Doubly excited singlets

The vexed question of the mechanism of the direct and sensitized cis-trans isomerization of stilbene has been the subject of recent papers.546 The studies of the two groups differ in the favoured mechanism for the direct cis-trans isomerization, one proposing the conventional triplet-state mechanism, the other plausibly invoking the intervention of a doubly excited singlet potential surface with a deep potential minimum at a 90° configuration. 

As with anthracene, no delayed excimer fluorescence is observed for 1,2-benzanthracene under these conditions (225), and the deviation of Ps from Vi> may well be due to decay from the relaxed doubly excited singlet pair state as shown in Scheme 5. A relatively smallp = 0.092 value for 1,2-benzanthracene has been estimated from photodimerization results in cyclohexane (229). The 1,2-benzanthracene case illustrates some of the difficulties associated with the evaluation of published TTA data. Undoubtedly, depending upon the availability of quintet, triplet, and excited singlet monomer states at or below the energy of initially formed triplet encounter pairs, exceptions to Eq. 70 and Scheme 5 will be found. The details of the mechanism proposed here may also need to be modified even for anthracene and 1,2-benzanthracene. 

FIGURE 2. Schematic potential energy surfaces for [2+2] cycloaddition in the ground (G), singlet (S), and doubly excited (D) states. E is the exciplex and P the pericyclic minimum. 

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