WO1988003667A1

WO1988003667A1 - Recording media incorporating complex metal azide explosives and dye-azide explosives

Info

Publication number: WO1988003667A1
Application number: PCT/US1987/002904
Authority: WO
Inventors: Paul C. P. Thomson
Original assignee: Optical Recording Corporation; Cohn, Ronald, D.
Priority date: 1986-11-07
Filing date: 1987-11-09
Publication date: 1988-05-19

Abstract

A recording medium (42) utilizing an explosive compound and a dye. The dye is selected to absorb incident light radiation (40) from a chosen laser source, thereby becoming heated and triggering the explosion of the explosive material in the vicinity of the incident light radiation, to form optically detectable indicia in the medium (42). In one embodiment, the explosive is a complex transition metal azide such as a cupric azide-amine complex. In other embodiments, the dye is modified to have a thermally unstable azide group chemically associated therewith to provide the dye's absorptive capability and the explosive feature in the same molecule. The embodiments of the invention provide improved solubility of the explosive material in the dye.

Description

RECORDING MEDIA INCORPORATING COMPLEX METAL AZ:IDE EXPLOSIVES AND

DYE-AZIDE EXPLOSIVES

BACKGROUND OF THE INVENTION

The present invention relates to the recording of binary information on laser recording media. More particularly, the invention relates to laser recording media containing thermal energy amplifying substances and the preparation of such media.

In recent years, data storage systems have been developed to store data permanently on a recording material by a beam of radiant energy. Recording with beams of high-intensity light, such as with an intensity modulated laser beam, is advantageous due to the small size of the spots which can be created.

While laser recording equipment for data storage systems has been developed to an advanced state, a precise and convenient recording media has not previously been disclosed. Conventional photographic plates have been used but are disadvantageous because they require special handling and must be chemically processed before the data can be retrieved.

Some recording media, including those that contain chemical dyes which change color in response to heat or light and those that are thin films or tapes which are perforated by pulses of intense laser light, do not require chemical development. However, to record on such media it has been necessary to use high-intensity lasers in order to generate enough energy to make recording marks. The production of high-intensity laser light is much more expensive than the production of lower intensity laser light. It is, therefore, desirable to provide a recording medium which will record information when subjected to comparatively low intnnsity laser light.

One approach to low intensity laser recording is disclosed in U.S. Patent No. 3,787,210, dated January 22, 1974, of Roberts. This patent describes a medium which employs a transparent substrate coated with heat-absorbing particles dispersed in a self-oxidizing binder. Specifically, Roberts refers to a recording film that comprises a substrate of an organic material coated with a material having heat-absorbing characteristics, such as carbon black particles, dispersed in a self-oxidizing binder such as nitrocellulose. As laser light strikes the medium, it is absorbed by the carbon black particles, and heat builds up to the point where combustion of the nitrocellulose is initiated. The combustion propels an area of the coating away from the substrate, leaving a clear spot surrounded by a dark background.

Because carbon black particles are used, the Roberts medium has a grain structure. This limits the resolution of the medium. Also, Roberts' use of a self-oxidizing binder further limits resolution and presents other problems. Nitrocellulose is a poorly defined substance formed by treating cellulose with mixtures of nitric and sulfuric acid. Widely different nitrocellulose products are obtained by varying the source of cellulose, strengths of acids, temperature, time of reaction, and the acid cellulose ratio. Exact reproduction of a particular nitrocellulose has proven to be difficult due to the numerous variables in the reaction process. Thus, a medium of the type described by Roberts would be difficult to manufacture in commercial quantities due to uniformity and other quality control problems. This is undesirable as it is important that the media obtained from different batches possess a constant sensitivity to laser light in order to ensure that the spots generated are of uniform size and can be spaced closely to achieve maximum data density.

Another approach to low intensity laser recording is described in U.S. Application Serial No. 143,827, filed April 25, 1980, of Moore et al. and its foreign counterparts, such as U.K. Patent No. 1,592,390. The described recording medium is a polymer material containing particles of reactant substance. In one example, reactant particles are made by combining a metal reducing agent with an oxidizing agent such as barium chromate to form a thermite type mixture. The reactant particles react exothermically upon exposure to laser light of sufficient intensity. This reaction chars the polymer material, thus creating dark areas which can be read by an optical digital scanner. Thus, in the recording method of Moore et al., a polymer binder is charred. Also, the reactant particles utilized in Moore et al. will produce a grain structure similar to that of the Roberts media, thus limiting resolution.

Rubner, U.S. Patent 4,287,294, discussed in context in more detail below, describes a photographic medium in which organic azides are used as photo-initiators.

de Bont et al., U.S. Patent 4,230,939, describes an information-recording element having a dye-containing auxiliary layer. It provides a transparent substrate, an auxiliary layer provided on the substrate, and a laser-reflective information recording layer provided on the auxiliary layer. This auxiliary layer or activating layer includes a laser light-absorbing dye. In the process of using the elements of the de Bont et al. patent, laser light is incident on and passes through the transparent substrate, traverses and is partly absorbed by the auxiliary layer, is then reflected against and partially absorbed by the information recording layer, again traverses and is partly absorbed by the auxiliary layer and ultimately leaves the element on the side of the substrate. The resultant temperature increases in the laser incident areas of the auxiliary layer and the information recording layer cause melting and hole formation in the information recording layer and in the auxiliary layer. The auxiliary layer may also include an endothermal material (i.e. a material capable of conversion via exothermal decomposition into thermal energy and pressure build-up) namely a polymeric endothermal material such as nitrocellulose or nitroglycerine-nitrocellulose mixtures which are used as explosive binders for the dye in the auxiliary layer. Picrates, for example the picric acid salt of the dye used in the auxiliary layer, are also suggested. However, none is specifically exemplified.

BRIEF SUMMARY OF THE INVENTION

It has now been discovered that these and other problems of the prior art may be significantly reduced, according to a first embodiment of the present invention, by providing a recording medium matrix with a photosensitive material that includes a selected metal azide complex explosive material and a compatible, appropriately light absorptive dye. The explosive material exhibits a suitably rapid exothermic decomposition at conditions achievable in localized areas under laser radiation of a wavelength and intensity commonly encountered in optical recording equipment, to cause ablation of the photosensitive layer and hence leave optically detectable indicia. The material also has the added benefit that the metal azide complex is more readily solubilized, and is solubilized to a greater extent, than a discrete azide compound.

