Photoexcitation and Electron Transfer Properties of Rod- and Coil-Type Oligo(thienylene−ethynylene)s

Abstract

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.

Odd–Even Effect of Thiophene Chain Lengths on Excited State Properties in Oligo(thienyl ethynylene)-Cored Chromophores

“…These lifetimes probed at emission maxima are found to be single and two exponential in CHX and THF, respectively. The half-lives within a few hundred picoseconds were roughly in agreement with those in oligo­(thienyl ethynylene)­s (∼0.31 ns), suggesting that this part of emission is mainly contributed by the (TE) n chromophore. The long-lifetime emission only observed in THF is assumed to derive from a symmetry-broken excited state because of the fast solvation effect in polar THF, whereas there almost no solvation occurred in nonpolar CHX. , The following femtosecond time-resolved transient absorption measurements further help to identify the solvent-dependent excited state dynamical behaviors.…”

Synthesis and properties of oligo(2,5-thienylene-ethynylene)s

“…Pd(0) is inserted into the C−Hal bond of R The relatively rigid conjugated OTE chains 3 (X=S) soon become (n≥4) insoluble unless solubilizing side chains are attached. Therefore, alkyl, 11,13,14,18,19,[21][22][23][25][26][27][28][31][32][33] aryl, 17 alkoxy 34 or alkylsulfanyl 34 groups are normally attached to one or both free positions of the thiophene rings.…”

Molecular Wires Based on Thienylethynylene: Synthesis, Photophysical Properties, and Excited-State Lifetime

“…In comparison with their corresponding OTEs, the obvious red-shift in absorption λ max of these molecular wires was achieved due to the introduction of truxene units to improve the effective conjugation length of the whole molecular wire. For example, the absorption λ max red-shifted about 77 nm for Tr(TE) 2 Tr , 49 nm for Tr(TE) 3 Tr , 39 nm for Tr(TE) 4 Tr , 22 nm for Tr(TE) 5 Tr relative to those of (TE) 2 (317 nm), of (TE) 3 (360 nm), of (TE) 4 (377 nm), and of (TE) 5 (400 nm), respectively. 4a, All PL spectra in solution showed a maximum peak with a shoulder on the red side. It was observed that the PL λ max in dilute THF solution red-shifted from Tr(TE) 2 Tr to Tr(TE) 6 Tr .…”

“…Moreover, it was observed that both absorption and PL λ max of Tr(TE) 6 Tr red-shifted only 2 nm in comparison with those of Tr(TE) 5 Tr , which indicated that the saturation of the effective conjugation length of this series of molecular wires was achieved around Tr(TE) 6 Tr . Normally, the saturation of the effective conjugation length for OTEs is found at around the octomer range . The fluorescence quantum yields (Φ PL ) of these molecular wires in dilute THF solution were measured to be 0.25 for Tr(TE) 2 Tr , 0.22 for Tr(TE) 3 Tr , 0.20 for Tr(TE) 4 Tr , 0.20 for Tr(TE) 5 Tr , and 0.18 for Tr(TE) 6 Tr , respectively, using 9,10-diphenylanthracene as the standard .…”