Effect of Coulombic Interactions on Rotational Mobility of Guests in Sol-Gel Silicate Thin Films

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 (pKa = 3.69 (plus_minus) 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