Photochromic chromophores

For two-photon memories, a number of media types and reading mechanisms have been used (165). Generally, media comprise two photon-absorbing chromophores dissolved within a soHd polymer matrix. Suitable reversible photochromic dyes are, for example, spiropyrans. Although photochromic materials often suffer from photobleaching, as well as from instability leading to self-erasure, new materials and host environments are under development (172). Bacteriorhodopsin (BR) also has been proposed as a two-photon memory material. 

Luin S, Voliani V, Lanza G, Bizzarri R, Nifosi R, Amat P, Tozzini V, Serresi M, Beltram F (2009) Raman study of chromophore states in photochromic fluorescent proteins. J Am ChemSoc 131 96-103... 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Provided that an optically active molecular aggregate is photochemically perturbed to change the state of molecular alignment, the effect of a chiral environment on an achiral chromophore incorporated in the molecular aggregate will be also altered. It has been known that polypeptides bearing photochromic side groups change their optically active properties as a result of photochromic reaction(10-12). This phenomenon is likely to be related to non-linear photoresponsiveness. 

Photochromic dyes can also be found among the various chromophores that have been implemented in molecular glasses. The free volume available in the amor-... 

The efficiency of energy conversion in the polymers, i.e. how much the change in shape of the chain is induced by a photon, is, however, rather low. They need many photons and a high content of pendant photochromic chromophores to induce a large conformation change. To make a sensitive photo-responsive polymer, i.e. one which responds more efficiently to fewer photons, it is necessary to introduce an amplification mechanism to the system. One possible way of achieving this is to utilize the phase transition of polymers [7,8]. 

The contribution of fluorescence to the deactivation of the excited singlet state of the phytochromobilin chromophores in Pr and Pfr (see Section II.D) is negligible in quantitative terms. The total fluorescence quantum yield of the photochromic P and P3 components amounts only to from Tables 1 and 4). This means that deactivation proceeds predominantly through nonradiative channels, i.e., via internal conversion back to the electronic ground state of Pr and via primary photoreaction(s). Nevertheless, the fluorescence efficiency suffices to serve as a sensitive tool to monitor certain aspects of the competing primary reaction(s) of P (see Sections III.A and III.C). 

The insensitivity of so many of the photophysical and photochemical properties to proteolytic degradation down to about half of the original molecular weight is remarkable. It is particularly noteworthy when the location of the bilatriene chromophore near the N-terminus—one of the sites of preferential endogenous proteolytic attack—is taken into account and also the fact that the photochromic behavior of chromopeptides obtained by further protein degradation progressively deteriorates and is eventually lost [8b],... 

These conclusions are still consistent with the finding that significant conformational differences between Pr and Pfr do in fact exist ([65,147] for reviews see [8c, 148]). They can be rationalized—albeit not with conclusive rigour—by a conformational adaptation of the apoprotein part located around the bilatriene-binding pocket, following the Z E photoisomerization of the chromophore. This local change then should suffice to determine through bilatriene chromophore-protein interactions the spectroscopic characteristics of the chromophore as well as stability and reactivity of the two photochromic forms of phytochrome. 

The excited singlet Pr is composed of three species possessing bilatriene-type chromophores approx. 91% are undoubtedly the functional component, P , with a lifetime of 44 ps, approx. 8% represent a close to 200-ps component, P, which still exhibits the essential photochromic properties of the active chromoprotein, and finally a very minor component, Pj , which is not photochromic and is classified as an impurity. 

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. 

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. 

Such thermally irreversible photochromic chromophores represent the other class, classified as P-type (photochemically reversible type). Although many photochromic compounds have been so far reported, P-type chromophores are very rare. Only two families, furylfulgide derivatives and diarylethene derivatives, exhibit this reactivity.19 101 The photogenerated isomers of these derivatives are thermally stable and never revert to their initial isomers even at elevated temperatures (-100 °C). The thermally stable photochromic compounds offer potential for various applications in photoswitching and memory devices. 

A crucial point that must be addressed concerns the thermal stability and the fatigue phenomenon observed in the chromophores. It is a fact that many photo-chromic compounds are irreversibly degraded upon long exposure to light, thus limiting their use for various applications. Major advances in the preparation and performance of photochromic materials have been made in the past five years. Irie et al.11271 have recently developed new photochromic compounds, 1,2-diarylethenes, which display photochromic behavior with unchanged intensity even after 104 coloration decoloration cycles. 

In order to see how soon this photochromism was developed during irradiation, sheets of peroxide-bleached groundwood were photoaged for different periods of time and then stored for 24 hours. As shown in Figure 3, the photochromism is apparent after only a few minutes of irradiation indicating that the chromophoric groups responsible for this photochromic behaviour are rapidly and completly developed in the first half hour of the photoageing. 

In a series of experiments with a spruce chemimechanical pulp, it was established that, regardless of the initial irradiation time, a second irradiation period of five minutes with UV-containing radiation after 24 hours storage was sufficient to eliminate the photochromic brightness increase, as shown in Figure 4. If the chromophoric structures are present in the pulp from beginning in their uncoloured or potential chromophore form or whether they are formed by reactions during the initial irradiation is not known and may in fact also depend on the prehistory or the type of pulp. 

A large number of SCLC polymers with azoaryl photochromic chromophores have been studied [74] mainly with a view to applications in optical recording [2]. 

Stumpe and his co-workers [ 122-124] synthesized several SCLC methaciy-late copolymers containing the photochromic N-salicylideneaniline (NSA) chro-mophore together with several other mesogenic and nonmesogenic groups. Copolymers, 58, had from 0 to 100% of the NSA chromophore. Most of these polymers had an unspecified 5 mesophase. The NSA chromophore undergoes an excited state proton transfer (Fig. 17) to the Z-keto (often called cw-keto form)... 

The photochromic mechanism of fulgides can be separated into four parts in this discussion chromophore and excited state, E - Z isomerization, photocycliza-tion, and heliochromic reaction. 

It was only recently that these phenomena were detected photochemically using photochromic molecules whose chromophores undergo a reversible photoisomerization reaction. The photochemical reactions involved in these processes are mainly the trans-ds isomerization of aromatic azo compounds and stilbene derivatives as well as the ring opening/dosure reaction of spirobenzopyran derivatives . ... 

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