Photoinduced electron transfer in a phenothiazine-riboflavin dyad assembled by zinc-imide coordination in water

The known electron acceptor systems whereby the redox centers are linked by reversible noncovalent interactions are in most cases restricted to organic solvents. A kinetically labile coordinative bond has been designed for self-assembly of an electron donor (phenothiazine) and a photoinducible electron acceptor (riboflavin) in water at neutral pH. The pH dependent formation of the donor-acceptor complex in water was investigated by potentiometric titrations showing a binding constant of log K = 5.9. The strong binding constant supports the observed large fluorescence deactivation of the riboflavin emission by the phenothiazine zinc complex. The riboflavin fluorescence lifetime was found to be constant (tau = 4.7 ns) whatever the quencher concentration, clear evidence fur a static quenching mechanism. A strong thermodynamical driving force and the observation of the riboflavin radical anion and phenothiazine radical cation by transient spectroscopy provide evidence for intramolecular electron transfer as the likely mechanism for the: fluorescence quenching.