Intramolecular energy transfer in the 4-R-1,2,4-triazoline-3,5-diones: Relation to theory and photochemical reactivity

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Abstract
Various aspects of the intramolecular radiationless decay of the A1A′(1B1) excited electronic state of the 4‐R‐1,2,4‐triazoline‐3,5‐diones (R‐TAD) have been investigated using both laser excited time resolved fluorescence and conventional techniques. The lifetimes of selected vibronic levels of methyl and ethyl TAD under collision free conditions were found to vary approximately linearly from ≃110 to 55 nsec over 2000 cm−1 of excess vibrational energy in the A1A′(1B1) state. Using a somewhat unique technique based upon the fact that vibrational relaxation rather than electronic quenching occurs upon collisions with argon, the quantum yields of fluorescence of selected vibronic levels were determined for methyl TAD and were found to be ≃0.02. Nonradiative rate constants for the selected vibronic levels in A1A′(1B1) of methyl TAD could then be determined and these were found to vary approximately linearly from 8.5×106 to 16.9×106 sec−1 over 2000 cm−1 of excess vibrational energy. Using the experimentally determined variation of the nonradiative rate constant, solution lifetimes, features in the absorption spectrum, and various aspects of the theory of radiationless processes, a choice of the most probable radiationless pathway was made. This choice was intersystem crossing to a lower lying triplet state, either A1A′(1B1) → 3A″(3B1) or A1A′(1B1) → 3A′(3B2) depending upon the R substituent. This choice of nonradiative pathway was found to explain adequately the relative photochemical hydrogen abstraction reactivity of various R‐TAD's. It was also found that methyl, ethyl, and n‐butyl TAD all display the same fluorescence lifetime of 92±3 nsec at high pressures of inert gas. This is discussed in terms of the radiationless transition theory, which indicates that some vibrational modes do not accelerate the radiationless decay rate over that of the vibrationless level. This is the first reported experimental verification of the presence of vibrational motions that do not participate in radiationless decay.