SYNTHESIS, CHARACTERIZATION, PHOTOPHYSICS, AND PHOTOSENSITIZATION STUDIES OF SQUARAINE-BIPYRIDINIUM DIADS

Two squaraine-bipyridinium diads designed to study electron transfer b etween the squaraine chromophore and the bipyridinium group have been synthesized. The two diads, denoted as C(4)Sq-By and C(12)Sq-By, consi st of amphiphilic squaraines covalently linked to a bipyridinium group by a trimethylene chain. The squaraine chromophores in the diads do n ot form charge-transfer complexes with the bipyridinium group in the g round state. Both C(4)Sq-By and C(12)Sq-By show fluorescence yields ab out 100 times weaker than those of alkyl substituted squaraines, which is attributed to quenching by an intramolecular electron transfer fro m the excited squaraine chromophore to the bipyridinium; the intramole cular electron-transfer rate is estimated to be 4 x 10(11) s(-1). Assu ming the short lifetime for the undetectable transient produced is due to ver?, fast back electron transfer to the radical cation of squarai ne from the reduced bipyridinium ion, the rate of the back intramolecu lar electron transfer is estimated as >3 x 10(10) s(-1). The two diads synthesized in this work have been used as photocurrent enhancers for the monolayers of DSSQ ylamino)phenyl-4'-(dimethylamino)phenylsquarai ne). C(4)Sq-By as a solution additive is 5 times more effective in sen sitizing the cathodic photocurrent generation of the DSSQ monolayer-mo dified SnO2 electrode compared to methylviologen. Analogously, C(12)Sq -By is 3 times more effective in sensitizing the DSSQ monolayer-modifi ed SnO2 electrode compared to 3-tetradecyl-4'-methylbipyridinium dichl oride when it is incorporated upon spreading in the monolayer of DSSQ. The increase in cathodic photocurrent generation efficiency is postul ated to result from fast electron transfer from the excited squaraine to the bipyridinium group in the diad. The fast electron transfer faci litates electron separation in the photogeneration step and consequent ly increases the quantum efficiency of the cathodic photocurrent gener ation process.