Molecular Alignment in Monolayer of n-Alkyloxy-Cyanobiphenyl and Clear Evidence of the Odd–Even Effect

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Abstract
Systematic observations are carried out to determine molecular alignments of monolayers of homologous series in n-alkyloxy-cyanobiphenyl (nOCB) liquid crystals in the range of n=7 to 10 on both molybdenum disulfide (MoS2) and highly oriented pyroritic graphite (HOPG) substrates using a scanning tunneling microscope. Our observations determined the following. For odd n of carbon atoms in alkylchains such as 7OCB and 9OCB, the molecular alignments on both substrates (MoS2 and HOPG) are mainly due to the interaction between alkylchains and substrates. For even n, on the other hand, the alignments are basically due to the dipole-dipole interaction between liquid crystal molecules on the substrates. These differences occur due to the situation of the end-carbon in an alkylchain. However, 10OCB on HOPG shows an exceptional alignment resulting from the alkylchain-length effect. Excluding this case, all other seven combinations clearly indicate the odd–even effect. The validity of our previous model for molecular alignments of nCB monolayers has been confirmed for nOCB.