Excited state properties of an aza-analogue of distyrylbenzene. Solvent polarity and hydrogen-bonding effects

Abstract

The steady-state and time-resolved fluorescence of phenylene-1,4-ethylene bis(4-quinoline), PhQ, have been measured in some organic solvents. The fluorescence quantum yield and lifetime are highly sensitive to the solvent nature—they decrease markedly with increasing the hydrogen bond donating ability and polarity of the solvent. The radiative rate constant is less sensitive to the solvent polarity while the nonradiative decay rate constant of the excited PhQ increases significantly from 4.9 × 10⁸ to 26.4 × 10⁸ s⁻¹ on going from 1,4-dioxane to ethanol. The fluorescence quenching of PhQ by some alcohols, water, trifluoroacetic acid, phenol and anisole has been studied in 1,4-dioxane. The quenching efficiency is related to the hydrogen bond donating ability of the quencher. From the results of the quenching experiments and the large increase in the basicity of excited PhQ (ΔpK_a = 6.65), excited state proton transfer, from the quencher to excited PhQ molecules, was suggested as a mechanism for fluorescence quenching of PhQ in protic solvents and acid solutions.