Fluorescence Quenching and the Charge Transfer in the Excited State. III. Flash Photolytic Studies

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Abstract
It has been established by the flash photolysis that the N,N-dimethyl β-naphthylamine (DMN) cation is produced by the interaction of excited DMN with dimethyl isophthalate (DMP). The assignment has been made by comparing the transient spectrum with that obtained by the γ-radiolysis of DMN in s-butyl chloride. The decay of the DMN cation is first order in acetone (DK, 20.7), ethanol (24.3) DMF (36.7), second order in formamide (109.5) and it is the superposition of first and second order in acetonitrile. This strongly suggests that the radical cation exists as the solvent-shared ion pair in medium polar solvents while as free species in highly polar solvents. The decay of the former has been found to be not essentially affected by oxygen. The finding that the transient spectrum is observed even in the aerated solution supports the view that it is produced at the singlet excited state. From the effect of [DMP] on the yield of triplet DMN, it has been inferred that the radiationless process occurring in the ex-CT complex is mainly internal conversion. The reactivity of the triplet DMN against DMP has been found very small as compared with that of the signlet excited state; the rate constant for (DMN)T+DMP→DMN+DMP was k10\simeq105 M−1 sec−1. The erroneous discussion given in the previous paper has been corrected; the decrease both in the quenching constant (for normal fluorescence) and in the emissivity of the ex-CT emission with the solvent polarity (in slightly polar solvents) is most likely to be attributed to the solvent-solute interaction, the increase of which (with the increase in solvent polarity) is unfavorable both to the formation of the ex-CT complex and to the emissivity of the ex-CT emission.