Liquid Crystals as Reaction Media

Liquid crystals possess physical properties which lie somewhere between those of solids and liquids cf. Section 3.1 and [725]. The rigidity which is present in a solid matrix is absent in liquid crystals, thus permitting molecular motion as well as conformational flexibility of the dissolved solute molecules. At the same time, due to the order in the liquid-crystalline phase, the randomness in motion and conformational flexibility of the dissolved solute molecules is to some extent restricted. If the structures of the solute and solvent molecules are compatible, then solute molecules can be incorporated into the liquid-crystalline phase without disrupting its order. Thus, the reactivity of substrate molecules incorporated into liquid crystals without destroying their order should be different from that in isotropic solvents. Apart from the first report on the influence of liquid crystals on chemical reactions by Svedberg in 1916 [726], the use of liquid crystals as 

The rates and activation parameters for the thermal cis tram isomerization of A,A -distearoylindigo have been determined in both isotropic and liquid-crystalline solvents [727]. 

Similar rate decelerations in liquid-crystalline solvents have been observed for the thermal cis trans isomerization of a bulky tetrasubstituted ethene in cholesteric phases [728], On the other hand, the activation parameters for the thermal cis trans isomerization of less-dipolar substituted azobenzenes show no dependence on the solvent order. This indicates that the cis isomers and their corresponding activated complexes present a similar steric appearance to the solvent environment [729]. This result is more consistent with an isomerization mechanism which proceeds by inversion rather than by rotation cf. Eq. (5-40) in Section 5.3.2 and [527-529, 561]. The latter reaction represents a nice example of the use of liquid-crystalline solvents as mechanistic probes [729]. 

As a consequence of the alignment of solute molecules in liquid-crystalline solvents, the ratio of products formed in competitive reaction pathways can be different from that observed in isotropic liquids. This is illustrated by the Norrish type II photolysis of alkyl phenyl ketones with varying alkyl chain length in the isotropic, smectic, and 

The small increase in product ratio observed on changing the solvent from n-heptane to n-butyl acetate is caused by the increase in solvent polarity. When the photolysis was carried out in solid n-heptadecane, no specific change was observed in the ketone/cyclobutanol product ratio. 

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Not only reaction rates, but also the positions of chemical equilibria can be influenced by liquid crystals as reaction media. A nice example is the ionization equilibrium of chloro-tris(4-methoxyphenyl)methane according to Ar3C-Cl Ar3C+ -I-C1 , which is more shifted in favour of the nearly planar triarylcarbenium ion in nematic liquid crystals as compared to in an isotropic reaction medium [868], Obviously, the discshaped carbenium ion fits better into the rod-Hke nematic phase than the tetrahedral covalent ionogen, which distorts the internal structure of the nematic liquid crystal. 

Nakano, T. and Hirata, H., Liquid crystals as reaction media. 1. Photochemical dimerization of acenaphthylene in cholesteric hquid crystal. Bull. Chem. Soc. Jpn., 55, 947-948,1982. 

Weiss, R. G., Thermotropic liquid crystals as reaction media for mechanistic investigations. Tetrahedron, 44, 3413, 1988. 

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