Effects of Divalent Ligand Interactions on Surface-Induced Ordering of Liquid Crystals

We report the synthesis of a mesogen that contains two nitrile groups (4-pentyl-3′,4′-dicyanobiphenyl, DCB), and the use of the mesogen in an investigation of the effects of multivalent ligand interactions on the ordering of liquid-crystalline phases at surfaces decorated with copper (Cu 2+ ) ions. Differential scanning calorimetry (DSC) confirmed that DCB was miscible with nematic phases of 4-pentyl-4′-cyanobiphenyl (5CB). Quantitative measurement of the optical retardance of mixtures of DCB and 5CB in contact with surfaces decorated with Cu 2+ (characterized by X-ray photoelectron spectroscopy, XPS) revealed a continuous ordering transition in the nematic LC as a function of increasing concentration of DCB (0.9−2 wt %). In contrast, Fourier transform infrared (FTIR) spectroscopy of 5 mM Cu 2+ dissolved in bulk benzonitrile revealed no evidence of a change in nitrile coordination around the Cu 2+ upon addition of 5CB or DCB, thus leading to the proposition that the effect of DCB on the ordering of the LC phases at Cu 2+ decorated surfaces is due to divalent coordination interactions of DCB with the immobilized Cu 2+ ions. This hypothesis was further tested by measurements of FTIR spectra and surface-induced ordering transitions of LC phases of DCB/5CB upon introduction of dimethylmethylphosphonate (DMMP), an organophosphonate that binds competitively with nitrile groups for Cu 2+ . These results, when combined, lead us to conclude that it is possible to substantially tune surface-induced ordering of LCs by manipulating the valency of the mesogens and thus coordination interactions of the LCs with metal ions immobilized on surfaces. Overall, the results of this study guide the design of multifunctional liquid crystals for use as chemoresponsive materials.