Excited-State Electron Transfer in Anthocyanins and Related Flavylium Salts

Abstract

Anthocyanins constitute the major red and purple pigments in plants. In this work, it is shown that the red flavylium cation of anthocyanins forms ground-state charge transfer complexes with several naturally occurring electron donors, such as flavones (e.g., quercitrin) and even benzoic and cinnamic acid derivatives. Excitation of these charge transfer complexes results in efficient static fluorescence quenching due to fast electron transfer from the copigment to the flavylium cation. The complete inhibition of the fluorescence of the excited base form of anthocyanin implies that the rate constant for electron transfer is substantially larger than that for deprotonation (ca. 1011 s-1). The high efficiency of electron transfer results from the high oxidizing power in the excited state of the red form of anthocyanins. In uncomplexed anthocyanins, ultrafast proton transfer efficiently dissipates the absorbed light energy. Although ultrafast electron transfer could play a similar role in flavylium cation−polyphenol complexes, it may also serve as a source of free radicals if recombination is not 100% efficient, which might eventually be important for understanding the role in vivo of anthocyanin−polyphenol complexes.