Optical recording medium containing polymethine compound

An optical recording medium including a recording layer containing a polymethine compound which can be expressed by the following formula: ##STR1## where R.sub.1 and R.sub.2 represent aryl groups substituted by alkyl groups, or aryl groups substituted by alkoxy groups; R.sub.3 and R.sub.4 represent substituted or unsubstituted aryl groups, substituted or unsubstituted heterocyclic groups, or substituted or unsubstituted styryl groups; m is 0, 1 or 2; and X.crclbar. is an anion.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical recording medium and, more 
specifically, to a record blank suitable for recording information with a 
laser beam, in particular, a semiconductor laser beam, and to an 
information record allowing optical reproduction of information recorded 
by a laser beam. 
2. Description of the Related Art 
Generally, information can be recorded on an optical recording medium, such 
as a series of optical discs or optical cards, by forming an optically 
detectable minute pits (for example, of approximately 1 .mu.m) arranged in 
such a manner as to define a spiral, concentric or linear track on a 
recording layer formed on a substrate of the recording medium, thereby 
making it possible to store information at high-density. 
As described, for example, in "Review and Analysis of Optical Recording 
Media" of Optical Engineering, Vol.15, No.2, March-April 1976, 
pp99.about., a known optical recording method consists in applying a light 
beam, such as a laser beam, to the recording layer of an optical recording 
medium so as to generate deformation or pits on the recording layer. In 
other known methods, the application of such a light beam causes the 
generation of bubbles, changes in phase, discoloration, discolorization or 
the like. 
Various materials have been proposed for the recording layer of such an 
optical recording medium. Examples of such materials include inorganic 
materials, such as a metal film consisting of an aluminum deposit film or 
the like, a bithmuth film, a tellurium oxide film, or a chalcogenite-type 
non-crystalline glass film. Generally, such thin films are sensitive to 
light having a wavelength of approximately 350 to 800 nm and exhibit high 
reflectance to laser beams. One disadvantage of these films is that they 
provide a rather poor laser-beam-utilization factor. Further, such an 
inorganic material thin film is usually formed as a recording layer by 
sputtering or the like. Such a film formation method, however, requires a 
production line equipped with a vacuum system, so that these inorganic 
material thin films have higher production costs. 
In view of the above problems, optical recording mediums using organic 
coloring matter whose optical properties can be changed by the energy of 
light having a relatively long wavelength (for example, 780 nm or more) 
are being carefully studied. Optical recording mediums using such organic 
coloring matter are effective in that they allow the formation of pits by 
a semiconductor laser having an oscillation wavelength of around 780 nm or 
830 nm. Further, organic coloring matter allows film formation by wet 
coating and can be easily treated, making it possible to easily 
mass-produce recording mediums on equipment costing less. 
In this context, for example, in Japanese Patent Laid-Open No. 1 -26879, an 
optical recording medium which uses a polymethine dye for the recording 
layer and which allows high S/N recording by a semiconductor laser and 
exhibits satisfactory heat stability is shown. 
As stated above, employment of an organic coloring matter for the recording 
layer helps to reduce production costs of optical recording mediums 
because it allows film formation by wet coating. Such an organic coloring 
matter, however, must exhibit a maximum absorptivity in a range near the 
wavelength of the recording light beam, excellent heat stability, and high 
solubility in solvent. In a case where an organic coloring matter having 
poor solubility in solvent is used, a slight change in the concentration 
of the solution to be applied would likely cause solid particles, e.g., 
minute crystals of the organic coloring matter, in the solution. In that 
case, a wet coating process would lead to mixing the solid particles in 
the solution for preparing recording layers to the recording layer, 
resulting in low-quality optical recording mediums exhibiting a high noise 
level at the time of reproduction. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the above problems. It is an 
object of this invention to provide an optical recording medium comprising 
a recording layer containing an organic coloring matter exhibiting high 
absorptivity to light having a wavelength range of 600-1200 nm, excellent 
stability even under high-temperature/humidity conditions, and further, 
improved solubility in solvent, and which has high recording sensitivity, 
excellent durability, and allows reproduction of recorded information at a 
high S/N ratio. 
