STERIC HINDRANCE INFLUENCE OF A DIAZO LINK UPON MESOGENIC PROPERTIES OF SOME LIGANDS AND RELATED COPPER-COMPLEXES

Some new polymethylated compounds containing three rings and four ring s have been synthetized. The crystalline structures of 2,3-dimethyl-4- ethoxy enzoyloxy-4'-(4-methoxysalicylaldimine)-azobenzene (DIM) and 2, 3,6-trimethyl-4-methylben zoyloxy-4'-butylazobenzene (TRIM) are descri bed. Due to larger steric interactions, the diazo linkage makes a larg er dihedral angle in TRIM than in DIM. The effect of steric hindrance of the diazo linkage upon the mesophase stability is discussed by the use of isotropic Carbon-13 chemical shifts, molecular modelling and tr ansition temperatures, T-NI. For the three rings core, the thermal sta bility of the. mesophase decreases with substitution pattern dissymetr y. The effect of the conjugation loss is evidenced by the decrease of T-NI versus the carbon number within the chain. For the four rings cor e,the effect on the transition temperatures of polymethylated ligands is nearly independent of the length of the mesogenic core and depends only on the lateral substituent positions within the core. For the rel ated copper complexes, the T-NI transition temperatures exhibit a smal l dependence upon the methyl substitution.