The emergence of a polar nematic phase: a chemist's insight into the ferroelectric nematic phase.

The discovery of a new polar nematic phase; the ferroelectric nematic, has generated a great deal of excitement in the field of liquid crystals. To date there have been around 150 materials reported exhibiting the ferroelectric nematic phase, in general, following three key archetypal structures with these compounds known as RM734, DIO and UUQU-4N. In this review, the relationship between the molecular structure and the stability of the ferroelectric nematic, NF, phase will be described from a chemist's perspective. This will look to highlight the wide variety of functionalities which have been incorporated into these archetypal structures and how these changes influence the transition temperatures of the mesophases present. The NF phase appears to be stabilised particularly by reducing the length of terminal alkyl chains present and adding fluorines laterally along the length of the molecular backbone. This review will look to introduce the background of the ferroelectric nematic phase before then showing the molecular structures of a range of materials which exhibit the phase, describing their structure–property relationships and therefore giving an up–to–date account of the literature for this fascinating new mesophase

Chalcogen bonding and liquid crystallinity: understanding the anomalous behaviour of the 4'-(alkylthio)[1,1'-biphenyl]-4-carbonitriles (nSCB).

The synthesis and characterisation of the first eleven members of the 4'-(alkylthio)[1,1'-biphenyl]-4-carbonitriles series is reported. All eleven members show monotropic liquid crystal behaviour. The first six members are exclusively nematogenic, the seventh member shows nematic and smectic A phases, and the higher homologues only smectic A behaviour. A comparison of their transitional behaviour with that of the corresponding alkyl and alkyloxy-cyanobiphenyls reveals a new pattern of behaviour. Specifically, the nematic-isotropic transition temperatures show a large decrease on passing from the methylthio to ethylthio homologues. This unexpected behaviour is interpreted in terms of chalcogen bonding

Liquid crystal dimers and the twist-bend nematic phase: on the role of spacers and terminal alkyl chains.

The synthesis and characterisation of four series of liquid crystal dimers based on benzylideneaniline mesogenic units, and in which the lengths of terminal alkyloxy chains are varied are reported. The series differ in terms of their flexible spacers, namely, heptamethylene, nonamethylene, hexyloxy, and oxypentyloxy chains. The heptamethylene- and nonamethylene-linked dimers both show conventional nematic, N, and twist-bend nematic, NTB, phases with short terminal chains, and smectic behaviour emerges on increasing terminal chain length. This is attributed to increased molecular inhomogeneity driving microphase separation. The dimers containing the shorter heptamethylene spacer show a smectic A phase whereas those with the longer nonamethylene spacer exhibit an anticlinic smectic C phase. Smectic behaviour is not observed for the dimers containing either a hexyloxy spacer which exhibit nematic and twist-bend nematic phases, or with an oxypentyloxy spacer which show only a conventional nematic phase. A general observation is that TNTBN and TNI alternate in the same sense in a homologous series on varying the length of the terminal alkyl chains suggesting that the spatial uniformity of the molecular curvature is an important factor in stabilising the NTB phase. The transitional properties of the four corresponding dimers possessing nitrile terminal substituents are also described. These show enantiotropic nematic phases, and in addition, for those containing either polymethylene or hexyloxy spacers, a twist-bend nematic phase is observed. Differences in the thermal behaviour of the dimers may be attributed largely to changes in molecular shape arising from the nature of the link between the spacer and mesogenic units

Molecular shape, electronic factors, and the ferroelectric nematic phase: investigating the impact of structural modifications.

The synthesis and characterisation of two series of low molar mass mesogens, the (4-nitrophenyl) 2-alkoxy-4-(4-methoxybenzoyl)oxybenzoates (NT3.m) and the (3-fluoro-4-nitrophenyl) 2-alkoxy-4-(4-methoxybenzoyl)oxybenzoates (NT3F.m), are reported in order to investigate the effect of changing the position of a lateral alkoxy chain from the methoxy-substituted terminal ring to the central phenyl ring in these two series of materials based on RM734. All members of the NT3.m series exhibited a conventional nematic phase, N, which preceded the ferroelectric nematic phase, NF, whereas all the members of the NT3F.m series exhibited direct NF-I transitions except for NT3F.1 which also exhibited an N phase. These materials cannot be described as wedge-shaped, yet their values of the ferroelectric nematic-nematic transition temperature, TNPN, exceed those of the corresponding materials with the lateral alkoxy chain located on the methoxy-substituted terminal ring. In part, this may be attributed to the effect that changing the position of the lateral alkoxy chain has on the electronic properties of these materials, specifically on the electron density associated with the methoxy-substituted terminal aromatic ring. The value of TNI decreased with the addition of a fluorine atom ortho to the nitro group in NT3F.1, however, the opposite behaviour was found when the transition temperatures of the NF phase were compared which are higher for the NT3F.m series. This may reflect a change in the polarity and polarizability of the NT3F.m series compared to the NT3.m series. Therefore, it is suggested that, rather than simply promoting a tapered shape, the role of the lateral chain in inhibiting anti-parallel associations and its effect on the electronic properties of the molecules are the key factors in driving the formation of the NF phase

The ferroelectric nematic phase: on the role of lateral alkyloxy chains.

