PROCESS FOR OPTICALLY STORING INFORMATION

METHODE DE STOCKAGE OPTIQUE D'INFORMATIONS

English Abstract

FM/6-18881/A
Process for optically storing information
Abstract of the Disclosure
A process for optically recording information on a recording material comprising a
substrate coated with a layer of a dithioquinacridone, between which substrate and the
dithioquinacridone layer or on said layer there may be provided a further layer of an
organic hydrazone, or the dithioquinacridone layer is mixed with a hydrazone. The
information is recorded by iradiation with laser light in the NIR range and can be read out
once or repeatedly by measuring the shift in the absorption by the transmission or
refection method. The system is an information carrier of WORM type.

Claims

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What is claimed is:
1. A process for optically recording and storing information in the form of bits, wherein a
recording material comprising a substrate coated with at least one layer of a dithioquin-
acridone as recording layer is irradiated point by point or linearly with laser light in the
near NIR range.
2. A process according to claim 1, wherein the substrate is selected from the group
consisting of metals, alloys, glass, minerals, ceramics and thermoset and thermoplastic
materials.
3. A process according to claim 2, wherein the substrate is selected from glass and homo-
and copolymeric plastics materials.
4. A process according to claim 3, wherein the plastics materials are selected from the
group consisting of thermoplastic polycarbonates, polyamides, polyesters, polyacrylates
and polymethacrylates, polyurethanes, polyolefins, polyvinyl chloride, polyvinylidene
fluoride, polyimides, thermoset polyesters and epoxy resins.
5. A process according to claim 1, wherein the substrate has a thickness of 0.01 to 1 cm.
6. A process according to claim 1, wherein the dithioquinacridones are of formula I or Ia
or mixtures of said dithioquinacridones
<IMG> (I),

- 13 -
<IMG> (Ia),
wherein R1 is F, Cl, Br, C1-C8alkyl or C1-C3alkoxy, and n is 0 or 1, 2 or 3.
7. A process according to claim 69 wherein R1 is -F, -Cl, -Br, -CH3 or CH3O- and n is 0 or
1.
8. A process according to claim 1, wherein the layer consists of dithioquinacridone.
9. A process according to claim 1, wherein the dithioqninacridone layer has a thickness of
100 to 5000 .ANG..
10. A process according to claim 1, wherein the dithioquinacridone layer has a thickness
of 200 to 3000 .ANG..
11. A process according to claim 1, wherein the dithioquinacridone layer or the substrate
is coated with a reflective layer.
12. A process according to claim 11, wherein the reflective layer has a thickness of 100 to
5000 .ANG..
13. A process according to claim 1, wherein the reflective layer consists of a metal of the
third, fourth and fifth main group and the subgroups of the Periodic Table of the Elements.
14. A process according to claim 13, wherein the reflective layer is of aluminium or gold.
15. A process according to claim 1, wherein the topmost layer is coated with a protective
layer.
16. A process according to claim 1, wherein a further layer of an organic hydrazone is
provided between the substrate and the dithioquinacridone layer or on the dithioquinacri-

- 14-
done layer.
17. A process according to claim 16, wherein the layer consists of a pure hydrazone or is
in the form of a solid solution of the hydrazone in or as a homogeneous mixture with a
transparent binder.
18. A process according to claim 16, wherein the layer of a pure hydrazone has a thickness
of 50 to 2000 .ANG..
19. A process according to claim 17, wherein the layer of the solid solution or mixture has
a thickness of 0.1 to 100 µm.
20. A process according to claim 17, wherein the layer of a hydrazone and a binder is the
transparent substrate of the recording material.
21. A process according to claim 1, wherein the recording material consists of at least one
layer of a homogeneous mixture of a dithioquinacridone and a hydrazone.
22. A process according to claim 16, wherein the hydrazone contains 6 to 40 carbon
atoms.
23. A process according to claim 16, wherein the hydrazone has the formula II
<IMG> (II)
wherein R2 is H, C1-C6alkyl or phenyl which is unsubstituted or substituted by F, Cl, Br,
C1-C6alkyl, C1-C6alkoxy, di(C1-C6alkyl)amino, diphenylamino, dibenzylamino,
phenylbenzylamino, (C1-C6alkyl)phenylamino or (C1-C6alkyl)benzylamino,
R3 is phenyl, naphthyl, anthryl, styryl, pyridyl, furyl or thiophenyl which are unsubstituted
or substituted by F, Cl, Br, C1-C6alkyl, C1-C6alkoxy, di(C1-C6alkyl)amino, diphenyl-
lamino, dibenzylamino, phenylbenzylamino, (C1-C6alkyl)phenylamino or (C1-
C6alkyl)benzylamino, or R2 is H and R3 is a carbazole radical of formula

