Fluorescence and Phosphorescence of Flavylium Cation Analogues of Anthocyanins

Fluorescence spectra and lifetimes were determined for 16 synthetic flavylium cation analogues of anthocyanin plant pigments in dry acetonitrile acidified with trifluoroacetic acid (TFA). Phosphorescence was also observed from the lowest excited triplet state all cations at 77 K a rigid TFA-acidified isopropanol glass. The fluorescence quantum yields depend systematic manner on nature position substituents chromophore three specific substitution patterns associated significant decreases yield identified. A 4′-bromo or 4′-iodo substituent B-ring produced small but normal heavy-atom effect, reducing phosphorescence lifetime relative to without halogen atom. In contrast, 3′-bromo exhibited an “inverse” heavy atom i.e., increase rather than decrease, which rationalized basis nodal properties natural transition orbitals (NTOs) involved S0→S1 radiative transition.