Excimer minimum

Because of the especially high symmetry and because all the electrons are involved in binding interactions, the conditions in H4 are evidently optimal for the formation of an excimer minimum. The situation will be different for photocycloadditions in which n bonds are converted into a bonds and were additional electrons in mutually repelling closed shells make an intimate approach of the two reactants difficult. In these cases, the excimer minimum will be flatter and will occur only at larger nuclear separations, where the molecules are barely touching. (Cf. Section 5.4.2 and Figure 6.7.)... 

According to Figure 6.17, the height of the barrier between the excimer minimum and the pericyclic funnel depends both on their depths and on the relative placement of the two excited-state surfaces S and D. The depth of the diagonally distorted pericyclic funnel is determined by the nature of the biradical its dependence on molecular structure, on the head-to-head and head-to-tail orientation of the components, and on reaction medium can be discussed using the principles outlined in Section 4.4.1. 

Figure 6.17. Schematic correlation diagram for photodimerization, showing the crossing (f) that determines the barrier between excimer minimum (E ) and peri-cyclic minimum (P ), an earlier crossing (—), and a later crossing ) (by permission from Caldwell, 1980). Effects of a likely diagonal distortion are not shown.

There may well be systems in which the excimer minimum occurs in the S2 rather than the S, surface (Figure 7.27b). The approach to the pericyclic funnel P on S, may then be barrierless, and an excimer intermediate will... 

Figure 7.27. Schematic representation of the state correlation diagram for a ground-state-forbidden pericyclic reaction with an excimer minimum E a) at geometries well before the pericyclic funnel P is reached, and b) at geometries similar to those of P. ...

S is greatly stabilized compared to the others, through exciton as well as CT interactions—that is, exactly by the factors responsible for the stability of excimers and exciplexes. (Cf. Sections 5.4.2 and 5.4.3.) The S state has a minimum at the biradicaloid square geometry that may be referred to as the excimer minimum. A strong absorption band in the near-IR region corresponding to a transition between the S and S states of the fluorene excimer has been observed by photodissociation spectroscopy (Sun et al., 1993). 

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