Photochemical Reactions of 4-Acylstyrenes. Effects of Acyl Substituents on Photocyclodimerization via an Excited Triplet State

Article abstract of DOI:10.1246/bcsj.65.837

The photochemical reactions of 4-acylstyrenes (1a-e; acyl=acetyl, benzoyl, 1-naphthoyl, 2-naphthoyl, and 9-phenanthrylcarbonyl, respectively) were investigated, and their excited state behavior and photoreactivities are discussed in relation to the aryl ketone structures.Photoirradiation of 1a-e in benzene gave two isomeric cyclodimers of trans- and cis-1,2-bis(4-acylphenyl)cyclobutanes (2a-e and 3a-e) as main products, accompanied by the formation of some oligomers and polymers.The fractions of the trans-form in the cyclodimers were ca. 80percent; this indicated that the cyclodimerization proceeded via the ?,?^* triplet state.The quantum yields of the photodimerization for 1a-e were almost the same irrespective of the acyl substituents (Φ=0.03-0.04 at <1a-e>=0.1 mol dm^-3).Phosphorescence spectra of 1c (τ=0.48 s) and 1d (τ=0.91 s) observed in a rigid solution at 77 K, which coincided with those of the respective model aryl ketones, were ascribed to the low-lying excited ?,?^* triplet states localized on the naphthalene moiety of the molecules.Phosphorescence emissions of 1a and 1b could not be detected.The transient absorption spectrum of 1d (τ=23 ns) observed in a fluid solution at 293 K was tentatively assigned to the ?,?^* triplet state mainly localized on the styrene moiety of the molecule.The photoreaction mechanism is discussed in terms of the intramolecular triplet energy transfer.