Photoprocesses of Thiacarbocyanine Monomers, Dimers, and Aggregates Bound to Polyanions

The photophysical and photochemical properties of thiacarbocyanine 1 and the methyl 2 and ethyl 3 meso derivatives in aqueous solution were studied in the presence of poly(styrenesulfonate) (PSS), poly(acrylic acid) (PAA), or poly(methacrylic acid) (PMA). When the ratio of polyanion residue to dye concentrations ( r ) is increased, a monomer/dimer equilibrium in neat water is shifted toward bound H-aggregates (high dye loading conditions), dimers, and monomers (low dye loading). The photodeactivation properties of 1 − 3 depend significantly on these ground-state equilibria. The fluorescence quantum yield (Φ f ) is strongly enhanced for low dye loading and reduced when bound aggregates and dimers are present, r = 3−30 and (0.2−2) × 10 3, respectively. For 1 the quantum yield of trans → cis photoisomerization (Φ t - c ) is also reduced by aggregation. The quantum yield of intersystem crossing (Φ isc ) is enhanced for bound dimers, the value is largest for 3 . In the redox reactions via the triplet state of 3, ascorbic acid and p -benzoquinone serve as electron donor and acceptor, respectively. The photoinduced electron transfer is less efficient for bound monomers than for bound dimers. For the three polyanions comparable effects were found; especially, the ground-state equilibria and the Φ f, Φ isc, and Φ t - c values depend in a well-defined manner on the parameter r .