Exciplex potential energy surface

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. 

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. 

Fig. 2.3 Potential energy surface description of exciplex formation between two molecules M and N. MN> and M N> are the ground- and excited-state curves. rw-N stands for the intermolecu-...

Let us now consider a potential energy surface description of exciplex formation (Fig. 2.3). At large separations of the (ground-state) molecules M and N, the absorption spectrum of either component would be identical to that of each monomer, i.e. neither component would influence the other. As M and N approach, the absorption spectrum remains constant. Eventually, M and N undergo collisions. Since there are no substantial attractions between M and N in their ground-states, steric hindrance will repel the molecules and very few (dissociative) complexes will exist at any given time. As a result, no new absorptions will be observed.1J... 

Chemical reactions, particularly photochemical interactions, are characterized by distance-dependent rate constants rather than sharp spherical reaction boundaries. Furthermore, the detailed shapes of the potential energy surfaces for the kinds of interactions discussed in this chapter are largely unknown. Nevertheless if one is interested in a more qualitative assessment of the capture radius prediction, evidence should be available from a comparison of the cyclization rates of Py-poly-styrene-Py and DMA-polystyrene-Py. In polar and polarizable solvents, exciplex formation in the latter polymer should be preceded by rate-limiting electron transfer to generate the solvent separated ion pair. Estimates for this interaction distance are typically 7 A to 10 A (16), larger than the 5 A normally invoked for pyrene excimer formation (6). 

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