In a second embodiment of the invention, the recording medium includes an appropriately light absorptive dye compound having an explosive group chemically associated therewith, bonded to the dye molecule ionically or covalently, optionally through a transition metal azide complex. Suitably, the explosive group is an azido group. Dye compounds having explosive azido groups ionically bonded thereto, i.e. having azido counter-ions (N₃-), can be prepared from available ionic dyes, by replacement of their normal anions (commonly but not exclusively halogen, perchlorate, etc.) with azide anion by processes of ion exchange, in a manner which does not significantly change the absorption characteristics of the dye molecule. Dye compounds having explosive azido groups non-ionically bonded thereto can be prepared from available dyes containing in their molecular structure replaceable chemical groups non-ionically bonded to the dye molecule skeleton, by chemical substitution processes which do not significantly change the absorption characteristics of the dye molecule. Dye compounds having a transition metal azide compound bound thereto to provide the explosive group in the molecule of the complex may be prepared by reaction of an amino group containing dye with a transition metal azide, e.g. cupric azide. In each case, in use, the light radiation absorbed by the dye is converted by the dye to thermal energy which causes the chemically associated explosive group to decompose exothermically, to create optically detectable indicia in the recording media .

A significant advantage of this embodiment is that the dye compound having explosive azide compounds bonded thereto is more readily solubilized and is solubilized to a greater extent than discrete azide compounds and dye mixtures. A solubility improver, which is a substance to enhance the compatibility of metal azide and dye and which by its nature reduces the inner sensibility of the recording medium, is not required.

One particularly preferred example of explosive materials containing suitable materials for use in the present invention is transition metal azide complexes, especially those having explosion temperatures above about 100° C and below about 350° C, and which are capable of giving significant energy output on explosive decomposition. Thus the photosensitive material suitably includes a molecular dispersion of a metal azide complex. Such a matrix proves to be a very precise recording medium. When a pulse of moderate intensity light, e.g. laser light, strikes such a medium, that volume of the photosensitive material which is in the path of the pulse absorbs the light energy and is heated. This causes the molecules of the reactant substance, i.e. metal azide complex, in the heated volume predictably to react exothermically. The additional heat energy and gases liberated during the reaction are sufficient to create a void or hole in the photosensitive material. Similarly, in the case of recording media which incorporate dyes having explosive groups chemically associated therewith, the dye portion of such molecule absorbs the light energy and is heated, to cause the explosive groups in the heated volume of the medium and associated with the dye molecule predictably to react exothermically, and thereby create a void or hole in the photosensitive material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A particularly advantageous medium according to the present invention includes a substrate material which is coated with a photosensitive material, and a protective transparent covering which overlies the photosensitive material. The photosensitive material is a solid solution which includes a metal azide complex and a dye. When a beam of light is directed at such a medium, light energy is absorbed by the dye which in turn transfers thermal energy to the azide. The azide explosively reacts, liberating heat and nitrogen and creates a void or a hole in the photosensitive material. The void has different optical characteristics than the surrounding material which contains the dye and thus can be read by an optical scanner (i.e. the void may be a dark spot on a light background, or for a reflective substrate, a light spot on a dark background).

The principle advantage of using complexes or dye-azide compounds, rather than discrete azide compounds, is that the solubility thereof is enhanced by the presence of organic ligand, so that the likelihood of the production of "grains" or precipitation occurring is considerably reduced.

The photosensitive material of the invention thus does not utilize "grains" but rather includes homogeneously dispersed molecules of the metal azide and the absorbing dye. Because there are no "grains", the medium has very high resolution. Some patents describe photographic media which form an image by the decomposition of organic azides. The images are formed by mechanisms that do not utilize heat released during decomposition. Moreover, the metal azide organic complexes of the present invention have different characteristics from uncomplexed metal azides. Certain organic azides have been previously used as cross-linking agents, dye couplers, or initiators for polymerization. For example, in U.S. Patent No. 4,287,294 of Rubner, organic azides are used as photo-initiators. The organic azide in the Rubner patent absorbs the radiant energy and transfers this energy to an olefxnically unsaturated polymer which initiates further polymerization or cross-linking. This is distinct from the present invention in which a void is created by heat and gas given off as a metal azide reacts exothermically. The photosensitive material in the present invention is ablated, in contrast with the charring of the polymeric binder which occurs in the method of Moore et al.

One object of this invention is to provide a digital recording medium which is able to utilize comparatively low intensity laser light as a recording source.

Another object of the invention is to provide a digital recording medium which, after being recorded upon, will provide high resolution when read by an optical scanner.

Another object of this invention is to provide a digital recording medium which, after being recorded upon, will provide a high signal to noise ratio when read by an optical read-out device.

These and other objects, advantages, and features of this invention will be apparent from the following detailed description. BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a block diagram of an analog to digital optical recording system capable of producing a permanent record on the recording medium of the present invention; and

FIG.s 2 and 3 are side, cross section views of recording media according to the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a typical device is shown for recording a binary data pattern by means of a radiant energy source, particularly a pulsed light source. The system includes a recorder unit 10 having its input connected to an audio-visual analog signal source 12, such as a microphone or television camera. This analog input signal 23 is applied to the input of an analog-to-digital signal converter 24 provided in the recorder unit 10 and which produces a digitally encoded electrical output signal 26. The output of the analog-to-digital signal converter 24 may be directly connected to an electrical optical digital signal recorder 28 through an amplifier 29 if it is desired to record the digital signal in real time simultaneously as it is generated. However, it may be desirable temporarily to store the digital signal 26 on the magnetic tape or other memory device of a digital computer 30 and to record such signals later at a more convenient time.

The electrical to optical digital signal recorder 28 converts the digital electrical signal into a digital light signal and records such light signal by scanning a pulsed light beam 40 of small spot size on a photosensitive recording medium to produce a track of digitally encoded spots which can be less than one micron in diameter. The spots are transparent in an opaque background, thus providing the ones and zeros of a binary code. The recording medium is supported, in a fashion which may be conventional, for movement in a path perpendicular to the optical axis and is mechanically coupled to a recording medium positioning mechanism 44 adapted for moving the medium to produce the track of digitally encoded spots. The recording medium positioning mechanism 44 may include a drive motor (not shown) which is energized selectively in response to signals transmitted by the optical digital signal recorder 28.

