Photoinduced Electron Transfer vs. Trans -> Cis Photoisomerization of Quaternary Salts of 4-Substituted 4'-Azastilbenes (R = CN, H, CH3, and OCH3) and Their Quinolinium Analogues.

Article abstract of DOI:10.1021/j100291a042

A series of quaternary salts of trans-1-alkyl-4-<4-R-styryl>pyridinium and -quinolinium (A_t⁺X^-; R = CN, H, CH3, and OCH3, X^- = I^- and ClO4^-) were studied in solution by using steady-state irradiation and pulse excitation methods.A transient, observed by pulse radiolysis (iodide or perchlorate) and laser flash photolysis (248 nm; iodide only) in polar solvents, is assigned to the radical A, formed by one-electron reduction of A_t⁺.A radical was also produced by laser excitation (353 nm) of A_t⁺ in the presence of electron donors (D: I^-, SCN^-, and amines, e.g.DABCO, triethylamine, and dimethylaniline) at high concentrations.The quantum yield of trans -> cis photoisomerization (Φ_t->c) decreases on increasing and the yield of A radical shows the corresponding increasing dependence.On direct excitation of A_t⁺ the electron-transfer process occurs mainly via static fluorescence quenching and on sensitized excitation via dynamic triplet quenching.In solvents of moderate polarity (e.g. dichloromethane) where the salts are present as ion pairs, Φ_t->c is stongly reduced for the iodides as compared to the perchlorates.Photoinduced electron transfer and trans -> cis photoisomerization via the singlet mechanism are competing processes also for the ion pair.At lower temperatures the yield of formation of A radical is reduced and Φ_t->c correspondingly enhanced.