Aggregation of thiacyanine derivatives on polyelectrolytes

The electrostatic binding of a series of anionic disulfopropyl-thiacyanine (THIA) derivatives to poly(allylamine hydrochloride) (PAH) in aqueous solution is investigated by optical spectroscopy. From strong variations of the absorption, fluorescence, and excitation spectra of the dyes upon binding to PAH, conclusions are drawn about the binding modes. The unsubstituted THIA forms a ground state complex with PAH which exhibits a blue shift in absorption, like H-aggregates, but also a strongly enhanced fluorescence with respect to the free dye. The methyl derivative behaves similarly. The absorption of the methoxy derivative also exhibits a hypsochromic shift upon addition of PAH but its fluorescence is quenched. In the case of the dichloro-derivative, addition of PAH induces the formation of J-aggregates. The fluorescence quantum yield is strongly enhanced from 0.008 for the monomer to 0.48 for the PAH-J-aggregate. The monochloro-THIA shows a competition between the J-aggregate formation and the dye–PAH complex formation. Similarly to the methoxy-THIA, the fluorescence is quenched upon addition of PAH. The stability of the PAH-J-adduct to replacement by polyanions is larger than for the H-adduct, although the aggregation tendency of the dyes in pure water displays an opposite behaviour. Results show that the optical properties and the aggregate structures of sensitizing cyanine molecules can be tuned widely by interaction with polyelectrolytes.