Comprehensive investigation of the photophysics, photochemistry, and kinetics of a wide variety of photochromic hydrazones in various solvents

Article abstract of DOI:10.1021/ja00030a036

The photophysics, photochemistry, and kinetics of two major families of photochromic hydrazones have been studied by means of microsecond and nanosecond laser flash techniques. In addition, picosecond fluorescence spectroscopy was used in some cases in order to aid in the elucidation of the photochromic mechanisms. Several derivatives of benzaldehyde (hydrazones) have been studied. All of them showed anti to syn isomerization. The derivatives of 2-hydroxybenzaldehyde hydrazones were found to undergo intramolecular proton transfer originating from the hydroxyl group. In the latter type of compounds, anti to syn isomerization and proton transfer were found to be competitive processes originating in the lowest excited singlet state. This result plus other data on the risetime for proton transfer dictates that the isomerizations occur in <5 ps. The photophysics and photochemistry of the excited-state intramolecular proton transfers were found to be strongly dependent upon the substituents and the hydrogen-bonding character of the solvent. For all of the 2-hydroxybenzaldehyde derivatives the excited-state intramolecular proton transfer results in the nonadiabatic formation of a cis-keto form. The ground-state thermally reverse proton transfer occurs on the microsecond time scale. In hydrogen-bonding solvents, it was found that the excited-state intramolecular proton transfer results also in the nonadiabatic formation of a trans-keto form. Several 1-naphthoquinone 2-phenylhydrazones have been shown to undergo syn to anti isomerization except for 1-naphthoquinone 2-diphenylhydrazone which showed an anti to syn isomerization. In addition, competitive photochemical processes including intramolecular proton transfer (from hydrogen on nitrogen) are also found to occur in some cases depending upon the nature of the solvents. The same is true for the 1-phenylhydrazone of 1,2-naphthoquinone. The hydrazone form of the 4-phenylhydrazone of 1,4-naphthoquinone shows direct syn ? anti isomerization in contrast to that found by earlier researchers. In all of the foregoing, the processes occur in the lowest excited singlet state.