In accordance with the present invention, there is provided an optical 
recording medium comprising a recording layer containing a polymethine 
compound which is shown by the following formula (I): 
##STR2## 
(where R1 and R2 represent aryl groups substituted by alkyl groups, or 
aryl groups substituted by alkoxy groups; R3 and R4 represent substituted 
or unsubstituted aryl groups, substituted or unsubstituted heterocyclic 
groups, or substituted or unsubstituted styryl groups; m is 0, 1 or 2; and 
X .crclbar. represents an anion). 
In accordance with the present invention, the polymethine compound 
expressed by formula (I) exhibits high absorptivity to light having a 
wavelength in the near-infrared range, and having excellent solubility in 
solvent. Further, the polymethine compound exhibits a high oxidization 
potential in cyclic voltammetry, and is relatively free from deterioration 
by oxidization. Thus, it is expected that the polymethine compound makes 
it possible to obtain an optical recording medium exhibiting high 
recording sensitivity with respect to semiconductor lasers, with excellent 
durability, and allowing the reproduction of recorded information at a 
high S/N ratio.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will now be described in detail. 
The optical recording medium of the present invention has a recording layer 
which absorbs electromagnetic radiation to record information as a result 
of thermal action causing optical changes. The recording layer contains a 
polymethine compound which is shown by the following formula: 
##STR3## 
In the above formula (I), R1 and R2 represent, for example, aryl groups 
which are substituted by substituted or unsubstituted alkyl groups as 
shown in the following formula (II), or aryl groups which are substituted 
by substituted or unsubstituted alkoxy groups as shown in the following 
formula (III): 
##STR4## 
In the above formula (II), R5 represents a substituted or unsubstituted 
alkyl group. Examples of the alkyl group include: methyl group, ethyl 
group, n-propyl group, iso-propyl group, n-butyl group, sec-butyl group, 
iso-butyl group, t-butyl group, n-amyl group, t-amyl group, n-hexyl group, 
n-octyl group, and t-octyl group. 
Examples of the substituted alkyl group includes: 2-hydroxyethyl group, 
3-hydroxypropyl group, 4-hydroxy butyl group, 2-acetoxyethyl group, 
carboxymethyl group, 2-carboxyethyl group, 3-carboxypropyl group, 
2-sulfoethyl group, 3-sulfopropyl group, 4-sulfobutyl group, 
3-propylsulfate group, 4-butylsulfate group, 
N-(methylsulfonyl)-carbamylethyl group, 3-(acetylsulfamyl) propyl group, 
and 4-(acetylsulfamyl) butyl group. Further, the alkyl group may be 
cyclic, for example, be a cyclohexyl group or the like. 
When an aryl group is bonded with a plurality of alkyl groups, the mutual 
alkyl groups may be the same or differ from each other. 
In the above formula (III), R represents a substituted or unsubstituted 
alkoxy group. Specifically, it may be a methoxy group, ethoxy group, 
propoxy group or the like. 
Further, when, in formulas (II) and (III), R5 and R6 are linear or branched 
substituted or unsubstituted lower alkyl groups of a carbon number ranging 
from 1 to 3, or alkoxy groups bonded with substituted or unsubstituted 
lower alkyl groups, these compounds exhibiting an optimum solubility in 
solvent. The resulting layer exhibits a satisfactory environmental 
stability, indicating that compounds with such substituent groups are 
especially suitable for forming the optical recording medium of the 
present invention. Examples of such lower alkyl groups and alkoxy groups 
include: methyl group, ethyl group, n-propyl group, iso-propyl group, 
methoxy group, ethoxy group, n-propoxy group, and iso-propoxy group. 
Next, R3 and R4 may represent: substituted or unsubstituted aryl groups 
(for example, phenyl groups, naphthyl groups, tolyl groups, xylyl groups, 
methoxyphenyl groups, dimethoxyphenyl groups, trimethoxyphenyl groups, 
ethoxyphenyl groups, dimethylaminophenyl groups, diethylaminophenyl 
groups, dipropylaminophenyl groups, dibenzylaminophenyl groups, 
diphenylaminophenyl groups, or ditolylaminophenyl groups, 
dimethoxyaminophenyl groups etc.); substituted or unsubstituted 
heterocyclic groups (for example, pyridyl groups, quinolyl groups, lepidyl 
groups, methylpyridyl groups, furyl groups, thienyl groups, indolyl 
groups, pyrrole groups, carbazolyl groups, or N-ethylcarbazolyl groups); 
or substituted or unsubstituted styryl groups (for example, styryl groups, 
methoxystyryl groups, dimethoxystyryl groups, trimethoxystyryl groups, 
ethoxystyryl groups, dimethylaminostyryl groups, diethylaminostyryl 
groups, dipropylaminostyryl groups, dibenzylaminostyryl groups, 
diphenylaminostyryl groups, 2,2-diphenylvinyl groups, 
2-phenyl-2-methylvinyl groups, 2-(dimethylaminophenyl)-2-phenylvinyl 
groups, 2-(diethylaminophenyl)-2-phenylvinyl groups, 
2-(dibenzylaminophenyl)-2-phenylvinyl groups, 2,2-di(diethylaminophenyl) 
vinyl groups, 2,2-di(methoxyphenyl) vinyl groups, 2,2-(ethoxyphenyl) vinyl 
groups, 2-(dimethylaminophenyl)-2-methylvinyl groups, or 
2-(diethylaminophenyl)-2-ethylvinyl groups). 
