THE PHOTOCHEMISTRY OF TRIARYLMETHANE DYES BOUND TO POLYELECTROLYTES - PHOTOINDUCED ELECTRON-TRANSFER INVOLVING BOUND DYE MONOMERS AND DIMERS

The photochemistry of two triarylmethane dyes, ethyl violet and crystal violet, bound to poly(methacrylic acid) or poly(acrylic acid) in aqueous solution was investigated. With the use of a large molar excess of polymer (P) residues (P/D > 1000), the dyes (D) were bound as monomeric species. Encapsulation of dye in polymer microdomains was important for solutions with pH < 4, due to the importance of hypercoiled conformations for largely uncharged polyelectrolyte. This compact form provided an effective environment for observation of increased fluorescence and intersystem crossing quantum yields for the bound dye. The semi-reduced and semi-oxidized dye radicals produced on electron transfer between the bound dye monomer triplet and the co-bound electron donor and acceptor reactants (e.g. ascorbic acid and tetranitromethane respectively) were identified by laser flash photolysis. Under alternative conditions of pH and dye loading, for which the polyelectrolytes were substantially charged and maintained less compact, more rod-like conformations (ph 5-9, P/D = 5-100), dyes were shown to be bound as dimer aggregates. On laser excitation (532 nm), the bound dye dimers again produced semi-oxidized and semi-reduced radicals; the semi-oxidized dye radical in turn oxidized the carboxylate group of the polymer at higher pH. The relative quantum yields of radical formation, which varied with pH and the apparent local concentration of the dye, showed non-linear behavior on variation of excitation intensity. A mechanism involving dimer triplet annihilation as a radical source is proposed.