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

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 degrees 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.