TRIPLET-STATE ELECTRON-TRANSFER FROM ANTHRACENE AND PYRENE COVALENTLY BOUND TO POLYELECTROLYTES IN AQUEOUS-SOLUTION

The electron-transfer quenching of the triplet state of an aromatic chromophore (anthracene, pyrene) covalently bound to polyacids or polystyrenesulfonate has been studied. The quencher is a zwitterionic viologen (4,4'-bipyridinio-1,1'-bis(propanesulfonate)) that becomes anionic upon reduction. The efficiency of charge separation was found to be largest (ca. 0.8) at high pH for the anthracene-tagged polymer, in contrast with earlier results for the singlet state (J. Phys. Chem. 1989, 93, 1928). For pyrene a number of different polymer backbones were studied, and it was found that the state of ionization of polyacids had little effect on the yield of ion pairs. (The yield was between 0.4 and 1.0, depending on the polymer.) This is also different than the singlet-state results, where no charge separation occurs at any pH (J. Phys. Chem. 1988, 92, 2934). It is proposed that the local polymer structure has a specificity for the covalently bound chromophore, which in turn modifies the quenching mechanism. In all cases the ion pairs have a long lifetime, in excess of 2 ms. The T-T extinction coefficients were determined for polymer-bound anthracene and pyrene and were found to be much smaller than the unsubstituted species in organic solvents. The conditions for photoionization in these systems are also discussed.