The recording medium 42 of the present invention includes a matrix interspersed with one or more explosives such as metal azide complexes or dye-azide compounds, which are stable when exposed to light of first level of intensity, but which react exothermically when exposed to light at a second, higher level of intensity.

In FIG. 2, such a recording medium is shown as an active or recording layer 50 of photosensitive material, normally including a metal azide complex and a dye, or a dye-azide compound, on a smooth surface of a substantially transparent substrate 54. Binders are not necessary when using dye-azide compounds. As described below, the active layer 50 can be a composite of two or more layers with a complex metal azide and dye in separate layers of the composite. A protective layer 56 may, optionally, be provided over the active layer 50 so that the protective layer 56 and substrate 54 protect opposite sides of the active layer from dust and other physical contaminants. Optionally, a thin reflective coating could be included on a surface 58 of the active layer 50 to reflect incident radiation 40a back into the active layer.

FIG. 3 shows a closely related embodiment for use with a beam 40b of incident radiation directed toward an active layer 60 rather than a substrate layer 64. In this embodiment, any protective layer 66 would be substantially transparent. And, if a reflective layer were used, it would best be located between the active layer 60 and the substrate 64. In either embodiment, one or more subbing layers can be provided between the substrate and the active layer or between the protective layer and the active layer for the purposes described below. Such subbing layers must be substantially transparent if they are located in. the path of the incident beam.

PHOTOSENSITIVE LAYER

A recording medium according to the first embodiment of the present invention comprises a photosensitive layer which includes an explosive material, particularly a metal azide complex, and a dye. The dye absorbs laser light, efficiently converting the light to heat which is transferred to the metal azide complex. The metal azide complex reacts exothermically, when initiated by heat from the dye, to amplify the energy from an incident radiation beam. The explosive material is preferably one which gives a high velocity of escaping gases on decomposition.

Specifically, at the location where the beam strikes the medium, the exothermal reaction of the metal azide complex causes a heat build up and the formation of a visible mark or spot in the medium.

Cupric, lead, and silver azide complexes are well suited since they react highly exotherically and yet can easily be incorporated in a recording medium. Transition metal azides complexes, especially cupric azide complexes, constitute the most preferred class of explosive compounds for use in this embodiment of the present invention.

The medium may be formed by depositing a layer containing the metal azide complex on a substrate material from a solution thereof followed by spin-drying. Alternatively. the metal azide complex may be applied directly by vapor deposition. A typical medium according to the invention will have an active layer of photosensitive material that is 0.05-2.0, preferably 0.05-0.15 microns thick.

The metal azides used in this embodiment of the present invention are explosive complexes of such metal azides. The use of such explosive complexes shows significant advantages over the use of the metal azide itself in many instances. It can adjust the explosion temperatures into the desired range of 100-350°C (preferably 150-250°C). Most importantly, however, it permits adjustment and suitable arrangement of the solubility of the explosive material, in desired solvents, in which the selected dye is also soluble, to allow ease and safety of manufacture of these materials by deposition of the active layer on the substrate from solution.

Transition metal azide compounds will form complexes with chemical groups (ligands) containing atoms having one or more spare electron pairs. In the case, for example, of such complexes with aromatic or aliphatic amine ligands, which have a spare electron pair on the N group, a suitably explosive metal azide complex may be formed. However, not all amine ligand metal azide complexes produce suitably explosive materials. Since the most preferred metal azide for use in the complexes in the compositions of the present invention is cupric azide, the suitably explosive complexes will be further discussed and exemplified with reference to cupric azide complexes, but it is to be understood that in many cases analogous complexes with other transition metal azides, particularly lead azide and silver azide, can be formed.

Since cupric azide forms a variety of complexes with nitrogen containing ligands, it is convenient to divide such complexes into two general classes, namely: non-ionic, where the copper atom is complexed with two azide groups and one or two amine groups

(e.g. Cu(Amine)₂ (N₃)₂, Cu(Amine)(N_{3 )2} or (N₃)₂Cu(Amine)Cu(N₃)₂);

and ionic, where the copper atom is complexed with three or four azide groups, and the resulting anion or dianion is associated with one or two amine cations respectively,

(e.g. [Cu(N₃)₄]2-_{[Amine R+ ]2 ,}

[Cu(N₃)₃]- [Amine H⁺] or [(N₃)₂Cu(N₃)Cu(N₃)₂]- [Amine H⁺].

Examples of suitably explosive and suitably soluble complexes can be found in both classes.

Examples of non-ionic complexes of the general structure [Cu(N₃)₂ (Amine)₂] suitable for use in the present invention are:

Complex Explosion Temperature °C

di-(methylamine) diazidocopper 180-190 di(n-propylamine) " " 187 di(ethylenediamine) " " 210 di(pyridine) " " 205 di(3-methylpyridine) " " 207-215 di(2,6-dimethylpyridine) " 202-203 di(2,4,6-trimethylpyridine) " 198-202 di(iso-quinoline) " " 197-200

Examples of non-ionic complexes of the general structure [Cu(N₃)₂ (Amine)] suitable for use in the present invention are: Complex Explosion Temperature °C

(o-toluidine) diazidocopper 123

(m-toluidine) " " 157-160

(p-toluidine) " " 135

(o-anisidine) " " 125

(1,4,5-xylidine) " " 130

The following are less preferred examples of non-ionic complexes suitable for use in the present invention due to their sensitivity to shock. They, therefore, require careful handling:

Complex Explosion Temperature °C

(2-methylpyridine) diazidocopper 204-205

(3-methylpyridine) " " 210-211

(2,4-dimethylpyridine) " " 208-209

(quinoline) " " 207-208

(pyridine) " " 202-203

(p-chloroaniline) " " 135-136

An example of non-ionic complexes of the general structure [(N₃)₂Cu(Amine)Cu(N₃)₂] is:

