4466system only when the Cr center bears LiL, or stronger ligands but form Cr"P with weaker ligands. The Cr"P products are relatively stable in solution but are sensitive to the presence of 02, which oxidizes them immediately to the Cr'I'P state. The C r l W species, however, are short-lived in protic solvents and undergo disproportionation and protonation to form chlorins in neutral and acid solutions or phloM anions under alkaline conditions. The phlorins are also readily oxidized by O2 to Cr"'P(LIL2). These studies J. Phys. Chem. 1992, 96, 4466-4469 establish the conditions for production of Cr"-porphyrins and permit us to study the reactions of these products with various alkyl radicals that lead to formation of Cr-C bonds. This will be the topic of a future publication.Acknowledgment. This research was supported by the Office of Basic Energy Sciences of the US Department of Energy. We thank Dr. P. Hapiot for helpful discussions.Compounds containing a viologen linked to a zinc porphyrin via methylene groups have been synthesized, and photoinduced intramolecular electron transfer between porphyrin and viologen was observed. The photoexcited singlet state of the porphyrin was quenched by the bonded viologen, the photoexcited triplet state was quenched, and the lifetime of the charge separated species was about 1 ps.
Compounds containing a viologen linked to a porphyrin via a number (n = 4-7) of methylene groups have been synthesized and photoinduced long-range electron transfer between porphyrin and viologen was observed. The photoexcited singlet state of the porphyrin was quenched by the bonded viologen in viologen-linked porphyrins. The fluorescence decay profiles of the viologen-linked porphyrins consist of a sum of two first-order decays with shorter and longer lifetimes. The shorter lifetime arises from the direct quenching of the photoexcited singlet state of the porphyrin by the linked viologen. The intramolecular electron transfer process from the porphyrin to the bonded viologen becomes less effecthe when the methylene chain length becomes longer.
Platinum-loaded Langmuir-Blodgett and cast films of a porphyrin are prepared, and steady photoinduced hydrogen evolution is observed using both films under steady-state irradiation.
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