Complexation of Flavonoids: Spectral Phenomena, Regioselectivity, Interplay with Charge and Proton Transfer

The review compiles information on the spectral classification of flavonoids, the changes in their electronic structure upon complex formation, and the manifestation of these changes in the absorption and emission spectra. Part of the review is devoted to the regioselectivity of the complex formation process, including types of complexation sites, the structure of chelates and ′open′ complexes, and the correlation between the structure of complexes and their spectral properties. The interplay between complex formation and other processes occurring in flavonoids during electronic excitation is also considered, such as intramolecular inter‐fragment charge transfer (ICT) and intramolecular proton transfer (ESIPT). The review also contains systematic data on the study of regioselectivity and spectral properties of flavone complexes, obtained by the author and their colleagues over the past decades. The review includes an analysis of the spectral properties of different groups of flavonoids and describes how these properties change when flavonoids form complexes with metal ions. It demonstrates the structures of complexation sites of flavonoids and discusses the regioselectivity of complex formation. The review provides examples of the interplay between complexation and other photochemical and photophysical processes, such as interfragmental charge transfer and excited‐state intramolecular proton transfer.