- 15 -
<IMG>
which is unsubstituted or substituted by F, Cl, Br, C1-C6alkoxy or
di(C1-C6alkyl)amino, and R6 is C1-C6alkyl,
R4 and R5 are each independently of the other C1-C6alkyl, phenyl, naphthyl or benzyl, or
phenyl, naphthyl or benzyl which are substituted by F, Cl, Br, C1-C6alkyl, C1-C6alkoxy or
di(C1-C6alkyl)amino,
and n is 0 or 1.
24. A process according to claim 16, wherein the hydrazones have the formulae
<IMG> ,
<IMG> ,
<IMG> ,
<IMG> ,
(C6H5)2N-N=CH-CH=C(p-CH3O-C6H5)2 .
25. A process according to claim 1, wherein the material has the structure comprising (a)
transparent substrate, (b) recording layer and (c) transparent protective layer.

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26. A process according to claim 1, wherein the material has the structure comprising (a)
transparent substrate, (b) recording layer, (c) reflective layer and (d) transparent protective
layer.
27. A process according to claim 1, wherein the material has the structure comprising (a)
substrate, (b) reflective layer, (c) recording layer and (d) transparent protective layer.
28. A process according to claim 1, wherein the laser light has an energy of 0.1 bis
10 nJ/marking (bit).
29. A process according to claim 1, wherein the stored information is read out once or
repeatedly by measuring the shift in absorption by the reflection or transmission method
using laser light in the NIR range.
30. A process according to claim 29, wherein the stored information is read out at the
wavelength used for recording said information.
31. A material for the optical recording and storage of information, comprising at least one
layer of a dithioquinacridone as recording material coated on a transparent and dielectric
substrate.
32. A material according to claim 31, wherein a layer of an organic hydrazone, with or
without a transparent binder, is provided between the recording material and the substrate
or on the recording material.
33. A material according to claim 31, wherein the recording material is a homogeneous
mixture of a dithioquinacridone and an organic hydrazone.
34. A material according to claim 31, the topmost layer of which is coated with a
reflective layer which may be coated with a protective layer.
35. A material containing written information, wherein the recording layer of a recording
material comprising a substrate coated with at least one layer of a dithioquinacridone as
recording layer contains the written information in the form of bits which have higher
absorption in the NIR range than in the unchanged environment and thus have reduced

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reflectivity and transmission.
36. A material according to claim 35, which has the structure comprising (a) transparent
substrate, (b) recording layer and (c) transparent protective layer.
37. A material according to claim 35, which has the structure comprising (a) transparent
substrate, (b) recording layer, (c) reflective layer and (d) transparent protective layer.
38. A material according to claim 35, which has the structure comprising (a) substrate, (b)
reflective layer, (c) recording layer and (d) transparent protective layer.
39. A material according to any one of claims 35 to 38, wherein the recording layer
consists of at least one layer of a dithioquinacridone and at least one layer of an organic
hydrazone, or of at least one layer in the form of a homogeneous mixture of a
dithioquinacridone and an organic hydrazone.

