Optical Data Storage using Dyes

The mechanism of optical recording with organic dyes involves converting the excitation energy, generated by the focused laser beam, into thermal energy by non- 

As well as meetiug the uormal commercial requiremeuts, i.e. cost aud availability, dyes for use iu optical data recordiug ueed to meet a set of techuical criteria  

Reflectivity, a crucial property, is related directly to the refractive iudex of a dye film, which itself is fuudameutally related to the molar absorptiou coefficieut of the dye. It 

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Other dye-based technologies evaluated for optical data storage include photo-chromic dyes for rewritable systems and azo dyes for holographic data storage. Spirobenzothiopyran dyes such as (40) absorb in the red/near-IR in their colored form and are suitable for erasible optical data storage [34]. Dyes for holographic data storage, such as (41), are similar to those used in nonlinear optics [35] (see below). 

Several methods of optical data storage using LCPs are known."" " The preferred thermo-optical system uses a thin (5 to 7 pm) film of a high (60 to 80°C) polymer which has a subsequent liquid crystal phase (either nematic or smectic) up to 20 to 30°C above the Tg. Either bonded to the polymer backbone or simply dispersed in the polymer is a dye, chosen to absorb the laser light used to write on the film. 

Photochromic Organic Dyes. Intensive investigations into this category of substances have led to numerous patent appHcations. Copper—phthalocyanine pigments, organic dyes based on cyanine (Ricoh, Pioneer), naphthochinone (Nippon Denki), and ben2othiopyrane (Sony) (123) have been described. They did not lead, however, to any commercial use. Surveys on the possibiUties of optical data storage with photochromic dyes can be found (124,125). 

The chapters cover the following areas (i) use of coordination complexes in all types of catalysis (Chapters 1-11) (ii) applications related to the optical properties of coordination complexes, which covers fields as diverse as solar cells, nonlinear optics, display devices, pigments and dyes, and optical data storage (Chapters 12-16) (iii) hydrometallurgical extraction (Chapter 17) (iv) medicinal and biomedical applications of coordination complexes, including both imaging and therapy (Chapters 18-22) and (v) use of coordination complexes as precursors to semiconductor films and nanoparticles (Chapter 23). As such, the material in this volume ranges from solid-state physics to biochemistry. 

These dyes are used predominandy as photographic spectral sensitizers, but some years ago a new application in optical data storage media was found. Compound 10 dyes paper in brilliant pink shades [6] ... 

Cationic dyes which absorb in the near-infrared region of the spectrum are used in optical data storage disks, e.g., 34 [32] ... 

Cyanine dyes (3) are the best known polymethine dyes. Nowadays, their commercial use is limited to sensitizing dyes for silver halide photography and as infrared absorbers for optical data storage and other (bio)imaging applications. However, derivatives of cyanine dyes provide important dyes for polyacrylonitrile. 

Optical data storage media fall into three main classes [26] (Figure 6.9). Read only media comprise the CD in its audio and CD-ROM formats and DVD, also of the CD format but having about six to ten times the data capacity of a CD-ROM, depending upon whether one or two recording layers are used. These read only media are mainly dedicated to entertainment. As seen from Figure 6.9, the systems which use organic dyes are WORM (write-once-read-many), CD-R, and DVD-R. 

Dithiolene complexes, and more specifically the nickel derivatives, are involved in materials used for optical data storage, such as compact disc or laser disc read-only memory (CD- or LD-ROM), and also in copiers or photography related devices. In the latter case, it is the IR-absorbing property that is exploited. Some of these compounds can be found in reviews by Mueller-Westerhoff et al. (353, 354). Recent patents are given as examples in references (458—481). In the field of optical storage, short reviews were published in 1988 (482) and 1990 (362, 483). Here, the dithiolene complexes act as inhibitors of the laser-induced fading of the colored thin layers of the optical discs. They also act as an antioxidant and increase the photostability of the cyanine dyes that constitute the recording layer. Contrary to what was observed for the two... 

Second, in addition to the above, the fact that many expanded porphyrins are highly colored makes their use as dyes an obvious possibihty. Here their planar nature makes them particularly attractive as chromophores for use in liquid crystals and optical data storage applications. Also, these properties could make them of interest as photo-sensors in various clinical or pseudo-clinical situations. For instance, the high affinity by certain sapphyrins for enveloped viruses and cholesterol rich liposomes suggests that expanded porphyrins could be used to detect and/or destroy a variety of unwanted biological targets, including arterial sclerotic plaque. 

Due to possible utilization of photoinduced orientation in polymeric films in optical data storage, this phenomenon and the quadratic nonlinear optical effects were extensively investigated in the last few years. It was reported, for instance, that to study photoisomerization in a polymeric environment, a series of polymers containing azo dyes with large differences in the second order transition temperature were compared. Particular emphasis was placed on the relationship between photoisomerization, Tgof the polymers, and their molecular structure. As a result, it was shown that light-induced nonpolar orientation in very high Tg polyimides (Tg up to 350 °C) can take place even at room temperature. The polymers used in one of these studies can be illustrated as follows... 

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