Anti-Parallel Dimer Formation of 4-Cyano-4’-Alkyl Biphenyls in Isotropic Benzene Solution - Seeds of Liquid Crystalline Phases -

Abstract

We do not have to explain the importance of liquid crystalline (LC) materials in our daily life. Especially, without the application of liquid crystalline materials to display technologies in many practical tools such as television sets, personal computers, mobile phones, and so on, ubiquitous information technologies sustaining our convenient network societies are not considered. 4-cyano-4’-alkyl biphenyls (nCBs), such as 4-cyano-4’-pentyl biphenyl (5CB) and 4-cyano-4’-octyl biphenyl (8CB), have been well known as LC substances widely used in LC displays. Pure 5CB is thermo-tropic LC substance, and demonstrates isotropic to nematic LC phase transition at TI-N = 35 oC and nematic LC phase to solid phase transition (melting point) at TM = 22.5 oC. On the other hand, pure 8CB possesses TI-N = 40 oC, nematic LC to smectic LC phase transition at TN-S = 32.5 oC and TM = 22 oC. 7) In the case of a smectic LC phase formed by 8CB in a temperature range between TN-S and TM, it has been revealed via several scattering experiments and scanning tunneling microscopic (STM) observation on a graphite surface that 8CB molecules form anti-parallel dimers and the formed dimers arranged in a direction (uniaxial director) and also form periodically well-defined layers that can slide over one another. The reason for the anti-parallel dimer formation should be the strong dipole-dipole interaction between cyano groups. Moreover, Smith et al. reported that a solid phase structure of some longer nCBs (n ≥ 8) deposited on flat graphite surfaces showed two-dimensional molecular arrangements constructed by the anti-parallel dimers. Consequently, an anti-parallel dimer, (8CB)2, is an intrinsic Anti-Parallel Dimer Formation of 4-Cyano-4’-Alkyl Biphenyls in Isotropic Benzene Solution Seeds of Liquid Crystalline Phases -