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The photobleaching of alloxazine in buffered aqueous solution has been studied by means of flash photolysis using conventional and laser excitation sources. Several transient species have been characterized. The alloxazine triplet state (Amax 420 nm and 550 nm, T = 9 ps) was identified with the aid of low-temperature comparison experiments in ethanol. Transient absorption with A, .. 440 nm, which appears after decay of the triplet state, and whose second-order decay is pH-dependent, is postulated to be due to the semiquinone radical (AH,') and a radical derived from alloxazine by addition of water and loss of a hydrogen atom (HAOH'), which are in equilibrium with their conjugate cation radicals. The results of experiments in the presence of oxygen indicate that these species are not primarily formed from the triplet state. The enhanced second-order decay of the flavin radicals in oxygen-containing solutions is interpreted in terms of their reaction with the peroxy radicals. The proposed mechanisms account for the production of hydroxylated alloxazines.

Semiempirical charge self-consistent MO calculations have been carried out for a large number of benzene dimers of different geometric conformations (e.g., D6h, C2h, S2, Ce, S12, C2v, etc.). It has been found that the most stable excimer conformation is not the most symmetric (Le., D6h), but rather one in which the rings are rotated relatively about the D6h axis and/or tilted, one with respect to the other. The lifetime of excimer fluorescence can be satisfactorily explained in two ways: (1) The transition is made vibronically allowed in much the same way as the IB2u-+lAlg transition of benzene; or (2) the excimer possesses C1 or C2v symmetry, in which case a very small relative tilting of the benzene planes can induce transition moments of proper magnitude without invoking vibronic effects at all. A number of suggestions are made concerning solvent effects, and the presence of underlying continua in discrete absorption spectra, etc_ In particular, a concept of contact excimer absorption is broached and rationalized. The results of computation are in excellent agreement with the general run of available experimental data.

We have investigated the rise lifetimes and decay lifetimes of the long-lived luminescences of the following systems: a naphthalene-ds, a phenanthrene, and a triphenylene solution in a rigid-glass matrix; a phenanthrene-d10, a phenanthrene, a naphthalene-ds, and a chrysene solution in a biphenyl crystal; and chrysene in solution in a phenanthrene crystal. It has been found that the luminescence rise time is invariably shorter than the luminescence decay time. This phenomenon is satisfactorily explicable on the basis of the following attitudes: In rigid glasses, triplet-triplet absorption events contribute to depopulation of the triplet state and decrease the rise time; in mixed crystals, on the other hand, triplet-triplet annihilation events are dominantly responsible for abbreviating the rise time. The effects of excitation intensity and excitation wavelength on the delayed luminescence rise times of rigid-glass solutions, and the effects of excitation intensity, concentration of guest species, and temperature on the delayed luminescence rise times of mixed-crystal systems are discussed. The activation energies for the annihilation process were obtained from a first-order analysis of the luminescence decay and rise times. The agreement between calculated activation energies and T1H − T1G spectroscopic energy gaps is satisfactory and validates the postulated kinetic processes.