Photoexcitation and electron-transfer properties of two series of oligo(thienylene−ethynylene)s, in which
thiophene rings were connected with ethynylene groups at 2,5 or 2,3 positions (nαTE or nβTE; n denotes the
number of the repeating unit), have been studied. From MO calculations and steady-state absorption spectra,
expanded π-electron systems were expected for rod-type nαTE in the ground states, while limited π-electron
systems were expected for coil-type nβTE. On the other hand, because nβTE shows a substantial red shift of
the fluorescence band similar to that of nαTE with increasing n value, a conformational change expanding
π-conjugation of nβTE was suggested in the excited state. From the picosecond laser flash photolysis, the
time scale for the conformational change was evaluated to be ca. 30 ps. The triplet state properties of nαTE
and nβTE were estimated by means of the nanosecond laser flash photolysis. Furthermore, electron donor
abilities of the present oligomers were investigated by studying the photoinduced electron-transfer processes
with fullerenes, C60 and C70. It was revealed that the present oligomers donate an electron to the triplet excited
C60 or C70 generating the radical cations and anions of oligomers and fullerene, respectively. The electron-transfer rate constants were as small as 0.07−0.0008 of the diffusion-controlled limit, indicating the longer
range electron-transfer processes due to larger size of the oligomers and fullerenes. On the other hand, back-electron-transfer processes proceeded at the diffusion-limiting rate.
Photoinduced intramolecular processes in a dyad of azulene and C60 (Az−C60) were compared with those of
a dyad of naphthalene and C60 (Naph−C60) on the basis of laser flash photolysis experiments. Upon
photoexcitation of C60 in the presence of azulene, intermolecular electron transfer proceeded from azulene to
the triplet state of C60 (C60(T1)), although the rate constant was small (107 M-1 s-1), because of the small
free-energy change for electron transfer via C60(T1). In Az−C60, it was revealed that the S2 state of the Az
moiety (Az(S2)−C60(S0)) donates the excited energy to the C60 moiety, effectively generating Az(S0)−C60(S1).
In polar solvents, a charge-separated state (Az•+−C60
•-) was generated from Az(S0)−C60(S1), from which the
S1 state of the Az moiety (Az(S1)−C60(S0)) was also generated by competitive energy transfer. The lifetimes
of the charge-separated states were on the order of nanoseconds. Successive energy-transfer processes {Az(S2)−C60(S0) → Az(S0)−C60(S
n
), Az(S0)−C60(S1) → Az(S1) −C60(S0), where n ≥ 2} demonstrate that the multiple
energy transfer is achieved in a simple dyad molecule. On the other hand, Naph−C60 dyad did not show
charge separation upon excitation of the C60 moiety, but deactivated via intersystem crossing, generating
almost quantitatively the C60(T1) moiety. These findings indicate the favorable donor ability of azulene compared
to that of naphthalene, even though both azulene and naphthalene have the same 10-π-electron system.
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