Complex Explosion Temperature °C

hexamethylenetetramine tetra-azidodicopper 180-185

Examples of ionic complexes of the general structure [Cu(N₃)₄] 2- _{[Amine H+ ]2} suitable for use in the present invention are: Complex Explosion Temperature °C

di-(dimethylammonium) tetraazido cuprate 210 di-(N-butylammonium) tetraazido cuprate 178-180 di-(iso-butylammonium) tetraazido cuprate 203 di-(piperidinium) tetraazido cuprate 200-205

Examples of ionic complexes of the general structure [Cu(N₃)₃]- [Amine H⁺] suitable for use in the present invention are:

Complex Explosion Temperature °C

(piperidinium) triazidocuprate 178-180 (di-isobutylammonium) " " 195-196

Examples of ionic complexes of the general structure [(N₃)₂Cu(N₃)Cu(N₃)₂]- [Amine H]⁺ suitable for use in the present invention are:

Complex Explosion Temperature °C

(dimethylammonium) pentaazido dicuprate 201

(diethylammonium) pentaazido dicuprate 184-186

(trimethylammonium) pentaazido dicuprate 201

(o-chloroanilinium) pentaazido dicuprate 210-213

(m-chloroanilinium) pentaazido dicuprate 210-213 (N,N-dimethylanilinium) pentaazidodicuprate 174

(N,N-diethylanilinium) pentaazidocuprate 198-199

Less preferred examples (due to shock sensitivity) of ionic complexes of the general structure [(N₃)₂Cu(N₃)Cu(N₃)₂]- [Amine H]⁺ are:

Complex Explosion Temperature °C

(ethylanilinium) pentaazidodicuprate 187-189 (pyrrole) " " 155-162 (quinaldine) " " 190-197 (8-methylquinoline) " 219-220

When the size of the organic component (ligand) of the complex becomes large, the complexes tend to produce less energy on decomposition. Thus it is preferable to utilize ligands that have a low molecular weight.

A further class of suitable explosives is the cobalt and nickel hydrazine complexes, for example cobalt azide hydrazine complex of formula:

Co(N₃)₂ (NH₂NH₂)

and cobalt perchlorate hydrazine complex of formula:

Co(ClO₄)₂ (NH₂NH₂)

In general, to be suitable for use in the present invention, an explosive material should be a primary explosive as opposed to a secondary explosive, and should have an explosion temperature in the approximate range 100°C-350°C, and preferably 150°C-250°C. It should have a reasonable degree of solubility in at least one organic or inorganic solvent, preferably an oxygenated organic solvent. It should be capable of generating significant amounts of heat on decomposition. In practice, the energy supplied by the incident laser radiation on making the optical record should be at least doubled by the thermal decomposition or explosion of the explosive material, and preferably is increased by at least 10 fold. It is further preferred that the granularity of the explosive material, and all other materials, in the photosensitive layer should be small, preferably less than 0.1 micron, to provide images of high optical resolution.

Advantageously, the photosensitive layer will also include a dye to facilitate absorption of energy from the energy source and conversion of the radiant energy into thermal energy. The dye should be selected for its ability to absorb energy. In the case of a medium to be written by a laser beam, the dye should be selected to absorb light at the wavelength of the irradiating light. The dye efficiently absorbs the incident radiation of the laser, and converts the radiant energy to thermal energy to trigger the decomposition of the explosive material.

The recording media, in accordance with the present invention, can be used in conjunction with light radiation in the infrared, visible or ultraviolet regions, emanating from suitable laser sources emitting in those regions. It is only necessary to choose a dye which has strong absorption characteristics at the wavelength of the chosen laser radiation. Suitable such dyes are known for use with laser radiation in the ultraviolet (250-350 nm) range, the visible (350-750 nm range) and the infrared (750-1150 nm) range.

A large number of dyes are suitable for use in conjunction with the metal azide recording medium. Those dyes which efficiently absorb the irradiating light and are soluble in suitable solvents are ideal. It is preferable to choose an explosive material and a dye which are significantly soluble in the same solvent or in mutually compatible organic solvents.

Useful dyes will be readily determinable by those skilled in the art, bearing in mind the wavelength of the selected laser for recording purposes. Identification of available dyes by their chemical class and structure, along with their radiation absorption characteristics, can be made from the available scientific literature.

For use in sensitizing a medium to the 514 nm output of an argon ion laser, the dyes erythrosin, erythrosin B, sudan III, rhodamine 6G and rose bengal are examples of suitable dyes.

For use in sensitizing a medium to the output of common, low intensity, infrared semiconductor diode laser radiation sources (780-830 nm), useful dyes for the present invention will be found among the infrared dyes of the polymethine class, the squarylium class, the quinone class, and the metal complex class.

Since the chemical structures are often complicated, the dyes are commonly known by trivial names. From the guidelines and examples given herein, the available scientific literature and skill in the art, the operator should have no difficulty in selecting and using suitable dyes for the present invention in association with selected lasers and primary explosive.

Of the polymethine dyes, dicarbocyanines and tricarbocyanines are particularly preferred.

The following are specific examples of suitable infrared dyes for use in the present invention, in connection with infrared low-intensity laser radiation sources (750-1100 nm):

IR 125, of formula

HDITC;

IR 140;

IR 132;

IR 144;

HITC (iodide or perchlorate);

DTTC;

Squarylium dyes, for example:

(Gravensteijn et al., Proceedings of S.P.I.E. 1983, 420, 327)

Quinone dyes, for example

Metal Complex dyes, for example

m = H, H , Cu, 2n, ALa , VO. λ_max 770-830nm

(All fr om K.A. Bello , "Near infra-red absorbing dyes", Ph.D thesis, University of Leeds, 1986)

In the case where the chosen dye contains one or more amine groups, it may be chemically bonded- to the transition metal azide compound, so that the dye functions as the ligand of the complex. Then the explosive group is part of the same molecule as the dye with consequent ease of energy transfer there between. Examples of such complexes are discussed below. The photosensitive material may be formed to include the dye in at least two ways. The metal azide complex and dye can be applied as separate, but adjacent layers or can be intimately mixed in a single layer.

