Optical devices isomerization

A fundamental requirement for the system to be suitable for the application of the photochromic probe method is that the polymer is sufficiently transparent to UV or visible light in the spectral range of the photochromic probe molecules, first of all to induce the photo-isomerization reaction upon irradiation of the sample, and secondly to allow the determination of the absorption spectrum of the isomers before and after the irradiation, and thus to calculate the degree of isomerization. In this respect, Hyflon AD is an excellent polymer because it is highly transparent in the visible spectrum and in most of the UV spectrum, the reason for its original application in optical devices. 

Poly(azobenzene) and its derivatives have applications in optical devices [17]. A novel polyaniline containing azo groups was synthesized by the HRP-catalyzed oxidative coupling of 4,4 -diaminoazobenzene (Scheme 1). The polymerization was carried out at pH 6.0 in tris buffer with 70% yield. The polymer analyzed by GPC (80 000, PD = 4.8) was soluble in DMSO and DMF. Azo groups were detected in the main chains as well as in the side chains. Photoexcitation studies indicated that cis-trans isomerization of the chromophore may be the result of structmal constraints in the polymer [ 18). Photodynamic properties of the azo functionahzed polyaniline in their relaxed or constrained conformations were quite different. 

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]. 

Most of the phenomena described in this monograph on photoreactive organic thin films are based on the isomerization of units deliberately built into molecules, molecular assemblies, or polymers. Most especially, the spectroscopic and isomerization behavior of these units determines the switching and triggering properties of the photoreactive systems and devices. Information storage and nonlinear optical properties, as well as photo-control of equilibria and of polymer, membrane, and other properties are exploited in applications. 

A 3D optical memory device can be based on different principles. Among them are the write once and read many times principle, based on TP initiated photopolymerization resulting in a photopolymer structure [242] or photo-bleaching of a fluorescent material [250]. The rewritable principle is based mainly on TP induced molecular change. This TP induced molecular change can be an isomerization reaction in the writing process and OP induced fluorescence in the reading process [119, 144, 247, 258],... 

---