The induction of the Ntb phase in mixtures

ABSTRACT We report the induction of the Ntb phase over a wide temperature and concentration range in a binary system. This was achieved by addition of a flexible dopant without LC properties to a flexible dimer exhibiting only a nematic LC phase. The Ntb phase was identified by Polarising Optical Microscopy (POM), Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD) techniques. Graphical Abstract


Introduction
The Ntb phase, characterized by spontaneous formation of chiral domains with a helical pitch on the 10 nm scale and pseudo-layer structure, formed by chemically non-chiral compounds is one of the recently observed examples of chiral symmetry breaking in liquid crystals [1][2][3][4][5][6][7]. Previously this effect has been more associated with higher ordered liquid crystal (LC) phases, such as lamellar, columnar and cubic arrays [8][9][10][11][12]. Ntb phase formation is most commonly associated with systems containing two mesogens separated by flexible odd-numbered spacers. However, examples of bent-core mesogens, as well as oligomeric and main-chain polymers have been reported too [13][14][15][16]. The Ntb mesophase is relatively robust to the addition of linear mesogens and it can even be stabilised by adding nematogens [17,18]. Moreover, it can be formed by supramolecular association of mesogenic groups [18,19] and has been observed in multicomponent mixtures [20,21]. Until now, the question of whether Ntb phase behaviour can be induced systematically by adding a dopant to a conventional nematic and hence modulating the assembly behaviour of the nematogen has not been investigated. 3 Here, we show that it is possible to achieve Ntb phase formation systematically by adding a non-LC dopant to a nematic dimer. The molecules used in this study were selected for their structural simplicity in order to ease future theoretical and experimental studies, to link conceptually to existing systems and to obtain relatively low transition temperatures, making physical investigations more accessible.

Experimental
The known compound CBOC5OCB (Figure 1(a)) with a nematic phase range of almost 85 o C was selected as the matrix [22]. The material CBC3CB (Figure 1(a)), which is not liquid crystalline was selected as a dopant. The structural similarity to CBOC5OCB was selected to ensure full miscibility over a wide concentration range.
For the first step in the investigation, a phase diagram between these two materials was constructed. Mixtures were investigated by Polarizing Optical Microscopy (POM), Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD). The experimental details and the instrumentation used in this work are presented in the ESI.

