Effects of Terminal Substituents on Mesomorphic Properties: Phenyl 444-X-Substituted phenyliminomethyl)benzoate and its Isomeric Compounds

Abstract

The nematic-isotropic transition temperature of phenyl 4-(4-X-substituted phenyliminomethy1)benzoate and its seven isomeric compounds carrying only one terminal substituent; i.e., methoxyl, ethoxyl, nitro, or cyano 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 temperature is as large as 31 to 54°C between the cyano and methoxy derivatives and 23 to 44°C between the nitro and methoxy derivatives. 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 relationship between molecular features and mesomorphic behavior, the compounds were classified into four pairs of the isomeric compounds with the same molecular skeleton, those with a carbonyloxy linking group inverted, and those with an azomethine linking group inverted and their N-I transition temperatures were compared with each other.

Key Words:

• nematic
• terminal substituents
• linking groups