Microheterogeneity in Dye-Doped Silicate and Polymer Films

Abstract

Single molecule confocal microspectroscopic methods are used to characterize individual molecular-scale environments in silicate thin films for the first time. Rhodamine dyes doped into the materials at nanomolar levels are used as probes of the physicochemical environment in which each molecule is entrapped. The results are compared to those obtained from dye-doped organic polymer films. Static fluorescence spectra and time-dependent fluorescence signals recorded for a large number of single molecules show the silicate materials to be highly inhomogeneous in comparison to the polymer films. Histograms of the fluorescence maxima for encapsulated Rhodamine B show a full width at half-maximum of 13.3 nm for the silicate host framework and 6.7 nm for the polymer film. The integrated fluorescence signal from single molecules, recorded with millisecond time resolution, under continuous illumination conditions, is also sensitive to the local environment. The time-dependent signal traces show dramatic intensity fluctuations for some molecules and none for others. The fluctuations occur most frequently for the silicate-entrapped dye. In the present work, the signal fluctuations are proposed to result from time-dependent variations in the molecular environment, which in turn cause changes in the excitation and emission characteristics of the molecules. The photophysical phenomena behind these fluctuations include quantum yield variations, intersystem crossing to a long-lived dark state, and, to a lesser extent, spectral diffusion. All such effects are highly dependent upon the physicochemical properties of the molecular-scale environment, as shown by comparison to results obtained for the polymer samples. The results are used as further evidence for the heterogeneous nature of entrapment in silicate host structures. The dynamic nature of many of the molecular-scale environments in these materials is demonstrated as well.