Optical fading

As mentioned in sections 1.2.2.2 and 1.2.3.2, the photochromic reactions of spirobenzopyran and spironaphthoxazines show a marked solvent dependency and this is also the case with benzo and naphthopyrans. Consequently, spectral data collected from the literature is only comparable within any one study or where the same solvent has been used. This accounts for any discrepancies between one set of results and any other one listed in this and related sections of this chapter. The data normally quoted when discussing the properties of photochromic materials relate to the absorption maximum (2. ) of the coloured state, the change in optical density (absorbance) on exposure to the xenon light source (AOD) and the fade rate which is the time in seconds for the AOD to return to half of its equilibrium value. 

The use of photochromic compounds for optical data storage has been proposed. One of the unsolved problems for practical application is the development of techniques that allow nondestructive reading since the optical data are usually read at the same wavelength as that used in the recording process, the data may fade during the reading process. Sisido et al. 101-104 have proposed the use of photochromic... 

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... 

Practical applications of photochromism at first (ca. 1955-70) concentrated on the spiropyrans, and especially on BIPS compounds, because of their ready availability, photosensitivity, convenient thermal fade rates, and good color contrast when perceived by the human eye. However, applications in which the dye was required to cycle very many times (e g., an optical binary switch for a photochemical computer memory) or be irradiated continuously (e g., sunglasses, vehicle windshields) were impractical because of the rapid fatigue of these dyes.196... 

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