Exciplex Minima and Barriers

Photudimerizations and photocycloadditions are important examples of bi-molecular reactions. For such reactions an encounter complex has to be first formed, which in the following will be treated as a supermolecule. Correlation diagrams can be constructed for this supermolecule in the usual manner and can be utilized to discuss the course of the reaction. This was demonstrated in Chapter 4 for the exploration of pericyclic minima using H4 as an example. 

The formation of the encounter complex will in general be diffusion controlled. When two (or more) molecules D and A form a sufficiently stable aggregate in the ground state, this aggregate will be a new chemical species commonly referred to as a CT complex and can be identified by its characteristic long-wavelength absorption. (Cf. Section 2.6.) 

Stable complexes can form in excited states even when this is not possible in the ground state. These exciplexes or excimers can freqi ently be identified by their characteristic fluorescence. (Cf. Sections 5.4.2 and 5.4..1.) In view of the formation i f tin exciplex, ground-slate-forbiddcn pholocycload-dilions typically involve an intermediate, F2 and ictiirn through a funnel. 

Which of several possible products will be formed and whether photocycloaddition will occur at all will depend on the depth of the various minima on the excited surface S, on the height of the barriers between them, and on the partitioning between starting materials and products upon return to S . Additional complications result if the diagonally distorted funnels give rise to products of crossed cycloadditions. 

Formation of a bound excimer ordinarily proceeds without an activation barrier other than that imposed by the need for diffusion to bring the reaction partners together in a medium of nonzero viscosity. It is then reasonable to assume preferential formation of the most stable excimer. This is also reasonable if an equilibrium is established among the various possible excimers. The relative stabilities of the excimers or exciplexes can be estimated from orbital interactions or from experimental data (exciton splitting and CT interactions, cf. Section 5.4.2). In the absence of steric complications, the syn head-to-head excimer is usually preferred. (Cf. Section 7.4.2.) 

---

Although the relative orientations of the two addends must be different for ortho and meta addition, it is conceivable that both processes should proceed via the same exciplex. One may speculate that the exciplex does not have one favorite rigid geometry, but that it is in a double-minimum energy well on the excited-state potential-energy surface, with the minima separated by a small barrier. 

---