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Picometer Detection by Adaptive Holographic Interferometry
Published in Klaus D. Sattler, Fundamentals of PICOSCIENCE, 2013
squares are formed by four single DBV 2+ dications giving rise to a cavitand with a diameter of 1.1nm; the inset in Figure 15.18a shows a 3D representation of this structure. The individual molecules can be arranged either in a right-handed or left-handed order (see Figure 15.19), which gives rise to a planar chirality of the cavitand structure. Since neither the molecules nor the substrate show any chirality, both enantiomers should be observable on the surface, which is indeed the case. The enantiomers do not mix on the surface but form mirror domains instead (see Figure 15.19), with an angle of 32∘ with respect to each other. If the electrode potential is decreased below -600mV, the DBV 2+ dications are reduced to the radical cation DBV *+, which is indicated by the current wave in the cyclic voltammogram at -700mV (Figure 15.17b; the current peak at -800mV arises from chloride desorption). This reduction is accompanied by a significant structural change as well as an increase in surface coverage which can be observed in the STM images. The resultant stripe structure in Figure 15.18b is formed by π-π stacking of the radical cations, and is characteristic of the monocation radicals of most bipyridinium derivatives [75-78].
An experimental and computational study of calamitic and bimesogenic liquid crystals incorporating an optically active [2,2]-paracyclophane
Published in Liquid Crystals, 2018
Richard J. Mandle, John W. Goodby
There are three types of chirality relevant to chemistry and molecular structure; point chirality (e.g. citronellol), axial chirality (e.g. BINOL) and planar chirality, the later arising for a case of chirality that arises from two (or more) disymmetrically substituted non-coplanar rings. Materials containing stereogenic centres (point chirality, e.g. cholesterol benzoate) are ubiquitous in liquid crystals, as are axially chiral materials [1,2]. In a nematic liquid crystal, the addition of a small quantity of a chiral solute leads to the formation of a helical chiral nematic (N*) phase with a helical pitch, P, which is inversely proportional to the solute concentration. Different chiral solutes at the same concentrations can give different pitch lengths, and this is accounted for by defining helical twisting power (HTP) as HTP = (P.c.r)−1, where c is the concentration and r is the optical purity. The introduction of dopants with large HTP values can lead to novel behaviour, such as wide temperature range blue phases [3] and unusual modulated nematic phases in dimeric materials [4]. [2,2]-Paracyclophanes with a single substituent on one aromatic ring exhibit planar chirality, and cannot be converted into their mirror image via rotation as shown in Figure 1. Chiral [2,2]-paracyclophane (such Phanephos) [5] have been employed in enantioselective synthesis, [6] as well as in highly conjugated materials for emission of circularly polarised light [7]. There are few liquid crystals that contain chiral paracyclophanes and the HTP of these materials are rather low (~6–10 μm−1) [8,9]; however, in these examples the bulky paracyclophane protrudes from the mesogenic unit. We envisaged that the steric bulk of the [2,2] paracyclophane group might lead to large values of HTP were it included as a terminal unit rather than as part of the mesogenic core. Both enantiomers of 4-hydroxy[2,2]paracyclophane are conveniently obtained via the enzymatic resolution reported by Cipiani et al. [10], and from this building block we prepared compounds 1 and 2 as shown in Scheme 1.