Switchable applications

To understand how chirality is expressed, it is important to first describe the different thermotropic mesophase assemblies which can be formed by chiral discotics. Even though expression of chirality has been observed in thermotropic mesophases, the chiral expression occurs in a rather uncontrolled manner, and systems which are suitable for applications, for example, easily switchable columns/ferroelectric discotic liquid crystals, consequently have not yet been developed. Hence, the assembly of discotics in solution has received considerable attention. Supramolecular assemblies of discotic molecules in solution are still in their infancy and have not yet found commercial application, but they are of fundamental importance since they allow a detailed and focused investigation of the specific interactions that are required to express chirality at higher levels of organization. As such, the fundamental knowledge acquired from supramolecular assemblies in solution might formulate the design criteria for thermotropic chiral discotic mesophases and provide the necessary tools for the creation of functional systems. 

Ferromagnetic and ferroelectric materials are only two examples of a wider group that contains domains built up from switchable units. Such solids, which are called ferroic materials, exhibit domain boundaries in the normal state. These include ferroelastic crystals whose domain structure can be switched by the application of mechanical stress. In all such materials, domain walls act as planar defects running throughout the solid. 

These types of switchable electrode surfaces have been used to selectively pattern two different cell populations onto a surface [151] and additionally these surfaces can selectively release different cells at different applied potentials [152]. However, it is important to recognize that electrochemically switching a surface from inactive to conjugation and active to conjugation has been well explored with nitro-terminated aryl diazonium salts. In such studies, the application where very anodic potential resulted in a six-electron reduction to an amine [139], to which proteins could be attached [153-155]. The key difference is that the interaction of the biological medium with the surface is controlled by the presence of the antifouling layer. In many ways these electrode surfaces developed by Mrksich and coworkers [150-152, 156] are very similar to the antifouling surfaces with molecular wires discussed in Section 1.4.2 [131, 132, 138, 142]. In both cases the electrode is... 

The main application areas for electrochromic systems are in electrically switchable rear-view car mirrors for anti-dazzle, in glazing units for temperature and light control and in visnal displays. The main advantage offered by electrochromic systems in the first two of these applications is the fact that the cells can be made very large, i.e. one cell can comprise a whole glazing unit. 

Figure 6. (a) Schematic representation of the four-module format of an ionically switchable photoactive dyad. This requires a shield module owing to the exceptional sensitivity of its emissive state towards collisional quenching. However, this emission can be put to good use, as discussed in the text. In many respects however, the situation in Figure 1 is applicable here. Note that the relative position of the dyad with respect to the shield can depend on whether the receptor is ion-bound or not. (continued)... 

Figure 9. (a) Schematic representation of the five-module format of a photoactive triad which is switchable only by the simultaneous presence of a pair of ions. This design involves the multiple application of the ideas in Figure 1. The four distinct situations are shown. Note that the presence of each guest ion in its selective receptor only suppresses that particular electron transfer path. The mutually exclusive selectivity of each receptor is symbolized by the different hole sizes. All electron transfer activity ceases when both guest ions have been received by the appropriate receptors. The case is an AND logic gate at the molecular scale. While this uses only two ionic inputs, the principle established here should be extensible to accommodate three inputs or more, (b) An example illustrating the principles of part (a) from an extension of the aminomethyl aromatic family. The case shown applies to the situation (iv) in part (a) where both receptors are occupied. It is only then that luminescence is switched "on". Protons and sodium ions are the relevant ionic inputs.

Another redox switchable system is based on dyad 21 in which 2-chloro-1,4-naphthoquinone is covalently attached to 5-dimethyl-aminonaphthalene via a non-conjugated spacer. The intrinsic fluorescence of the dansyl excited state in dyad 21 is strongly quenched, due to the intramolecular electron transfer from the excited dansyl to the adjacent quinone acceptor. However, the fluorescence can be switched on by addition of a reducing agent. Apart from chemical switching, the fluorescence of dyad 21 can also be switched electrochemically. This can be realized using a photoelec -trochemical cell, and the solution starts to fluoresce upon application of a reductive potential.31... 

The use of T2D-IR spectroscopy in its various modes is not limited to MLCT. It is applicable to all kinds of photo-triggered processes. Besides the investigation of electronically excited states, photo-chemical reactions like isomerizations and dissociations can be explored. Application to a photo-switchable peptide has already been demonstrated [10]. Conformational dynamics of biomolecules that are triggered by laser-induced T-jump or the control of pH by photo-acids, can also be investigated. 

Irie et al. reported on the absolute reversible photocyclization reactions of achiral diarylethene 52 in chiral crystal leading to 53 in up to 99% ee. The asymmetric photochromic material has potential application in nonlinear optics, for example, in switchable second harmonic generation devices. [32]... 

At present metal hydrides, owing to their unique properties, find applications (Figure 2) in many fields of science and technology, associated with hydrogen storage, thermal compression, separation, electrochemistry, switchable mirror, etc. 

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