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|Title:||Photochemical probes of conformational mobility in thermotropic liquid crystals|
|Authors:||Workentin, Steven Mark|
|Advisor:||Leigh, William J.|
|Abstract:||<p>Phenyl ketone photochemistry has been used to probe the effects of thermotropic liquid crystals on the conformational mobility of dissolved solutes. In addition, concentration and temperature dependent deuterium (²H) NMR spectroscopy studies of deuterated analogues of the probe ketones have afforded detailed information on the solubilisation behaviour of the solutes in the nematic and smectic liquid crystalline phases of various mesogens. Results from the ²H NMR studies have allowed for the construction of binary phase diagrams of the solute/solvent mixtures and have provided a method for the accurate determination of the solubility limits of these probes in smectic phases. The Norrish II reactions of butyrophenone (BP), valerophenone (VP), hexanophenone (HP), and γ-cyclohexylbutyrophenone (CHBP) have been carried out at 30℃ as a function of concentration in the isotropic, nematic and smectic liquid crystalline phases of trans,trans-4'-alkyl-[1,1'-bicyclohexyl]-4-carbonitrile (CCH-n) liquid crystals. The distribution of fragmentation and cyclisation products from photolysis of these ketones in the nematic phase is similar to that obtained from photolysis in model isotropic solvents. The product distribution from photolysis of a homogeneous smectic phase solution of BP is also similar to that in the isotropic phase, but the fragmentation/cyclisation ratios from photolysis of homogeneous smectic phase solutions of VP, HP, and CHBP are significantly affected by the ordered environment. For ketone/mesogen mixtures of bulk solute concentration above the solubility limit, the observed effects of the smectic phase on the photochemistry of these solutes is lowered. The rates of intramolecular triplet decay of β-phenyl-4-methoxy-propiophenone (1) in the isotropic and liquid crystalline phases of CCH-n, trans-4-alkylcyclohexyl-trans-4'-alkyl-cyclohexylcarboxylate (OS-nm), and trans,trans-4'-alkyl-[1,1'-bicyclohexyl]-4-alkylether (CCH-nOm) mesogens have been investigated using nanosecond laser flash photolysis techniques. The average triplet lifetimes measured for homogeneous smectic solutions of 1 range from a factor of ca two to four times longer than those in homologous nematic or isotropic phases at the same temperature. The magnitude of the change in lifetime that occurs at the nematic to smectic (N→Sm) phase transition is a function of the relative amount of order associated with each of the smectic phases. A study of the photochemistry of a series of benzoyl- and β-aryl substituted derivatives of 1 in the CCH-n and CCH-nOm mesogens has shown that the nature of the effect on β-phenyl quenching is mainly the result of a change in the equilibrium constant for trans/gauche interconversion in the smectic phase compared to that in the nematic/isotropic phases. This latter study also investigated the effects of solute structure on the ability of thermotropic liquid crystals to control the conformational mobility of this probe. Slight variations in the structure of the alkoxy substituent reduces the ability of the smectic phase to slow triplet deactivation, while substitution at the para-position of the β-phenyl ring magnify the effect of solvent order on the triplet lifetime. (Abstract shortened by UMI.)</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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