Mix and match: twist-bend nematic behaviour in liquid crystal dimer mixtures of the CBOnOCB

Abstract

The twist-bend nematic (NTB) phase, a spontaneously chiral liquid crystalline state formed by achiral molecules, features a nanoscale helical structure with potential applications in advanced polarisation optics and ultra-fast electro-optical switching. Despite early theoretical predictions and the first experimental confirmation in the CB7CB dimers, optimising the NTB phase properties remains a challenge. In this work, the two-component mixtures of dimers based on cyanobiphenyl with two ether bridges (CBOnOCB) were investigated. The constituent nematogens of the CBOnOCB series practically do not form the NTB phase individually, and our research focuses on mixtures of these dimers to investigate whether the NTB phase can be realised by combining them. We made 15 different dimer mixtures from the odd-numbered homologous the CBOnOCB series (n = 3–11). We present the results for the mixture CBO3OCB/CBO7OCB for which stable NTB phases were obtained in the entire sample volume in specific proportions. Equimolar odd-odd mixtures consistently exhibited the NTB phase, while longer-spaced dimer pairs showed rapid crystallisation, limiting the NTB phase range. Differential Scanning Calorimetry, Polarized Optical Microscopy, Fourier Transform Infrared Spectroscopy supported by quantum chemical calculations, and birefringence measurements were used to investigate this specific induction of the NTB phase.