Single Molecule Polarity Studies

Using what is perhaps the most solvent sensitive fluorescent dye (Nile Red) known, we have developed and demonstrated unique SMS methods for characterizing the static and dynamic polarity properties of nanoscale environments in organic polymer films[28] and in organically- modified silica films[36,40] prepared by the sol-gel process. We showed that the position and width of the emission spectrum for each molecule provide a measure of the static polarity of its local environment. The emission bandwidth itself provides a measure of the dynamic local polarity (i.e., the reorganization energy). We demonstrated these measurements by comparing results from polar and nonpolar organic polymer films[28] and later made similar measurements in silica films containing covalently attached butyl and cyano functional groups.[36,40] These results showed that the nanoscale environments became distinctly less polar based on their static properties as organic groups were added into the silica matrix. Interestingly, the same results reveal an increase in dynamic polarity, i.e., increasing reorganization energies, across the same series of samples. These studies led us to develop and synthesize an unique form of Nile Red that could be polymerized into the silica matrix. The results of fluorescence correlation spectroscopy (FCS) studies performed with this dye showed that the organically modified silica films were actually comprised of viscous, liquid-like oligomers, rather than of solid silica frameworks.[52] Since this early work, we have moved on to studies of silica film gradients. Aside from a single early report, sol-gel methods had not previously been employed to prepare gradient films, to our knowledge. We have since demonstrated a dip-coating method (termed infusion-withdrawal dip coating) for preparing organically-modified silica film gradients[75] and later showed that these materials comprised mobility gradients, with Nile Red dye molecules exhibiting increased diffusivities along the gradient direction (i.e., with increasing film organic content).[78] The silica work was performed in collaboration with Maryanne Collinson, with her group reporting XPS studies of materials composition along similar gradients.[76,81,88]

Reference numbers refer to articles in the Higgins Group publication list