Isomerization of dewar benzenes involving electron donor-acceptor exciplexes

Valence photoisomerization of hexamethyl (Dewar benzene) (HMDB) is sensitized by aromatic singlet photosensitizers 1,4-dicyanobenzene, 1-cyanonaphthalene, 9-cyanoanthracene, and 9,10-dicyanoanthracene with a limiting quantum efficiency of 1.0 in cyclohexane solvent. Quenching of the fluorescence of the aromatic sensitizers leads to exciplex emission which is identical to that obtained by quenching with the isomer, hexamethylbenzene (HMB). The emission is identified as HMB exciplex emission on the basis of relative lifetime and dual quenching experiments. The relative yield of HMDB-derived (“adiabatic”) emission is 20–50% depending on the excitation energy of the HMB exciplex product. Neither biacetyl singlet or triplet nor 1-cyanonaphthalene triplet photosensitization is successful in bringing about isomerization of HMDB. Dimethyl 1,4,5,6-tetramethylbicy-clo2.20]hexa-2,5-diene-2, 3-dicarboxylate undergoes valence isomerization on quenching electron donor fluorophores, with a quantum efficiency of 0.2. The aromatic valence isomer is not produced in an excited state in this case. Factors which govern the efficiency of adiabatic and diabatic isomerization of the Dewar benzenes are discussed, including sensitizer redox properties, configuration, and multiplicity, the excitation energy and binding characteristics of exciplexes, and the Dewar benzene substituent pattern.