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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 (TIN) 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. [ABSTRACT FROM AUTHOR]

To describe the effects of the alkylthio groups on the phase transition behavior of organic liquid crystal (LC) molecules and their causes, we herein scrutinize the melting points (Tm) and nematic (N)–isotropic phase transition temperatures (TNI or TIN) of alkylthio-, alkoxy-, and alkyl-based LC homologues. To begin with, phase transitions are compared for three homologous series of 4-cyanophenyl benzoates bearing 4-alkyl-, 4-alkoxy-, and 4-alkylthio groups (denoted by nCPB, nOCPB, and nSCPB, respectively) with different alkyl chains (CnH2n+1; n = 1–8). In contrast to nCPB and nOCPB, some of which exhibit enantiotropic N phases, all alkylthio-based nSCPB are found to display monotropic N phases. The nSCPB homologues exhibit higher Tm than not only their corresponding nCPB for all n but also nOCPB counterparts in the case of long chains (n ≥ 6); a similar trend is confirmed also for the Tm values of the structurally similar biphenyl- and cyanobiphenyl-based homologues. Meanwhile, the TNI or TIN values of nSCPB and nCPB are lower than those of nOCPB for all n, and higher for nSCPB than for nCPB with the short chains (n = 1 and 2) but comparable with one another for the longer chains (n ≥ 3). In addition, our discussion addresses more anisotropic 1,4-phenylene bis(benzoate) LCs with alkylthio and alkoxy groups. Furthermore, single-crystal structures of three LC molecules with methylthio groups are examined. Several interatomic contacts around the sulfur atoms such as S⋯HC contacts are indicated, which differ for respective molecules. The present study draws the following conclusions. The alkylthio groups mostly play the role of decreasing the phase transition temperatures (especially TNI or TIN) and hindering LC formation for organic rod-like molecules, primarily due to conformational flexibility, steric bulkiness, and lower molecular anisotropy imposed by C–S–C. The sulfur atom contributes to intermolecular attractive interactions, which are reflected in phase transitions as spatially averaged phenomena (i.e., dispersion force and more polarizable π-systems). The influence of those interactions become more pronounced with lengthening alkyl chains. [ABSTRACT FROM AUTHOR]

We herein report phase transitions, mesomorphism, birefringence behavior and photoluminescence properties of symmetric liquid crystal (LC) dimers based on diphenylacetylene or tolane. A homologous series of 4-methoxy tolanes linked with oligomethylene spacers consisting of carbon numbers (m) of 5–12 via ether linkages (1OTOmOTO1) were developed. The 1OTOmOTO1 series exhibited only a nematic phase, in contrast with the previously reported homologues with long terminal alkoxy chains, which exhibit layered smectic phases. We revealed that even the longest even-numbered 1OTO12OTO1 exhibits higher birefringence (Δn) than the shortest odd-numbered 1OTO5OTO1 at similar shifted temperatures. This fact suggests that the parity effect in the spacer that enhances Δn in an even-m surpasses the dilution effect that decreases Δn in large m for LC dimers. In addition, the photophysical measurements found aggregation-induced emission of the dimer concomitant with the emergence of an abnormally structured fluorescence band, which perhaps arises from a unique excited state enabled by aggregation. Single-crystal structural analysis revealed that diphenylacetylene moieties in neighboring molecules have face-to-edge orthogonal packing with one another, supporting the strong face-to-edge preferences of diphenylacetylene moiety and its enhanced fluorescence in aggregated states. [ABSTRACT FROM AUTHOR]