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The hydrosilylation of four isomers of tetra(dimethylsiloxy)tetraphenylcyclotetrasiloxane with (4´ cyanobiphenyl 4 yl)undec 10 enoate in the presence of a platinum catalyst afforded four liquid crystalline stereoisomers of phenylcyclotetrasiloxane with mesogenic cyanobiphenyl groups. The reaction was performed for the first time. The influence of the spatial structure of the stereoisomers of tetraphenylcyclotetrasiloxanes with mesogenic groups on the formation of the liquid crystalline state by thermooptical, X ray diffraction, and calorimetric methods. The temperatures and enthalpies of phase transitions were determined, and their reversibility was shown. Differences in both the textures of melts formed on cooling and the phase state and in interplanar distances were found by optical microscopy and X ray diffraction methods. Based on these data, the spatial structure of the isomers was concluded to affect the phase composi tion and type of packing in the liquid crystalline state.Data on the influence of the structure of stereoisomers of cyclosiloxanes on the properties and type of packing in the liquid crystalline (LC) state are lacking in literature. This is related, first of all, to the fact that methyl(hydri do)cyclosiloxanes used for the synthesis of LC compounds usually represent mixtures of isomers (regardless of the cycle size). The influence of the flexibility of the central core and spacer on the type of the forming LC phase has been revealed 1,2 to the present time by computer simula tion. Weak interactions between mesogens, a sufficiently long spacer, and a flexible core favor the formation of nematic phases, whereas discotic molecules with a rigid core, a short spacer, and strongly interacting mesogenic groups form smectic or columnar phases. For example, studying methylcyclosiloxanes, the authors assumed the existence of several types of packing: cylindrical, 3-5 sheaf like, and disk like. 6 Since several stereoisomers of methyl cyclosiloxanes can exist, the types of packing in the LC state for the cis isomer and r 2,t 4,t 6,t 8 isomer should differ due to different intramolecular interactions of mesogenic groups. Unfortunately, none of the published works discusses the influence of the spatial structure of stereoisomers or the composition of a mixture of isomers of methylcyclosiloxanes on the type of packing in the LC state. 3- 10 We earlier 11 attempted to study this influence for cyclohexasiloxane. It was found that methylcyclosiloxanes enriched in the trans isomer form different LC textures on cooling of the isotropic melt. 11 cis Isomers of cyclo tetra and cyclohexasiloxanes with the same mesogenic groups also form different types of packing in the LC state. 12 The purpose of this work is to synthesize structural isomers of cyclosiloxanes with a certain position of mesogenic cyanobiphenyl groups and to identify phase transitions and the type of packing in the LC state. Results and DiscussionSynthesis of LC stereoisomers of tetraphenylcyclo tetrasiloxanes with cyanobiphenyl mesogenic groups. A...

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