Solvent effect on mobility and photostability of organic dyes embedded inside silica sol-gel thin films

Polarization resolved fluorescence from single molecules was measured from rhodamine 6G (R6G), sulforhodamine B (SRB), Ore-on Green 514 (ORG), and didodecyl-3,3,3',3'-tetramethylindocarbocyanine (DilC(12)) embedded in silica sol-gel thin films in contact with water and ethanol. Positively charged R6G was found to be firmly immobilized, regardless of the solvent medium. Despite its net negative charge, zwitterionic SRB was found mostly immobilized in both solvent media. The immobilization of SRB was attributed to a delicate balance of Coulombic interaction, hydrogen bonding, and molecular imprinting effect. While R6G showed a significant decrease in photostability in a solvent medium because of enhanced solvent interactions, hydrogen bonding appeared to effectively retard the molecular motions of solvated SRB such that its photostability exceeded those in dry films. The mobility of negatively charged ORG was found to be moderately enhanced through repulsion, but Coulombic force was not as influential as hydrophobic interaction, which was responsible for the high mobility found for DiIC(12) in dry films. When in water and ethanol, the hydrophobic chains of DiIC(12) were pushed by the solvent toward the less polar silica matrix, forcing DiIC(12) to the solid-liquid interface and transforming a substantial amount of tumbling DiIC(12) into intermediate DiIC(12). Since the photostability of DiIC(12) is closely related to the dynamics associated with its hydrophobic chains, DiIC(12) in both water-covered and ethanol-covered films displayed comparable photostability because of the similar silica environment surrounding the hydrophobic chains.