The photophysics and dynamics of diphenylbutadiene in alkane and perfluoroalkane solvents

Absorption and emission spectral characteristics, quantum yields, radiative and nonradiative rates, and rotation times of trans,trans-1,4-diphenyl-1,3-butadiene (DPB) in perfluoroalkane and alkane solvents are reported. The emission spectra and radiative rates show a continuous change in the character of the emitting state of DPB with solvent polarizability, presumably due to solvent-induced changes in the relative energies of the 2A_g and 1B_u states. Rotation times measured in these nonpolar solvents do not conform to any available theory of rotational friction; however, deviations from hydrodynamic predictions can be roughly correlated with the solute/solvent size ratio. Nonradiative decay rates are assumed to reflect an adiabatic trans-cis isomerization process. The rates measured here, together with collected literature data in alkane and alcohol solvents, are analyzed using a Kramers' barrier crossing model coupled to a simple dielectric description of how the reactive potential varies with solvent. By using estimates of friction obtained from rotation times, we can reasonably model the data in all solvents if it is assumed that the scaling between reactive friction and rotational friction (ζ/ζ_rot) differs among the various solvent classes: alkanes (ζ/ζ_rot) ≈ 1, perfluoroalkanes (ζ/ζ_rot) ≈ 0.2, and n-alcohols (ζ/ζ_rot) ≈ 0.4.