Photoreactions of 3-(2-hydroxyphenyl)-1-(substituted phenyl)-2-propen-1-ones (substituted 2-hydroxychalcones) in organic solvents in the presence andabsence of acid

Absorption spectra and photochemical reactivities of a dozen 3-(2-hydroxyph enyl)-1-(substituted phenyl)-2-propen-1-ones (substituted 2-hydroxychalcone s) have been investigated in organic solvents in the presence and absence o f added acid. Upon UV irradiation in the presence of 5 mM perchloric acid, flavylium ions were formed in high quantum yields around 0.34, regardless o f the nature of the substituents. Upon irradiation in neutral solution, on the other hand, amino-substituted chalcones gave photo-Z-isomers as thermos table products, while the majority of other chalcones gave cyclic hemiaceta ls (2-hydroxy-2-phenyl-2H-1-benzopyranes). The quantum yields for the photo chemical E/Z isomerization in the neutral solution were comparable to those in the acid solution, except for the amino-substituted chalcones which rev ealed lower quantum yields. Remarkable hypsochromic shifts were observed fo r the absorption spectra of the amino-substituted chalcones upon addition o f acid, reflecting protonation on the amino group.