Photoswitching molecular

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. 

This review surveys our studies devoted to the photoswitchable molecular receptors based on photochromic crown ethers. Photochromic systems described in the review may be classified into three groups according to the reaction types E,Z-isomerization, [2+2]-photocycloaddition reactions and electrocyclic reaction. It has proved the groups to be an especially suitable basis for photochromic systems, and promising for the industrial applications. 

Ushakov, E.N., Gromov, S.P., Buevich, A., Baskin, I.I., Fedorova, O.A., Vedernikov, A.I., Alfimov, M.V., Eliasson, B., Edlund, U. (1999) Crown-containing styryl dyes cation-induced self-assembly of multiphotochromic 15-crown-5 ethers into photoswitchable molecular devices, J. Chem. Soc., Perkin Trans. 2, 601-607. 

The photochromic chromophores can be classified into two categories, depending on the thermal stability of the photogenerated isomers. When photogenerated isomers are unstable and revert thermally to their initial isomer state in the dark, the chromophores are classified as T-type (thermally reversible type). Most photochromic chromophores belong to this type. The photogenerated blue color of 6-nitro-l, 3, 3 -trimethylspiro-[2H-l-benzopyran-2,2 -indoline], for example, disappears in less than half an hour even in high Tg polymer matrices.181 Such thermally unstable photochromic chromophores cannot be applied in photoswitchable molecular systems, because the switched states are unstable. For those applications, the characteristic of persistence, or in other words thermal irreversibility, is indispensable. 

Properties which change concomitantly with diarylethene derivative photoisomerization are geometrical structures, electronic structures, refractive indices, and chiral properties (when the molecules have chiral substituents). Table 1 shows how the above property changes are applied to various photoswitching molecular systems. Details of these photoswitching functions are described in Sections 2.3 to 2.6. 

The following section describes various photoswitching molecular systems employing diarylethenes as the switching units. 

Irie M. Photoswitchable molecular systems based on diarylethenes In Feringa BL, Ed. Molecular Switches. Weinheim Wiley-VCH, 2001 37-62. 

Irie, M. (2001) Photoswitchable molecular systems based on diarylethenes, in Molecular Switchings (ed. B.L. Feringa),... 

Reversible energy transfer between monomeric and dimeric forms of rhoda-mine 6G in ethylene glycol has been observed" and the concentration dependence of the overall fluorescence quantum yield has been modelled by Monte-Carlo simulations. Triplet energy transfer in disordered polymers has been analyzed on the basis of Bassler s model in which the trap energies have a Gaussian distribution." Energy transfer has also been observed in mono-layers and for photoswitchable molecular triads." The structural requirements for efficient energy transfer from a carotenoid to chlorophyll have been... 

Fig. 11 Organometallic photoswitchable molecular wires. (Reprinted with permission from [81])...

The selected examples in this chapter demonstrate the success of the development of photoswitchable molecular materials through the rational designs based on the coupling of photochromic moieties into molecules with different functions, such as luminescence, nonlinear optics, liquid crystal, molecular machine, receptor, electrical con-ductor/semiconductor, and many others. It is important to emphasize that photoswitchable materials are not confined only to the selected areas in these examples. With the understanding of both the photoswitching processes and the functional properties at the molecular levels, materials that possess different varieties of photoswitchable functional properties can be readily designed and synthesized. 

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