Molecular function photochromism

Nonadiabatic transitions definitely play crucial roles for molecules to manifest various functions. The theory of nonadiabatic transition is very helpful not only to comprehend the mechanisms, but also to design new molecular functions and enhance their efficiencies. The photochromism that is expected to be applicable to molecular switches and memories is a good example [130]. Photoisomerization of retinal is well known to be a basic mechanism of vision. In these processes, the NT type of nonadiabatic transitions play essential roles. There must be many other similar examples. Utilization of the complete reflection phenomenon can also be another candidate, as discussed in Section V.C. In this section, the following two examples are cosidered (1) photochromism due to photoisomerization between cyclohexadiene (CHD) and hexatriene (HT) as an example of photoswitching molecular functions, and (2) hydrogen transmission through a five-membered carbon ring. 

The formation of hydrogen bonds between the carboxylic acid groups of the functionalized liquid crystal copolymers and the pyridine portion of the dopants leads to stable, none separating mixtures. In mixtures containing up to 30 % of the dopants no separation was observed. Induction of a nematic mesophase is observed in the case of a smectic polymer matrix doped with low molecular weight photochromic dyes. 

Product 129 containing a cyclopentene-based photochromic fragment was proposed as an axis of pseudorotaxane structures, which has potential for use in the construction of a prototype of molecular machines active as a functional stopper (08DP294). 

The term photochromism can be defined as a light-driven reversible transformation between two isomers possessing different absorption spectra.111,21 The two isomers differ from one another not only in their absorption spectra, but also in their geometrical structures, oxidation/reduction potentials, refractive indices, and dielectric constants. When such photochromic chromophores are incorporated into functional molecules, such as polymers, host molecules, conductive molecular wires, or liquid crystals, the functions can be switched by photoirradiation.[3 61 Photostimulated reversible changes in refractive index can also be applied to optical waveguide switching.171 This chapter reviews applications of photochromic chromophores, especially diar-ylethene derivatives, in various photo switching molecular systems. 

Because of the multifunctional nature of these photochromic systems, the change in chirality simultaneously triggers the modulation of some particular function, such as fluorescence, molecular recognition, or motion. In most cases, this is the result of a change in the geometry or the electronic properties of the system. 

In a photochromie system all of the refractive-index change is a result of photoinduced reactions of isolated molecules, and there is no mass transport over distances larger than molecular dimensions. Since each molecule functions independently, the spatial frequency response of photochromic systems extends from zero to the diffraction limit of the recording light. (This is frequently referred to as "molecular resolution.") While our definition of a photochromic system does not require that the process be reversible, many photochromic systems are reversible, optically and/or thermally (31). In fact, it is in general only with photochromic processes that one can obtain, reversible image recording. 

Photochromic molecules provide a unique opportunity for the practical application of theoretical chemistry, molecular modeling, and molecular simulation, since their molecular properties directly determine the function of devices.1 Recent remarkable developments of computer hardware and software have opened a new horizon in material design, namely, the combination of computational studies and experimental approaches to offer an understanding of function on a molecular level.2 This does well for the future design of functional materials. 

The great demand for miniaturization of components in electrotechnical, medicinal or material applications has led to the development of a highly multidisciplinary scientific and technological field called nanotechnology to produce devices with critical dimensions within the range 1 100 nm. The ultimate solution to miniaturization is logically a functional molecular machine, an assembly of components capable of performing mechanical motions (rotation or linear translation) upon external stimulation, such as photoactivation.1103,1104,1239-1244 This motion should be controllable, efficient and occur periodically within an appropriate time-scale therefore, it involves photochromic behaviour discussed in the Special Topic 6.15. Such devices can also be called photochemical switches (Special Topics 6.18 and 6.15). Here we show two examples of molecular machines a molecular rotary motor and a molecular shuttle. 

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