Reversible optical data-storage

The application of nonlinear optical recording techniques for reversible optical data storage based on the excitation of photochromic molecules by two-photon processes also has been described (154). 

Solid films of spiropyrans are important in optical data storage. Thin films of spirobenzopyran (1.0 (tm) have been prepared by vacuum deposition, and its reversible photochromism has been confirmed.39 The J-aggre-... 

The problem with the application of these changes in optical properties for optical data storage is that the significant drop in reflectivity upon the first UV irradiation is related to an irreversible structural change. So the film can be used a as write-once medium only. Furthermore, the sensitivity of the system is low, so writing the information takes a rather long time. The changes in the optical properties in subsequent irradiation cycles are reversible, but they are connected with the unstable cis-isomer, which renders the inscribed information volatile. 

Feringa, B. L., Jager, W. F., Delange, B., Organic Materials for Reversible Optical Data Storage, Tetrahedron 1993, 49, 8267 8310. 

One of the most interesting photoresponsive properties of azo polymers is the photoinduced birefringence and dichroism (Xie et al., 1993). The photoinduced anisotropy is caused by the disparity of the repeated trans-cis isomerization of azo chromophores under linear polarized light irradiation. The most efficient excitation occurs in the polarization direction, which can force the chromophores to continually change their orientation and to be eventually stabilized at the direction perpendicular to the polarization (Natansohn and Rochon, 2002). The effect shows potential applications in areas such as reversible optical data storage, optical switching and sensors. 

The idea to use this effect for reversible optical data storage goes back to Todorov [77,78] and Rings-dorf [79], which is now addressed as photo-mode recording. The advantage of photo-mode recording over heat-mode recording lies in superior resolution. 

Tetrasubstituted alkenes 118, which contain two ring systems, can be efficiently synthesized by the domino-Heck double cyclization of aryl bromide 117, appended with an allylsilane and alkyne (Scheme 58) (108). As a result of their remarkable switching properties under UV light, these products are crucial elements for the development of reversible optical data storage. 

It has been demonstrated that the photo-induced orientation of dye-containing SCLCPs can be used for reversible optical data storage [85 90]. FTIR polarization spectroscopy has been applied to monitor the orientation of the main chain and the mesogenic side-chains which are of critical importance in these storage processes [15, 91-94]. Here, some results obtained by laser irradiation of a homologous series... 

Figure 2-31. General structure and nomenclature (PnRm) of the investigated SCLC polyesters used for reversible optical data storage (e.g., P6al2 polyester with 12 CH2 groups in the acid part of the main chain and six CH2-groups in the side-chain spacer and a CN-substituent on the mesogen).

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