Photosensitized [2+2] cycloreversion reactions of arylated cage compounds in nonpolar solvents. Highly efficient adiabatic exciplex isomerization

Abstract

Photosensitized [2+2] cycloreversion reactions of arylated cage compounds 1 to dienes 2 using several cyanoaromatics as sensitizers are reported. In acetonitrile, 1a, isomerized to 2a via an efficient cation-radical chain process. In contrast, the isomerization of 1a to 2a proceeded via an exciplex in benzene. Interestingly, the exciplex emission maximum observed in the fluorescence quenching by 1a was the same as that by 2a. Such a coincidence was also observed in a series of arylated cage compounds 1a-d and dienes 2a-d in various solvents. These observations together with the results obtained from reaction and exciplex emission quenching experiments clearly demonstrated the occurrence of adiabatic exciplex isomerization of 1 to 2. Efficiencies (A e ) of the adiabatic process obtained by comparing the intensities of the observed exciplex emissions with those of diene exciplex emissions were quite high and became almost quantitative in some cases. A e values were found to increase as (1) the oxidation potentials (E 1/2 ox ) of cage compounds 1a-d decreased, (2) the reduction potentials (E 1/2 red ) of sensitizers increased, and (3) the solvent polarity (E T ) increased