The number m is 0, 1 or 2. 
Further, the symbol X .crclbar. represents an anion, such as a chlorine 
ion, bromine ion, iodine ion, perchlorate ion, nitrate ion, 
benzenesulfonate ion, p-toluenesulfonate ion, methylsulfate ion, 
ethylsulfate ion, propylsulfate ion, tetrafluoroborate ion, 
tetraphenylborate ion, hexafluorophosphate ion, benzenesulfonate ion, 
acetate ion, trifluoroacetate ion, propionacetate ion, benzoate ion, 
oxalate ion, succinate ion, malonate ion, oleate ion, stearate ion, 
citrate ion, 1-hydrogen-2-phosphate ion, 2-hydrogen-1-phosphate ion, 
pentachlorostannate ion, chlorosulfonate ion, fluorosulfonate ion, 
trifluoromethanesulfonate ion, hexafluoroarsenate ion, 
hexafluoroantimonate ion, molybdate ion, tungstate ion, titanate ion, or 
zirconate ion. 
The following are specific examples of the polymethine-type dyes used in 
the present invention, which, however, should not be construed 
restrictively. 
__________________________________________________________________________ 
Compound No. 
R.sub.1, R.sub.2 
R.sub.3, R.sub.4 m X 
__________________________________________________________________________ 
##STR5## 
##STR6## 1 ClO.sub.4 
2 
##STR7## 
##STR8## 1 ClO.sub.4 
3 
##STR9## 
##STR10## 1 BF.sub.4 
4 
##STR11## 
##STR12## 1 ClO.sub.4 
5 
##STR13## 
##STR14## 1 ClO.sub.4 
6 
##STR15## 
##STR16## 1 I 
7 
##STR17## 
##STR18## 0 ClO.sub.4 
8 
##STR19## 
##STR20## 0 ClO.sub.4 
9 
##STR21## 
##STR22## 0 
##STR23## 
10 
##STR24## 
##STR25## 2 ClO.sub.4 
11 
##STR26## 
##STR27## 1 ClO.sub.4 
12 
##STR28## 
##STR29## 1 ClO.sub.4 
13 
##STR30## 
##STR31## 1 ClO.sub.4 
14 
##STR32## 
##STR33## 1 ClO.sub.4 
15 
##STR34## 
##STR35## 1 ClO.sub.4 
16 
##STR36## 
##STR37## 1 SbF.sub.6 
17 
##STR38## 
##STR39## 0 ClO.sub.4 
18 
##STR40## 
##STR41## 1 SbF.sub.6 
19 
##STR42## 
##STR43## 1 ClO.sub.4 
20 
##STR44## 
##STR45## 1 ClO.sub.4 
21 
##STR46## 
##STR47## 1 ClO.sub.4 
22 
##STR48## 
##STR49## 1 ClO.sub.4 
23 
##STR50## 
##STR51## 1 ClO.sub.4 
24 
##STR52## 
##STR53## 1 ClO.sub.4 
25 
##STR54## 
##STR55## 1 ClO.sub.4 
26 
##STR56## 
##STR57## 0 ClO.sub.4 
27 
##STR58## 
##STR59## 1 ClO.sub.4 
28 
##STR60## 
##STR61## 1 I 
29 
##STR62## 
##STR63## 1 ClO.sub.4 
30 
##STR64## 
##STR65## 1 ClO.sub.4 
31 
##STR66## 
##STR67## 1 ClO.sub.4 
__________________________________________________________________________ 
As shown in FIG. 1, an optical recording medium according to the present 
invention can be formed by providing on a substrate 1, a recording layer 2 
containing a polymethine-type dyes which is shown by the above formula 
(I). 
