EVIDENCE FOR LATERAL CHAIN CONFORMATION CHANGE IN THE SOLID-PHASE OF SUBSTITUTED NEMATOGENS

A new homologous series of hyl-4'-(4-trans-pentylcyclohexanoyloxy)azob enzenes has been synthesized. These compounds contain four rings in th e main core, a lateral alkoxy branch and an adjacent lateral methyl gr oup on one inner ring. They present a large nematic range and a solid- solid phase transition for the first members of the series. The crysta l structures of two similar compounds containing a lateral butoxy chai n have been solved. The HM4 compound (C31H33N2O5Cl) is derived from th e series, but does not have the terminal pentyl chain while the PM4 co mpound (C36H43N2O5Cl) possesses this terminal chain. HM4 and PM4 cryst allize, respectively, in P2(1)/c (Z=4) and P (1) over bar (Z=2) space groups. The final reliability factors are R=0 . 138 for HM4 (the termi nal cyclohexyl ring is largely disordered) and R=0 . 041 for PM4, for 2204 and 1963 observed reflections, respectively. The four ring centra l core is linear and rigid for both compounds. The conformation of the butyloxy side chain branched on one of the central phenyl rings is ve ry dependent on the terminal substituent, as it is found in a largely bent conformation in HM4 and in an extended conformation in PM4. To mo nitor the lateral chain conformation in the solid and nematic phase, w e have used the -OCH2- resonance in the C-13 MAS NMR spectra. The temp erature dependence of this resonance has indicated that the solid-soli d transition encountered for the first members of the series is associ ated with a change of the lateral chain conformation within the solid phase. At the solid-nematic transition, the change of the chain confor mation involving the first segment of the lateral chain is far less pr onounced. When this solid-solid phase transition disappears for the la st members of the series, this change in the conformation occurs at th e solid-nematic transition.