Yoshiko Sasaki

Solvent effect on photoinduced electron transfer between C60 and 3,3′,5,5′-tetramethylbenzidine studied by nanosecond laser photolysis

Photoinduced electron transfer from 3,3′,5,5′-tetramethylbenzidine (TMB) to C60 in polar and non-polar solvents has been investigated by nanosecond (ns) laser photolysis in the visible and near-IR regions. The transient absorption bands of triplet C60(TC60*), the C60 radical anion (C60˙–) and the TMB radical cation (TMB˙+) were observed in both non-polar and polar solvents. The rate constants of electron transfer from TC60* to TMB, which were evaluated from the decay of TC60* and growths of TMB˙+ and C60˙–, decrease with increasing solvent polarity. In non-polar solvents, TMB˙+ and C60˙– seem to be produced immediately after the ns laser excitation at the same time as the formation of TC60*, suggesting a weak interaction between C60 and TMB, before electron transfer. On decreasing the solvent polarity, the kinetics of the back-electron-transfer reaction from C60˙– to TMB˙+ change from second-order to first-order, indicating that C60˙– and TMB˙+ exist as ion pairs in non-polar and less-polar solvents. The back-electron-transfer rates also tend to decrease with the solvent polarity.

Generation of long-lived radical ions through enhanced photoinduced electron transfer processes between [60]fullerene and phenothiazine derivatives

Photoinduced electron transfer processes between fullerenes (C60) and four phenothiazine derivatives (PTZs) in the absence and presence of hexylviologen dication (HV2+) have been studied by the transient absorption method in the visible and near-IR regions. Electron-transfer takes place from PTZs to the triplet states of fullerenes (3C60*) giving the radical anion of fullerenes (C60.-) and the radical cations of PTZs (PTZ.+). The rate constants and efficiencies of electron transfer are quite high, because of the high electron-donor abilities of PTZs as elucidated by their low oxidation potentials. On addition of HV2+ to the C60 and PTZ systems, the electron-mediating process occurs from C60.- to HV2+, yielding the viologen radical cation (HV.+). In the presence of a sacrificial donor, HV.+ persisted for a long time.

Competition between triplet energy transfer and electron transfer of photoexcited C60in the presence of [beta ]-carotene by changing solvent polarity

The electron transfer and triplet energy transfer from β-carotene to photo-excited C60 in polar and nonpolar solvents have been investigated by 532 nm laser flash photolysis with observation of the transient absorption bands in the visible and near-IR regions. The transient absorption band of the triplet state of C60, which appeared immediately after nanosecond laser exposure, was effectively quenched by β-carotene. With the decay of the triplet state of C60, the intense absorption band of the cation radical of β-carotene appeared at 980 nm with a weak absorption shoulder of the anion radical of C60 at 1070 nm in addition to the triplet state of β-carotene at 550 nm. This indicates that electron transfer takes place from β-carotene to the triplet state of C60 in addition to energy transfer from the triplet state of C60 to β-carotene. In nonpolar solvents, triplet energy transfer predominantly takes place. The relative fraction of the two routes was varied by changing the solvent polarity. After electron transfer, the cation radical of β-carotene disappeared by back electron transfer.

Electron transfer and energy transfer of photoexcited C60 in the presence of retinols

Electron transfer and energy transfer between the photo-excited triplet state of C60 (3C60*) and retinols, retinal and retinoic acid have been investigated by laser flash photolysis. In polar solvent, with the decay of the transient absorption band of 3C60* at 740 nm, the rise of the anion radical of C60 (C60-•) at 1070 nm and cation radicals of retinols [(retinols)+•] at 600 and 940 nm were observed in addition to the triplet state of retinols [3(retinols)*] at 400 nm. For retinal and retinoic acid, energy transfer occurs predominantly even in polar solvent, nevertheless the electron donor abilities of retinal and retinoic acid evaluated from the oxidation potentials are similar to those of retinols. The quantum yield and rate constant of electron transfer for trans-retinol are slightly faster than that for cis-retinol. After electron transfer, the cation radicals of retinols disappeared mainly by back electron transfer; however, some reactions of (retinols)+• such as intra- and inter-molecular ring closing reactions also occur, yielding persistent C60-•.

Photoinduced electron transfer and electron-mediating systems from aromatic amines to triplet states of C60 and C70 in the presence of a viologen dication

Photoinduced electron transfer between fullerenes (C60 and C70) and various aromatic amines (AAs) in the absence and presence of a viologen dication has been studied by the transient absorption method in the visible and near-IR regions. Electron-transfer takes place from AAs to the triplet states of fullerenes (3C60* and 3C70*) giving the anion radicals of fullerenes (C60*- and C70*-) and the radical cations of AAs (AA*+). The rate constants and efficiencies of electron transfer are quite high, because of the high electron-donor abilities of AAs as their low oxidation potentials indicate. The absorption bands of AA*+ appeared also in the near-IR region indicating that the radical-cation center (hole) delocalizes over the entire region of each AA. On addition of an octylviologen dication (OV2+) to C60/C70-AA systems, the electron-mediating process from C60*- and C70*- to OV2+ occurs yielding the viologen radical cation (OV*+) with longer lifetime.

Photoinduced electron-transfer reaction between C60 and cyclic silicon compounds

Electron transfer from cyclic silicon compounds to photo-excited C60 in polar solvents has been investigated by laser photolysis with observation of the transient absorption bands in the near-IR region. For three-, four- and five-membered cyclic silicon compounds in benzonitrile, the rise of the radical anion of C60 [C60−] was observed with the rapid decay of the triplet state of C60 [3C60*], indicating that electron transfer takes place via 3C60*. The rate constant (ket) and quantum yield (Φet) of electron transfer decrease with an increase in the number of silicon units. The ket and Φet values of the cyclic silicon compounds are smaller than those of the corresponding cyclic germanium compounds; an especially prominent difference was observed for four-membered cyclic compounds.

Photoinduced electron transfer from tetraethoxyethene to C60and C70 studied by laser flash photolysis

The electron transfer from tetraethoxyethene (TEOE), which has high electron-donor ability, to photo-excited C60 or C70 in polar and less polar solvents has been investigated by 532 nm laser flash photolysis with observation of the transient absorption bands in the near-IR region. The transient absorption bands of the triplet states of fullerenes (TC60* and TC70*), which appeared immediately after nanosecond laser exposure, were effectively quenched by TEOE. With the decay of TC60* and TC70*, the absorption bands of C60˙- and C70˙- appeared at 1070 and 1380 nm, respectively, showing that the electron transfer takes place from TEOE to TC60* or to TC70*. Then, C60˙- and C70˙- disappear by back electron transfer to TEOE˙+. The efficiencies and rates of the electron-transfer reactions vary with solvent polarity.