Synthesis, Characterization, Photophysics, and Photosensitization Studies of Squaraine−Bipyridinium Diads

Abstract

Two squaraine−bipyridinium diads designed to study electron transfer between the squaraine chromophore and the bipyridinium group have been synthesized. The two diads, denoted as C4Sq−By and C12Sq−By, consist of amphiphilic squaraines covalently linked to a bipyridinium group by a trimethylene chain. The squaraine chromophores in the diads do not form charge-transfer complexes with the bipyridinium group in the ground state. Both C4Sq−By and C12Sq−By show fluorescence yields about 100 times weaker than those of alkyl substituted squaraines, which is attributed to quenching by an intramolecular electron transfer from the excited squaraine chromophore to the bipyridinium; the intramolecular electron-transfer rate is estimated to be 4 × 1011 s-1. Assuming the short lifetime for the undetectable transient produced is due to very fast back electron transfer to the radical cation of squaraine from the reduced bipyridinium ion, the rate of the back intramolecular electron transfer is estimated as >3 × 1010 s-1. The two diads synthesized in this work have been used as photocurrent enhancers for the monolayers of DSSQ (4-(distearylamino)phenyl-4‘-(dimethylamino)phenylsquaraine). C4Sq−By as a solution additive is 5 times more effective in sensitizing the cathodic photocurrent generation of the DSSQ monolayer-modified SnO2 electrode compared to methylviologen. Analogously, C12Sq−By is 3 times more effective in sensitizing the DSSQ monolayer-modified SnO2 electrode compared to 4-tetradecyl-4‘-methylbipyridinium dichloride when it is incorporated upon spreading in the monolayer of DSSQ. The increase in cathodic photocurrent generation efficiency is postulated to result from fast electron transfer from the excited squaraine to the bipyridinium group in the diad. The fast electron transfer facilitates electron separation in the photogeneration step and consequently increases the quantum efficiency of the cathodic photocurrent generation process.