JPS6398493A - Optical recording medium - Google Patents

Abstract

PURPOSE:To obtain an optical recording medium free of deterioration of light resistance even after long-time preservation and having excellent reliability and mass-producibility, by providing a recording layer comprising a cyanine coloring matter and stable radicals, on a base. CONSTITUTION:The weight ratio of a cyanine coloring matter to stable radicals is set in the range of 1:0.4-1.2. If the amount of the stable radicals is below the range, deterioration of the light resistance of a recording layer becomes conspicuous, whereas if the amount of the stable radicals is too large, the reflectance of the recording layer is lowered, and C/N haracteristic is deteriorated. The cyanine coloring matter and a stable-radical material are dissolved in a solvent so that the amounts of the coloring matter and the stable radicals are in the weight ratio range, and the resultant solution is applied to a transparent base 2 to provide the recording layer 1. The recording layer 1 is irradiated with laser light L from the side of the base 2 to melt, decompose of sublime the cyanine coloring matter, whereby record pits are formed. Reproduction of recorded information is performed by irradiating the recording layer with the laser light L from the base side, and reading an information signal as a difference in reflectance between pit areas and space areas. Since the cyanine coloring matter has a high absorptivity and a high reflectance in the wavelength region of the laser light, the optical recording medium thus obtained shows favorable optical recording and reproduction characteristcs, and the addition of the stable radicals enhances light resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an optical recording medium typified by an optical disk, and particularly to improvements in so-called organic dye-based write-once optical recording media.

[Summary of the invention]

The present invention aims to improve the light resistance of an optical recording medium having an optical recording layer formed on a substrate by composing the optical recording layer with a cyanine dye and a stable radical.

[Conventional technology]

Optical recording methods have the characteristics of being able to record and play back without contact, being easy to handle, and being resistant to scratches and dust.
Furthermore, since it has the advantage of having a storage capacity several tens to hundreds of times larger than magnetic recording methods, it is expected to be used for large-capacity files such as code information and image information.

Among these, there is a high level of interest in so-called write-once optical recording media that users can directly read after writing, and in order to meet the demands for high sensitivity, high stability (chemical and mechanical), and high quality, research on recording materials is particularly important. is being actively carried out.

By the way, the recording material used in this type of optical recording medium is required to have appropriate light absorption coefficient and reflectance, and conventionally, metal or metal alloy thin films such as Te and Ti, metal compound thin films such as Teox thin film, silver lead thin film, etc. Thin films, etc. were used. However, since these metal thin films are formed using vacuum thin film forming techniques such as sputtering and vacuum evaporation, there are problems in terms of mass production, and the resulting optical recording medium is expensive. there were.

Under these circumstances, organic dye materials are used as recording materials,
Optical recording media have been proposed in which a recording layer is formed by coating means. In this way, optical recording media using organic dye materials as recording materials can be said to have practicality in terms of mass production and product cost, since the recording layer can be coated by means such as a spin code.

These organic dye-based optical recording media include two types: one in which a reflective film is provided between the substrate and the recording layer and the difference in reflectance on the reflective film is read as a signal, and the other type in which the difference in reflectance on the reflective film is read as a signal. There is a type that reads directly through a transparent substrate, but in the former, the reflective film must be formed by a method such as vapor deposition, which reduces the process advantages, and the underlying reflective film has high thermal conductivity. Therefore, it has drawbacks such as large energy loss and the need to increase the output of the laser for irradiation more than necessary.

Therefore, since dyes with high reflectance have appeared due to advances in dye synthesis technology, optical recording media in which the reflectance of a recording layer containing the organic dye can be directly read through a transparent substrate have been actively employed.

[Problem that the invention seeks to solve]

By the way, the above-mentioned optical recording medium utilizes the fact that the temperature of the recording layer in the irradiated area increases rapidly when the organic dye absorbs laser light. It forms recording pits. Therefore, the organic dye material used must be able to effectively absorb the laser light in the wavelength range of the laser light,
At the same time, it is required to have properties such as good reflection of laser light in this wavelength range and solubility in an appropriate solvent.

Cyanine dyes have large absorption in the near-infrared region, which is the wavelength range of semiconductor lasers, for example, and also have high reflectance in the near-infrared region, so they are considered promising as recording materials for write-once optical recording media. .

However, cyanine dyes generally have poor light resistance, and there remains concern about their reliability as recording media when long-term storage of information is assumed.

