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

Systematic observations are carried out to determine molecular alignments of monolayers of homologous series in n -alkyloxy-cyanobiphenyl ( n OCB) liquid crystals in the range of n =7 to 10 on both molybdenum disulfide (MoS 2 ) 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 (MoS 2 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 n CB monolayers has been confirmed for n OCB.