Description

FM16-18881tA
Process for o~ticallY storing information
The present invention relates to a process for optically recording and storing information
in the form of bits by irradiating a recording material comprising a subshate coated with
at least one layer of a dithioquinacridone as reco~ding material, which recording material
may be coated with a reflective layer or a reflective layer ma~ be provided between ~e
substrate and the dithioquinacriclone layer~ and an additional layer containing an organic
hydrazone may be provided between dle substrate or between the reflective layers and the
dithioquinacridone layer, or the dithioquinacridone layer may be mixed with a hydrazone,
and the topmost layer may be coated with a protective layer, the irradiation of said
recording material being carried out with laser light in the near IR range. The invention
further relates to a recording material and to a material which contains the recorded
information.
The use of dyes that absorb in the near infra-red range (NIR) for recording information in
WORM ~rite once _ead many) systems has been known for some time and is described,
inter alia, by M. Emmelius in Angewandte Chernie, No. 11, pp. 1475-1502 (1989). By
irradiatingsuch recording materials with laser light it is possible to effect the shift in
absorption necessary for recording information in the form of bits by physical changes
(e.g. by sublimation or diffusion) or by chemical changes (e.g. photochromism,
isomerisation or thermal decomposition). Systems containing organic pigments devoid of
metal ions which, when irradiated with laser light, undergo a phase change (change in the
arrangement of the molecules) and an associated bathochrornic or hypsochromic shift of
the absorption bands, are not yet known in the art.
In EP-A 0 401 791 it is disclosed that dithioquinacridones treated locally with a solvent
exhibit changes in absorption caused by phase changes and are therefore suitable for
recording and storing information by recording in the form of bits (corresponds to the
local treatment) with an inkjet printer.
In US-A-4 760 004 there are disclosed layer materials comprising a layer of a
dithioquinacridone on a metallic and electrically conductive and opaque substrate and a

- 2 -
further layer of a hydrazone thereon. These layer systems are suitable for use as
electrophotographic photoreceptors for producing latent images by selectively reducing
charges with laser light after an integral charge. The latent image can then be converted by
known reprographic methods into a visible image.
It has now been found that dithioquinacridones which do not have high light absorption in
the NIR range undergo a phase change on exposure to laser light in the NIR range (e.g.
with a diode laser), the phase change temperature being c. 230C. The phase change is
associated with a pronounced shift in the light absorption. It has further been found that
this phase change is also induced by contact with a hydrazone compound and the phase
change temperature can in ~his case be lowered to to c. 180C. Substrates coated with
dithioquinacxidones and which may comprise a further layer of a hydrazone compound are
there~ore erninently suitable for use as materials for recording, storing and reproducing
information (WORM systems), for example in the form of compact discs.
In one of its aspec~s the invention relates to a process for optically recording and storing
inforrnation in the form of bits, wherein a recording material comprising a substrate coated
with at least one layer of a dithioquinacridone as recording layer is irradia~ed poin~ by
point or linearly with laser light in the near NIR range.
Suitable substrates are typically metals, alloys, glass, ~r~inerals, ceramics and thermoset or
therrnoplastic materials. The substrate may have a thickness of 0.01 rnm to 1 cm,
preferably of 0.1 rnrn to 0.5 cm. Preferred substrates are glass and homopolyenc or
copolymeric plastics materials. Suitable plastics materials include thermoplastic
polycarbonates, polyamides, polyesters, polyacrylates and polymethacrylates,
polyurethanes, polyolefins, polyvinyl chloride, polyvinylidene fluoride, polyimides,
therrnoset polyesters and epoxy resins.
Dithioquinacridones are known and described, inter alia, in US-A-4 760 004. These
compounds may be of formula I or Ia or rnixtures thereof

S H
(Rl)n I I ~ (Rl)l, (1),
H S
S S
~N~ (Rl)n (1~)~
H H
wherein Rl is F, Cl, Br, Cl-Cl8aL~cyl or Cl-C3aLIcoxy, and n is 0 or 1, 2 or 3.
Rl as aL1cyl may be linear or branched and alkyl preferably contains 1 to 12, more
particularly 1 to 6 and, most preferably 1 to 3, carbon atoms. Typical examples are methyl,
ethyl, n- and isopropyl, n-, iso- and tert-butyl, pentyl, hexylj octyl, nonyl, decyl, dodecyl,
tetradecyl and octadecyl. Methyl and ethyl are particularly preferred. Rl as aLtcoxy is
typically methoxy, ethoxy and n- and isopropoxy. ~ formulae I and Ia n is preferably 0, 1
or 2 and is most preferably 0 or 1. In a preferred embodiment of the invention, the
dithioquinacridones are those of formulae I and Ia, wherein Rl is -F, -Cl, -Br, -CH3 or
CH30-, and n is 0 or 1. Dithioquinacridone is especially preferred.
The thickness of the dithioquinacridone layer is typically 100 to 5000 ~, preferably 200 to
3000 A and, most preferably, 300 to 150û ~.
The substrate may be provided with one or more than one layer of dithioquinacridone,
typically with 1 to 10, preferably 1 to 5 and, most preferably, 1 ~o 3, layers. The number of
layers and further layers will depend mainly on the optical density of the layerarrangement, which must ensure a suf~lcient transmission at the wavelength used for
recording.
The dithioquinacridone layer or the substrate can be coated with a reflective layer which
has a thickness of typically 100 to 5000 A, preferably 100 to 3000 A and, most preferably,
300 to 1500 A. Particularly suitable reflective materials are metals which reflect the laser