Therefore, the present invention was proposed in order to solve the above problems, and provides an optical recording medium that does not deteriorate in light resistance even when stored for a long period of time, has excellent reliability, and is excellent in mass production. The purpose is to provide.

[Means for solving problems]

In order to achieve the above object, the present inventors have conducted intensive research and found that a recording layer that combines a cyanine dye and a stable radical has a reflectance and light intensity in a predetermined wavelength range of laser light. We have come to the conclusion that since the absorption coefficient is large, the light resistance does not deteriorate over a long period of time.

The optical recording medium of the present invention was completed based on the above findings, and is characterized by forming a recording layer containing a cyanine dye and a stable radical on a substrate.

The cyanine dye used in the recording layer in the present invention is
The following formula (1) (where n represents the number of methine chains and is a natural number,
I? ,, R2 is an alkyl group, and X- is Cj! -, Br-
.

and Cβ04-, respectively. ), and has a structure in which nitrogen-containing heterocycles are bonded to both ends of a methine chain. That is, it represents the above-mentioned nitrogen heterocycle, and examples of such nitrogen-containing heterocycle include i) ii) and the like. Note that these nitrogen-containing heterocycles may have some of their hydrogen atoms substituted with an alkyl group or the like.

Specific examples of such cyanine dyes include, for example: etc.

On the other hand, the stable radicals used in the recording layer in the present invention have the effect of stabilizing the cyanine dye and improving light resistance. Specifically, (a) unpaired electrons pass through the conjugated system to Something that can be delocalized. For example, (b) Recombination of radicals is subject to steric constraints. For example, (1) tetramethyl-1-piperidinyloxy0.

The above-mentioned stable radicals have good compatibility with cyanine dyes and excellent film-forming ability, so they are uniformly dispersed in the cyanine dyes.

Here, the weight ratio of the cyanine dye and stable radical (cyanine dye bistable radical) is (1:Q, 4
) to (1:2). That is, if the amount of stable radicals is less than the above range, the light resistance of the recording layer will deteriorate significantly, whereas if the amount of stable radicals is too large, the reflectance of the recording layer will decrease and the C/N characteristics will deteriorate, which is not preferable.

In the optical recording medium of the present invention, as shown in FIG. 1, the cyanine dye and stable radicals are dissolved in a solvent so as to have the composition range described above, and then coated on a transparent substrate (2) to form a recording layer. (1) is formed. To record information signals on the obtained optical recording medium, the recording layer (1) is irradiated with laser light focused to a diameter of several μm from the transparent substrate (2) side, and the cyanine dye is melted, decomposed, and sublimated. and form recording bits.

Also, during reproduction, a laser beam of a predetermined wavelength is irradiated from the substrate (2) side in the same way, and the information signal is read based on the difference in reflectance between the focused area and the space area (the area where no focus is formed). The laser light emitted is
It may be selected appropriately depending on the absorption wavelength of the recording material (i.e., cyanine dye). Specifically, semiconductor laser, C
Examples include a 02 gas laser, an Ar gas laser, a He-Ne gas laser, a ruby laser, and a dye laser.

The substrate (2) may be of any type as long as it is used for ordinary optical discs, and preferably is a transparent substrate having rigidity such as a glass substrate or a plastic substrate such as polycarbonate resin.

Solvents used to form a recording layer by coating cyanine dyes and stable radicals include alcohols such as methanol, ethanol, and isopropyl alcohol, ketones such as acetone, methyl ethyl ketone, and cyclohexanone, ethyl ether, Ethers such as dioxane and tetrahydrofuran, ethyl acetate, n-acetic acid
Esters such as butyl, as well as Hensen, toluene,
xylene, n-hexane, cyclohexane, acetonitrile, dimethylformamide, dimethyl sulfoxide,
Examples include various general-purpose solvents such as chloroform and mixed solvents thereof.

Furthermore, as a coating method, usual methods are used, such as spin-coating method, spray coating method, roll-coating method, disopioning method, etc. Among them, spin coating method is used because of its excellent uniformity of the coating film obtained. The law is preferred.

[Effect]

Since the cyanine dye used as the recording material of the optical recording medium of the present invention has a high optical absorption coefficient and reflectance in the wavelength region of laser light, the resulting optical recording medium exhibits good optical recording and reproducing characteristics.

Furthermore, although cyanine dyes still have problems with light resistance, the light resistance can be improved by adding stable radicals.

Generally, oxidation of organic compounds (CyH) such as cyanine dyes proceeds through the following process.

