Ultra-Sensitive Detection Methods of the Orientation of Surfaced-Adsorbed Molecules
AuthorMantha, Jordan Howard
AdvisorCline, Joseph I
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In this Ph.D. dissertation a detection method was developed that can sensitively detect the spatial orientation of sub-monolayer coverages of molecules adsorbed to a surface. The method is a special case of the technique known as cavity ring-down spectroscopy (CRDS), an ultra-sensitive absorption method that has been used widely for gas-phase detection. It was extended for use withcondensed-phase samples (both liquid and polymer thin film) and polarization resolution in order to sensitively detect the orientation of surface adsorbed molecules.A photochromic spiropyran known as 6,8-dibromoBIPS was used to test the sensitivity and applicability of the technique. The minimum detectable absorbance without signal averaging was determined to be 1–10 × 10<super>-6</super>. This means the instrument is able to detect less than one monolayer surface coverage of analyte molecules with reasonable molar extinction coefficients at the detection wavelength.The molecular orientation sensitivity of the technique was investigated by determining that 6,8-dibromoBIPS showed orientational ``memory'' effects when dispersed in a polymer thin film. It was shown that parallel orientation of excitation and probe polarizations produces a 15–25% absorption difference for the horizontal and vertical probe polarizations.The kinetics of the photo-induced ring-opening isomerization and thermal ring-closing of 6,8-dibromoBIPS in liquid toluene and polymer thin films was studied. The photoisomerization quantum yield and ring-closing rate constant were compared to literature values for 6,8-dibromoBIPS in liquid toluene. The quantum yield was in good agreement, but the ring-closing rate constant was not found to be in agreement with the value reported in the literature. However, therate constant determined in this thesis was consistent with the overall solvent trend reported in the literature. In the polymer thin film the photoisomerization quantum yield for 6,8-dibromoBIPS was found to be consistent with that obtained in liquid toluene. In contrast, the ring-closing rate constant was an order of magnitude slower. This was interpreted to be an effect of the restricted polymer matrix environment.A separate study was the calculation of the torsional potential energy surface of the photofragmentation of a chiral nitrosoalkane that had previously been investigated experimentally in the Cline group. Both singlet and triplet state surfaces were calculated using the UB3LYP level of theory. The torsional barriers were examined in order to predict a preferred sense of rotation for thenitric oxide photofragment as it recoils from the parent molecule.