Liquid-Crystalline Solvents as Mechanistic Probes. 17. Influence of Cholesteric and Smectic Mesophase Order on the Isomerization of Some N,N'-Diacylindigos

Article abstract of DOI:10.1021/jo00363a024

The rates and activation parameters for cis -> trans thermal isomerization of four N,N'-diacylindigos have been compared in isotropic and liquid-crystalline phases.The results indicate that cholesteric solvent order has no perceptable influence on the isomerization of N,N'-diacetylindigo.The shape changes attendant upon isomerization of N,N'-diacylindigos are inhibited by smectic phase order when the acyl chains are incorporated into smectic layers: the indigoid portion of the molecule, per se, does not sense on a microscopic level the macroscopic order of the smectic phase.Thus, the activation parameters for isomerization of cis-N-acetyl-N'-stearoylindigo and cis-N,N'-distearoylindigo in the isotropic phase of n-butyl stearate (BS) are within experimental error of each other and are similar to the values obtained when benzene is solvent; in the smectic B phase of BS, the activation enthalpy and activation entropy of N,N'-distearoylindigo are, respectively, 7 kcal/mol larger and 18 eu more positive than the values of N-acetyl-N'-stearoylindigo.