EFFECTS OF TERMINAL SUBSTITUENTS ON MESOMORPHIC PROPERTIES - PHENYL 4-(4-X-SUBSTITUTED PHENYLIMINOMETHYL)BENZOATE AND ITS ISOMERIC COMPOUNDS

The nematic-isotropic transition temperature of phenyl 4-(4-X-substitu ted phenyliminomethyl)benzoate and its seven isomeric compounds carryi ng only one terminal substituent; i.e., methoxyl, ethoxyl, nitro, or c yano group, has been compared. The temperature is influenced not only by the terminal substituent but also by the arrangement of the linking groups and the effects of these two features are mutually dependent. The efficiency order of substituents for nematic thermal stability was found to be CN > NO2 > C2H5O > CH3O. The difference in the temperatur e is as large as 31 to 54-degrees-C between the cyano and methoxy deri vatives and 23 to 44-degrees-C between the nitro and methoxy derivativ es. The latter finding is in sharp contrast with the order CH3O > NO2, which has been generally quoted. In order to find some clue to the re lationship between molecular features and mesomorphic behavior, the co mpounds were classified into four pairs of the isomeric compounds with the same molecular skeleton, those with a carbonyloxy linking group i nverted, and those with an azomethine linking group inverted and their N-I transition temperatures were compared with each other.