Rotational dynamics of electronically excited aniline in solution from picosecond fluorescence anisotropies

Abstract

Ultraviolet fluorescence upconversion gating has been used to measure the fluorescence anisotropy of aniline in four hydrocarbon and alcoholic solvents. The deconvoluted anisotropy decay time of 1.2 ps in isopentane is only about a factor of 2 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 factor of 2. These results imply that the orientational motion of electronically excited aniline in hydrocarbons may be significantly nondiffusive and that unhindered inertial rotation through large angles may be possible. The experimental anisotropy decays do, however, differ significantly from those calculated for completely unhindered free rotation. The anisotropy decays much more slowly in methanol and sec-butanol than in the hydrocarbons, presumably due to dipole–dipole and/or hydrogen bonding interactions in the hydroxylic solvents. The rotational relaxation of N,N-dimethylaniline is slower than that of aniline in hydrocarbon solvents but faster in methanol, perhaps reflecting reduced hydrogen bonding. Fluorescence anisotropy decays of 4-cyclohexylaniline and 2,4,6-tri-tert-butylaniline have also been examined in order to estimate the true initial rotationless anisotropy [r(0)] for aniline.