1. Field of the Invention
The present invention relates to a photochromic material used as various kinds of recording material, displaying material, photographic printing material, optical filter, photometer, masking material and the like. In addition, the present invention relates to an optical recording medium used in for example an optical disk recording and regenerating apparatus (hereinafter referred to singly as recording and regenerating apparatus), in particular to a rewritable optical recording medium capable of not only writing in and reading out informations by the use of laser beams but also rewriting informations.
2. Description of Related Art
An optical disk, which is a disk optical recording medium used in a recording and regenerating apparatus usually has recording function but an addition-type optical disk incapable of erasing recorded informations after recording them and a rewritable optical disk capable of erasing recorded informations after recording them to record other informations again have been known. The recording and regenerating apparatus using such the optical disks have been usually used as an outside memory of an electronic computer.
The addition-type optical disk comprises a substrate made of glass or plastic and a metallic vapor deposited layer, such as a tellurium (Te) vapor deposited layer, having a low melting point and formed on the substrate as a recording medium. In a recording and regenerating apparatus using this, the addition-type optical disk is rotated at a high speed to collect laser beams optically modulated depending upon informations to be written in onto the metallic vapor deposited layer as a light absorptive layer, whereby subjecting the metallic vapor deposited layer to a thermal processing, in short the metallic vapor deposited layer is punched to write in the informations. In the addition-type optical disk, the informations are written in the metallic vapor deposited layer by the thermal processing, as above described, so that it is impossible to erase the informations recorded once.
On the other hand, in the rewritable optical disk, a photomagnetic effect, for example Kerr effect, and a crystalline-amorphous phase transition have been utilized according to circumstances. For example, in a recording and regenerating apparatus using the rewritable optical disk utilizing the phase transition, the rewritable optical disk provided with a tellurium family amorphous metallic layer formed as the recording medium on the surface of the substrate is rotated at a high speed to collect laser beams optically modulated depending upon informations to be written in onto the amorphous metallic layer, whereby producing the phase transition in the amorphous metallic layer to write the informations in the amorphous metallic layer. And, in order to erase the recorded informations, the phase transition is produced again in the amorphous metallic layer by the utilization of a heat of laser beams to return the amorphous metallic layer to the original condition (Japanese Patent Application Laid-Open No. Sho 62-165749 and Japanese Patent Application Laid-Open No. Sho 62-165750). As above described, in case of the conventional rewritable optical disk, the writing in piles can be conducted by merely irradiating with lights and characteristics of a noncontact optical recording can be perfectly kept, which are preferable.
A photochromic material composing such the rewritable optical recording medium is a material, which is colored by irradiating ultraviolet rays and decolorized by an irradiation of visible rays and a heat, and is made by dispersing a photochromic compound in a solution or a high molecular medium or carrying the photochromic compound on the high molecular medium by a chemical bond. It has been known that in particular the material made by carrying the photochromic compound on the high molecular medium by a chemical bond can suppress an elusion of the photochromic compound from the medium and thus a heat stability of a colored body can be improved [refer to for example Kobunshi Ronbunshu., 33, 649 (1976)].
In addition, it has been disclosed in Mol. Cryst. Liq. Cryst., 1988, Vol. 155, pp 221-230 that a side chain-type high molecular liquid crystal photochromic material exhibits a photochromism and a thermophotochromism. On the other hand, sulfur-containing spiropyran has been proposed as a photochromic compound having a sensitivity to a near infrared range [refer to for example Journal of Physical Chemistry., Vol. 72, No. 3 (1968), pp. 997-1001]. This photochromic compound is reversibly colored and decolorized in a photon mode.
The photochromic material usable in the rewritable optical recording medium must be superior in repeated applicability of at least colorless.fwdarw.colored and colored.fwdarw.colorless and have an increased color-changing speed when irradiated with a light not less than a threshold value but not be discolored when irradiated with a light less than the threshold value and superior in preservative stability under the colorless condition and the colored condition, so that it is limited by a spirobenzothiopyran derivative compound. However, the spiropyran compound usually exhibits the following basic problems:
1. The sulfur-containing spiropyran is remarkably inferior in durability to the repeated application of coloring and decolorization and the colored body is unstable to heat. In addition, in the case where sulfur-containing spiropyran is used for the optical recording material, the recorded informations are broken by a reading-out light according to circumstances because of the photon-mode recording.
2. The spiropyran compound is crystalline under the monomeric condition and a coating is associated with it, so that a resolution power of the optical recording medium (as for the optical disk, a diameter of 1 bit is about 1 .mu.m) can not be sufficiently enhanced.
3. The colored spiropyran compound is usually thermally unstable to be isomerized into a colorless body, so that it is insufficient in preservative stability.