Rhodamine 19 Fluorescent Dimers Resulting from Dye Aggregation on the Porous Surface of Sol−Gel Silica Glasses

Abstract

Rhodamine 19 (R19) doped silica glasses were prepared through the sol−gel process with dye concentrations ranging from 4.0 × 10-6 to 6.4 × 10-4 M. The role that the porous surface of the silica glasses plays on the R19 aggregation process was studied through excitation and emission fluorescence spectroscopy, absorption spectroscopy, and fluorescent lifetime measurements. The photophysical behavior of R19 at intermediate concentration values revealed the formation of fluorescent J-dimers in an in-plane oblique angle configuration. The exciton theory was used for the elucidation of the adsorption angle (φ), the angle between the monomer units (α), and the separation distance between the molecules (R) of the R19 fluorescent dimers. Further increase of R19 concentration was observed to promote the conversion from fluorescent to nonfluorescent dimers (H-dimers) through a rearrangement of the monomeric constituents to obtain a parallel plane twist angle configuration.