Comparison of the bandshape and lifetime data analysis of temperature-dependent fluorescence measurements for the investigation of an excited state conformational equilibrium. The case of differently twisted donor–acceptor biphenylsDedicated to Professor F. Dörr on the occasion of his 80th birthday.

Abstract

Steady-state and time-resolved fluorescence measurements are performed to gain information about the influence of temperature on excited state conformational relaxations of three differently twisted donor–acceptor biphenyls (I–III) in nonpolar and polar solvents. The band-shape analysis related to Marcus theory and the determination of the fluorescence rate constants as a function of temperature indicate that the relaxation towards a planar structure in II and III is not stopped at low temperature in the liquid phase of the solvents. Only compound III in a medium polar solvent (diethyl ether) exhibits a nonlinear dependence of fluorescence quantum yields (Φf), reorganization (λs + λ′i) and charge recombination (ΔG−CT) energies on temperature, which is due to a temperature dependent population ratio of two conformationally different charge transfer species CT and CTR.