Photophysical properties of quaternary salts of 4-dialkylamino-4′-azastilbenes and their quinolinium analogues in solution: IX

Abstract

The decay processes of excited quaternary salts of trans-1-alkyl-4-[4-dialkylaminostyrl]-pyridinium and trans-1-alkyl-4-[4-dialkylaminostyryl]-quinolinium (A t +X −; X − ≡ I − or ClO 4 − were studied by pulse and steady state methods as a function of temperature, quenchers (ferrocene and azulene) and solvent polarity. Introduction of the dialkylamino group markedly reduces the quantum yield Ø t→ c of trans → cis photoisomerization and slightly enhances the quantum yield Ø f of fluorescence. In polar solvents deactivation of the first excited singlet 1*A t + predominantly by internal conversion via an activated step is suggested by the temperature dependence of Ø f and a low yield for intersystem crossing. In solvents of moderate polarity ( e.g. dichloromethane) a major short-lived transient (Tr T) (λ max ≈ 400 nm, ⪆ 700 nm; τ T ≈ 1 – 10 μs) and a minor long-lived transient (Tr R) (λ max ≈ 400 nm; t 1 2 > 10 μs) were observed for the iodides by laser flash photolysis. Pulse radiolysis studies permit the assignment of Tr R to a radical A . Tr T, which was also observed for iodides and perchlorates in polar solvents either on addition of excess I − or on sensitized excitation, is assigned to the lowest triplet state 3*A t + of the trans configuration. On the basis of results from energy transfer experiments with a range of sensitizers, the energy of 3*A t + is estimated to be 180 kJ mol − for the pyridinium and quinolinium salts respectively. In solvents of moderate polarity Tr T is assigned to the triplet state ( 3*A t + … I −) of the ion pair, the population of which is enhanced by the heavy atom effect.