Structural and photophysical properties of a water-soluble porphyrin associated with polycations in solution and electrostatically-assembled ultrathin films

A water-soluble porphyrin, meso-tetra(4-sulfonatophenyl)porphyrin (TSPP), has been associated with two different polycations, poly(diallyldimethylammonium chloride) (PDDA) and poly(ethyleneimine) (PEI), to investigate the effects of polymer binding upon the TSPP structure and excited-state dynamics both in solution and in ultrathin (similar to 10-30 Angstrom) films deposited on glass slides by electrostatic assembly. Association of the porphyrin with PEI intrinsically quenches the singlet state dynamics of TSPP, both in solution and in films, while quenching is observed upon association with PDDA only for high concentrations of porphyrins or in films where TSPP aggregates are observed. For PDDA:TSPP films without significant aggregate content, the fluorescence decay time (tau(1/e) similar to 5-6 ns) approaches that observed for monomeric or polymer-bound porphyrins in dilute solution (tau(1/e) = 10.2 and 11.3 ns, respectively). However, rapid (< 1 ns) deactivation of the singlet states can be observed whenever appreciable aggregates are present, indicating that efficient energy transfer between the porphyrins leads to quenching at aggregate sites. A number of phenomenological observations regarding the presence of aggregates are presented. In particular, the extent of aggregation in the films depends on the concentration, pH, and temperature of the deposition solutions and also the age of the films, with older samples exhibiting reduced effects due to aggregation.