-4- ;~
light used for recording and reproduction well, for example the metals of the third, fourth
and fifth main groups and the subgroups of the Pe~iodic Table of the Elements.
Particularly suitable metals are Al, In, Sn, Pb, Sb, Bi, Cu, ~g, Au, Zn, Cd, Hg, Sc, Y, La,
Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Wj Fe, Co, Ni, Ru, Rh, Pd, Os, 1~, Pt and the lanthanide
metals Ce, Pr, Nd, Pm, Sm, Eu, C-,d, Tb, Dy, HO, Er, Tm, Yb and Lu. A reflective layer of
aluminium or gold is especially preferred on account of the high reflectivity and the ease
with which it can be prepared.
The topmost layer, depending on the layer structurç for example the reflechve layer, the
dithioquinacridone layer or a further auxiliary layer (conveniently of a hydrazone), may
conveniently be coated with a protective layer that may have a thickness of 0.1 to 100 ~lm,
preferably 0.1 to 50 ~Lm and, most preferably, 0.5 to 15 ~lm. Mainly suitable for use as
protective matelial are plastics matenals that are coated in a thin layer either direct or with
the aid of adhesive layers on to the substrate or the topmost layer. It is expedient to choose
mechanically and thermally stable plastics materiais which have good surface properties
and may be additionally modified, for example marked. The plashcs materials may be
thermoset and thermoplastic materials. Radiation-cured (e.g. UV cured) protective layers
which are particularly easy and economical to prepare are preferred. A host of
radiation-curable materials are known. Exemplary of radiahon-curable monomers and
oligomers are acrylates and methacrylates of diols, hiols and tetrols, polyimides from
aromatic tehracarboxylic acids and aromatic diamines containing Cl-C4aLkyl groups in at
least two ortho-positions of the amim) groups, and oligomers containing diaLcyl groups,
conveniently dimethylmaleimidyl groups. Specific examples are UV-crosslinkable
polymers derived from polyacrylates, such as RENGOLUX~' RZ 3200/003 or 3203/001,available from Morton International-Dr. Renger.
In a preferred embodiment of the process of this invention a further layer of an organic
hydrazone is provided between the substrate which may be coated with a reflective layer
and the dithioquinacridone layer or is provided on said layer, which hydrazone preferably
contains an aromatic group attached to a N atom. The layer may be of pure hydrazone or
in the form of a solid solution of the hydrazone in or as a homogeneous mixture with a
transparent binder, for example a plastics material. The thickness of pure hydrazone layers
is typically 50 to 2000 A, preferably 100 to 1000 A and, most preferably, 100 to 500 A.
The layer thickness of the solid solution is typically 0.1 to 100 ~m, preferably 0.5 to
50 ~um and, most preferably, 0.5 to 5 llm. The solid solution may contain 0.1 to 95 % by
weight, preferably 1 to 80 % by weight and, most preferably, 1 to 60 % by weight of a

