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. For comparison, a series of N-alkylpyridiniumyl-1,8-naphthalimide “parent” compounds was prepared and photophysically characterized. Relative to the parent compounds, the electronically excited singlet state of the 1,8-naphthalimide was found to be quenched by the covalently attached viologen. From Stern−Volmer analyses of the steady-state fluorescence spectra, along with the singlet-state lifetime of the pyridinium-substituted 1,8-naphthalimide, the rate constants for intramolecular quenching were calculated to range from 1.5 × 1010 s-1 (2 intervening methylenes) to 8.3 × 107 s-1 (6 intervening methylenes) in aqueous buffered solution. For comparison, the intermolecular reactivity of the excited singlet state of N-alkylpyridiniumyl-1,8-naphthalimides with methylviologen was assessed. In 0.5 M phosphate buffer (pH 7.0), the bimolecular rate constant was found to be 3.2 × 109 M-1 s-1. Nanosecond laser flash photolysis studies were carried out to identify the quenching products. From these studies, reduced methylviologen was identified as a singlet-state quenching product. From these results, we attribute both the intra- and intermolecular quenching process to electron transfer 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 intervening methylenes. Linear least-squares analysis of the results yielded an apparent β value of 1.04 Å-1 for electron transfer through the polymethylene bridge.