Characterization of molecular scale environments in polymer films by single molecule spectroscopy

Single molecule spectroscopic methods are used to obtain detailed informati on on the polarity and rigidity of molecular-scale environments found in th in poly(vinyl alcohol) (PVA) and poly(methyl methacrylate) (PMMA) films. Ni le Red is employed as a highly sensitive spectroscopic probe of environment al properties in these experiments. Fluorescence spectra are recorded fur n umerous single molecules and their peak positions and widths determined by fitting the spectra to Gaussian functions. The spectral data are analyzed u sing a new model for the dependence of the Nile Red charge-transfer transit ion on the properties of the surrounding medium. This model is based on pre vious work by Marcus (Marcus, R. A, J. Phys. Chem. 1990, 94, 4963). Additio nal information required for the analysis is obtained fi om extensive bulk solution-phase absorption and fluorescence studies. A broad inhomogeneous d istribution of environments is found fur PVA. The results are shown to depe nd significantly on PVA film water content, with the results for hydrated f ilms indicating the presence of less rigid environments. In contrast to the PVA results, two distinct classes of environments are found in the PMMA fi lms. On the basis of an analysis of the data using the aforementioned model , it is concluded that the two environments differ in rigidity but have nea rly identical polarity.