Study of Odd-Even Effect in Molecular Alignments of nOCB Liquid Crystals: STM Observation and Charge Density Distribution Analysis

Differences between the structures of n-alkyl-cyanobiphenyl (nCB) and n-alkyloxy-cyanobiphenyl (nOCB) monolayers are investigated by charge density analysis using ab initio molecular orbital calculations to confirm the odd-even dependence of the alignment of nCB and nOCB liquid crystal molecules on the length of the alkyl chain n. The results of the calculations reveal large charges around the oxygen atom in nOCB, explaining discrepancies in the behavior of nOCB. Moreover, the alignments of mixtures of different length molecules observed by scanning tunneling microscope (STM) clearly demonstrate the effects of this charge. Based on the results, the charge around the oxygen atom is concluded to be a secondary factor in determining alignment configurations, with the odd-even effect being the main factor.