Organic dyes usually exhibit enhanced photostability when trapped inside sol-gel silicates. The enhanced photostability is attributed to the reduction of intramolecular motions that facilitate photodegradation. We report the simultaneous detection of mobility and photostability of sol-gel encapsulated didodecyl-3,3,3',3'-tetramethylindocarbocyanine (DiI) using single molecule spectroscopy. Fluorescence from DiI was resolved into parallel and perpendicular polarization components and separately detected. On the basis of the calculated fluorescence polarization, single DiI molecules were classified into "tumbling" and "fixed". Out of 212 molecules investigated, 52% were found to be fixed. For the first time, the mobility of a guest molecule in sol-gel silicate can be directly correlated with its own photostability. Both tumbling and fixed molecules have shown to exhibit nonuniform photostability, indicative of the very heterogeneous guest-host interactions within each subgroup. The survival lifetimes for the majority of the tumbling and fixed molecules were found to be 4.3 and 13.1 s, respectively, demonstrating unequivocally that fixed molecules exhibit a higher photostability than tumbling molecules. These results are in accordance with a recent study on rhodamine B encapsulated in dried sol-gel silicates.

We investigate the effect of Coulombic interactions on the mobility of rhodamine 6G (R6G) and Oregon Green 514 (ORG) in sol−gel silicates by measuring their mobility distributions using single molecule polarization measurements. While R6G is a cationic dye, ORG (pK a = 3.69 ± 0.08) is an anionic dye at neutral pH. The presence of more tumbling ORG molecules in sol−gel silicates indicates that R6G and ORG experience opposite Coulombic interactions with a predominately anionic silica sol−gel surface. On the other hand, the fact that tumbling ORG only represents a minor portion of ORG investigated, even at pH 7, clearly illustrates that Coulombic interaction alone does not control the mobility of an encapsulated guest molecule in sol−gel silicates.

Polarization resolved fluorescence from single molecules was measured from rhodamine 6G (R6G), sulforhodamine B (SRB), Oregon 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.

We investigated the effect of charge-charge interactions on the mobilities of rhodamine 6G (R6G), Nile Red, sulforhodamine B, and Oregon Green 514 (ORG) guest molecules within a silica sol-gel host as the guest charge progressed from positive to neutral to negative. Through classification of the mobility as fixed, tumbling, or intermediate behavior, we were able to distinguish differences in surface attraction as the guest charge was varied. On the basis of our results, an attractive charge (as tested by cationic R6G) does not contribute significantly to mobility within dry films. However, an increase in the cationic influence is observed in water-equilibrated environments. A comparison of ORG in dry and water- and phosphate-buffer-equilibrated films indicates that charge repulsion does significantly increase dye rotational mobility (to a maximum of 24 +/- 3% tumbling molecules). However, in view of the percentage of tumbling molecules found, charge-charge interactions do not appear to be the dominant force controlling guest mobility.