Liquid-crystal dimer

Fig. 7. Snapshots from a smectic-A phase of liquid crystal dimer molecules composed of two Gay-Berne mesogens linked by a flexible alkyl chain. Data from [112] supplied by Dr M R Wilson, University of Durham...

The dimer model proposed by Wilson [83] was found not to exhibit a nematic phase but smectic A and B phases and so, although semi-realistic features are included, the model is not able to reproduce the behaviour of real dimers such as the a,co-bis(4 -cyanobiphenyl-4-yloxy) [87], which exhibit nematic phases, although liquid crystal dimers exhibiting smectic phases are also known [88]. In these simulations, the GB-GB interaction appears to be too strong and so the system forms a smectic phase in which the GB and LJ units... 

Note 2 A liquid-erystal dimer or mesogenic dimer is sometimes known as a twin mesogen. Use of the terms dimesogenic compounds and Siamese-twin mesogen for liquid-crystal dimer or mesogenic dimer is not recommended. 

A specific example of type (c), a tail-to-tail liquid-crystal dimer, is... 

Note 4 A liquid-crystal dimer with different mesogenic groups linked by a spacer is known as an asymmetric liquid-crystal dimer. 

Note 5 A liquid-crystal dimer with flexible hydrocarbon chains having an odd number of carbon atoms is called an odd-membered liquid-crystal dimer, whilst one with hydrocarbon chains having an even number of carbon atoms is called an even-membered liquid-crystal dimer. 

Note 4 The tilt direction varies in a random manner from layer to layer in conventional smectic C mesophases. However it can alternate from layer to layer, as in an antiferro-electric chiral smectic C mesophase (see Definition 5.9, Note 7) and in the smectic C mesophase formed by certain liquid crystal dimers with an odd-number of carbon atoms in the spacers. The recommended symbol for this type of mesophase is SmCa. 

Note 1 The recommended subscript to designate an intercalated smectic mesophase is c Note 2 Intercalated smectic mesophases are commonly observed for liquid-crystal dimers. Note 3 At present intercalated smectic A (SmAc) and smectic C (SmCc) as well as intercalated crystal B (Be), G (Gc) and J (L) mesophases have been observed. 

Note 4 A thin sample of a smectic C phase with the layers parallel to the sample surfaces gives schlieren textures with centers that have four brushes. However, a smectic C phase formed by odd-membered liquid-crystal dimers (see Definition 2.11.2.9, Note 5) has schlieren textures with two or four brushes. 

A liquid crystal dimer is composed of molecules containing two conventional mesogenic groups linked via a flexible spacer. These materials show quite different behaviour to conventional low molar mass liquid crystals and in particular their transitional behaviour exhibits a dramatic dependence on the length and parity of the flexible spacer. In this review a comprehensive overview of the relationships between molecular structure and liquid crystallinity in dimers is provided. This includes a description of the novel modulated and intercalated smectic phases exhibited by dimers. 

Keywords Liquid crystal dimers. Structure-property relationships. Modulated smectic phase. Intercalated smectic phase... 

Structure Property Relationships in Liquid Crystal Dimers. .. 152... 

Non-Symmetric Discotic-Calamitic Liquid Crystal Dimers. 172... 

Laterally Linked Symmetric Liquid Crystal Dimers. 173... 

Liquid crystal dimers are, however, by no means a new class of materials, having been discovered some 70 years ago by Vorlander [4]. This first series of liquid crystal dimers were the a,co-bis(4-alkyloxyphenyl-4 -azophenyl)alkane dioates,... 

The importance of this discovery was apparently overlooked and liquid crystal dimers did not re-emerge until the 1970s and their rediscovery by Rault et al. [5]. Again this report made little impact. Indeed, it was not until Griffin and Britt [6] described the transitional properties of a series of diesters. 

The initial interest in liquid crystal dimers was triggered, therefore, by the similarity of their behaviour to that of the semi-flexible main chain liquid crystal polymers. It soon became apparent, however, that the dimers are of significant fundamental interest in their own right and exhibit quite different behaviour to conventional low molar mass liquid crystals. These studies, for example, have resulted in the discovery of a new family of intercalated smectic phases. This review focuses upon the novel behaviour of dimers and how it may be understood at a molecular level. 

Fig. la-e. Sketches of the possible molecular architectures for liquid crystal dimers a symmetric calamitic dimer b non-symmetric calamitic dimer c symmetric discotic dimer d non-symmetric discotic-calamitic dimer e laterally linked symmetric dimer. Ellipses represent rod-like mesogenic groups, circles disc-like mesogenic units and wavy lines denote the flexible spacers... 

The most extensively studied series of liquid crystal dimers are the a,m-bis(4 -cyanobiphenyl-4-yloxy)alkanes [ 18-28],... 

Fig. 4a,b. The molecular shape of a an even b an odd membered liquid crystal dimer with the spacer in the all-trans conformation... 

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