Charge separation in photoinitiated electron-transfer systems with polyviologen polyelectrolytes as quenchers

Abstract

Various polyviologen polyelectrolytes which may participate in photoinitiated electron-transfer reactions have been investigated as quenchers. A spectral study was made of the reduced polyviologen radicals on reducing the polyviologen species using the 2-propanol radical. Some of the mono radicals produced, possessing absorption spectra similar to that of the methylviologen radical cation, are found to produce multiradical species in the time scale of seconds and minutes. The nature of the final reduced products and the types of reactions generating them are discussed. The abilities of the polyviologens to quench the emission of the lowest excited states of the two photosensitizers, Ru(bpy)/sub 2/(CN)/sub 2/ and Ru(bpy)/sub 3//sup 2 +/, were investigated together with the quantum yields of electron transfer and the rates of their back-reactions by using the laser flash photolysis technique. The highest yield of electron transfer was found for the Ru(bpy)/sub 3//sup 2 +/-poly(o-xylylviologen) system where the quantum yield of photoinitiated electron transfer was determined to be 0.57. The results were compared to those obtained previously with methylviologen and other polymeric viologen systems. The higher than expected rates of quenching and back-reactions were attributed to hydrophobic interactions between the bipyridine groupings of the photosensitizer and quencher which may overcome themore » repulsive Coulombic forces between them. 45 references, 5 figures, 5 tables.« less