Examples of the material for the substrate 1 include: plastics, such as 
polycarbonate, polyester, acrylic resin, polyolefine resin, phenol resin, 
epoxy resin, polyamide or polyimide; glass; and metals. 
In forming the recording layer 2, one or more polymethine-type dyes 
expressed by formula (I) may be combined. Further, it is also possible to 
mix and disperse the compound, or stack it, with other types of coloring 
materials, such as polymethine-type coloring materials other than those 
expressed by formula (I), naphtholactam-type dyes, azulene-type, 
pyrylium-type, squalinium-type, croconium-type, triphenylmethane-type, 
xanthene-type, anthraquinone-type, cyanine-type, phthalocyanine-type, 
dioxazine-type, tetrahydrocholine-type, triphenothiazine-type, 
phenanthrene-type, or metals or metal compounds, such as Al, Te, Bi, Sn, 
In, Se, SnO, TeO; As or Cd. 
Polymethine compounds of the present invention can be easily obtained by 
synthesis methods disclosed, for example, by Bernard S. Wildi et al., 
"Journal of American Chemical Society" Vol. 80, 3772.about.3777 (1958 ), 
by H.Schmidf et al., "Liebig Annalen der Chemie" Vol. 623. 
pp.204.about.216 (1959 ), or by R. Wilzinger et al., "Helvetica Chimica 
Acta", Vo. 24, p.369 (1941). 
Further, to improve the light stability of the recording layer 2, it is 
possible to mix a stabilizer with the recording layer 2. Examples of the 
stabilizer include: various metal chelate compounds, in particular, 
comprising Zn, Cu, Ni, Cr, Co, Mn, Pd or Zr as the central metal, and 
polydentate ligands, for example, four-dentate ligands, such as N4, N2O2, 
N2S2S4, O2S2 and O4, or combinations thereof; and various amines, 
diamines, nitrogen containing aromatic compounds and onium salts thereof, 
for example, aminium salt, diimonium salt, pyridinium salt, imidazolinium 
salt, and quinolium salt. Further, pyrylium salt, which is a salt of an 
oxygen containing aromatic compound, may be used. 
In particular, aminium salt which is expressed by the following formula 
(IV), diimonium salt which is expressed by the following formula (V), or 
the like exhibits satisfactory compatibility with the polyimethine 
compound in the coating solution when the recording layer is formed by wet 
coating, thus making it possible to obtain a high-performance optical 
recording medium with excellent durability and light stability. 
##STR68## 
(where A represents 
##STR69## 
R7.about.R14 are substituents of a carbon number ranging from 1 to 8; at 
least one of R7.about.R14 is an alkoxyalkyl group, an alkenyl group or an 
alkynyl group; and X .crclbar. represents an anion.) 
##STR70## 
(where R15.about.R22 are substituents of a carbon number ranging 1 to 8; 
at least one of R15.about.R22 is an alkoxyalkyl group, an alkenyl group or 
an alkynyl group; and X .crclbar. represents an anion.) 
The amount of such a stabilizer added to the organic coloring matter in the 
recording layer is preferably (1/100.about.50/100), in particular, 
(10/100.about.45/100 ) by weight with respect to the organic coloring 
matter. 
The recording layer 2 can be formed on the substrate 1 by various methods, 
for example, by wet coating or chemical vapor deposition such as 
evaporation. When using the wet coating method, the layer can be formed by 
applying a solution prepared by dissolving or dispersing a polymethine 
compound in an organic solvent to the substrate 1. Further, in 
consideration of film formation characteristics and stability, a binder 
may be mixed with the recording layer as needed. 
Various organic solvents can be used in the wet coating varies depending 
upon whether the polymethine compound is dispersed or dissolved. Generally 
speaking, the solvent used may be of an alcohol type, ketone type, amide 
type, ether type, ester type, aliphatic-hydrocarbon-halide type 
aliphatic-hydrocarbon type, fluorine type, or the like. 
Ideally the polymethine compound of the present invention is completely 
soluble in the selected solvent. 
However, when forming the recording layer directly on a resin substrate by 
wet coating, only an organic solvent which will not melt the surface of 
the resin substrate or generate cracks therein may be used. 
