Symmetry Breaking in the Relaxed S1 Excited State of Bianthryl Derivatives in Weakly Polar Solvents

The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to investigate the nature of the relaxed S1 state of 9,9‘-bianthryl (AA), 10-cyano-9,9‘-bianthryl (CAA), and 10,10‘-dicyano-9,9‘-bianthryl (CAAC). Changes in both the real, Δe‘ (dielectric constant), and imaginary, Δe‘ ‘ (dielectric loss), components of the complex permittivity have been measured. The dielectric loss transients conclusively demonstrate the dipolar nature of S1 for all three compounds in the pseudopolar solvents benzene and 1,4-dioxane, and even in the nonpolar solvents n-hexane and cyclohexane. The required symmetry breaking is considered to result from density and structural fluctuations in the solvent environment. The dipole relaxation times for AA (CAAC) are ca. 2 ps for the alkanes and 7.9 (5.3) and 14 (14) ps for benzene and dioxane, respectively. The time scale of dipole relaxation for the symmetrical compounds is much shorter than that for rotational diffusion and is attributed to intramolec...