Reorientation of molecules induced by intramolecular twisting

Reorientation of triarylpyrylium (TAP) cations in solvents resulted from the excited state twisting and back-twisting was observed and investigated by polarised picosecond pump-probe absorption spectroscopy. The reorientation effect was independent of the solvent viscosity. On this base, reorientation due to orientational diffusion accelerated by the local molecular heating was rejected. Coupling between two nearly perpendicular motions, molecular rotation and intramolecular twisting, is discussed.