The polymethine compound of the present invention, however, has excellent 
solubility in a variety of solvents. As a result, a wider range of 
selection for the coating solvent or for the substrate material is 
available when the organic coloring matter is applied to the surface of a 
resin substrate by wet coating. As a result, less severe process 
conditions for preparing an excellent optical recording medium exhibiting 
a low noise level are required. 
Examples of the above-mentioned binder include: nitrocellulose, 
ethylcellulose, polystyrene, polyvinyl pyrolidone, polymethyl 
methacrylate, and polyamide. Further, wax, a higher fatty acid, or an 
amide (for example, oleylamide) may be used as an additive as needed. 
The above binder can be mixed with a plasticizer, such as dioctyl 
phthalate, dibutyl phthalate or tricresyl phosphate, an oil solution, such 
as mineral oil or vegetable oil, and further, a dispersing agent, such as 
alkyl benzene sodium sulfonate or polyoxyethylene alkyl phenyl ether and 
other additives, thereby enhancing the film formation characteristics and 
coating stability for the recording layer. 
The coating operation may be conducted by any appropriate coating method, 
such as immersion coating, spray coating, spinner coating, bead coating, 
Meyer bar coating, blade coating, curtain coating, roller coating or 
gravure coating. 
The amount of the polymethine compound contained in the recording layer 2 
is not less than 1%; usually, it ranges from 40 to 100 wt %, and more 
preferably, from 50 to 100 wt %. When the amount is 40 wt % or more, 
sufficient light absorptivity and satisfactory light reflectance with 
respect to a recording laser beam can be obtained for the recording layer. 
The thickness of the recording layer 2 ranges from 100 .ANG. to 20 .mu.m, 
more preferably, from 200 .ANG. to 1 .mu.m. The film should be made as 
thin as possible, while providing a stable film exhibiting satisfactory 
light reflectance with respect to a recording laser beam. 
Further, as shown in FIG. 2, it is possible to provide a protective layer 3 
which is transparent to recording and reproducing laser beams on the 
recording layer 2 of the optical recording medium of the present 
invention. The protective layer 3 may be opaque where the laser beam is 
applied from the side of the substrate 1. 
Further, as shown in FIG. 3, an under-coating layer 4 may be provided 
between the substrate 1 and the recording layer 2. As shown in FIG. 4, it 
is also possible to employ both the protective layer 3 and the 
under-coating layer 4. 
The under-coating layer 4 is formed with a view to (a) obtaining an 
improved adhesion for the recording layer, (b) providing a barrier to 
water and gas, (c) improving the stability of the recording layer, (d) 
improving the reflectance of the recording layer, (e) protecting the 
substrate from the solvent, (f) forming pregrooves, etc. To achieve the 
above object (a), various substances may be used, for example, a polymeric 
material, such as an ionomer resin, a polyamide-type resin, a vinyl-type 
resin, a natural high-molecular substance, silicone, or liquid rubber, or 
an silance coupling agent. Regarding the above objects (b) and (c), it is 
possible to use, apart from the above high-molecular material, an 
inorganic compound, such as SiO2, MgF2, SiO, TiO2, ZnO, TiN, or SiN, or a 
metal or a semimetal, such as Zn, Cu, S, Ni, Cr, Ge, Se, Cd, Ag, or Al. 
Regarding the object (d), it is possible to use a metal, such as Al or Ag, 
or an organic thin film having a metallic gloss, such as a cyanine dye or 
a methine dye. To achieve the objects (e) and (f), an ultraviolet curing 
resin, a thermosetting resin, or a thermoplastic resin may be used. A 
suitable thickness of the under-coating layer ranges from 50 .ANG. to 100 
.mu.m, more preferably, from 200 .ANG. to 30 .mu.m. 
The protective layer is provided to protect the recording layer from flaws, 
dust, contamination or the like, and improve the stability and the 
reflectance of the recording layer. The material of the protective layer 
may be the same as that of the under-coating layer. The thickness of the 
protective layer is not less than 100 .ANG., more preferably, not less 
than 1000 .ANG.. 
The under-coating layer and/or the protective layer may contain a 
polymethine-type compound as expressed by formula (I) of the present 
invention. Further, the under-coating layer or the protective layer may 
also contain a stabilizer, dispersing agent, flame retarder, lubricant, 
antistatic agent, surface-active agent, plasticizer or the like. 
