Thioether-linked azobenzene-based liquid crystal dimers exhibiting the twist-bend nematic phase over a wide temperature range

Abstract

Two homologous series of sulfur-containing thioether-linked 4-cyanoazobenzene-based liquid crystal (LC) dimers were developed: a symmetric series based on 4-cyanoazobenzene arms [(CN)AzoSnSAzo(CN)] and a non-symmetric series based on 4-cyanoazobenzene and 4-cyanobiphenyl arms [(CN)AzoSnSCB]. They contained different flexible oligomethylene spacers (C n H 2n) consisting of odd numbers of carbon atoms (n = 3, 5, 7, 9, and 11). The symmetric (CN)AzoSnSAzo(CN) dimers with n = 5, 7, 9, and 11 showed an elusive monotropic twist-bend nematic (N TB) phase, while all the non-symmetric (CN)AzoSnSCB dimers exhibited the N TB phase. Notably, the N TB phase observed for non-symmetric (CN)AzoS7SCB could be supercooled to room temperature, leading to a glassy N TB phase. (CN)AzoS9SCB showed a similar tendency accompanied by partial crystallization. These are the first reported examples of azobenzene-based LC dimers exhibiting a broad temperature range of N TB phases supercooled to room temperature. Such prominent stability of the N TB phase is due to the supercooling effect assisted by the synergy of molecular bend arising from thioether linkages and non-symmetric mesogenic arms. The phase-transition temperatures were overall higher for symmetric (CN)AzoSnSAzo(CN) than for non-symmetric (CN)AzoSnSCB. It is assumed that the difference in the mesogenic arm structures hardly influenced their molecular geometry or anisotropy in terms of entropy changes. File list (2) download file view on ChemRxiv Manuscript_for_ChemRxiv.pdf (710.37 KiB) download file view on ChemRxiv Supporting information_ChemRxiv.pdf (732.08 KiB) Thioether-linked azobenzene-based liquid crystal dimers exhibiting the twist-bend nematic phase over a wide temperature range

Methylene- and thioether-linked cyanobiphenyl-based liquid crystal dimers CBnSCB exhibiting room temperature twist-bend nematic phases and glasses

“…On the other hand, in our previous work, we revealed that thioether-linked LC dimers have a beneficial effect in the realization of the N TB phase over a wide temperature range and the room-temperature N TB and N TB G phases. 61,64,67,68,70 The asymmetrical methyleneand thioether-linked CB-based dimers bearing ethylene or hexamethylene spacers, namely CB2SCB and CB6SCB, respectively, was also reported previously. 64 The latter exhibited an N TB phase, which gave the N TB G phase upon supercooling to room temperature.…”

“…As shown in the magnified image of CB8SCB in the inset of used in the present study, second-order-like transitions were observed in the LC dimers. 61,67,68,70,92 In addition, in a few cases of the LC dimers homologs, it was reported that the DSC peak on the N-N TB phase transition alters from second-order-like to first-order-like with decreasing the spacer chain length, 57,61,70,71 which could be associated with a larger structural difference between the N and N TB phases for shorter spacer homologs than for the longer counterparts, implying the molecular rigidity and packing manner in the N TB phases for shorter spacer LC dimers. 70 In the present study, the N-N TB transitions of CBnSCB gradually transformed from second-order-like to first-order-like peak shapes…”

“…61,64 In addition, C-O-C has a higher rotation barrier compared with those of C-CH 2 -C and C-S-C. 97 Hence, the less bent (or more straightened) CBOnSCB, which is assumed to have a lower steric hindrance or molecular biaxiality, exhibits higher phase. 67,68 The intermediate molecular bend (111°) of CBnSCB may also contribute to the stability of the N TB phase, since bent molecules fill the space, forming Sm layers. 98,99 and CBOnSCB.…”

“…35 However, the formation of room-temperature N TB materials from their single-component systems indeed remains a rarity. 50,61,64,67,68,70,72 This has restricted deep structural analyses and examination of the physical properties of the N TB phases, in addition to the development of new applications. Therefore, novel LC dimers that can maintain N TB phases at room temperature and produce the corresponding glasses are in high demand.…”

Mirror Symmetry Breaking and Network Formation in Achiral Polycatenars with Thioether Tail

“…[ 15 , 16 ] More recently, the thioether linkage was shown to be advantageous for the design of mesogenic dimers with wide ranges of the twist‐bend nematic phase. [ 17 , 18 ] Herein the effect of replacing alkyloxy chains of polycatenar LCs by alkylsulfanyl (alkylthio) chains on LC self‐assembly and spontaneous mirror symmetry breaking is reported. Polycatenar LCs are rod‐like mesogens with multi terminal chains, providing a transition between lamellar and columnar self‐assembly by modification of the chain volume.…”