No abstract

No abstract

Photoreduction of methyl viologen (MV2+) by eosin-Y (EY2-) in the presence of triethanolamine (TEOA) has been investigated in water-methanol mixture by means of steady-state photolysis and laser-flash photolysis in the visible/near-infrared regions. The complete conversion to the persistent methyl viologen radical cation (MV.+) was observed in the presence of lower concentrations of EY2- and excess TEOA. By laser-flash photolysis measurements, electron transfer was confirmed to occur from the triplet state of EY2- [3(EY2-)*] to MV2+ in the rate constants of ca 2.0 x 10(10) M-1 s-1. The rates and efficiencies of production of MV.+ were found to be dependent on solvent compositions and concentrations of MV2+, ionic salt and TEOA. The back electron transfer reaction from MV.+ to EY.- was retarded in the presence of TEOA, which supports that EY2- is reproduced by accepting an electron from TEOA. In the presence of excess TEOA, the indirect formation of MV.+ from EY.3-, which was produced by accepting an electron from TEOA, was confirmed. The contributions of both the oxidative and reductive routes of 3(EY2-)* for the MV.+ formation have been confirmed.

No abstract

No abstract

No abstract

The photoinduced electron transfer of C 60 in the presence of stable free radicals such as nitoxides has been studied by nanosecond laser photolysis by measuring the transient absorption spectra in the visible and near-IR regions. For the N-oxy-piperidine radical, the rise of the radical anion of C 60 was not observed, although the decay rate of the triplet excited state of C 60 was quite fast. For free radicals with 4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (nitronyl nitroxide), electron transfer via the triplet excited state of C 60 was confirmed from the rise of the anion radical of C 60 , although the quantum yield of electron transfer is not high. From the rates and efficiencies of electron transfer, which are strongly affected by the donor ability of the substituents on the phenyl moiety of the nitronyl nitroxide radical, a specific spin-spin interaction of the triplet spin state of C 60 with the doublet spin state of the nitronyl nitroxide radical was revealed. In the longer time scale measurements, the radical anion of C 60 persisted for a long time, suggesting that the nitronyl nitoxide cation, which was formed by donating an electron to the triplet state of C 60 , forms the complex with the nitronyl nitoxide radical. An electron-mediating process from the radical anion of C 60 to the viologen dication was confirmed by the rise of the radical cation of the viologen, with the concomitant decay of the radical anion of C 60 . Final back electron transfer from the viologen radical cation to the nitronyl nitroxide cation was observed by the transient absorption measurements on additional long time scales.

The rate constants for quenching of the triplet states of eosin and erythrosin (3D*) with various substrates have been evaluated using nanosecond laser flash photolysis while observing the transient absorption bands in the visible/near-IR regions. Intense transient absorption bands due to 3D* were observed at about 1040 nm. An energy-transfer reaction from 3D* to anthracene-9-carboxylic acid was confirmed by the appearance of the absorption band of the triplet state of the acid at 425 nm. The ion radicals of electron donors and electron acceptors were also observed with the decay of 3D*, indicating the occurrence of an electron-transfer reaction. High hydrogen-abstraction abilities of 3D* were also confirmed for substituted phenols and benzenethiols. The negative slopes of the Hammett plots indicate that the reaction center of 3D* has a highly electrophilic nature.