Switching molecular hysteresis

Molecular hysteresis results when the interconversion between the states does not follow the same pathway on sweeping the external stimulus or field in one or the opposite directions [8.240a, 8.241]. A hysteresis loop has, for instance, been described for the electrochemical interconversion of redox states in a dinuclear metal complex it involves the switching between two different binding sites [8.240b]. Spin... 

We now describe linkage isomerization that is triggered electrochemically, an essential element of the manifestation of molecular hysteresis which we will be introducing later. A redox process can direct various charges in structure and the reverse (redox switching) and provides a basis for molecular machines and engines. 

The main features of the antiferroelectric switching in FLCPs are a third state, which shows an apparent tilt angle of zero, a less marked threshold between the three states when compared to the low molecular weight antiferroelectric liquid crystals, a hardly observed hysteresis, and an anomalous behavior of the spontaneous polarization with temperature (Fig. 24), which is not encoun-... 

Ferroelectric materials are a subclass of pyro- and piezoelectric materials (Fig. 1) (see Piezoelectric Polymers). They are very rarely foimd in crystalline organic or polymeric materials because ferroelectric hysteresis requires enough molecular mobility to reorient molecular dipoles in space. So semicrystalline poly(vinylidene fluoride) (PVDF) is nearly the only known compoimd (1). On the contrary, ferroelectric behavior is very often observed in chiral liquid crystalline materials, both low molar mass and poljuneric. For an overview of ferroelectric liquid crystals, see Reference 2. Tilted smectic liquid crystals that are made from chiral molecules lack the symmetry plane perpendicular to the smectic layer structure (Fig. 2). Therefore, they develop a spontaneous electric polarization, which is oriented perpendicular to the layer normal and perpendicular to the tilt direction. Because of the liquid-like structure inside the smectic layers, the direction of the tilt and thns the polar axis can be easily switched in external electric fields (see Figs. 2 and 3). 

With the input intensity changing slowly, the hysteresis loop is well reproducible. The upward transition BC in Fig. 2 is much faster than the downward transition DA The switching times are up— 1 s and down—15 s, respectively. The observed threshold power for up transition at point B was Pth 110 mW. With a beam cross section of 5x 10 cm at the sample, the corresponding threshold intensity was 7th 2.2 kW/cm. This transition should result from the field-induced molecular reorientation usually known as the Freedericksz transition. We found that the observed threshold for a circularly polarized input beam was twice that for a linearly polarized input beam, as expected from theory." However, no polarization rotation of the output was observed in the latter case. 

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