-4-
light used for recording and reproduc~ion well, for example the metals of the third, fourth
and fifth main groups and the subgroups of the Periodic Table of the Elements.
Particularly suitable metals are Al, In, Sn, Pb, Sb, Bi, Cu, Ag, Au, Zn, Cd, ~g, Sc, Y, La,
Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt and the lanthanide
metals Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, HO, Er, Tm, Yb and Lu. A reflective layer of
aluminium or gold is especially preferred on account of the high reflectivity and the ease
with which it can be prepared.
The topmost layer, depending on the layer structure for example the reflective layer, the
dithioquinacridone layer or a further auxiliary layer (conveniently of a hydrazone), may
conveniently be coated with a protective layer that may have a thickness of 0.1 to 100 ~,lm,
preferably 0.1 to 50 ~Lm and, most preferably, 0.5 to 15 ~lm. Mainly suitablç for use as
protective material are plastics materials that are coated in a thin layer either direct or with
the aid of adhesive layers on to the substrate or the topmost layer. It is expedient to choose
mechanically and thermally stable plastics materials which have good surface properties
and may be additionally modified, for example marked. The plastics materials may be
thermoset and thermoplastic materials. Radiation-cured (e.g. UV cured) protective layers
which are particularly easy and economical to prepare are preferred. A host of
radiation-curable materials are known. Exemplary of radiation-curable monomers and
oligomers are acrylates and methacrylates of diols, triols and tetrols, polyimides from
aromatic tetracarboxylic acids and aromatic diamines containing Cl-C4alkyl groups in at
least two ortho-positions of the amino groups, and oligomers containing diaL~yl groups,
conveniently dimethylmaleimidyl groups. Specific examples are UV-crosslinkable
polymers derived from polyacrylates, such as RENGOL~JX(~) RZ 3200/003 or 3203/001,
available from Morton International-Dr. Renger.
In a preferred embodiment of the process of this invention a further layer of an organic
hydrazone is provided between the substrate which may be coated with a reflective layer
and the dithioquinacridone layer or is provided on said layer, which hydrazone prefçrably
contains an aromatic group attached to a N atom. The layer may be of pure hydrazone or
in the form of a solid solution of the hydrazone in or as a homogeneous mixture with a
transparent binder, for example a plastics material. The thickness of pure hydrazone layers
is typically 50 to 2000 ~, preferably 100 to 1000 ~ and, most preferably, 100 to 500 ~.
The layer thickness of the solid solution is typically 0.1 to 100 ~Im, preferably 0.5 to
50 l,lm and, most preferably, 0.5 to 5 ~,lm. The solid solution may contain 0.1 to 95 % by
weight, preferably 1 to 80 % by weight and, most preferably, 1 to 60 % by weight of a

f3\N' ~
R6
which is unsubstituted or substituted by F, Cl, Br, Cl-C6aL~cyl, Cl-C6aLkoxy or
di(cl-c6alkyl)amino~ and E~6 is Cl-~6alkYl~
R4 and Rs are each independently of the other Cl-C6alkyl, phenyl, naphthyl or ben~yl, or
phenyl, naph~yl or benzyl which are each substituted by ~, Cl, Br, Cl-C6aLIcyl,
Cl-C6aL~oxy or di~Cl-C6alkyl)amino,
andnisOor 1.
In formula II n is preferably O. R2, R3, R4, Rs, R6 and substituents as aLkyl may be linear
or branched and contain preferably 1 to 4 carbon atoms, most preferably 1 or 2 carbon
atoms. Typical examples are methyl, ethyl, n- and isopropyl, n-, iso- and tert-butyl, pentyl
and hexyl. Methyl and ethyl are prefeIred.
The alkoxy substituent preferably contains 1 to 4 carbon atoms and may be linear or
branched. Typical examples are methoxy, ethoxy, n- and isopropoxy, n-, iso- and
tert-butoxy, pentoxy and hexoxy. Methoxy and ethoxy are preferred.
Typical examples of aLkyla nino substituents are dimethylarnnio, diethylamino,
methylethylamino, di-n-propylamino or diisopropylamino, di-n-butylamino,
n-propylmethylamino, n-butylmethylamino, n-propylethylarnino, n-butylethylamino,methylphenylamino, ethylphenylamino, methylbenzylamino and ethylbenzylamino.
Typical preferred hydrazones are those of formulae
~CH=N-N(C6~ls)2
C2H5

- 7 - 2~ r~
~,J~l~ CH= N-N(C6H5)(C~13)
l 2H5
(c6Hs)2N-N=Hc ~ N(C2H5)2 .
(c1oH7)(c6Hs)N-N=llc ~=~ N(C2H5)2,
(C6~5)2N-~=CH-CEI=C(P-CH30-c6Hs)2
The recording materials suitable for use in the prac~ice of this invention can be prepared
by methods which are known per se. Dependin~ on the materals used and their mode of
use, different coating techniques can be applied.
Substrates containing a hyclrazone can be prepared by the blencling and shaping techniques
commonly used t`or thermoset and therrnoplastic resinx, typically by casting, compression
moulding, injection moulding and extrusion methods.
Suitable coating techniques include imrnersion, casting, brushing, doctor coating,
centrifugal casting, and vapour deposition methods which are carned out under vacuum.
If, ~or example, casting methods are employed, solutions in organic solvents will normally
be used, which solutions may additionally contain a bincler if a hyclrazone is used. When
using solvents, care must be taken that the substrates are inactive to these solvents. It is
preferred to prepare all layers by vapour deposition, especially under vacuum. Suitable
coating techniques are described, inter alia, in EP-A-0 401 791.
The recording layer or layers and the metallic reflective layers are preferably applied by
vapour deposition under vacuum. The material to be applied is ~lrst put into a suitable
vessel, which may be equipped with a resis~ance heating, and placed into a vacuum
chamber. The substrate on to which the material is to be deposited is clamped above the
vessel with the material to be vapourised. The clamp is constructed such that the substrate

