Photochromic azobenzene functionalised banana–calamitic dimers and trimers: mesophase behaviour and photo-orientational phenomena

Abstract

Azobenzene functionalised banana-calamitic dimers and a novel type of banana-calamitic-banana trimers have been synthesised. Their thermotropic behaviour and the structure of the mesophases have been characterised by differential scanning calorimetry, polarising optical microscopy and X-ray diffractions measurements. Electro-optical studies evidence the polar properties of the smectic phases. A rare anticlinic-synclinic smectic CP (SmCP) phase transition could be proven for two of the twins upon cooling without an electric field although the molecules are not optically active. Remarkably, the phase sequence SmC a -SmC s has been observed upon cooling, the inverse sequence to that reported for non-chiral compounds up to now. A comparative study of the photochemical properties of dilute solutions and of thin films of two compounds has been performed. Irradiation with polarised ultra-violet (365 nm) and visible light (473 nm) induces a photo-orientation process of the chromophores in a direction perpendicular to the polarisation plane of the exciting light. For a trimeric compound bearing an azobenzene bridge between both bent-core mesogenic units this process is associated with cycles of the E-Z-E isomerisation of the azobenzene groups followed by rotational diffusion of molecules in the films. For a bis(azobenzene) containing dimer the E-Z isomerisation in films is strongly suppressed and the photo-induced orientation has a rotational diffusion mechanism.

Liquid crystal dimers and the twist-bend nematic phase. The preparation and characterisation of the α,ω-bis(4-cyanobiphenyl-4′-yl) alkanedioates

“…In particular, liquid crystal dimers proved to be a rich source of new types of intercalated smectic phases (see, for example,references [8][9][10][11][12]) and, rare and unusual phase transition sequences (see, for example, references [13,14]). More recently, interest has focussed on, for example, their flexoelectric behaviour, [15][16][17] T-and H-shaped dimers, [18][19][20][21] non-alkyl spacers, [22][23][24] cholesterylbased dimers, [25][26][27][28][29][30] discotic dimers, [31][32][33] carbohydrate-based dimers, [34] bent core-calamitic nonsymmetric dimers, [35,36] the effects of linking groups, [37] dimers containing new types of mesogenic units and functionalities, [38][39][40][41][42][43][44][45][46] as well as on higher oligomers. [47][48][49][50] Perhaps the most exciting recent discovery involving liquid crystal dimers, however, has been the observation of nematic-nematic transitions for a small number of odd-membered dimers.…”

Mesophase behaviour of binary mixtures of bent-core and calamitic compounds

“…Recently synthesis of banana-calamitic dimers or trimers opened an alternative way of tuning the physical properties [27,28]. These oligomers may in some sense be regarded as analogues of mixtures with specific concentrations: dimers correspond to a 50-50 mol%, while trimers correspond to a 66.6-33.3 mol% or 33.3-66.6 mol% binary mixture of a bentcore and a calamitic compound.…”

Effect of substituent on UV–visible absorption and photostability of liquid crystals: DFT study

“…[1] An exciting recent development is mainly concerned with coupling of these two areas, in nematic LCs, to produce photorefractive systems/devices. [2,3] An appropriate sample will allow the absorbing chromophores to dissipate some of their energy through charge separation. Furthermore, the absorbing chromophore must be tuned to match the wavelength for the desired application.…”