Correlation between Molecular Structure and Helicity of Induced Chiral Nematics in Terms of Short-Range and Electrostatic−Induction Interactions. The Case of Chiral Biphenyls

Abstract

The helical structure of the chiral nematic phases induced by chiral dopants in nematic solvents provides a macroscopic image of the molecular chirality of the dopant promoted by the orientational order. Chiral biphenyls are challenging systems because their twisting ability shows a strong dependence on the molecular structure, which does not conform to empirical correlation rules. This points out the need for adequate interpretative tools, able to establish a link between molecular properties and macroscopic response. In this paper the twisting ability of chiral biphenyls is reviewed, by reporting examples taken from the literature together with some new experimental results. The microscopic origin of the observed behavior is explained in terms of chirality and anisotropy of short-range and electrostatic-induction interactions. These are described, respectively, by a shape model and a reaction field method, having the common characteristics of a realistic representation of the structure and properties of the chiral dopants in terms of molecular surface, atom charges, and distributed polarizabilities.

A Chiroptical Molecular Switch with Distinct Chiral and Photochromic Entities and Its Application in Optical Switching of a Cholesteric Liquid Crystal

“…The observed variations of the HTP can be explained as follows; either one or two photoinduced trans to cis isomerizations of the azobenzene moieties in (S)-2 give rise to a compound with an overall helicity opposite to the helicity of the binaphthyl core resulting in a reversal of the handedness of the cholesteric phase. An alternative explanation (following again Ferrarini, Spada et al) [27] could be that azobenzene isomerization changes the alignment of the nematic solvent around the solute resulting in a different cholesteric phase.…”

“…Ferrarini and Spada et al have in fact recently demonstrated that biphenyl compounds with the same absolute configuration ((P), for instance) can induce a (P)-or (M)-cholesteric phase in E7, depending on their disk-like or rod-like behaviour, which is, in turn, related to the molecular shape of these derivatives. [27] On the contrary, the HTP of (S)-11, the model compound for the core of (S)-2, in K15 (a nematic solvent structurally similar to E7) is only + 1.5 mm À1 . [14b] Therefore the calculated HTP values of b + 10.9, À8.4 and À39.3 mm À1 of tt-, ctand cc-(S)-2, respectively, could largely be attributed to the geometry around the azobenzene moieties which changes upon photoisomerization.…”

Predictions of molecular chirality and helical twisting powers: A theoretical study

“…However, recent success has been found through calculations of the difference in chemical potential for enantiomers in a twisted nematic host, 4,5 in calculations of a scaled chiral index, 6,7 using a mean field approximation that takes into account chiral dispersive interactions 8 and with the chirality order parameter approach of Nordio, Ferrarini, and co-workers. [9][10][11][12][13][14][15][16] In order to determine a relationship between the molecular shape and a pseudoscalar property such as the HTP, these studies have, by necessity, been limited to relatively rigid chiral dopants where the molecular conformation is unlikely to vary significantly from the minimum energy structure that can be found from x-ray diffraction experiment and by theoretical techniques such as molecular mechanics and density functional theory. However, many important chiral dopants have a much higher degree of molecular flexibility.…”

“…In this paper we have combined the chirality order parameter approach of Nordio et al [9][10][11][12][13][14][15][16] with Monte Carlo simulations to study a range of systems. The simulation approach used is described in Sec.…”

Progress in computer simulations of liquid crystals

“…There are also possible applications in polymer-stabilised blue phases [6], which may provide a new generation of devices: fast light modulators and tunable photonic crystals. Nordio, Ferrarini and co-workers [109][110][111][112][113][114][115][116]. have developed a useful theoretical model to link structure with HTP.…”