In other words, the radical (CyOO・) that reacted with oxygen is
The reaction proceeds in a chain manner by abstracting hydrogen from the organic compound (CyH).

hν CyHe cy・+H・cy・+02−1→cyoo・cyoo・+ cyn -ncyoon +(:y
・Here, when a stable radical is added, this stable radical recombines with the radical (Cyoo・) generated by the reaction of the cyanine dye with oxygen, thereby stopping the above chain reaction, and as a result, the fading of the dye occurs. It is thought that this will be suppressed.

〔Example〕

The present invention will be described below based on specific examples.

Creation Example 1 First, cyanine dye (manufactured by Nippon Kanko Shokuryo Co., Ltd., trade name NK-125) and BDPA (stable radical) were weighed at a weight ratio of (2:1), and then the cyanine dye was Two types of solutions were obtained by dissolving each in methyl ethyl ketone at a concentration of 1% by weight. Thereafter, each solution was applied onto a glass substrate by spin coating and dried to produce an optical recording medium (sample 1) with a recording layer having a thickness of 1000.

The structure of the cyanine dye used in this example is shown by the following formula (II).

Preparation Example 2 In Preparation Example 1 above, the cyanine dye and BDPA were weighed so that the weight ratio was (1:1), and the rest was the same as Preparation Example 1.
An optical recording medium (sample 2) was prepared in the same manner as described above.

Preparation±U In Preparation Example 1 above, DPPA was used as a stable radical, and the weight ratio of cyanine dye and DPPA was (2
An optical recording medium (Sample 3) was prepared in the same manner as in Preparation Example 1 except that: 1).

In Example 1 of creating a swindler, DPPA is used as a stable radical, and the weight ratio of cyanine dye to DPPA is (1).
An optical recording medium (Sample 4) was prepared in the same manner as in Preparation Example 1 except that: 1).

Preparation Example 5 For comparison, an optical recording medium (Sample 5) was prepared in the same manner as in Preparation Example 1, except that the recording layer was formed using only the cyanine dye.

Next, each of the samples was irradiated with light of 10,000 lux using a tungsten lamp, and the state of deterioration of the absorbance of the recording layer at this time was examined. Figure 2 shows the results when BDPA is used as a stable radical (curve 1 in the figure), when DPPH is used as a stable radical (curve m in the figure), and when no stable radical is contained (curve n in the figure). are shown respectively.

As is clear from FIG. 2, it was found that by incorporating stable radicals into the recording layer, the fading of the cyanine dye was suppressed.

Further, the decomposition rate (ko/k) of the cyanine dye in each of the above samples is summarized in Table 1.

In addition, in Table 1, the decomposition rate (k) of each optical recording medium is shown.

/k) indicates the relative decomposition rate when the decomposition rate of the comparative example is set to 1.

(The following is a blank space) Table 1 As is clear from Table 1, it was found that by incorporating stable radicals into the recording layer, the lifespan of cyanine dyes could be extended by about 5 times.

In particular, optical recording media whose recording layers are composed of cyanine dyes and stable radicals have significantly improved light resistance, and can thus maintain the reliability of information signals for a long period of time.

Further, for the optical recording media of Samples 1 to 4, the writing μm laser light output was 8 mW.

A predetermined signal was written at a linear velocity of 2 m/sec and a modulation frequency of IM/Hz, and reproduced with a readout laser light output of 0.21. When the C/N of the reproduced signal was examined, it was found to be 50 dB or more (
Maximum 55. dB). As a result, it was found that the optical recording medium of the present invention exhibited recording and reproducing characteristics with little noise and high practicality.

〔Effect of the invention〕

As is clear from the above explanation, in the optical recording medium of the present invention, since the recording layer uses a dispersion system of cyanine dye and stable radical, the light resistance is significantly improved, and at the same time, the C/N will also be significantly improved.

Therefore, since the reliability of the information signal can be maintained for a long period of time, an extremely practical optical recording medium can be provided.

In addition, the optical recording medium of the present invention has a recording layer formed by a coating technique that has been widely used in the past, so it is advantageous in terms of mass production and productivity, and it is said that manufacturing costs can be reduced. It also has advantages.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a main part showing the structure of an optical recording medium. FIG. 2 is a characteristic diagram showing the deterioration rate of the recording layer as a change in absorbance over time. 1...Recording layer 2...Substrate

Claims (1)

[Claims] An optical recording medium characterized in that a recording layer containing a cyanine dye and a stable radical is formed on a substrate.