Photophysical properties of polyads containing a fluorescein moiety

A series of dyads, triads and tetrads containing fluorescein as photosensitizer, carbazole as electron donor, viologen and anthraquinone as electron acceptors, and their model compounds were synthesized and characterized. Their absorption spectra, fluorescence spectra, fluorescence lifetimes, the free energy changes of photoinduced intramolecular electron transfer and recombination in methanol were measured and estimated. In all the polyads, the fluorescence of fluorescein is strongly quenched in methanol. The quenching by carbazole and anthraquinone is dominantly a dynamic electron transfer process, while viologen readily forms a non-emitting complex with fluorescein in polyads, possibly due to its double positive charges. When fluorescein in the triads and tetrads is excited, the primary photoinduced intramolecular interaction process is mainly electron transfer from carbazole to fluorescein. Estimation of the free energy changes of the electron transfer reactions indicates that in the triads and tetrads, after the initial charge separation, the subsequent electron transfer processes can occur, and longer-lived charge-separated products can eventually be obtained.