The excited state equilibrium between two rotational conformers of a sterically restricted donor-acceptor biphenyl as characterized by global fluorescence decay analysis

A global fluorescence decay analysis approach has been employed to characterize the conformational relaxation in the excited state of a sterically hindered twisted donor-acceptor biphenyl (III) dissolved in diethyl ether (EOE). A temperature dependent population ratio of two conformationally different charge-transfer species CT and CTR is found for which the thermodynamic equilibration (reversible photoreaction) is reached above 265 K. The separated fluorescence bands of CT and CTR as well as kinetic and thermodynamic parameters associated with the adiabatic (forward and backward) photoreaction could be determined (DeltaH = -2.5 kJ/mol, DeltaS = -0.7 J/(mol K) with activation barriers E-a(CT - CTR) = 14.3 kJ/mol and E-a(CT<--CTR) = 16.8 kJ/mol). The derived excited state dipole moments (mu(CT) = 26D; mu(CTR) = 30D) and their k(dagger) ratio (k(f)(CTR)/k(f)(CT) approximate to 0.7) are consistent with a photoreaction from a more planar (CT) to a more twisted (CTR) conformer.