The photophysical and photochemical properties of organic dyes bound to polyelectrolytes in water

The principal focus of this research has to do with the use of organic dyes as fluorescence probes of microenvironments of domain-forming polymers or biological macromolecules. 7-Aminocoumarins possessing azole derivatives are examined for their potential use as fluoroprobes of microdomains. The photophysical properties of coumarins 6, 7, and 30 attached to a carrier “template” polyelectrolyte poly(methacrylic acid) (PMAA) in aqueous solutions have been investigated. Alterations in the fluorescence wavelength, quantum yield, lifetime, and polarization are observed for these dyes, when they are bound within compact hydrophobic microdomains of the polymer. The spectral trends as well as the influences of more polar solvents on fluorescence quantum yields and lifetimes are consistent with the importance of a low-lying intramolecular charge transfer (ICT) state for these chromophores. The effect of protonation in different solvent mixtures on the photophysical properties of these heterocyclic systems has been studied. Sites of protonation involving either imidazole/thiazole or aniline-type nitrogens have been inspected by NMR. The conjugate acid forms of the modified coumarins show enhanced absorptivity in the red and altered photophysical properties; these cationic coumarin dyes display fluorescence that is particularly sensitive to bulk viscosity or microviscosity for domain forming polymers.; Another key aspect of this research is the demonstration of a self-assembling photochemical electron transfer system that consists of dye chromophores and ancillary redox active agents co-bound to a charged polyelectrolyte. The light harvesting and charge separation properties are designed to resemble elementary steps that take place in photosynthetic reaction centers. Triarylmethane dyes, crystal violet, ethyl violet and malachite green are induced to aggregate (predominantly as dimers) on binding with poly(acrylic acid) (PAA) and PMAA under conditions in which the water-soluble polymers are substantially ionized (pH > 6.0). Addition of secondary counter-ion species, methyl, benzyl or heptyl viologen leads to the formation of complexes consisting of dye dimers and viologen bound within the polymer domain. The onset of photoinduced electron transfer involving dye aggregates serving as PMAA-bound electron donors and viologen acceptor groups is monitored using laser flash photolysis techniques.; The association of ethyl violet and rhodamine 60 with humic acids (HA) in aqueous solution has also been studied. The binding behavior of these cationic dyes to HA exhibits a different pattern to that of PMAA. The quenching of dye fluorescence for HA-bound dye indicates an electron transfer from donor dye to electron acceptor groups native to HA. Competitive binding experiments were carried out that demonstrated partitioning of rhodamine 6G between HA and PMAA. This polyelectrolyte-dye exchange system could serve as a probe of mechanisms of transport of dye pollutants in the environment.