Photoisomerization of Charge-Transfer Complexes of Hexamethyl(Dewar benzene). Contrasting Paths for Rearrangement Involving Adiabatic Reaction and Ionic Photodissociation

Article abstract of DOI:10.1021/ja00343a033

The charge-transfer (CT) complexes of hexamethyl(Dewar benzene) (HMDB) with electron acceptors, fumaronitrile, diethyl 1,2-dicyanofumarate, and 1,2,4,5-tetracyanobenzene, have been characterized and compared to similar complexes of hexamethylbenzene (HMB).Irradiation of HMDB CT bands in the 313-435-nm region under a variety of conditions leads to HMDB->HMB isomerization.The quantum yield of rearrangement in a nonpolar solvent is low (e.g., 0.06), although the relative yield of adiabatic isomerization, monitored by emission from excited complexes of HMB, is high (0.72).Quantum efficiencies for isomerization of complexes in polar media generally exceed unity, consistent with a radical-ion chain mechanism for ring opening.The quantum chain reaction depends on the polarity of the solvent, the reduction potential of the acceptor, the extent of conversion, and the wavelength of irradiation.The wavelength effect is associated with excitation to upper vibrational levels of a CT band with enhancement of ionic photodissociation.Comparison of the quantum yield results for excited CT complexes with the findings for rearrangement of HMDB via exciplexes reveals generally different patterns of reactivity.