Further, in another possible construction of the optical recording medium 
of the present invention, two recording mediums having the same structure 
shown in one of FIGS. 1 to 4 are combined (in some cases, one of the two 
consists of a substrate only) to form an air-gap structure with the 
recording layers 2 positioned inside, i.e., a so-called air-sandwiched 
structure. Or, two recording mediums may be glued together with the 
recording layer 2 positioned inside, thus forming a so-called 
close-contact (glued) structure. 
A modulated electromagnetic radiation for recording is applied to an 
optical recording medium (a record blank with no information recorded 
thereon) according to the present invention, produced as described above, 
so as to cause the recording layer discolorization or discoloration, or 
generate a recess (pit) thereon, thereby producing an information record 
on which information has been recorded. 
Recording on the optical recording medium of the present invention can be 
effected by applying a gas laser such as a helium-neon laser (of an 
oscillation wavelength of 633 nm). Especially suitable, however, is a 
method according to which recording is effected by forming a recess on the 
recording layer by applying thereto a laser beam having a wavelength of 
750 nm or more, and in particular, a laser beam having an oscillation 
wavelength which is in the near-infrared or infrared range, such as a 
gallium-aluminum-arsenic semiconductor laser (of an oscillation wavelength 
of 830 nm). To perform reading, a laser beam of a type as mentioned above 
which have been adjusted to such an intensity as will not cause any change 
in the recording layer may be used. Further, it is also possible to 
conduct writing and reading by using lasers of the same wavelength or 
lasers having different wavelengths. 
As described above, in accordance with the present invention, 
long-wavelength absorptivity is obtained in the recording layer. 
Accordingly, high-sensitivity recording is possible even when a 
long-wavelength oscillation laser, such as a semiconductor laser, is used 
thus making it possible to obtain an optical recording medium exhibiting a 
high C/N ratio. 
Further, in accordance with the present invention, the noise level is 
always low, even when the recording layer is formed by wet coating. 
Moreover, it is possible to obtain an optical recording medium with 
excellent heat stability. 
EXAMPLES 
The present invention will now be described in detail with reference to 
specific examples, which, however, should not be construed restrictively. 
Synthesis Example 1 
A polymethine compound according to the present invention, compound No.(5 
), was synthesized in the following manner: 
4.4 g of 4,4'-diaminobenzophenon, 30.9 g of P-iodotoluene, 19.5 g of 
potassium carbonate anhydride and 10 g of copper powder were added to 70 
ml of o-dichlorobenzene, and the mixture thus obtained was refluxed with 
stirring for twenty-four hours at 180.degree. C. After the reaction, the 
mixture was filtered and washed with ethyl acetate. Then, the mixture was 
separated by silica gel column chromatography using chloroform, thereby 
obtaining 11.3 g of 4,4'-ditolylaminobenzophenon. 
Subsequently, 9 g of the 4,4'-ditolylaminobenzophenon was reacted with 15.7 
g of an ether solution (2 mol/l) of methylmagnesium iodide in 72 ml of 
tetrahydrofuran for sixteen hours at 60.degree. C. 
Next, this reaction liquid was processed by adding thereto dilute 
hydrochloric acid and an aqueous solution of sodium hydroxide. It was then 
filtered and washed with water and recrystallized in ethanol, thereby 
obtaining 7.1 g of 1,1-bis(p-ditolylaminophenyl)-ethylene. Subsequently, 7 
g of the 1,1-bis(p-ditolylaminophenyl)-ethylene and 2.5 ml of ethyl 
orthoformate were dissolved in 25 ml of acetic anhydride, and while the 
solution was being cooled with ice, a liquid mixture of 1.5 ml of 
perchloric acid and 25 ml of acetic anhydride was added dropwise thereto. 
The resulting solution was then made to react for two hours at 90.degree. 
C. Afterwards, 700 ml of isopropyl ether was added to the reaction liquid 
to precipitate the reaction product. Subsequently, the crystals were 
separated by filtration and washed with isopropyl ether and water. The 
precipitate was recrystallized in a mixture of acetone and methanol to 
obtain, after drying, 7.2 g of polymethine compound No.(5 ). 
The compound No.(5 ), synthesized in the manner described above, exhibited, 
in acetonitrile, a maximum absorptivity to light having a wavelength of 
848.2 nm, which is in the near infrared range. The yield was 35 %. 
