4′-Alkylseleno-4-cyanobiphenyls, nSeCB: synthesis and substituent effects on the phase-transition and liquid crystalline behaviors

Abstract

The synthesis and phase-transition behavior of a novel homologous series of 4′-alkylseleno-4-cyanobiphenyls [nSeCB, where n represents the number of carbon atoms in the alkyl chains (n = 1–8)] is reported. Polarized optical microscopy and differential scanning calorimetry revealed that the nSeCB homologs with n = 1–6 exhibited monotropic nematic (N) phases at low temperatures, which appeared below 10 °C for nSeCB with n = 3–6. However, no liquid crystal (LC) phase was observed in the nSeCB with n = 7 and 8. In addition, we compared the phase-transition behaviors of the nSeCB homologs with those of the alkyl-, alkoxy-, and alkylthio-based homologous series (nCB, nOCB, and nSCB, respectively). The LC phases of certain nCB and nOCB were enantiotropic, whereas those of all nSCB and nSeCB were monotropic. The longer-chain nCB, nOCB, and nSCB homologs formed layered smectic A (SmA) phases, whereas only N phases were observed for the nSeCB homologs within the tested range of the chain lengths. The nSeCB series showed the lowest isotropic-to-N phase-transition (T_IN) temperatures for all chain lengths except 1SeCB. The low mesogenic ability and LC phase-transition temperatures of nSeCB were attributed to the higher flexibility, steric bulk, and lower molecular anisotropy due to the C–Se–C bond, which has lower rotational barriers, longer bond lengths, and smaller bond angles than the other bonds compared herein. The varying alkyl-chain lengths had a greater influence on the phase-transition temperatures of the nSeCB homologs than those of the other homologs. Furthermore, the single-crystal structural analysis of 1SeCB was conducted to obtain insights into the phase-transition behaviors.