Photoreactions of 3-(2-Hydroxyphenyl)-1-(substituted phenyl)-2-propen-1-ones (Substituted 2-Hydroxychalcones) in Organic Solvents in the Presence and Absence of Acid

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Abstract
Absorption spectra and photochemical reactivities of a dozen 3-(2-hydroxyphenyl)-1-(substituted phenyl)-2-propen-1-ones (substituted 2-hydroxychalcones) have been investigated in organic solvents in the presence and absence of 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 of the nature of the substituents. Upon irradiation in neutral solution, on the other hand, amino-substituted chalcones gave photo-Z-isomers as thermostable products, while the majority of other chalcones gave cyclic hemiacetals (2-hydroxy-2-phenyl-2H-1-benzopyranes). The quantum yields for the photochemical E/Z isomerization in the neutral solution were comparable to those in the acid solution, except for the amino-substituted chalcones which revealed lower quantum yields. Remarkable hypsochromic shifts were observed for the absorption spectra of the amino-substituted chalcones upon addition of acid, reflecting protonation on the amino group.

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