The fate of electronically excited molecules: Photochemical reactivity and excitonic interaction
Once excited by ultraviolet light to an electronically excited state, a molecule can undergo a number of processes. Some of these processes occur on the excited state surface while others return the molecule to the ground state. Still others involve coupling or energy transfer between excited states. Many result in the scission or formation of bonds. The present work describes the behavior of several systems upon electronic excitation. The spectroscopy of the three isomers of diethynylbenzene is investigated, with a particular emphasis on the possibility of an excited state Bergman cyclization in the ortho isomer. Ultraviolet excitation is used to probe the unimolecular dissociation of three small hydrocarbons, 1,3-butadiene, isoprene, and vinylacetylene. Each of these molecules dissociates to radical products upon photoexcitation, and products of subsequent radical-radical and radical-molecule reactions are also studied. Another molecule known to undergo an excited state reaction is anthranilic acid. Upon electronic excitation, anthranilic acid exhibits a substantial hydrogen atom dislocation. The effects of water complexation and the spectroscopy of the carbonyl stretch vibration are studied in the present work. In addition, a model is presented for the electronic coupling in anthranilic acid dimer, a bichromophore which exhibits localized electronic excitation. Two other bichromophores, diphenylmethane and 4-methyldiphenylmethane, are studied in the present volume. The coupling between electronic states is examined, particularly in terms of the relatively low barriers to phenyl ring torsion. Finally, a new supersonic jet discharge design is described. The products of a 1,3-butadiene discharge are identified and compared to the photochemical products found with ultraviolet excitation.