Intersystem vibronically induced

Thayer et al. (240-242) on propynal are the only reasonably complete nonradiative rate calculations done on a carbonyl. Their values for the intersystem crossing and internal conversion rates are low by factors of 10 and 80, respectively, for the vibrationless excited state when compared to the experimental values. They correctly predict the energy dependence of the decay channels, although they fail to predict the large enhancement of the intersystem crossing rate for three vibronic levels. Also, the energy dependence of the collision-induced nonradiative transitions seems to be well reproduced. 

This may be due to a destabilization caused by twisting around the C32-C13 bond. Direct excitation isomerization yields (<)>jgo) are usually higher in systems characterized by high intersystem crossing. This suggests that a triplet isomerization mechanism plays an important role following direct excitation, a fact that was quantitatively confirmed for retinals in nonpolar solvents. With the exception of 11-cis PRSB, the lowest triplet state (T3) appears to possess a substantial isomerization barrier so that partition between cis and trans isomers takes place in higher vibronic states. An O2-induced mechanism, which is probably associated with a triplet pathway, characterizes the fluorescent derivatives ROH, RAc, and RBA. 

A previous review provides a description of the theory of electronic relaxation in polyatomic molecules with particular emphasis on the vibronic state dependence of radiationless transition rates. A sequal review considers the general question of collisional effects on electronic relaxation, while the present one covers only the special phenomenon of collision-induced intersystem crossing. It departs from the other collisional effects review in presenting only a qualitative description of the theory the full theoretical details can be obtained from the previous review and the original papers.As a review of the basic concepts of radiationless transitions theory is necessary as a prelude to a discussion of collision-induced intersystem crossing, considerable overlap exists between this section and Section II of the previous collision effects review. However, since many concepts from radiationless transition theory, such as the nature and criteria for irreversible decay, the role of the preparation of the initial state, the occurrence of intramolecular vibrational relaxation, etc. pervade the other papers on laser chemistry in these volumes, it is useful to recall the primary results of the theory of electronic relaxation in isolated molecules and its relevance to the material in the present volume as well as to this review. 

At the same time collisions induce the thermally equilibrated phosphorescence from the lowest vibronic levels of the T, state, but the overall triplet character of the system (monitored by the T-T absorption or the T-T transfer yield) remains unchanged. The phosphorescence induction may thus be considered as resulting from vibrational relaxation, transferring the molecules from short-lived and weakly fluorescing mixed states to the pure triplet states, with a relatively higher phosphorescence yield (Lahmani et ai, 1974 van der Werf, 1976). We consider, therefore, that the term collision-induced intersystem crossing, often used in this case, is not appropriate. Collisions do not induce, but only sample (by transfer to phosphorescent levels) inherent triplet character of states prepared by optical excitation and unimolecular evolution. 

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