Studies of micro-environments in polymer and silicate thin films by single molecule spectroscopy

As a result of recent advances in optical microscopy, the detection and spectroscopic characterization of single molecules is now possible. This dissertation demonstrates the use of single molecule fluorescence spectroscopy as a means to characterize the nanometer-scale environment in polymers and sol-gel derived silicates. In the first chapter, general information on the principles of single molecule spectroscopic methods, and the molecular probes employed in this thesis, are introduced. A brief review of recently published applications of these methods in material science is also included.; In Chapter 2, a model based on the work of Marcus was used to obtain more quantitative information on the nanoscale environmental properties found in thin polymer films. The method was demonstrated in the experiments on poly (methylmethacrylate)(PMMA) and poly (vinyl alcohol)(PVA) thin films. Specifically, parameters related to the local polarity and rigidity of the material were obtained, using the highly solvent-sensitive emission characteristics of Nile Red molecules doped into these films. The statistical results indicate a broad inhomogeneous distribution of environments exists in PVA while two distinct classes of environments are found in PMMA films. The analysis shows these environments differ in rigidity, rather than polarity.; In Chapter 3, the effect of nanoscale environment on the time-dependent single molecule emission characteristics is explored. It is demonstrated that the hydration level of PVA films plays an important role in determining the rate of observed “blinking” phenomena, as well as molecular rotation, translational diffusion, and other dynamics (i.e. spectral diffusion). These experiments were performed by changing the relative humidity of the atmosphere surrounding the sample. Confocal and broad illumination images are used to study molecular translational diffusion. Time dependent polarization signal transients and dichroism analysis are used to obtain quantitative information on molecular rotational diffusion. Autocorrelation functions are used to quantify the fluorescence fluctuations and extract dynamic information on Nile Red single molecules and their environments. It was found that triplet blinking, translational diffusion, rotational diffusion, and spectral diffusion of the Nile Red molecules in PVA films are not the major contributors to the observed fluorescence fluctuations. Rather, the results indicate these fluctuations arise primarily from time-dependent variations in the emission yield of Nile Red, due to participation of a TICT excited state in the photophysical dynamics.; In Chapter 4, two BODIPY dyes are employed to provide complementary information on the structure (i.e. phase separation) of isobutyltrimethoxy silane (BTMOS), propionitriletriethoxy silane (CNS), and aminopropyltrimethoxy silane (APS) from tetraethoxy silane materials. Oligomer formation in the early stage of the sol-gel process for BTMOS is hypothesized and used to interpret the observed experimental data. Autocorrelation analysis of fluorescence transients is used to estimate the diffusion coefficients of single BODIPY molecules in the organically modified silicates.