Ultrafast Rotational Dynamics of Aniline From Femtosecond Fluorescence Anisotropies

Rotational dynamics studies have been performed using ultraviolet fluorescence upconversion to probe aspects of rotational motion ranging from free rotation in vapors to restricted rotation in condensed matter. Fluorescence upconversion was used to measure the time evolution of the fluorescence anisotropy of stilbene vapor and of aniline in three hydrocarbon, alcoholic and aprotic polar solvents. The stilbene vapor fluorescence anisotropy showed free rotor behavior and the limiting anisotropy (at long times) indicated low rotational-vibrational energy transfer. The deconvoluted anisotropy decay time of 0.94 ± 0.05 ps in aniline/isopentane solution is only 54% longer than calculated for free rotation of collision-free molecules, and increasing the solvent viscosity by a factor of 15 by going from isopentane to hexadecane increases the rotational relaxation time by only another 72% to 1.6 ps. The extended diffusion fitting of aniline/isopentane anisotropy decay resulted in a collisional time of 205 ± 15 fs indication that electronically excited aniline in this hydrocarbon rotates on average 19.5 ± 15 degrees between collisions. These results indicate that the orientational motion of electronically excited aniline in hydrocarbons is significantly nondiffusive. The anisotropy decayed much more slowly in methanol and acetonitrile than in the hydrocarbons, presumably due to dipole­dipole interactions in acetonitrile and/or hydrogen bonding interactions in the hydroxylic solvent. The anisotropy decay in methanol presented double exponential behavior, probably due either to the coexistence of two slowly interconverting aniline species (H-bonded and not H-bonded) in solution or to the contribution of the hydrogen-bonded aniline molecules rotating around their a-axes. Free rotation, hydrodynamics models, extended-diffusion, Langevin, generalized Langevin and molecular dynamics calculations were used to evaluate the results and comparisons were made among the various approaches. From the molecular dynamics and generalized Langevin calculations an estimate was made concerning the nature of the torque the ant line molecule senses