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

Abstract

The twist-bend nematic (N_TB) phase is a new heliconical liquid crystal (LC) phase that is associated with spontaneous symmetry breaking for achiral bent LC molecules. Herein, we demonstrate a homologous series of LC dimers exhibiting the stable N_TB phases, which undergo vitrification to give N_TB glass (N_TBG) phases upon cooling below room temperature. Methylene- and thioether-linked cyanobiphenyl-based LC dimer homologs, i.e., CBnSCB (carbon number of the central oligomethylene spacer, n = 2, 4, 6, 8, and 10, for a bent molecular geometry) were developed for the first time. The phase-transition behavior and phase characteristics of CBnSCB were investigated by polarized light optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffractometry (XRD). All CBnSCB homologs were found to be mesogenic, wherein CBnSCB (n = 4, 6, 8, and 10) exhibited N_TB phases below the temperatures of conventional nematic (N) phases. Additionally, CB2SCB, which possesses the shortest spacer, showed a kinetically induced, monotropic mesophase that was formed directly from the isotropic (I) phase with a strong first order phase-transition property. Although POM observations of CB2SCB revealed the presence of optical textures similar to modulated N or pseudo-layered N_TB phases, the XRD results indicated its apparently non-layered and liquid-like nature. In addition, the mesophase of CB2SCB was assumed to be miscible with the N_TB phase of CB4SCB. It was therefore suggested that CB2SCB exhibited a N_TB phase formed directly from the I phase. It is noteworthy that the N_TB phases of CBnSCB (n = 2, 4, 6, and 8) underwent vitrification upon supercooling below room temperature, giving the N_TBG phases. In particular, CBnSCB (n = 6 and 8) exhibited remarkably stable N_TB and N phases, undergoing no crystallization upon re-heating from the N_TBG phase up to the I phase. Furthermore, we investigated the phase-transition behaviors of previously reported analogs, i.e., bis(thioether)-linked CBSnSCB and ether- and thioether-linked CBOnSCB LC dimers (odd numbered n = 3, 5, 7, 9, and 11, for a bent geometry) using POM, and observed N_TB phase formation from the small supercooled N domains of CBS11SCB, CBO3SCB, and CBO11SCB. It was therefore indicated that all reported bent CBSnSCB and CBOnSCB dimers (n = 3, 5, 7, 9, and 11) form the N_TB phase. It was found that the CBnSCB homologs exhibited remarkably stable N_TB phases and vitrifiable tendencies compared with the previously reported CBSnSCB and CBOnSCB analogs.