ENONE PHOTOCHEMISTRY - DYNAMIC PROPERTIES OF TRIPLET EXCITED-STATES OF CYCLIC CONJUGATED ENONES AS REVEALED BY TRANSIENT ABSORPTION-SPECTROSCOPY

Triplet pi,pi* excited states of a large number of cyclic alpha, beta-unsaturated ketones (enones) have been generated by pulsed laser excitation and detected by their characteristic ultraviolet absorption, lambda-max 280-310 nm. The large variation in the lifetimes of these enone triplets as a function of molecular structure and the changes in rate constants for triplet energy transfer from enone triplets to naphthalene and methylnaphthalene for rigid vs nonrigid triplets are consistent with triplet relaxation by twisting around the olefinic bond. This has been confirmed by studies using time-resolved photoacoustic calorimetry (PAC), which afford both lifetimes and energies of the relaxed enone triplet states. The PAC lifetimes are in excellent agreement with those measured by transient absorption spectroscopy (TAS). The effect of substitution on the C = C bond on both lifetimes and energies of enone triplets is compatible with pyramidalization at the beta-carbon atom of the triplet prior to intersystem crossing to the ground state. Rates of quenching by conjugated dienes are much lower than anticipated on consideration of the triplet energies of the enone as donor and the diene as acceptor. In systems in which enone-diene adducts are known to be formed, Stern-Volmer plots showed pronounced curvature. Self-quenching of enone triplets is particularly pronounced in the cases of cyclopentenone and some longer lived triplets but is not observed for simple cyclohexenones. In the latter case, the rate constant for self-quenching is too small to cause an unambiguous change in the short triplet lifetimes. The origin of the residual UV absorption observed for many enones following decay of the triplet has not been unambiguously established. Prominent candidates are dimeric triplet 1,4-biradicals en route to enone photodimers and deconjugated isomers of the starting enone.