Photochemical probes of conformational mobility in liquid crystals. The photochemistry of β-phenyl-4-methoxypropiophenone in the nematic and smectic B phases of CCH-n and OS-nm liquid crystals

The photochemistry of β-phenyl-4-methoxypropiophenone (1) in the isotropic and liquid crystalline (nematic and smectic B) phases of trans, trans-4′-alkyl-(1,1′-bicyclohexyl)-4-carbonitrile (CCH-n) and trans-4-alkylcyclohexyl-trans-4′-alkylcyclohexylcarboxylate (OS-nm) mesogens has been investigated using nanosecond laser flash photolysis techniques. Solubility limits of 1 as a function of temperature in the smectic phases have been rigorously determined by 2 H NMR spectroscopy, using α-, methoxy-, and β-phenyl-deuterated analogs of the ketone. Triplet decay of 1 in the smectic (Crystal B) phase of CCH-4 is multiexponential, suggesting that the ketone is solubilized in a distribution of conformations in the highly-ordered liquid crystalline lattice. The average triplet lifetime of 1 at various temperatures throughout the smectic phase is about a factor of three longer than that in homologous nematic and isotropic phases at the same temperature. The differences in the Arrhenius parameters for triplet decay in the smectic and nematic/isotropic phases are very small, however. The present results for 1 in the smectic phase of the CCH-n mesogens correct the interpretation of previously reported data obtained with higher concentration mixtures, and afford an accurate indication of the true effects of smectic B liquid crystalline order on the conformation-dependent triplet state behavior of this molecule. In OS-nm liquid crystals, triplet decay follows clean first order kinetics in all phases, but the variations in Arrhenius dependence with solvent phase are similar to those observed in the CCH-n liquid crystals. Key words: smectic, liquid crystals, 2 H NMR, nanosecond laser flash photolysis, intramolecular triplet quenching, photochemistry.