Probing Reactions in Monolayers Using Normal Incidence Cavity Ring-down Spectroscopy
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This dissertation describes a study that was conducted on a two-dimensional (2-D) polymer system to help characterize the formation and dissociation of the polymer. Normal incidence cavity ring-down spectroscopy (NICRDS) was used to probe the monolayer system. Cavity ring-down spectroscopy (CRDS) is an ultra-sensitive absorption technique which has been extensively used for detection of gas-phase species and has recently been extended to studies in condensed phases. To date, this technique had not been used to study reactions in monolayers and, more specifically, 2D polymers. The newly emerging field of 2D polymers is predicted to impact several areas of technological importance, one of which includes membrane separations. These materials are rationally synthesized and are crystalline in two dimensions forming a covalently linked sheet of molecules. This new class of materials has yet to be fully understood since characterization is difficult due to their delicacy and size. The carboxy fantrip 2D polymer system is an anthracene-based analog and was used in our studies since it is known to be photo and thermally activated. Photo polymerization and depolymerization and thermal depolymerization were monitored using NICRDS to help characterize the poly(carboxy fantrip) 2-D polymer. Relative absorbance values occurring from changes in the CRDS signal during the polymer formation or dissociation reactions are in good agreement with known absorbance values and the predicted monolayer film thickness. In addition to using NICRDS, we developed a new method of CRDS which is comprised of a dual cavity where two wavelengths of light are simultaneously used as probes. A probe at normal incidence geometry and a probe at the Brewster angle for fused silica overlap on the sample of interest. For our experiments, the Brewster angle probe served as an indicator for changes in the ring-down times for the thin film/optical flat system unrelated to the photochemistry of the 2-D polymer occurring over the course of the experiment. It was a useful check for substrate degradation during high temperature experiments.Experiments were also conducted on free-standing surfactant thin films using NICRDS and Brewster angle normal incidence cavity ring-down spectroscopy (BANI CRDS) to study self-ordering and aggregation of probe molecules that are known to form liquid crystals in aqueous solutions. Evidence from our studies suggested that no self-ordering was occurring in the free-standing surfactant films at the concentrations examined. Nevertheless, the development of the BANI CRDS technique should allow for expansion of ultrasensitive absorption measurements to a wider array of condensed-phase systems, including studying reactions in monolayers.