In a two layer system, for example, one can first form a layer consisting of a metal azide complex dispersed in a polyethylene glycol binder. A dye can then be applied as a coating using a 2% solution in methanol. For example, a solution of rose bengal can be poured on a metal azide complex layer, which is then spun at 1900 RPM until dry. The resulting photosensitive material is a homogeneous red layer with an absorption coefficient of about 1.5 at 514 nm.

IR 125 dye has been used to form plates sensitive at 830 nm. As with the red dyes above, for example the IR 125 dye is made into a 2% solution in methanol and coated on a metal azide complex/binder layer. The coated material is spun at 1900 RPM until dry, and a clear green photosensitive material results. Such material has a broad absorption band covering the 750-850 nm region of the spectrum. Thus, such photosensitive material can be used to manufacture media which are sensitive to the output of semiconductor lasers operating in this wavelength region.

While the use of a dye is the most convenient way to utilize laser radiant energy to initiate an exothermic reaction in the metal azide complex, there are other possibilities. A medium would operate without a dye if the incident radiant energy was of a wavelength absorbed by the metal azide complex or by an adjacent binder material or subbing layer. Thus, suitable choice of the organic ligand attached to the metal complex will facilitate absorption of the laser radiation, by having the organic ligand itself act as the dye. A second embodiment of the invention, as noted, provides a recording medium useful for the same or similar purposes, and including an appropriately light absorptive dye having one or more explosive groups chemically associated therewith. The group may be ionically or covalently bonded to the dye molecule, in a manner which does not significantly adversely affect its light absorptive properties. Suitably, the explosive group is an azide (N₃-) group or azido group (-N₃) respectively.

Dye compounds having explosive groups ionically bonded to the dye molecule skeleton, i.e. ionic dyes with azide (N₃-) counterions, can be prepared from known, available ionic dyes by replacement of the counterion thereof (commonly but not exclusively halogen, perσhlorate, perfluoroborate and the like) by ion exchange with azide, e.g. by use of sodium or potassium azide. The azide ions retain their thermal instability character in such molecules, and the decomposition thereof is readily triggered by the heat generated when the dye portion of the molecule is irradiated, to effect the desired change in optical characteristics of the recording medium in the locality of the incident radiation. The use of such a dye-explosive counterion compound shows significant advantages over the use of separate dye compounds and explosive compounds, in that the intimate association of the dye portion and explosive portion in the same molecule leads to a more efficient and rapid energy transfer to trigger the explosive decomposition. Moveover, such dye-explosive counterion compounds can be deposited onto the substrate in a single layer, from solution, and without the necessity of the use of a binder material. Consequently, potential problems of the interference of the binder substance in the energy transfer from the dye to the explosive group are avoided.

A specific example of a useful infrared dye to which an explosive azido counterion can be added is Rhodamine 6G, having normally a chloride counterion. On treatment with an aqueous solution of sodium azide, the chloride counterion is replaced with azide counterion, thus:

Rhodamine 6G chloride

Essentially, the same process can be adopted to convert a wide range of other ionic dyes to their azide counterion form, for use in this embodiment of the present invention. Further, the process is not limited to azide counterions. Other explosive ions such as fulminates, picrates and azide/diazonium ions can also be employed in ionic dye-explosives of this aspect of the invention.

This embodiment of the invention may also provide a recording medium useful for the same or similar purposes, and including an appropriately light absorptive dye having one or more explosive groups covalently bonded to the dye molecule. In such compounds, the explosive group, e.g. azido group, retains its ability to decompose thermally when a certain elevated temperature is reached, and such elevated temperature may be reached by absorption of appropriate light radiation by the dye portion of the molecule. Such a transfer of energy from the dye to the explosive group is particularly efficient in such a system, since the absorptive dye portion and the explosive portion are bonded together in the same molecule, through covalent bonds. Such materials can be applied to the substrate by simple coating from solution. No binder is required to maintain them in place on the substrate so that there is no loss of energy to the binder material in the operation of the optical recording process.

Dyes containing explosive groups of this covalent type can be prepared. An example is an amino-substituted benzothiazole compound, which can be reacted firstly with glutaconic dialdehyde anil, then with nitrous acid and then with sodium azide, to effect substitution of covalently bonded azido group onto the molecule in place of amine, thus*:

* (cf E.D. Sych, Ukrain. Khim. Zhur. 1952, 18 148)

Incorporation of one or more azido groups during synthesis of the dye can easily be accommodated. The invention may also provide a recording medium useful for the same or similar purposes, and including an appropriately light sensitive dye-transition metal azide explosive complex in which the dye is the ligand in the transition metal azide-complex explosive. Such complexes were referred to above. A great many of the dyes with absorption characteristics rendering them useful in the present invention also contain amine groups which provide free electron pairs. Cupric azide, for example, will, therefore, bond to such a dye (utilizing a dative covalent linkage) substantially in the same manner as the cupric azide will form complex explosives materials with amine compounds disclosed previously. In this way, by a simple contacting and mixing process of solutions of the amine containing dye and the transition metal azide, dye complexes may be formed, e.g. of general formula

or Mo

where R and R are H, CH₃, etc.

Such a complex retains the absorptive characteristics of the dye and the explosive decomposition property of the azide groups, and permits ready transfer of energy from the dye portion to trigger explosive decomposition of the azide.

SUBSTRATES

Virtually any nonporous material with a smooth surface can be used as a substrate to support the photosensitive material. Glass plates of 0.060", 0.090" and 0.250" thickness have been used as have polymethylmethacrylate (PMMA), polyester, polyacetate sheets and polycarbonate sheets or films.

Substantially transparent materials would be required if it is desired to record by directing the incident beam through the substrate, as in the embodiment of Fig. 2.

In the embodiment of Fig. 3, polished metal plates or any reflective or clear plastic could be used as a substrate. Glass or a polymeric material, such as those listed above, can be plated with a material, such as gold or aluminum, to provide a substrate with a reflective surface on which to adhere the photosensitive layer.

SUBBING LAYERS

Although a metal azide layer can be formed directly on any smooth surface, in many instances it is advantageous to provide a subbing layer between the photosensitive layer and the substrate. The subbing layer can improve the bonding of' the photosensitive layer to some substrates and may have one or more additional uses.

