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 organi c solvents. A kinetically labile coordinative bond has been designed for se lf-assembly of an electron donor (phenothiazine) and a photoinducible elect ron acceptor (riboflavin) in water at neutral pH. The pH dependent formatio n of the donor-acceptor complex in water was investigated by potentiometric titrations showing a binding constant of log K = 5.9. The strong binding c onstant supports the observed large fluorescence deactivation of the ribofl avin emission by the phenothiazine zinc complex. The riboflavin fluorescenc e lifetime was found to be constant (tau = 4.7 ns) whatever the quencher co ncentration, clear evidence fur a static quenching mechanism. A strong ther modynamical driving force and the observation of the riboflavin radical ani on and phenothiazine radical cation by transient spectroscopy provide evide nce for intramolecular electron transfer as the likely mechanism for the: f luorescence quenching.