Photochromic system, molecular

Pina, F., Maestri, M., and Balzani, V., Photochromic systems based on synthetic flavylium compounds and their potential use as molecular-level memory elements, in Handbook of Photochemistry and Photobiology, Abdel-Mottaleb, M.S.A. and Nalwa, H.S.Eds., American Scientific Publishers, Stevenson Ranch, 2003, 3, 411. 

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. 

In a chiral photochromic system (Figure 1), A and B represent two different chiral forms of a bistable molecule, and a reversible change in chirality occurs upon light irradiation. The left-handed (S or M) and right-handed (R or P) forms of a chiral compound 91 represent two distinct states in a molecular binary logic element. 

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. 

As discussed at the beginning of this chapter, photochromic systems represent potential molecular level memory devices. A number of problems, however, must be solved for practical applications. A challenge problem is to find systems with multiple storage and nondestructive readout capacity those in which the record can be... 

A remarkable photochromic system in which molecular and supramolecular chirality seem to communicate with each other has recently been described <2004SCI278>. The compound 16 (X = F1) shows exceptional stereoselectivity upon aggregation of the molecules during gel formation in toluene. This supramolecular chirality is translated into molecular chirality on photocyclization wherein a diastereoselectivity of 96% is obtained. 

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 processes are often observed both in solution and in the solid state, thus making for facile incorporation of photochromies in films, in membranes, and as dopants in host matrices—prerequisites for the construction of molecular optoelectronic devices. Section 2.3.1 focuses on the materials and supramolecular systems prepared from photochromic systems. For more comprehensive descriptions of the basic photochemical processes the reader is referred to any of the numerous reviews on the subject [47, 51, 89, 159-162]. 

There is no limit to the number of photochromic systems possible. The systems discussed are excellent candidates for integration into solid-state devices because nearly all retain their photochromic properties in the absence of solvent. The organization of these systems in tandem with other molecular systems is being pursued. For the switching applications many of these systems have much too slow a turnover rate to be explored as working devices. That is unless the connectivity in these systems can be increased. In the meantime, photochromic systems will probably be explored as possible optical memory devices. The most promising switches are those based on the much faster processes of electron and energy transfer. We will now examine research in these areas. 

In this chapter, crystalline-state photochromic dynamics of rhodium dithionite complexes are reviewed. The chemistries described here have been achieved not only by recent developments of the analytical technique but also by discovery of a new class of transition-metal based photochromic compounds. One of the advantages of transition-metal complexes is structural diversity. In order to find the rule of an exquisite combination of metal ions and ligands, we are currently synthesizing various dithionite derivatives with other metal ions and/or modified Cp ligands. As shown in this chapter, dithionite complexes are a very useful photochromic system to investigate crystalline-state reaction dynamics. We believe that dynamics studies of newly synthesized dithionite derivatives provide useful insight into the construction of sophisticated molecular switches. A dithionite complex may appear in a practical application field in the near future. 

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