The synthesis and characterisation of the 4-({[4’-nitro-(1,1'-biphenyl)−4-yl]oxy}carbonyl)phenyl 2-alkoxy-4-methoxybenzoates are reported: the 1O(m-On)PEPEBNO2 series. These materials include an additional phenyl ring compared to the extensively studied ferroelectric nematogen RM734, added to increase the liquid crystal transition temperatures. All members of the series exhibited both conventional nematic (N) phase and ferroelectric nematic (NF) phases; TNI and TNFN both decrease on increasing n, the length of the lateral alkoxy chain. The properties of this four-ring series are compared with analogous three-ring laterally substituted variants of RM734; the addition of the extra phenyl ring has a significantly more pronounced effect on the value of TNI than on TNFN. The increase in TNI may be attributed to the enhanced structural anisotropy and more favourable intermolecular interactions arising from the insertion of the phenyl ring, whereas the much weaker effect on TNFN may reflect a change in the shape of the molecule. We also report two materials with three-ring structures, the 4'-nitro-(1,1'-biphenyl)-4-yl 2-alkoxy-4-methoxybenzoates: 1O(m-On)PEBNO2 (n=1 and 2). The removal of the ester linkage between RM734 and 1O(m-O1)PEPNO2 extinguishes the NF phase and this appears to be consistent with a model in which the molecules are described in terms of a longitudinal surface charge density wave

The influence of molecular shape and electronic properties on the formation of the ferroelectric nematic phase.

The synthesis and characterisation of two series of ferroelectric nematogens based on RM734 having an additional methoxy group on the central phenyl ring are reported, the 3-methoxy-4-((4-nitrophenoxy)carbonyl)phenyl 2-alkoxy-4-alkoxybenzoates (7-m-n) and the 3-methoxy-4-((3-fluoro-4-nitro-phenoxy)carbonyl)phenyl 2-alkoxy-4-alkoxybenzoates (8-m-n). In order to compare the behaviour of these series to those of the corresponding materials that do not contain the methoxy group on the central phenyl ring, we also report the synthesis and characterisation of 4-[(4-nitrophenoxy)carbonyl] phenyl 4-methoxybenzoate (11-0-1), 4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 4-methoxybenzoate (12-0-1) and 4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 2,4-diethoxybenzoate (12-2-2). Two compounds in which a lateral ethoxy chain is attached to the central ring, 3-ethoxy-4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (18-2-1) and 3-ethoxy-4-[(3-fluoro-4-nitrophenoxy) carbonyl]phenyl 2,4-dimethoxybenzoate (19-2-1), are also described. The behaviour of these materials shows that the relative stabilities of the ferroelectric nematic, NF, and conventional nematic, N, phases are governed by a subtle interplay of steric and electronic factors. Furthermore, the electronic factors are better understood in terms of isolated regions of electron density rather than by a single large longitudinal dipole moment. In terms of molecular shape, to observe the NF phase it appears that the molecular structure must include one or more lateral substituents that enhance molecular biaxiality and destabilise the N phase

Multiple polar and non‐polar nematic phases.

Liquid-crystal materials exhibiting up to three nematic phases are reported. Dielectric response measurements show that while the lower temperature nematic phase has ferroelectric order and the highest temperature nematic phase is apolar, the intermediate phase has local antiferroelectric order. The modification of the molecular structure by increasing the number of lateral fluorine substituents leads to one of the materials showing a direct isotropic-ferronematic phase transition

Ferroelectric nematogens containing a methylthio group.

The synthesis and characterisation of eight nematogens containing a terminal methylthio group is reported. The compounds are based on RM734 and differ in the number and position of fluorine substituents, and in the position of the lateral methoxy substituent. Seven of these compounds exhibit a monotropic ferroelectric nematic phase, NF, including examples of NF-isotropic phase transitions. Two of the compounds show the anti-ferroelectric NX phase. Their transitional behaviour is discussed in terms of molecular shape and changes in electronic properties. Their phase behaviour is similar to that of the corresponding compounds containing a terminal methoxy group, but the methoxy terminated materials consistently show the higher transition temperatures. This is accounted for by the larger reduction in molecular shape anisotropy associated with the methylthio group, and the associated change in the electronic properties. The NX phase seen for the methylthio substituted compounds are thought to reflect the suppression of the NF phase by the methylthio group rather than any specific stabilising effect. Specific interactions between methylthio groups thought to stabilise nematic behaviour do not appear to stabilise the NF phase

Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

Acknowledgements EG and DP acknowledge the support of the National Science Centre (Poland): (Grant Number 2016/22/A/ST5/00319). We acknowledge the use of Beamline 5.3.1 and 11.0.1.2 of the Advanced Light Source supported by the Director of the Office of Science, Office of Basic Energy Science, of The U.S. Department of Energy under contract no. DE-AC02-05CH11231. We thank Alex Liebman-Pelaez for designing the heating stage used for TReXS measurement.Peer reviewedPostprin

The Influence of the Imine Bond Direction on the Phase Behaviour of Symmetric and Non-symmetric Liquid Crystal Dimers

Acknowledgements D. P. gratefully thanks the National Science Centre (Poland) under the grant no. 2021/43/B/ST5/00240.Peer reviewedPublisher PD