Photoinduced electron transfer in covalently linked 1,8-naphthalimide/viologen systems

A series of polymethylene-linked 1,8-naphthalimide/viologen diads has been synthesized. The number of intervening methylenes was varied from 2 to 6. F or comparison, a series of N-alkylpyridiniumyl-1,8-naphthalimide "parent" c ompounds was prepared and photophysically characterized. Relative to the pa rent compounds, the electronically excited singlet state of the 1,8-naphtha limide was found to be quenched by the covalently attached viologen. From S tern-Volmer analyses of the steady-state fluorescence spectra, along with t he singlet-state lifetime of the pyridinium-substituted 1,8-naphthalimide, the rate constants for intramolecular quenching were calculated to range fr om 1.5 x 10(10) s(-1) (2 intervening methylenes) to 8.3 x 10(7) s(-1) (6 in tervening methylenes) in aqueous buffered solution. For comparison, the int ermolecular reactivity of the excited singlet state of N-alkylpyridiniumyl- 1,8-naphthalimides with methylviologen was assessed. In 0.5 M phosphate buf fer (pH 7.0), the bimolecular rate constant was found to be 3.2 x 10(9) M-1 s(-1). Nanosecond laser flash photolysis studies were carried out to ident ify the quenching products. From these studies, reduced methylviologen was identified as a singlet-state quenching product. From these results, we att ribute both the intra- and intermolecular quenching process to electron tra nsfer from the singlet excited state of 1,8-naphthalimide to methylviologen . Within the covalently linked series, the rate constant for intramolecular electron transfer was found to vary exponentially with the number of inter vening methylenes. Linear least-squares analysis of the results yielded an apparent beta value of 1.04 Angstrom(-1) for electron transfer through the polymethylene bridge.