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.

Validation and Divergence of the Activation Energy Barrier Crossing Transition at the AOT/Lecithin Reverse Micellar Interface

“…The faster relaxation time constant of ∼0.1 ns does not show regular temperature dependence and can be recognized as inertial/ wobbling motion of the probe. 53 The time constant ∼1.5 ns in the anisotropy decay shows reasonably good temperature dependence and may be due to the rotation of the excited-state species of the probe ANS in RM, 21 close to the head group of lecithin. The longer time component (∼40 ns) depicts the residence of the dye at the reverse micellar interface revealing the overall rotation of the RM.…”

Molecular dynamics simulations of fluorescence polarization of tryptophans in myoglobin.

“…In principle, the slope of r(t) vs. t is zero at t = 0 because at sufficiently short times inertial effects must dominate and the anisotropy is determined by the properties of an ensemble of free librators or rotors (2,4). Theoretically, the short time decay function of r(t) should be gaussian (19).…”

“…The magnitudes and decay functions of the anisotropy can be related to models for the rotational diffusion (2). The current methods allow subpicosecond time resolution of fluorescence (3,4) and hence can expose diffusional and inertial motions of amino acid residues in proteins and other systems.…”

“…This average was calculated by successively choosing each point in the first half of the analysis trajectory as the time origin (r = 0), evaluating the integral starting from this point, and averaging over all chosen time origins. The fluorescence anisotropy was calculated using the expression (2) r(t) = 2Yi(exp{-fi[kr + kET(T)IdT}P2(/ie(t)OZa(0))/I(t), [4] where ,Ua and ,e are the absorption and emission dipole moments, respectively, of the tryptophans and P2 (x) is the second Legendre polynomial. The ensemble average was calculated in the same manner as for the intensity decay profile.…”

Time-resolved anisotropic coherent anti-Stokes Raman scattering: A new probe of reorientational dynamics

“…(O), (22) where we have dropped the label for the level i for simplicity. The polarizability tensor elements 13t(O) and akl(O) describe anti-Stokes and Stokes Raman scattering, respectively.…”