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.

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Relaxation processes in pyridine vapor have been studied using high-performance transient absorption spectrometry following excitation low in the S1 manifold. The ultraviolet spectrum of the lowest triplet state has been identified and measured, and its dependence on delay time and background pressure has been investigated for a variety of collision partners. It is concluded that formation of the triplet through S1 → T1 intersystem crossing proceeds in the statistical limit. Subsequent radiationless decay of the triplet population was found to show unusual pressure-dependent kinetics which apparently reflects collisional interconversion between two forms having very different intrinsic lifetimes. A simple model is proposed to explain the nonradiative behavior, collisional quenching, and spectra of the lowest triplet in terms of strong pseudo-Jahn–Teller vibronic coupling between nearly degenerate 3ππ* and 3nπ* states that leads to a double minimum in the T1 potential surface along the out-of-plane coupling coordinate. It is suggested that vibrationally relaxed T1 pyridine is nonplanar in structure whereas the vibrationally activated form is quasiplanar, and that the nonradiative T1 → S0 decay rates of these two forms are <105 s−1 and ∼5×106 s−1, respectively. Quenching of the triplet by ground state molecular oxygen was found to follow a sequential kinetic mechanism in which a transient intermediate was spectroscopically intercepted. This species is thought to be a weak complex formed between triplet pyridine and oxygen.

156. Bits and pieces, 50. A review of a graphically based modeling software which stimulates student interest in chemical kinetics.

Transient photophysical holeburning spectroscopy of the hydrated electron: A quantum dynamical simulation J. Chem. Phys. 90, 6916 (1989); 10.1063/1.456266The spectroscopy, photophysics, and photochemistry of the dimer of dimethyl tetrazine Nonradiative relaxation processes ofpyridazine's SI origin level have been investigated using several varieties of gas phase time-resolved spectroscopy. Measurements of ground state repopulation kinetics under vibrationally relaxing conditions showed rapid and nearly complete return of thermalized So, implying a quantum yield for SI ~ So internal conversion of at least 0.95. The room temperature photochemical quantum yield was found to be higher in the low pressure gas than in solution by a factor of at least 600. From these results the photochemically active state was deduced to be vibrationally energized So, rather than SI. The collision-free lifetime of the SI origin level was measured as 3--4 ns on the basis of near ultraviolet S _ S transient absorption and SI -+ So stimulated emission kinetics. Hot So molecules form~ as ;he product of SI internal conversion showed a broad, structureless transient absorption spectrum. Apart from its collision-free radiationless decay, the SI origin level was also found to undergo an unusual collisional quenching process induced by electronically inert collision partners at rates between 5% and 10% of gas kinetic. This collisional channel is suggested to involve promotion to the anomalous level 373 cm -I above the SI origin, which had been tentatively assigned as the origin of pyridazine's S2 state.