Other Photochromic Systems

Although there are many other interesting photochromic systems which are less common than the systems described up to this point, computational studies of such systems have been limited. Some exceptions will be briefly cited here. 

Nespurek et al.51 53 reported the mechanism of the photochromism of sydnone, a mesoionic compound. Since this system exhibits photochromism in the solid state, the origin of the colored form was assumed to be the formation of a colored center or intermolecular charge-transfer state. In contrast to this view, the combined evidence from experiments and MO calculations led Nespurek et al. to conclude that the nature of the photochromism was intramolecular. 

In a review of the photochromic transformation mechanism for quinone derivatives,54 the MO studies by Gritsan and co-workers55,56 using MNDO-AM1, CNDO/S, and PPP are mentioned as pertinent aids for the understanding of the mechanism in singlet as well as triplet potential surfaces. 

Photochromic Schiff bases were studied experimentally with the help of the MO calculations by Kownacki and co-workers.57-59 The proton-transfer mechanism along the internal hydrogen bonds was investigated, and it was shown that in molecules with two equivalent internal hydrogen bonds, only one proton translocated.59 

Other examples in which theoretical calculations were applied in the study of photochromism include an MNDO/3 study on dihydroindolizines,15061 the use of PPP electron densities as an aid for estimating the linear dichromism (LD) and magnetic circular dichromism (MCD) of symmetric dialkoxyanthracenes,62 and studies of the structures of unusual twisted cis zwitterionic azomethines,63 a 

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Photochromism is a reversible transformation of a single chemical species between two states, the absorption spectra of which are clearly different, the transition in at least one direction being induced by electromagnetic radiation [1], The widest and most important group of the photochromic system is based on electrocyclic reactions [2,3] a few have been commercially successful (polymer-based photochromic eyewear, novelty items and security printing inks). Several other photochromic systems based on E,Z-isomerization, cycloaddition reaction, electron or proton transfer have potential industrial applications [4],... 

Other photochromic systems are using the ring opening/ring closing of thienyl pyrans. Examples are 134 <20060L4931> as a biphotochromic system or 135. 

Major advantages of 41 and 42 over other photochromic systems are ... 

Benzo-2//-pyrans and their thia analogs, e.g., heterocycles not treated in this chapter, are photochemically labile and form reversible photochromic systems with open-chain and colored photoisomers. This behavior was observed for 2-benzyl-2,4,6-triphenyl-2//-species 467a,b (Y = H)399,409 and naturally occurring 2//-pyran 92a452a but only at about 77K. At room temperature, these as well as other 2//-pyrans are photochemically stable. Only 2,2,4,6-tetramethyl-2//-pyran (176) was photochemically changed to its exocyclic double bond isomer 308a.404 An irreversible photolysis of 2-azido-2//-thiopyran derivative 212b (R = H) has been discussed in reference 254. 

Other early observations of photochromic systems include ter Meer s potassium salt of dinitroethane (9) and Phipson s gate post painted with photochromic lithopone pigment (10). 

The physical property of primary concern in photochromic systems is the marked color change resulting from a major electronic rearrangement. Simultaneously with this color change, the other physical properties of the system are also altered to the extent of conversion to a new chemical species. In many cases, additional measurements aid the identification of the altered chemical species or in determining the mechanism for the photochromic activity. Occasionally, the reversible photochemi-... 

Most photochromic systems are not reversible indefinitely. However, very little careful analytical data have been accumulated to characterize the nature of the degradation products or to specify the degree of quantitative reversibility. The reasons for side reactions are inherent in the high photochemical reactivities of the systems. First of all, there must be an excited state formed by absorption this state is then transformed into other excited states or reactive species. The latter may include triplet states, carbonium ions, carbanions, free radicals, or other highly reactive intermediates. Certain of these are oxygen sensitive so that exclusion of the atmosphere and other potential sources of contaminants during irradiation is necessary. A second major route of degradation involves the excited state of the colored form which may already be... 

Other photochromic polypeptide systems have been described, in which the ability to photocontrol the specific conformation of polypeptides is an essential feature in the design of biomaterials for devices that can be photo switched. 

The properties of photochromic molecules such as DHI 7 and related compounds are dependent not only on structure but to a large extent also on their environment. Clear differences are to be expected if solution or solid phases of photochromic molecules are compared, i.e. (1) containing supramolecular anchor groups, (2) in liquid crystalline phases, or (3) polymer-containing photochromic systems. It is necessary to use the appropriate spectral methods for the detection of different properties. In addition to normal methods, photoacoustic spectroscopy (PAS), polarized light, and others may be applied. 

Organic photochromic systems have actual applications in variable transmission optical materials, authentication systems and novelty items. In addition, they offer great potential in erasable optical memories and many other fields where reversible changes of physical properties other than color are wanted. The domain is interdisciplinary and expanding. 

Photochromic systems have been described in which a 2,5-dihydrothiophene acts as the bridging ethene unit between two thiophene rings. The novelty here is that the core system has two 2-iodothiophene moieties which can be used to couple with a variety of other aromatic systems by standard reactions <2006SL737>. An example is given in Equation (6). 

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. 

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. 

There continues to be a high level of interest shown in photochromic systems. The optically active l,T-bi-2-naphthol gives an optically active intramolecular addition product (37) on irradiation, and on prolonged exposure, cycloreversion occurs which gives evidence for a photoequilibrium between the asymmetric molecules (Cavazza et al). A number of reports within the year describe a variety of aspects of the well-known thermally-reversible spiroindoline-oxazine to photo-merocyanine conversion. For example, the process is sensitised by triplet cam-phorquinone (Favaro et al.), the influence of complexation on indoline and phenanthroline spiropyrans with transition and rare earth metal ions has been described (Atabekyan et al.), and the photochromism of other derivatives in water using vesicles and y-cyclodextrin is reported to be faster than in methanol with the process most favoured in the vesicles (Ishiwatari et al.). 

Spiro compounds other than spirofindoline-benzopyrans] have been synthesized with the aim of finding photochromic systems endowed with both good color-ability and long durability. 

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. 

The number of papers on this topic is also small. The potential, limitations, and detailed comparisons with other typ>es of systems have been discussed for the newly introduced indolizines . Another published lecture describes the use of ps pulses to study primary photochemical steps in unsubsdtuted indolino-spiropyrans. A new photochromic system which has been described is mesoxaldehyde 1-allyl-l-phenyl-2-phenylosazone. ... 

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