Photochromic behavior materials

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]. 

Photochromism was first described in the scientific literature in 1876. 7 In his famous paper entitled The Photochemistry of the Future (1912), G. Ciamician19 discussed the importance of photochromic substances and mentioned the possibility of using such compounds for fashion purposes. Photochromism is also a natural phenomenon most biologic photoreceptors exhibit photochromic behavior. 101 Until the middle of this century research on photochromic compounds was mainly carried out in academic centers, but around 1960 it was recognized that this subject is also of considerable commercial interest and since then most of the research has been done in industrial laboratories. The applications of photochromic materials can been classified as follows 7 ... 

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. 

This chapter will classify and review photochromies according to the mechanism whereby the material manifests its photochromic behavior. This chapter is not intended to be a comprehensive review of every photochromic family excellent works that explicitly deal in great detail with photochromies are already available. 

Another class of three-membered heterocyclic compounds that exhibit photochromic activity via 1,3-electrocyclization reaction is the aziridines. For the most part, similar to the oxiranes, these materials require low temperatures to demonstrate photochromic behavior [6]. Also, some of the photochromic transformations are not completely reversible. However, several room temperature stable ylides have been reported for aziridines possessing the general form of Scheme 5 [I], where X and Y may be a mixed combination of nitro, methoxy, and hydrogen groups. The... 

The most intensively developed materials, however, are the azobenzene-containing side chain LC polymers, which show a unique combination of liquid crystallinity and photochromic behavior in a single macromolecule, whereas copolymerization of comonomers with different functionality allows fine tuning of phase behavior, photo-optical properties, and other parameters to the requirements of a particular application. Starting from the first publications of Eich et work in that area has been done by several research... 

Though the field of molecular electronics is still in its infancy with respect to the porous layered materials many of these phases have yet to be considered as hosts for conducting polymeric materials. The properties of intercalated sufonyl-spiropyran have been examined to determine any novel photochromic behavior. The parent... 

The many important developments of photochromic pigments and materials in recent years have necessitated that this collection of reviews be issued in two volumes. This volume 2 contains ten chapters complementing the nine chapters of Volume 1 devoted to specific families of photochromic compounds. The first eight chapters of this volume concern physicochemical studies and describe different methods for studying photochromic behavior through a variety of physicochemical approaches. The following topics are addressed ... 

The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. 

Aryl-substituted fulgides are the products of condensation of aromatic ketones with succinic anhydride, and form one of the oldest and most important groups of organic photochromic materials. First discovered in the early part of the last century by Slobbe- ", they are now well known to owe their photochromic behavior to reversible (comotatory)... 

Yang, X., Yang, B., Liu, Y., Zhu, H., 2012. Microwave-assisted synthesis of novel spiroox-azines and their photochromic behaviors in polymer matrices. Optoelectronics and Advanced Materials-Rapid Communications 6 (11—12), 1146—1152. 

Two novel classes of photochromic amorphous molecular materials based on azobenzene and dithioiylediene have been created. Photochromic behavior of these materials in solution and as amorphous films has beoi elucidated. These photochromic amorphous molecular materials based on azobenzene and didiienyledirae have found implications for SRG fcnmation and dual image formation, respectively. 

Nevertheless, on-press developable systems do not require a colorant. These materials prefer the addition of a color-on-demand material showing photochromic behavior on those parts where the laser shines on the plate. Patent literature mentions ... 

Scouting at DuPont continued for a brief period, during which Dr. Lawrence A. Cescon saw a reference to Japanese work that described photochromic behavior of certain oxidation products of triarylimidazoles (lophines). Soon other laboratories in Japan, Germany, England and the United States embarked on studies of these materials. 

Materials are being developed to exhibit both photochromic and photographic behaviors one such system is based on a substituted indolinospiro-hen opyrene embedded in a polystyrene matrix (26). This system acts as a photochromic system at low exposure in the uv range and at high exposure it functions as a photographic system. The image can be devisualized by heat and can be restored many times with uv irradiation. 

The methods of synthesis, the spectral and photochromic properties in solution, in polymer film and in vacuum-deposited thin films, and the structural determinations by X-ray diffraction are reviewed as is the electrochemical behavior of this family of switchable materials. 

Not all heterocyclic methylene bases give spiropyrans (at least under the usual conditions). l,3,4,5,5-Pentamethyl-2-methylene-2,5-dihydropyrrole (69)77 did not condense with 5-nitrosalicylaldehyde to give any photochromic material.7 However, l-methyl-2-methylene-3,3,4,5-tetraphenyl-2,3-dihydropyrrole (70), prepared by reaction of methyllithium with 1 -methyl-3,3,4,5-tetraphenylcrotonolactam, did condense with 5-nitrosalicylaldehyde to give a product exhibiting reverse photo-chromism the unexposed form was red and the exposed form yellow. This behavior probably represents a trans to cis isomerization of the open form 7... 

From the organic point of view, reversible contraction/dilation phenomena should be observed in photochromic networks below their glass transition temperatures, i.e. in the rubbery state where segment mobility is important. Considering that isomerization occurs in the rubbery state at a rate similar to that in solution, it should be expected that the use of isomerization reactions for photocontractile behavior should be optimal with easily deformable networks, i.e. swollen gels and rubbery materials. 

Monolayers (2D films) can be regarded as ultimate thin states of organic and polymeric materials and are quite fascinating for constructing molecular amplification systems for the following reasons. First, the simplest (thinnest) structure of a monolayer allows one to comprehend the details of molecular communications and motions, because the excessive complexities of three-dimensional materials are efficiently excluded. This can lead to many implications on the material design of wide varieties of related systems. Second, for surface-mediated transfer processes, which consist of molecule-to-molecule transfer of information based on molecular orientations inscribed on uppermost surface layers, the existence of monolayers on material surfaces is sufficient for performances of macroscale controls of materials. Note that the nature and behavior of buried photochromic moieties in thicker films play minor roles. 

Photochromism proceeds, in general, in condensed phases and is affected markedly by the nature of the matrix. Polymeric materials play a very crucial role in studies on photochromism, in particular from a practical viewpoint since various applications require photochromic materials in the form of films, sheets, plates, fibers, beads, and so on. Photochromic molecules have usually been incorporated into polymer matrices by binding them covalently to polymer backbones or by dissolving or suspending them in polymer solids. The photochemical as well as thermal behavior of organic photochromic molecules is influenced in various ways by the characteristics of the polymeric media so that photochromic polymers have attracted extensive interest, leading to interdisciplinary research.1,2... 

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