Comparison of results obtained from 1 ,I 'and 1,3-disubstituted ferrocenes clearly demonstrates the influence of structural isomerism on the mesomorphic properties.

(1 3.IV. 95) The thermal properties of two ferrocene derivatives, substituted by either one or two cholesteryloxycarbonyl units, were investigated. While the monosubstituted ferrocene derivative 1 was found to be non-mesomorphic, the disubstituted ferrocene derivative 2 exhibited a crystal smectic-B phase. This result shows that ferrocene-containing thermotropic liquid crystals, despite the bulkiness of the metallocene core, are not limited to disordered calamitic phases.

Two families of 1,3-disubstituted ferrocene derivatives have been synthesized and their liquid-crystal behavior investigated. Compounds of series I, (η5-C5H5)Fe[(η5-C5H3)-1,3-(COOC6H4OOCC6H4OCnH2n+1)2] (n = 1–14, 16, 18), exhibited remarkable mesomorphic properties. Indeed, nematic and/or smectic C phases, associated with large anisotropic domains, were observed. Derivatives of series II, (η5-C5H5)Fe[(η5-C5H3)-1,3-(COOC6H4OCnH2n+1)2] (n = 1-9), were found to be non-mesogenic. The crystal and molecular structure of Ih (n = 8) was determined by means of X-ray analysis. The crystallographic data confirmed both a highly anisometric structure for Ih and a compact arrangement of the molecules in the crystal. The present results have shown that a critical length/depth ratio of ca. 5-7 must be passed for obtaining liquid-crystal properties.