Photochromic properties of hydrolyzed benzopyrylium salts—the influence of substituents

Abstract

A series of systematically modified benzopyrylium salts was hydrolyzed to study the influence of donor substituents in 4-, 7- and 4′-position of the benzopyrylium cation on photochromic systems. A photochromic behavior was observed at the 7- and 4′-substituted flavylium salts, whereas the insertion of 4-phenyl group prevents the formation of a photochromic system.The photochemical quantum yield varies strongly depending on the substituents and the medium conditions. Good reaction quantum yields were found in highly viscous borax–glycerol mixtures. The exclusively OH- and OMe-substituted chalcones exhibit photochemical quantum yields up to 0.6, whereas the NR2-substituted ones show quantum yields usually below 0.1. As a rule the latter ones fluoresce more intensively, indicating that the S1 state of the trans-chalcone is the common precursor for the photochemical formation of flavylium cation. The differences in the reaction quantum yields can be referred to the different tendency of the formation of a highly polar state on the S1 hypersurface, presumably a twisted intramolecular charge transfer state (TICT).