The Role of Triplet States in the Trans - Cis Photoisomerization of Quaternary Salts of 4-Nitro-4'-azastilbene and Their Quinolinium Analogues. 6

Article abstract of DOI:10.1021/j100265a036

The quantum yields of fluorescence (φ_f) and of trans -> cis photoisomerization (φ_t->c) were measured for quaternary salts of 4-nitro-4'-azastilbene derivatives (A_t⁺X^-; A_t⁺ = trans-1-alkyl-4-<4-nitrostyryl>pyridinium and quinolinium; X(1-) = I(1-), ClO4(1-), and CH3SO4(1-)) as a function of solvent polarity, temperature, and concentration of quenchers.Transients (lifetime ca. 200 ns at 25 deg C, λ_max ca. 450 nm) observed by laser flash photolysis were identified as the trans triplet configuration of either the cation (^{3*_{At+) or the ion pair (}3*_{At+...X-) in highly and less polar solvents, respectively.In polar solvents at room temperature} 3*}A_t⁺ is quenched by oxygen, azulene, ferrocene, and I(1-) at rates close to the diffusion-controlled limit.The quenching measurements under pulsed and steady-state conditions suggest that ^3*A_t⁺ is an intermediate in the trans -> cis photoisomerization route.In contrast to the pyridinium salts, φ_t->c does not follow for the quinolinium salts linear Stern-Volmer dependences on , , or .It is concluded that for the pyridinium salts trans -> cis photoisomerization occurs only via the triplet mechanism, whereas a second route, bypassing the lowest triplet state, contributes to a certain extent (ca. 40percent) for the quinolinium salts (mixed singlet-triplet mechanism).