The study of photochromic compounds as erasable optical storage materials is one of the hotspots in the field of photochromism in recent years. Hershberg was the first to propose that photochromic reactions constitute a memory model that could potentially be used for data storage. Heller et al. summarized the basic requirements of photochromic compounds as erasable rewriting optical information storage materials, pointing out that there are five main problems that must be addressed in photochromic optical storage systems :(1) thermal stability at room temperature; (2) High sensitivity during optical writing and erasure; (3) Good fatigue resistance; (4) Matching of sensitive wavelength and laser; (5) Non-destructive readout. In recent years, the application of photochromic anhydride compounds in optical information storage has made great progress, and some of them have reached or approached the practical requirements. Polyanhydride has been valued for its good photochromism. For example, photochromic anhydrides can be used for information recording and holography. Photochromic 2,4, 5-triphenylimidazole dimers are used as photoinducers to produce holographic images. The cis-trans isomerization of azobenzene in liquid crystal media can be used as optical recording materials. Azobenzene covalently bonded to glass in the nematic and cholesterin phases records stable images with excellent resolution when exposed to sunlight and can therefore be used to produce holoimages26. On the basis of this research, further studies have combined photochromic molecules such as azobenzenes, spiropyranes, or carbonic anhydrides with polymers or liquid crystal polymers. This composite material is also used in non-destructive optical “readout” systems, or in holographic recording. The nonlinear optical recording process is a two-photon excitation process based on photochromic molecules. Spiropyrane has been used in non-destructive “write and read” systems. Recently, Fan Meigong et al. prepared and tested optical disc samples with pyrrole captured anhydride by glue spinning and vacuum coating, respectively. The forming and achromatic processes were irradiated with ultraviolet light and 632.8nm He-Ne lasers, respectively. After hundreds of write/erase cycles, no significant changes in photosensitivity and other properties were observed.
One of the difficult problems in practical applications is non-destructive readout. Yokoyama et al. found that the achromatic quantum yield of chromophore depends on the frequency of light wave. Using this property, non-destructive readout can be achieved, and the achromatic quantum yield is almost zero when illuminated with light with wavelengths greater than 750nm. However, the chromophores still have considerable absorption intensity in this spectral region for detection. Wilson suggested that liquid crystal anhydride be used as optical recording medium, and its anisotropy properties and the difference of refraction index before and after light provide a new detection method for the application of liquid crystal anhydride. The third method is to set up eleven photochromic compounds with achromatic thresholds. For example, picosecond flash photolysis of furanhydride shows that there is a small activation barrier in the ring-opening reaction with a giant excited state.
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