Photochemical reactions of aromatic compounds. Part 49. An exciplex chain mechanism for the 1,4-dicyanonaphthalene-photosensitised isomerisation of hexamethyl-(Dewar benzene)

Abstract

Mechanistic studies have been performed on the isomerisation of hexamethyl-(Dewar benzene)1 to hexamethylbenzene 2 photosensitised by 1,4-dicyanonaphthalene (DCN). Analysis of the emission spectra of the DCN–1 and DCN–2 pairs demonstrates that the adiabatic isomerisation of the DCN–1 exciplex to that of DCN–2 occurs with the probabilities 0.53 in cyclohexane and 0.40 in dibutyl ether. The limiting quantum yields for the net isomerisation are 1.5 in cyclohexane and 0.77 in dibutyl ether. Extensive kinetic analyses suggest that the isomerisation proceeds mostly through the adiabatic pathway from the DCN–1 exciplex to DCN–2 followed by sensitisation of the 1 isomerisation by the product (DCN–2) exciplex via the putative DCN–2–1 triplex, while the participation of the non-adiabatic isomerisation from the DCN–1 exciplex is not important.