ULTRAFAST ROTATIONAL-DYNAMICS OF ELECTRONICALLY EXCITED ANILINE MOLECULES IN SOLUTION FROM ULTRAVIOLET FEMTOSECOND FLUORESCENCE ANISOTROPIES

Fluroescence upconversion was used to measure the time evolution of the fluorescence anisotropy of aniline in three hydrocarbon, alcohol, and aprotic polar solvents. 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 70% to 1.6 ps. The extended diffusion theory applied to the aniline/isopentane anisotropy decay resulted in a collisional time of 205 +/- 15 fs, implying that electronically excited aniline in this hydrocarbon rotates on average 19.5 +/- 1.5 deg between collisions. These results indicate that the orientational motion of electronically excited aniline in isopentane is significantly nondiffusive. The anisotropy decayed much more slowly in methanol and acetonitrile than in the hydrocarbons, presumably due to dipole-dipole 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 (hydrogen and nonhydrogen bonded) in solution or to a fast contribution from the hydrogen-bonded aniline molecules rotating around their long axes. Free rotation, hydrodynamic models, extended diffusion, and Langevin calculations were used to evaluate the results and comparisons were made among the various approaches.