Susan L. Mattes

Exciplexes and Electron Transfer Reactions

Electronically excited molecules, being better electron donors and acceptors than their ground states, form charge-transfer complexes (exciplexes) which can lead to radical ions. Exciplex emission is widely used to probe polymers and organized media such as membranes and micelles. Exciplexes are also intermediates in photoreactions that lead to unique products. Photochemical electron-transfer processes, which are the basis of silver halide photography and electrophotography, are involved in many reactions of wide scope. Recent studies have led to the discovery of several electron-transfer photooxygenations with a diversity that will probably rival that of singlet oxygen. Both exciplex emission and photochemical electron transfer play important roles in organic photochemistry.

Photosensitized electron-transfer reactions of phenylacetylene

Phenylacetylene (A) reacts with excited sensitizers (S) to produce radical ions (S)·–, (A)·+, which react as geminate or separated pairs with (A) to give a dimeric radical cation (A+–·A); the latter reacts with nitriles to give pyridines (1) or with nitromethane leading to an oxidation product (2) and a dimer (1-phenylnaphthalene) is also formed via the geminate pair.

Photo-oxygenation via electron transfer and its susceptibility to catalysis

The photo-oxygenation of diphenylacetylene (1) is efficiently sensitized (via electron transfer) with dicyanoanthracene (Φ=ca. 0·15), but is sensitized very poorly (Φ < 0·001) with tetracyanoanthracene (TCA); catalysis by acids and nucleophiles substantially increases the quantum yield of photo-oxygenation through reactions competing with the reverse electron transfer between (TCA)–˙ and (1)+˙.