ROTATIONAL RELAXATION OF PERYLENE IN N-ALCOHOLS AND N-ALKANES STUDIEDBY 2-PHOTON-INDUCED ANISOTROPY DECAY

Two-photon-induced anisotropy decays have been measured for perylene i n a series of n-alcohols and n-alkanes and in 2-propanol and cyclohexa ne. Perylene fluorescence was excited at 570 nm and detected by time-c orrelated single-photon counting. The two-photon-induced anisotropy de cays r(1)(t) and r(2)(t) were measured for two-photon excitation with linearly and circularly polarized excitation pulses. Rotational correl ation times determined from single-exponential fits to anisotropy deca ys were shorter for linearly polarized excitation than for circularly polarized excitation in all cases with the possible exception of 2-pro panol at 20 degrees C. This result demonstrates the existence of at le ast two underlying rotational diffusion times, which are weighted diff erently in the decays of the anisotropic distributions generated with Linear and circular polarization as predicted by the theory of two-pho ton-induced anisotropy decay [J. Chem. Phys. 101 (1994) 10283]. The vi scosity dependencies of the rotational correlation time follow slip (f or alkanes) or sub-slip (for alcohols) boundary conditions. A solvent dependence in the ratio of rotational correlation times determined wit h linearly and circularly polarized excitation is shown to indicate no n-hydrodynamic behavior and can be described as a solvent dependence i n the relative values of the principal diffusion coefficients, i.e. th e effective rotor shape. The results are consistent with more anisotro pic diffusion in alcohols than in alkanes, with faster spinning about the axis normal to the plane of the molecule relative to tumbling abou t in-plane axes. The initial (t = 0) anisotropy values r(1)(0) and r(2 )(0) together with the two-photon polarization parameter Ohm were used to analyze the two-photon tensor governing the transition. (C) 1998 E lsevier Science B.V. All rights reserved.