Photochemical reactions of aromatic compounds. XXXVII. Solvent effects on fluorescence quenching and photoreactions of naphthonitrile-olefin and furan systems. Qualitative consideration on electronic structures and reactivities of nonemissive exciplexes.

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Solvent effects on the fluorescence-quenching rate constants (γkc) as well as on the quantum yields (φ∞−A) of photoreactions have been investigated for the nonemissive exciplex systems in which 1- or 2-naphthonitrile is the electron acceptor whereas the donors are some enol ethers, a few simple alkenes, furan, 2-methylfuran, and ethyl phenyl ether. On the basis of the assumption of the solvent as a continuous dielectric medium, the relationships between electronic structures and reactivities of nonemissive exciplexes have been qualitatively estimated from the plots of logγkc and log[φ∞−A⁄(1−φ∞−A)] vs. (ε−1)⁄(2ε+1). In nonpolar and moderately polar aprotic solvents, the nonemissive exciplexes have low charge-transfer contributions but relatively high reactivities in the cycloaddition while much larger charge-transfer contributions are induced in more polar alcoholic solvents; the reactivities in the protic solvents decrease with the increase of solvent polarity. For some exciplexes, the occurrence of complete electron transfer in very polar solvents such as ethanol, methanol, and acetonitrile is strongly suggested from the product distributions. The possible availability of ET(30) or Z value as a scale of solvent polarity has been tested for the solvent effects on the processes from the nonemissive exciplexes.
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