Switchable molecules

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. 

S. Shinkai, Switchable Guest-binding Receptor Molecules , in Comprehensive Supramolecular Chemistry (Eds. J. 

Figure 1.29 Scheme of sophisticated switchable surfaces where cells could be sequentially released and reattached. A self-assembled monolayer, using an O-silyl hydroquinone as the cleavable molecule, allows for the release and reattachment of cells onto a surface. (Reprinted with permission from Ref [150]. 2003 American Chemical Society.)... 

The transformation from spheres to cyhnders is a peculiar example for the self-adjustment of the molecular conformation. The switching shape can be regarded as an example for the principle of quasi equivalency established by A. Klug for the self-assembly of biomolecules and viruses [145] for the sake of an improved intermolecular packing, the molecules adopt a conformation different from the minimum energy one. This also demonstrates that shape control does not mean a fully constrained structure. Similar to biomolecules, the combination of flexible macromolecules and self-assembly principles is a powerful strategy for preparation of molecules with well-defined but switchable shape [23]. 

Methyl viologen (/V, /V - d i m e t h I -4,4 - b i p r i d i n i u m dication, MV2+ ) can function as an electron acceptor.34 When MV2+ is linked to electron donor, photoinduced electron transfer would occur. For example, within molecule 24 the 3MLCT excited state of [Ru(bpy)3]2+ is quenched by MV2+ through oxidative electron transfer process. The excited state of [Ru(bpy)3]2 + can also be quenched by MV" + and MV°. The transient absorption spectroscopic investigations show that the quenching of the excited state of [Ru(bpy)3]2+ by MV + and MV° is due to the reductive electron transfer process. Thus, the direction of the photoinduced electron transfer within molecule 24 is dependent on the redox state of MV2 +, which can be switched by redox reactions induced chemically or electrochemically. This demonstrates the potential of molecule 24 as a redox switchable photodiode.35... 

The insertion of a photosensitive group or of a redox active unit into the push-pull system 1 yields switchable molecular wires and push-pull molecules that contain a photo-switch or a redox switch S, as represented in 4. Compounds of such type containing for instance electroactive ferrocene groups and photosensitive metal complexes, have been synthesized. Some of them are shown in series 5 (Marczinke, B. Przibilla, K.J. Lehn, J.-M., unpublished data). 

Fig. 6.15 Different forms of inclusion of eo-sin guest molecules in (isomeric) photo-switchable POPAM host compounds. The number and position of the included eosin molecules vary according to the conditions...

There have been some impressive, relatively recent examples of molecular rectification. Compound 11.52 has proved to be an extremely efficient molecular rectifier, able to actually function as a rectification device by intramolecular tunnelling either as a monolayers or multilayer macroscopic film or on a nanoscopic level.51 Switchable rectification has been demonstrated for a related dye shown in Figure 11.36. The electrical asymmetry can be chemically switched, off and then back on, by treatment with acid and base, respectively. Protonation disrupts the intramolecular charge-transfer axis, destroying the rectification effect.52 Recent calculations, however, suggest that there may be relatively unpromising theoretical limits on the rectification possible by a single molecule.53... 

In principle any molecule able to exist in two reversible, switchable states can represents a molecular switch (bistable device) with potential to form part of molecular circuitry or act as molecular memory. An excellent component for switchable molecular devices is the 1,2-dithienylethene system, which has been exploited ingeniously by Lehn in a number of bistable systems.54 The core switching element is the transformation of the dithienylethene unit between two stable states as a function of the wavelength of incident radiation (Scheme 11.8). 

In remarkable work, Benjamin Coe of the University of Manchester, UK, has produced the first genuinely switchable NLO device. Coe and co-workers were able to show that the Ru(II) complex 11.74 exhibits a very large ji coefficient of 1112 X 10 30 esu associated with intense low-energy MLCT excitations. Furthermore, these / values may be tuned by manipulation of the substituents on the bipyridinium portion of the molecule and exchange of the amine ligands for, for example, pyridyls. The key to the complexes switching ability lies in the observation that the NLO behaviour of the system is reduced by approximately 10-20-fold by oxidation of the metal centre to Ru(III) by... 

Key questions ahead of us concern new concepts for addressing individual molecular switches and the construction of more complex systems which incorporate several switchable functions. Advances in scanning - probe techniques and single molecule spectroscopy as well as supramolecular chemistry will play an important role in this endeavor. 

---