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A series of seven closely related azobenzene derivatives are studied in their trans and cis form dissolved at a mole fraction of 1% and 7% in the nematic 4-cyano-4'-n-pentylbiphenyl (5CB) liquid crystal. Study results found that the presence of the azo-derivative caused a depression of the nematic-isotropic transition temperature ([T.sub.NI]) in all the cases.

The effects of the cis and trans form of the 4-OMe-phenylazobenzene at 1% and 7% mole fraction on order, mesophase stability and dynamics of the 5CB liquid crystal were studied using the ESR spin probe technique. The presence of the trans azo-derivative induced a downshift of the nematic-isotropic transition temperature TNI which increased with the concentration but did not change the dependence of the order parameter on the reduced temperature. The cis isomer caused even larger TNI shifts, determining a marked decrease of the order parameter with a peculiar dependence on the reduced temperature, quite different from the standard Haller-type behavior. The spin probe dynamics appeared instead to be essentially unchanged by the presence of the azo-derivative. [ABSTRACT FROM AUTHOR]