Effects of combining of different flexible spacers in liquid crystal trimers and tetramers on transition properties

Abstract

Liquid crystal trimers and tetramers containing two kinds of flexible spacers, namely O(CH 2 ) m O and COO(CH 2 ) n O, were divided into four classes according to the odd/even nature of the number of atoms in the flexible spacers: specifically, even-even, odd-odd, even-odd, and odd-even trimers, and even-even-even, odd-odd-odd, odd-even-odd, and even-odd-even tetramers. The transition properties of the four types of trimers and of tetramers were compared. Although the nematic-isotropic transition temperature and the associated entropy changes were primarily related to the number of the even-membered flexible spacers in these molecules, the different combinations of the flexible spacers significantly affected their transition properties.

Chiral liquid crystal tetramers

“…There are very few examples of smectogenic tetramers reported in the literature 27,28,34,35 and just one of these described fully intercalated smectic behaviour. Thus, Imrie et al reported tetramers containing electron rich and electron deficient aromatic-based groups and members of this series exhibited an intercalated smectic A phase with d/L z 0.27, and the driving force for this was attributed to a specific interaction between the unlike mesogenic groups.…”

“…These higher oligomers have yet to receive the same level of attention as dimers but the most studied to date are liquid crystal trimers which consist of molecules containing three liquid crystal groups interconnected by two flexible spacers (see, for example, [22][23][24]. By comparison, the transitional behaviour of very few liquid crystal tetramers containing four mesogenic units and three flexible spacers has been reported in the literature [25][26][27][28][29][30][31][32][33][34][35] and of these only a small number exhibit smectic behaviour. 27,28,34,35 It is interesting to note that tetramers have been reported containing four differing mesogenic groups, one of which is cholesterylbased, and these exhibit columnar phase behaviour.…”

“…By comparison, the transitional behaviour of very few liquid crystal tetramers containing four mesogenic units and three flexible spacers has been reported in the literature [25][26][27][28][29][30][31][32][33][34][35] and of these only a small number exhibit smectic behaviour. 27,28,34,35 It is interesting to note that tetramers have been reported containing four differing mesogenic groups, one of which is cholesterylbased, and these exhibit columnar phase behaviour. 29,30 In this paper, we report the synthesis and characterisation of two new sets of liquid crystal tetramers which have been designed to exhibit smectic phase behaviour in an attempt to enhance our understanding of structure property relationships in this largely unexplored class of liquid crystals.…”

Cholesteryl-based liquid crystal dimers containing a sulfur–sulfur link in the flexible spacer

“…These consist of molecules containing two or more mesogenic units interconnected via flexible spacers, usually in a linear fashion. The simplest oligomers, termed liquid crystal dimers, contain just two liquid crystal units interconnected by a single spacer and these have been by far the most widely studied oligomers [2,3] although higher oligomers such as trimers and tetramers have been reported (for recent examples, see [5][6][7][8][9]). The intense interest in liquid crystal dimers has arisen because they exhibit very different properties to conventional low molar mass liquid crystals and recent investigations have included the observation of nematic-nematic transitions [10][11][12][13]; siloxanebased dimers [14]; studies of their light emitting [15] and flexoelectric properties [16,17]; the effects of spacer length and linking groups [18,19]; bent corecalamitic dimers [20,21]; anthracene-based [22] and discotic dimers [23]; H-shaped [24,25] and cyclic dimers [26]; intercalated [27,28] and biaxial smectic phases [29].…”

Non-symmetrical liquid crystal dimers armed with azobenzene and 1,2,3-triazole-cholesterol

“…This phenomenon can be observed in other oligomers such as trimers and tetramers. [52] It should be noted, however, that the ΔS N*I /R values as shown by 6a-6g dimers are considerably lower than those are normally seen for liquid crystal dimers containing small terminal substituents or terminal linear alkyl chains. This can be attributed to the (i) two arms kinked at an angle with respect to each other and, therefore, give an overall bent shape to the molecule, [53] (ii) increased biaxiality of the bulky cholesteryl-based unit resulting from the decreased length-to-breadth ratio relative to conventional rodlike core such as the cyanobiphenyl, dimers containing branched terminal chains and other dimers containing cholesterol moiety, [54,55] (iii) in term of molecular geometry, the flexible spacer linked up two mesogenic groups by nitrogen in the triazole core and oxygen caused a bent conformation and dilutes the interaction strength parameter.…”