- 8 -
can be rotated (e.g. at 50 rpm) and heated. The vacuum chamber is evacuated to about
1.3 . 10-5 to 1.3 . 10-6 mbar (10-5 to 10-6 torr), and the heating is adjusted such that the
temperature of the material to be deposited rises to its vapourising temperature. The
deposition is continued until the layer applied has the desired thickness. Depending on the
system, first the recording material and then the reflective layer is applied, or conversely.
The application of a reflective layer can în some cases be disp~nsed with. This method of
vapour deposition is especially suitable for the simultaneous application of dithioquinacri-
dones and hydrazones to form homogeneous rnixed layers.
It is particularly preferred to apply the metallic reflective layer by the sputtering technique
on account of the good bonding to the substrate. The material to be applied (e.g.
aluminium) in the forrn of a plate is used as a "target" electrode, whereas the substrate is
mounted on the counter-electrode. First the vacuum chamber is evacuated to about10-6 torr and then inert gas, e.g. argon, is introduced until the pressure is about 10-3 torr.
Be~ween the target electrode and the counter-electrode a high direct current voltage or
radio-frequency voltage of seYeral kV is applied, optionally using perman-ent magnets
(magnetron sputtering) so as to produce Ar+ plasma. The metal particles sputtered by the
Ar~ ions of the target electrode are uniformly and f~y deposited on the substrate.
Coating is effected within a few to several minutes, depending on the target materials,
sputtering technique and sputtering conditions. This sputtering technique is described in
detail in the technical literature (e.g. W. Kern and L. Vossen, "Thin Film Processes",
Academic Press, 1978).
The thickness of the layer formed by vapour deposition can be monitored with the aid of
an optical system which measures the reflectivity of the reflective surface coated with the
absorption material. It is preferred to monitor the growth of the layer thickness with a
quartz resonator.
Protective layers are prefereably applied by spin coating and crosslinked with UV light
when using light-sensitive materials.
The material eligible for use in the practice of this invention is pre-eminently suitable for
recording information by irradiation with laser light in the NIR range. After irradiation a
markedly increased absorption of the laser light is observed, so that the reflection or
transmission after irradiation is substantially lower. The change in reflection or
transmission can therefore be used for reading out inforrnation without the stored

- 9-
infolmation being destroyed by the laser light used for reading out. The information can
there~ore be read out repeatedly.
The structure of the recording material of this invention will depend mainly on the method
of reading out: known techniques are measuring the change in transmission or reflection.
If the recording system functions according to a ch~nge in light transmission, the structure
may suitably comprise: transparent substrate/recording layer (one or more layers)/if
appropriate, transparent protective layer. The radiation for writing and reading out
information can be applied either from the substrate side of the system or from the
recording layer or protective layer side, the light detector always being on the adjacent
side.
If the recording process functions according to a change in reflectivity, then other layered
structures are possible for the substrate: transparent substrate/recording layer (one or more
layers)/reflective layer/if appropriate, protective layer (not necessarily transparent), or
subs~rate (not necessarily transparent)/reflective layer/recording layer/if appropriate,
transparent protective layer. In the former case, the radiation is applied from the substrate
side of the system, whereas in the latter case the radiation is applied from the recording
layer or, if present, from the protective layer side of the system. In both cases, the light
detector is on the same side as the light source. The first mentioned layer structure of the
inventive recording material is generally preferred.
Suitable lasers include comrnercial diode lasers, preferably semiconductor diode lasers,
for example GaAsAl, InGaAlP or GaAs lasers with a wavelength of 780, 650 and 830 nm
respectively7 or He/Ne lasers (633 nm) and argon lasers (514 nm). The information can be
written point by point or Linearly using a light modulator.
The energy of the laser light used for recording may be typically from 10 nJ/marking (bit~,
preferably from 0.2 to 5 nJ/marking (bit) and, most preferably7 0.8 to 3 nJ/marking (bit).
The amount of energy is essentially controlled by the irradiation time, for example by
pulses in the range from a few microseconds, typically 10 to 100 nanoseconds.
The method of this invention makes it possible to store information with a high degree of
reliability and durability, the information being distinguished by very good mechanical
and thermal stability as well as by superior light stability and clear edge definition. A
particular advantage is the surprisingly high signal-to-noise ratio of carrier material to

