SOLVENT EFFECT ON RADIATIVE AND NONRADIATIVE-TRANSITIONS IN ALL-TRANS-1,6-DIPHENYLHEXATRIENE

The spectral and luminescent properties of all-trans-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 < epsilon < 37.4). The maximum fluorescence quantum yield of DPH (phi-o = 0.85) was measured in quinoline, and the lowest quantum yield was observed in acetonitrile (phi-o = 0.17). The experimentally determined values of the fluorescence and non-radiative deactivation probabilities (k(fl) and k(d) of the lowest singlet excited state S1 are 2.9 x 10(8) s-1 (quinoline) and 2.0 x 10(8) s-1 (acetonitrile). The fluorescence from the S1(2(1)A(g)*) state (the transition to which from the ground state S0(1(1)A(g) is symmetry forbidden) is due to vibronic mixing with a higher S2(1(1)B(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 k(d) on the empirical parameter E(T)(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.