Rotational relaxation of perylene in n-alcohols and n-alkanes studied by two-photon-induced anisotropy decay Original Research Article

Two-photon-induced anisotropy decays have been measured for perylene in a series of n-alcohols and n-alkanes and in 2-propanol and cyclohexane. Perylene fluorescence was excited at 570 nm and detected by time-correlated single-photon counting. The two-photon-induced anisotropy decays r1(t) and r2(t) were measured for two-photon excitation with linearly and circularly polarized excitation pulses. Rotational correlation times determined from single-exponential fits to anisotropy decays were shorter for linearly polarized excitation than for circularly polarized excitation in all cases with the possible exception of 2-propanol at 20°C. This result demonstrates the existence of at least two underlying rotational diffusion times, which are weighted differently in the decays of the anisotropic distributions generated with linear and circular polarization as predicted by the theory of two-photon-induced anisotropy decay [J. Chem. Phys. 101 (1994) 10283]. The viscosity dependencies of the rotational correlation time follow slip (for alkanes) or sub-slip (for alcohols) boundary conditions. A solvent dependence in the ratio of rotational correlation times determined with linearly and circularly polarized excitation is shown to indicate non-hydrodynamic behavior and can be described as a solvent dependence in the relative values of the principal diffusion coefficients, i.e. the effective rotor shape. The results are consistent with more anisotropic diffusion in alcohols than in alkanes, with faster spinning about the axis normal to the plane of the molecule relative to tumbling about in-plane axes. The initial (t=0) anisotropy values r1(0) and r2(0) together with the two-photon polarization parameter Ω were used to analyze the two-photon tensor governing the transition.