It is difficult to form a smooth uniform layer of photosensitive material on some polymer substrates. Apparently the nature of the surface of some plastics affects the way films form. A subbing layer may serve to reduce the surface tension of the solution, resulting in a thin homogeneous coating. A subbing layer can also provide a smooth and receptive surface on which to apply the photosensitive material.

SPACER OR INSULATING LAYER

In order that the reflective layer (in Fig. 3) is protected from thermal damage, and that the heat loss from the active layer is minimized, it is advantageous to insert a subbing layer which functions as an insulating layer, between the reflective layer and the active layer, and, in some cases, between the active layer and the protective layer. Typically, this is a polymeric layer. Polymethylmethacrylate and polycarbonate layers have been used satisfactorily. Refractory materials such as silicon dioxide and zinc sulphide have also been used satisfactorily.

ANTI-REFLECTION COATING

It is also useful to provide a subbing layer on a substrate that has a reflective surface, e.g., a metallized film or plate. A subbing layer of the proper thickness will serve to maximize the reflection through a recorded spot on such a substrate due to constructive interference of the incoming and reflected beam, thus maximizing the signal to noise ratio (SNR) of the medium.

Gelatin, PMMA, polycarbonate and PVA are examples of materials suitable for use as subbing layers. Layers of such materials have been solvent coated from water solutions, a toluene solution in the case of PMMA, or a toluene methylene chloride solution in the case of polycarbonate. Any plastic or refractory material may serve as a subbing layer if it has the proper optical, thermal, and solvent characteristics.

PROTECTIVE LAYERS

A cover layer can be provided over the recording layer to protect the recording layer from scratches and to keep dust from the focal plane. Successful cover layers have been made of PMMA, PVA, polycarbonate, silicone rubber, and glass. The PMMA was solvent cast from a toluene solution. The PVA was cast from a water solution. The polycarbonate was solvent cast from a toluene/methylene chloride solution. The silicone rubber (RTV) was polymerized in place from a liquid monomer, and the glass was adhered to the recording layer using an optical adhesive. Cover layers of greater thickness (5/1000 - 47/1000 inch) comprising extruded polycarbonate sheet, cast PMMA sheet and glass have been laminated to the recording layer using optically transparent UV-curable adhesives.

If the recording beam is directed toward the active layer as shown in Fig. 3, the cover layer should be substantially transparent to the incident radiant energy beam. Recording with laser light has been performed through layers of each of the above materials. To protect the cover layer from scratching, an abrasion-resistant UV-curable coating 5-25 microns thick is used.

RECORDING

To record data on the medium, a beam of high-intensity radiant energy, such as laser light, is directed toward the medium as shown in Fig. 2 or 3. Radiant energy is absorbed by the photosensitive material, particularly any dye that is present, until there is sufficient energy to activate the exothermic decomposition of the metal azide complex.

Heat and gases released during the decomposition reaction blow a hole in the photosensitive material, which can later be read as part of a binary data pattern. When a dye coated on a reflective substrate is present, the reaction leaves a light area on a dark field. For a dye coated on a transparent substrate, the reaction leaves a dark area on a light field.

In tests of recording media according to the present invention, it has been possible to produce spots as small as 0.6 microns in diameter.

The invention is further described, for purposes of illustration only, in the following specific examples. EXAMPLE 1

Preparation of cupric azide Cu(N₃)₂ o-toluidine complex

The general synthetic procedure of A. Cirulis and M. Straumanis, J. prakt. Chem., 1943, 162, 307 was followed.

0.065 g copper sulphate (CuSO₄) was dissolved in 3 ml water. 0.167 g o-toluidine was dissolved in 3 ml methanol and added to the CuSO₄ solution. 0.083 g of sodium azide (NaN₃) was dissolved in 1 ml water. The azide solution was added dropwise while stirring. The mixture was stirred for 5 minutes then filtered. The brown needles of cupric azide o-toluidine complex were washed with water then dissolved in 5 ml concentrated ammonium hydroxide giving a 2% solution.

Cupric azide o-toluidine complex can be formed in bulk, dissolved in aqueous ammonia, mixed with a binder and solvent coated to form a homogeneous layer on a substrate. Subsequent coating with an appropriate dye dissolved in solvent renders the layer sensitive to laser radiation of choice.

EXAMPLE 2

2 ml of the cupric azide-o-toluidine complex, dissolved in ammonium hydroxide prepared as in Example 1, is mixed with 8 ml of a 2% of gelatin in water. Five drops of a surfactant, namely Photoflo (Kodak), are added to this mixture. The mixture is poured onto a 4" x 5" glass plate and spun at 1900 rpm until dry. A clear thin film of cupric azide-o-toluidine complex in a gelatin binder results, of about 0.25-0.5 microns in thickness. A dye layer can be overlaid on it, as previously described, to produce a recording medium according to the invention. EXAMPLE 3

Polycarbonate of thickness 10/1000 inch is metallized with 1000 Angstroms of aluminum. A spacer layer of polymethylmethacrylate 0.01-0.05 microns thickness is spin coated onto the metal layer from a PMMA/toluene solution. A binder layer 0.1-0.5 micron thick, consisting essentially of 8 ml 2% aqueous polyethylene glycol and 2 ml cupric azide o-toluidine/complex dissolved in ammonium hydroxide is spin coated onto the spacer layer.' The ammonia evaporates during spin coating, leaving a layer of polyethylene glycol containing a dispersion of cupric azide o-toluidine complex. A solution of IR 125, 2% in methanol, is spin coated onto the binder to give an absorbance of 1 to 2. U.V. curable adhesive is spin coated onto the dye layer, and then a transparent substrate (PC) of thickness 10/1000 inch to 47/1000 inch is laminated under pressure to the multilayer and U.V. cured.

The laminated material so prepared acted satisfactorily as an optical recording medium, using incident low intensity laser radiation from a semi-conductor laser diode.

