Mater. Chem., 1998, 8, 589.
absorption. It can be considered that the structureless band is
not only due to rotations around the inter-ring bond but also
to quite di†erent characteristics of the excited state of o-
PhBT from those of m- and p-PhBT . The result of the
2
2
StricklerÈBerg equation suggested one-to-one correspondence
of the ground state with the lowest singlet excited state. The
energy gap estimated from the crossing points of the absorp-
tion and Ñuorescence were 3.04, 3.22 and 2.93 eV at room
temperature and 3.04, 3.18 and 2.84 eV at 77 K for o-, m- and
10 K. Tamao, S. Komada, I. Nakajima, M. Kumada, A. Minato and
K. Suzuki, T etrahedron, 1982, 38, 3347.
11 J. Nakayama, T. Konishi, S. Murabayashi and M. Hoshino, Het-
erocycles, 1987, 26, 1793.
p-PhBT , respectively. A decrease in energy gap with decrease
12 S. Chodorowski-Kimmes, M. Beley, J-P. Collin and J-P. Sauvage,
T etrahedron L ett., 1996, 37, 2963.
2
in temperature was observed for m- and p-PhBT . These
2
13 I. G. C. Coutts, H. R. Goldshmid and O. C. Musgrave, J. Chem.
Soc. C, 1970, 488.
results indicate that the p-conjugation is more extended at low
temperature because of the planarity of the structures. On the
14 M. Meyer, J. C. Mialocq and M. Rougee, Chem. Phys. L ett., 1988,
150, 484.
15 S. J. Strickler and R. A. Berg, J. Chem. Phys., 1962, 37, 814.
16 R. Colditz, D. Grebner, M. Helbig and S. Rentsch, Chem. Phys.,
1995, 201, 309.
17 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart,
J. Am. Chem. Soc., 1985, 107, 3902.
18 J. J. P. Stewart, J. Comput. Chem., 1989, 10, 209; MOPAC
V ersion 7.
19 (a) M. Belletete, M. Leclerc and G. Durocher, J. Phys. Chem.,
1994, 98, 9450; (b) M. Belletete, N. Di Cesare, M. Leclerc and G.
Durocher, Chem. Phys. L ett., 1996, 250, 31; (c) C. Quattorocchi,
R. Lazzaroni and J. L. Bredas, Chem. Phys. L ett., 1993, 208, 120;
(d) S. Samdal, E. J. Samuelsen and V. Volden Hans, Synth. Met.,
1993, 59, 259.
other hand, the energy gap of o-PhBT did not change. The
2
energy gap of o-PhBT is intermediate between those of m-
2
and p-PhBT at both room temperature and 77 K. Hence, the
p-conjugation in o-PhBT is more extended than that of m-
PhBT . However, this extension is not due to an increase in
the planarity in the excited state, as shown by the structureless
2
2
2
Ñuorescence spectra. The overlap of p orbitals on the adjacent
sulfur atoms indicated that the excited state of o-PhBT could
2
be extended somewhat by interaction between the excited and
the unexcited bithienyl moieties. The structureless Ñuorescence
and decrease in lifetime with the solvent polarity suggested a
contribution from the charge-transfer state to the singlet
excited state.
20 P. Bucci, M. Longeri, C. A. Veracini and L. Lunazzi, J. Am.
Chem. Soc., 1974, 96, 1305.
21 K. Fukui and K. Tanaka, Bull. Chem. Soc. Jpn., 1977, 50, 1391.
22 I. B. Berlman, J. Phys. Chem., 1970, 74, 3085.
Conclusion
Three types of PhBT s were successfully synthesized and their
2
photophysical properties were investigated. Their photo-
Paper 8/02514B; Received 2nd April, 1998
2360
J. Chem. Soc., Faraday T rans., 1998, V ol. 94