Photoisomerization Reactions of Aromatic Compounds

Liquid benzene on photoirradiation gives a very small amount of benzvalene (tri-cyclo[3.1.0.0 ]-hex-3-ene) through formation of a diradical as an intermediate [1]. 

The presence of the diradical intermediate was supported by the fact that irradiation of benzene in acidic hydroxylic solvents gives addition products with solvent [2]. 

5-Tri-f-butylbenzene undergoes photoisomerization to give various products of valence isomers [3]. Possibly bulky fert-butyl group in the aromatic ring induces steric interaction to facilitate this isomerization. It is diflticult to predict the exact mechanism of formation of all these photoproducts. 

10-Dimethoxy-octamethylanthracene on photoirradiation isomerizes to 9,10-Dewar isomer [4]. 

---