EXAMPLE 4

Synthesis of dye containing azide counterion

1 g of Rhodamine 6G chloride dye is dissolved in 20 ml 1:1 mixture of acetone and methanol. 0.2 g sodium azide is dissolved in 2 ml water, and added slowly to the stirred, dry solution and stirred for a further 10 minutes to room temperature. Sodium sulphate is then added to dry the solution. After filtration, the solvent is removed under vacuum leaving green crystals of Rhodamine 6G azide. This compound can be solvent coated onto a suitable substrate, without a binder, to provide an optical recording material according to the invention. EXAMPLE 5

Di(Ethylene diamine) (Cu(N₃)₂) - IR-125 Coating

A substrate was formed by metallizing a polycarbonate film (4"x3"), having a thickness of 10/1000 inch with 1000 Angstroms of aluminum. A spacer layer of polymethyl methacrylate (PMMA) of a 0.01 - 0.05 micron thickness was spin-coated at 600 RPM onto the metallized polycarbonate from a 1% solution of PMMA in tolulene. Solution 1 and Solution 2 were prepared as outlined below and to 1.3g of Solution 2 was added 0.5g of Solution 1. The mixture was mixed briefly, filtered and spin-coated onto the PMMA layer of the substrate at 600 RPM to form a card. The sensitivity of the coating to writing with an 830 nm laser was increased by 1.9 times over the sensitivity of a card having IR- 125 dye alone.

Solution 1

Copper sulfate in the amount of 2.5g (10^-2mole) in 50 ml water was mixed with 1.4g sodium azide (2.15 x 10^-2 mole) in 100 ml water. The precipitate was filtered and washed with water. The filtered copper azide was dissolved in a methanolic ammonia solution (16% NH₃) containing 1.2g ethylene diamine (2.0x10^-2 mole). The total weight of the solution was 44.4g or 3.15% in Cu (N₃)₂. The solution was concentrated to 5.1% by evaporation of the excess volatiles in a stream of nitrogen. To redissolve some precipitated Cu (N₃)₂, methanolic ammonia was added (5.1g). The resulting solution was 4.2% in Cu(N₃)₂.

Solution 2

A solution of 1R-125, 0.060g in methanol, was made up with 0.225g of 20% polyethylene imine PEI 1200 (Virginia Chemicals) and 0.05g of a 2% solution of FC-170-C surfactant (3M company). The weight was adjusted to 1.3g with methanol. EXAMPLE 6

Di(diethylene triamine) - Cu(N₃)₂ - IR-125 Coating

2.5g copper sulfate (0.01 mole), 1.4g sodium azide (0.0215 mole) and 2.0g diethyenetriamine (0.02 mole) were made into a 7% Cu(N₃)₂ solution in methanolic ammonia-diethylene triamine as in Example 5.

Using the same procedure as in Example 5, 1.3g of the solution of IR-125 (Solution 2 in Example 5) was mixed rapidly with 15 drops of the 7% stock solution of Cu(N₃)₂ - diethylenetriamine to form a mixture solution. After filtration, a substrate similar to that of Example 5 was spin-coated with the mixture solution at 600 RPM. The sensitivity of the coating to writing with an 830nm laser was increased by 1.75 times.

EXAMPLE 7

Polyethylenle imine 600 - Cu(N₃)₂ - IR-125 Coating

Solutions 1 and 2 were prepared as outlined below. To 5g of Solution 2 was added lg of Solution 1. After fast stirring and immediate filtration, the resultant solution was spin-coated onto a substrate similar to that of Example 5 at 2000 RPM. The sensitivity of the coating to writing with an 830nm laser was increased by 1.4 times.

Solution 1

2.5g copper sulfate (0.01 mole) in water was added to 1.31g (0.02 mole) sodium azide in 100 ml water. Copper azide was precipitated from solution and was filtered, washed and dissolved on a filter with concentrated ammonia to give a 17.8% solution of Cu(N₃)₂. To this solution was added 5.51g polyethylene imine 600 (Virginia Chemicals), which was in a 33% solution in water. The total weight of the resultant solution was 56g (2.5% Cu(N₃)₂.

Solution 2

To 0.083g of IR-125 in methanol was added 0.071g polyethylene imine 1200 (Virginia Chemicals). The total weight of the solution was adjusted to 5g with methanol.

EXAMPLE 8

Polyethylene imine 1200 - Cu(N₃)₂ - IR-125 Coating

Solution 1 was prepared in the same manner as Solution 1 in Example 7. Solution 2 was prepared as outlined below.

Solution 2

To 0.100g IR-125 dissolved in methanol was added 2g of a 10% solution of polyethylene imine 1200 (Virginia Chemicals) in methanol and lg of a 2% solution of nonylphenolethoxylate (NPl) in methanol.

To Solution 2 (3.1g) was added 0.4g of Solution 1; after mixing and immediate filtration the filtrate was spin- coated onto a substrate similar to that of Example 5 at 600 RPM. The sensitivity of the coating to writing with an 830 nm laser was increased by 2.0 times.

Claims

WHAT IS CLAIMED IS:

1. A recording medium sensitive to light radiations of predetermined wavelength, said medium comprising a substrate and a photosensitive material deposited thereon, said photosensitive material comprising:

(a) an energy absorptive dye, and

(b) an explosive material having an appropriate temperature of explosive decomposition for use in optical recording medium and being capable of emitting significant quantities of energy upon explosive decomposition, said explosive material comprising a metal azide complex.

2. The medium of claim 1 wherein said explosive is a transition metal azide complex.

3. The medium of claim 2 wherein the explosive is an explosive having an explosive decomposition temperature in the approximate range 100° - 350° C.

4. The medium of claim 1 wherein the explosive is contained in an inert binder.

5. The medium of claim 3 wherein the explosive is a complex of cupric azide, lead azide or silver azide.

6. The medium of claim 5 wherein the explosive is a complex of said metal azide with an aromatic or aliphatic amine ligand.

7. The medium of claim 6 wherein the explosive complex is a non-ionic complex in which the metal azide is complexed with one or two amine groups.

8. The medium of claim 7 wherein the explosive complex corresponds to one of the general formulae : Cu(amine)₂ (N₃)₂;

Cu(amine) (N₃)₂; or

(N₃)₂Cu(amine)Cu(N₃)₂.

9. The medium of claim 8 wherein the explosive complex is selected from:

di-(methylamine) diazidocopper; di-(ethylenediamine) diazidocopper; di-(pyridine) diazidocopper; di-(3-methylpyridine) diazidocopper; di-(2,6-dimethylpyridine) diazidocopper; di-(2,4,6-trimethylpyridine) diazidocopper; anddi-(iso-quinoline) diazidocopper.