'(h~ d ~
- 10-
information marking, which permits the information to be read out easily. In addition, the
optical recording system is simple and inexpensive.
The information is read out by measuring the absorption by the reflection or ~ransmission
method using laser light. It is particularly advantageous that laser light of the wavelength
used for recording can be utilised, i.e. a second laser also need not be used. In a preferred
embodiment of the process, information is written and read out at the same wavelength.
The info~nation is normally read out by using low-energy lasers whose radiation intensity
is ten- to fi~ty-fold lower than the laser light used for recording. The inforrnation can be
read out once or repeatedly. The shift in the absorption spectrum and/or the stored
information can be read out with a photodetector using a low-energy laser. Suitable
photodetectors comprise PIN photodiodes as well as microscopic spectrophotometers, e.g.
UMSP 80 available from Carl Zeiss, which make it possible to measure the spectral
changes by transmission or absorption and, in particular, reflection.
A further object of the invention is the provision of a material ~or optical recording and
stonng information, comprising at least one layer of a dithioquinacridone as recording
material coated on a Iransparent and dielectric substrate. Between the recording material
and the substrate or on the recording material there may be provided a layer of an organic
hydra~.one together with a transparent binder, or the recording material may be a
homogeneous mixture of dithioquinacridone and an organic hydrazone. The topmost layer
may be coated wi~h a reflective layer that is desirably c,oated with a protective layer. In
other respects, the embodiments and preferences previously cited apply.
It is yet a further object of the invention to provide a material on which information is
written, the recording layer of which material contains the written information in the forrn
of bits which have higher absorption in the NlR range than in the unchanged environment
and thus have reduced reflectivity and transmission. In other respects, the embodiments
and preferences previously cited apply.
The material on which information is written is in particular an optical information
material of the WORM type. It may be used typically as a playable compact disc, as
storage material for computers or as an identity and security card.
The following Examples illustrate the invention in more detail.

9 ~
Example 1: Recording material having a recording layer of dithioquinacridone and a
hydra~one interlayer.
N'-Ethylcarbazol-2-aldehyde-N,N-diphenylhydrazone is vapourised under a high vacuum
(c. 4x10-6 mbar) to a layer thickness of c. 200 ~ on to a polycarbs)nate substrate (thickness
1.2 mm). Afterwards, dithioquinacridone is vapourised under a high vacuum on ~o this
layer to a layer thickness of c. 800 A. A gold reflective layer is then vapourised to a
thickness of c. 1000 A on to this dithioquinacridone layer by high vacuum vapourdeposition. Elec~ronic information is written dotwise with a GaAsAl diode laser (Toshiba)
of 780 nm (20 mW). The reflectivity measured through the substrate before and after
writing at the same wavelength of 780 nm is 73 % and 35 %, respectively.
Example 2: Recording material having a recording layer of dithioquinacridone and an
interlayer of a polycarbonate/hy~razone film.
1 g of p-diethylaminobenzaldehyde-N,N-diphenylhydrawne and 0.5 g of polycarbonate
(commercial Makrolon(g) are dissolved in 10 ml of cyclopentanone and the solution is
coated by centrifugal casting to a layer thickness of 0.8 ~lm on to a glass substrate
(thickness 1.0 mm~. As described in Example 1, a layer of dithioquinacridone is applied to
a thickness of 500 ~, followed by thç application of a gold reflective layer to a thickness
of 1500 ~. The reflectivity is 73 % before writing and 40 % after writing
Example 3: Recording material having a recording layer of dithioquinacridone.
In accordance wi~h the procedure of Example 11 layers of dithioquinacridone (thickness
700 A) and aluminium (thickness 2000 ~) are applied to a glass substrate (thickness
1.0 mm~. The reflectivity is 65 % before writing and 38 % after writing.