Organic reactions in liquid crystalline solvents. 8. Deuterium NMR studies of the solubilization of some aromatic ketones in the liquid crystalline phases of 4′-butylbicyclohexyl-4-carbonitrile

Abstract

The nature of the solubilization of β-phenyl-4-mefhoxypropiophenone (1) in the nematic and crystal-B liquid crystalline phases of trans,trans-4′-butyl-(1, 1′-bicyclohexyl)-4-carbonitrile (CCH-4) has been investigated by deuterium nmr spectroscopy. This has been carried out using deuterium quadrupolar splitting (ΔvQ) and T1 measurements on 1–15 mol% solutions of two selectively deuterated derivatives of the ketone in CCH-4 over the 25–110 °C temperature range. The nmr studies have been supplemented with differential scanning calorimetry (DSC) and thermal microscopic investigations of variously proportioned mixtures of the parent ketone with CCH-4, over a similar temperature range. For comparison, 2H nmr measurements have also been carried out for solutions of 4-methoxyacetophenone-α,α,α,d3 benzene-d6 and β-(4-cyclohexylphenyl)-4-methoxypro-piophenone-α,α-d2 in the same liquid crystal as a function of temperature. Our results indicate that the limit of solubility of 1 in the smectic phase of CCH-4 is less than 0.5 mol%. At temperatures above 35 °C, excess ketone is solubilized in a ketone-rich nematic phase that coexists with the bulk smectic phase, and the composition of the ketone-rich phase varies continuously with temperature throughout this range. At around 35 °C, the ketone-rich phase transforms into a metastable, non-birefringent phase which is characterized by isotropic nmr behaviour. Prolonged storage at room temperature causes crystallization of this phase, forming a stable binary mixture of ca. 9:1 CCH-4/1 that undergoes a K–Sm transition at 27–28 °C, a Sm–N transition at 43.5–44 °C, and a broad N–I transition at 58–63 °C. This stable modification can also be prepared by rapid cooling of a 10 mol% 1/CCH-4 mixure from the nematic phase. The 2H nmr studies of 4-methoxyacetophenone-α,α,α-d3 and benzene-d6 in CCH-4 indicate that these solutes have somewhat higher solubility in the crystal-B phase of this mesogen. The data necessitate a re-interpretation of our previously reported studies of the excited triplet state behaviour of 1 in the liquid crystalline phases of this material, and may be broadly applicable to numerous other studies of photochemical reactivity in this mesogen that have been reported. Keywords: liquid crystal, smectic, 2H nmr, photochemistry, aromatic ketone, bicyclohexyl-4-carbonitrile,4′-butyl.