Results and Discussion
The temperature-composition phase diagram, as a function of CBC3CB mol%, collected on cooling is presented in Figure 1(b). Composition information and transition temperatures are listed in the ESI ( Table 1). The LC behaviour of the mixtures was found to be monotropic, in other words not thermodynamically stable. However, on cooling the compositions were stable for at least a day, thus allowing for their detailed characterization. where the composition contains predominantly a material which is not liquid crystalline.
Additionally, the crystallization temperatures decrease with increasing CBC3CB content.
Interestingly, no crystallization of the samples was detected at concentrations higher than 22.3 mol% of CBC3CB (mixture 5) in DSC experiments. Optically detectable crystallization occurred after the samples were left at room temperature for at least 24h.
After induction of the Ntb phase at 11.8 mol% CBC3CB, a slight stabilisation of the phase was observed at 22.3 mol% (mixture 5). Further addition of the non-LC dimer destabilised the phase with the N-Ntb temperature transitions exhibiting an almost linear dependence on composition until the phase was no longer detected above 63.1 mol% of CBC3CB.
We note a much steeper decrease of the N phase stability on addition of further CBC3CB to CBOC5OCB than that of the Ntb mesophase. Looking at the phase diagram one could anticipate an intersection of the stability ranges of the N and the Ntb phases, with a direct Iso-Ntb phase transition, but this was not detected. On the contrary what was actually observed was large regions with co-existing isotropic, nematic and crystal phases, but no Ntb phase, at 68.1 mol% of CBC3CB (eg mixture 2;see Figure S1 in the ESI for POM micrograph). This is indicative of demixing of the 5 materials. As this region of the phase diagram is not the focus of this report, it will not be discussed further. With further increase of CBC3CB only the crystal phase was observed in the mixtures.
We note that recently the N-Ntb transition was reported on isolated droplets of pure CBOC5OCB under extensive supercooling using POM [24,25]. Unfortunately, we were not able to recreate this experiment under controlled conditions (see Figure S2 in the ESI for a POM micrograph).  Figure 3(b,c). A sample observed on untreated glasses under crossed polarizers exhibiting the Schlieren texture of the nematic phase ( Figure 3(a)) transformed into a blocky texture marking the N-Ntb transition (Figure 3(b)). On further cooling the texture developed into a polygonal defect pattern, (Figure 3(c)). In Figure S3  When mixture 4 was cooled into the Ntb phase at 80 °C (Figure 3(f)) no significant change was recorded for the small-angle intensities. The wide-angle diffraction arcs became narrower and spread further to the meridian, expanding on further cooling into a circle-type pattern at 40 o C ( Figure 3(g)). This suggests domain formation in the sample [18]. Moreover, it would be in line with a phase structure where mesogens in a helicoidal structure, but with a tilt to the helix, form right and left-handed domains which are oriented in the external field. The wide-angle peaks in 7 the θ-scans shifted slightly to smaller angles, suggesting a closer lateral molecular packing in the Ntb phase. The small-angle peaks yield values of 10.6 Å, consistent with the concept discussed above. Additionally, the slight reduction of the pseudo d-spacings, going from the N to the Ntb phase, could be due to increasing interdigitation of the molecules with either linear [5] or torsionally twisted hydrocarbon groups [32,33]. The correlation lengths (ξ) were calculated via the equation ξ=c/Δq, where c is the function used to describe the intensity profile and Δq is the full width at half maximum (FWHM), a variation of the Scherrer equation [34] (see Table 2 in the ESI for the tabulated ξ values). For both nematic phases these calculations gave quite small values ranging from 5.42 Å to 8.35 Å for the N phase to 7.35 Å to 9.83 Å for the Ntb phase. For the Ntb phase this value can be explained by the mesogens being at a tilt to the axis of a helix, with the helix axis being oriented overall parallel to the magnetic field [35]. Overall our XRD results are consistent with previous studies concerning the structure of the Ntb mesophase [32,33,[35][36][37][38][39][40].
Recently, duplex formation has been proposed [38,41,42]. We note that for this system at low concentrations of CBC3CB (roughly 12 mol%) only one in five duplexes would contain species promoting helicoidal structure formation, this would indicate that either CBC3CB is very efficient as a dopant or alternatively disfavours the concept of duplexes.
The structure of the nematic phases obtained in the studied mixtures is rationalized in the context of the shape and flexibility of the statistically achiral constituents. On mixing the two dimers, in the high-temperature N phase, the nematic environment of CBOC5OCB induces orientational order to CBC3CB molecules, absent in the pure CBC3CB fluid at the same temperatures. However, the CBC3CB present perturbs the ordering of CBOC5OCB and the temperature stability of the N phase decreases dramatically. 8 For the Ntb phase the arrangement is more complex. The introduction of moderate amounts of CBC3CB (11.8 mol%) are needed for Ntb formation and we observed a small increase of the mesophase stability at 22.3 mol% of the non-LC dopant. For higher concentrations a reduction of the Ntb stability occurs, before demixing starts after addition of more than 65 mol% CBC3CB.
These results indicate that phase formation is not due to specific attractive molecule-to-molecule [19] or in our case host-dopant interactions. Moreover, our results suggest that interdigitation of the two types of molecules occurs. A schematic representation of the assembly is shown in Figure   3(h) and a schematic representation of the molecules can be seen in Fig 3(i).
Given the smaller number of accessible conformational states of CBC3CB when compared to CBOC5OCB, due to its shorter spacer, we propose that CBC3CB acts as a template and that CBOC5OCB adjusts its conformational statistics so that CBOC5OCB is closer in shape to CBC3CB, becoming thus on average more bent and twisted; this can be achieved for example by a number of eclipsed conformations of the central hydrocarbon chain in order to optimize their interactions, or in other words to minimize excluded volume [43]. This view is supported too by the increasing enthalpy values at the N-Ntb transition (See Fig. 2) with increasing CBC3CB content.
The observed induction of a new liquid crystal phase is distinctly different from that where hydrogen bonding of species is employed for liquid crystal phase formation [44,45] or through halogen bonding [46] and different from liquid crystal phase formation due to excluded volume effects as reported recently [47] or by the balance between the charge-transfer interactions, dipolar interactions and excluded volume effects [48].

Conclusion
To summarise, based on POM, DSC and XRD techniques, we report for the first time the induction of the Ntb phase in binary systems on adding a non-LC dopant, CBC3CB, to a nematic dimer, CBOC5OCB. The Ntb phase is formed in a wide concentration range going from roughly 12-63 mol% of the dopant. We propose that Ntb phase induction is due to energy minimization by adjustment of molecular conformations of the non-chiral and flexible molecules, mainly of