10. The medium of claim 8 wherein the explosive complex is selected from:

(o-toluidine) diazidocopper; (m-toluidine) diazidocopper; (p-toluidine) diazidocopper; (o-anisidine) diazidocopper; and(1,4,5-xylidine) diazidocopper.

11. The medium of claim 8 wherein the explosive complex is selected from:

(2-methylpyridine) diazidocopper; (3-methylpyridine) diazidocopper; (2,4-dimethylpyridine) diazidocopper; (quinoline) diazidocopper; (pyridine) diazidocopper; and(p-chloroaniline) diazidocopper.

12. The medium of claim 8 wherein the explosive complex is hexamethylenetetramine tetra-azidodicopper.

13. The medium of claim 6 wherein the explosive complex is an ionic complex of cupric azide, in which the copper atom is complexed with three or four azide groups to form an anion or dianion, which is in turn associated with one or two amine cations respectively.

14. The medium of claim 13 wherein the explosive complex corresponds to one of the general formulae:

[Cu(N₃)₄]2- [Amine H⁺]₂;

[Cu(N₃)₃]- [Amine H⁺]; or

[(N₃)₂Cu(N₃)Cu(N₃)₂]- [Amine H⁺].

15. The medium of claim 14 wherein the explosive complex is selected from :

di-(dimethylammonium) tetraazidocuprate;

di-(n-butylammonium) tetraazidocuprate;

di-(isobutylammonium) tetraazidocuprate;

and di-(piperidinium) tetraazidocuprate.

16. The medium of claim 14 wherein the explosive complex is selected from:

(piperidinium) triazidocuprate; and (di-isobutylammonium) triazidocuprate

17. The medium of claim 14 wherein the explosive complex is selected from:

(dimethylammonium) pentaazidodicuprate

(diethylammonium) pentaazidodicuprate

(trimethylammonium) pentaazidodicuprate

(o-chloroanilinium) pentaazidodicuprate

(m-chloroanilinium) pentaazidodicuprate

(N,N-dimethylanilinium) pentaazidodicuprate

(N,N-diethylanilinium) pentaazidodicuprate

18. The medium of claim 14 wherein the explosive complex is selected from:

(ethylanilinium) pentaazidodicuprate

(pyrrole) pentaazidodicuprate

(quinaldine) pentaazidodicuprate

(8-methylquinoline) pentaazidodicuprate

19. The medium of claim 6 wherein the explosive material is cobalt azide hydrazine complex Co(N₃)₂ (NH₂NH₂) or the nickel analog thereof, or cobalt perchlorate hydrazine complex Co(ClO₄)₂ (NH₂NH₂) or the nickel analog thereof.

20. The medium of claim 5 wherein the absorptive dye is an infrared absorbing dye of the polymethine class or sguarylium class.

21. The recording medium of claim 20 wherein the dye is a dicarbocyanine or a tricarbocyanine

22. The recording medium of claim 5 wherein the dye has an infrared radiation absorption capacity in the range 750 nm - 1100 nm, to match the wavelength of semi-conductor diode laser emission.

23. The recording medium of claim 5 wherein the energy absorptive dye sensitizes the photosensitive layer to ultraviolet radiation in the wavelength range 250-350 nm.

24. The recording medium of claim 5 wherein the energy absorptive dye sensitizes the photosensitive layer to visible light radiation in the wavelength range 350-750 nm.

25. The recording medium of claim 24 wherein the energy absorptive dye sensitizes the photosensitive material to the output of an argon ion laser.

26. The recording medium of claim 23 wherein the dye is selected from the group consisting of erythrosin, erythrosin B, sudan III, rhodamine 6G and rose bengal .

27. The recording medium of claim 5 further including a subbing layer interposed between the substrate material and the photosensitive material, said subbing layer consisting essentially of a heat insulating material, to reduce heat losses from the photosensitive material and to protect the reflective layer against thermal damage.

28. The method of recording images on a recording medium which comprises contacting a beam of laser light with a medium containing (a) a substrate material, and (b) on the substrate material, a photosensitive material comprising an energy absorptive dye capable of at least partially absorbing the incident laser light and a metal azide complex explosive material, said explosive material having an explosion temperature in the approximate range of 100°-350°C, and being capable of emitting significant quantities of energy upon explosion, the dye and the explosive material being present in amounts sufficient that the beam causes the explosive material to react exothermically at the location of contact, form a void in the photosensitive material, thereby leaving a light area on a dark field, which area can be read by an optical readout device.

29. The method of forming a medium to record images, which comprises:

providing on a substrate material, a photosensitive material which contains an energy absorptive dye and a metal azide complex explosive material which has an explosive decomposition temperature in the approximate range 100° - 350°C and which is capable of emitting significant quantities of energy upon explosive decomposition.

30. A recording medium sensitive to laser light radiation of predetermined wavelength, said medium comprising a substrate and a photosensitive material disposed thereon, said photosensitive material comprising an appropriately light absorptive dye compound, having an explosive group chemically associated therewith and bonded to the dye molecule ionically or covalently.

31. The medium of claim 30 wherein said light absorptive dye compound has one or more explosive groups ionically bonded to the dye skeleton, as a counterion thereto.

32. The medium of claim 30 wherein said explosive group is the azide anion (N₃-).

33. The medium of claim 32 wherein the dye-azide counterion compound is Rhodamine 6G azide, of formula:

34. The medium of claim 30 wherein the light absorptive dye compound has one or more explosive groups covalently bonded to the dye skeleton.

35. The medium of claim 34 wherein the explosive group is an azido group -N₃.

36. The medium of claim 35 wherein the dye-azide compound is a di(benzothiazole) dye compound having -N₃ groups covalently bonded thereto.

37. The medium of claim 30 wherein said light absorptive dye compound comprises an appropriately light sensitive dye-transition metal azide explosive complex.

38. The medium of claim 37 wherein the light sensitive dye has amino groups in or attached to its molecular skeleton, to which the metal azide can bond.

39. The medium of claim 38 wherein the transition metal azide of the complex is cupric azide.