Solvent effect on radiative and non-radiative transitions in all-trans-1,6-diphenylhexatriene

Abstract

The spectral and luminescent properties of all-rans-1,6-diphenyl-1,3,5-hexatriene (DPH) were studied at room temperature in solvents with various refractive indices (1.33 < n < 1.63) and dielectric constants (1.9 < ϵ < 37.4). The maximum fluorescence quantum yield of DPH (φo = 0.85) was measured in quinoline, and the lowest quantum yield was observed in acetonitrile (φo = 0.17). The experimentally determined values of the fluorescence and non-radiative deactivation probabilities (kf1 and kd) of the lowest singlet excited state S1 are 2.9 × 108 s−1 (quinoline) and 2.0 × 108 s−1 (acetonitrile). The fluorescence from the S1(21A*g) state (the transition to which from the ground state S0(11Ag) is symmetry forbidden) is due to vibronic mixing with a higher S2(11B*u) state, which participates in the strongly allowed electron transition S2 ← S0. The matrix element of the vibronic interaction between the S1 and S2 states (V12) is 840 cm−1.Non-radiative transitions from the S1 state are essentially dependent on the solvent polarity. There is a linear dependence of ln kd on the empirical parameter ET(30), which characterizes the solvent polarity. The polar solvent quenches the fluorescence of DPH by inducing non-radiative transitions through intermediate twisted structures possessing dipole moments of a few debyes.