Elementary analysis values (C85H75N4ClO4), 
______________________________________ 
Analysis values: 
C: 81.56% H: 6.02% N: 4.46% 
(Theoretical values: 
C: 81.54% H: 6.04% N: 4.48%) 
______________________________________ 
Example 1 
A solution was prepared by dissolving 3 parts by weight of the polymethine 
dye compound No.(5 ) in 97 parts by weight of diacetone alcohol, and this 
solution was applied, by spinner coating, to polycarbonate (hereinafter 
referred to as "PC") substrate having a diameter of 130 mm and a thickness 
of 1.2 mm and equipped with pregrooves formed by injection molding. Then, 
after drying, an organic thin-film recording layer having a thickness of 
800 .ANG. was obtained. Further, two substrates were glued together by 
using an ultraviolet (UV) curing resin, using a spacer having a thickness 
of 0.3 mm, thereby obtaining an optical recording medium of an 
air-sandwiched structure. 
The optical recording medium, prepared in this way, was attached to a turn 
table, which was rotated by a motor at 1800 rpm. While the optical 
recording medium was being thus rotated, a laser beam was applied to the 
recording layer from the side of the substrate, by using a semiconductor 
laser having an oscillation wavelength of 830 nm, in a spot size of 1.5 
.mu.m, with a recording power of 8 mW and at a recording frequency of 3 
MHz, so as to form a pit on the recording layer, thereby writing 
information onto it. The information was reproduced by using same laser 
with a reading power of 0.6 mW, and the reproduction waveform thereof was 
analyzed by spectral analysis (using a scanning filter; band width: 30 
KHz) to measure the C/N ratio. 
Then, the information recorded under the above conditions was reproduced 
10.sup.5 times. After that, the C/N ratio was measured. 
Further, a recording medium prepared in the same manner was left to stand 
for 2000 hours at 65.degree. C. and 85% RH so as to test the medium for 
environmental stability. Then, the transmittance (measured with respect to 
a 830 nm laser) and C/N ratio after that were measured. The measurement 
results are given in Table 1. 
Examples 2.about.5 
Optical recording mediums of Examples 2 to 5 were prepared in the same 
manner as in Example 1 except that polymethine-type dye compounds No.(3 ), 
(4 ), (14 ) and (29 ) were used instead of compound No.(5 ). 
The optical recording mediums of Examples 2 to 5 thus obtained were 
measured in the same manner as in Example 1. The measurement results are 
given in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Reproduction 
Environmental 
repeated 
preservation stability 
Initial stage 
10.sup.5 times 
65.degree. C., 85% RH approx. 2000 hrs. 
Trans- 
C/N 
C/N Trans- C/N 
mittance 
ratio 
ratio mittance 
ratio 
Compound No. 
(%) (dB) 
(dB) (%) (dB) 
__________________________________________________________________________ 
Ex. 1 
(5) 
22.6 55 53 24.3 53 
Ex. 2 
(3) 
20.4 55 52 22.8 53 
Ex. 3 
(4) 
19.8 56 54 21.3 52 
Ex. 4 
(14) 
18.6 55 52 22.0 51 
Ex. 5 
(29) 
21.5 54 52 23.3 52 
__________________________________________________________________________ 
Examples 6 and 7 
The following compounds, No.32 and 33, were respectively mixed with the 
polymethine-type dye compound No.(5 ), each in a ratio by weight of 1:2, 
in diacetone alcohol. The solutions thus prepared were applied to 
substrates in the same manner as in Example 1 to form organic thin-film 
recording layers having a thickness of 850 .ANG. after drying, thereby 
preparing optical recording mediums of Examples 6 and 7. 
The recording mediums of Examples 6 and 7, prepared in this way, were 
measured in the same manner as in Example 1. The measurement results are 
given in Table 2. 
##STR71## 
Comparative Examples 1 and 2 
Optical recording mediums were prepared in the same manner as in Examples 6 
and 7 except that the compound No.(5), used in Examples 6 and 7, was 
excluded. The optical recording mediums thus prepared were measured in the 
same manner as in the above examples. The measurement results are given in 
Table 2. 
TABLE 2 
__________________________________________________________________________ 
Reproduction 
Environmental 
repeated 
preservation stability 
Initial stage 
10.sup.5 times 
65.degree. C., 85% RH approx. 2000 hrs. 
Trans- 
C/N 
C/N Trans- C/N 
mittance 
ratio 
ratio mittance 
ratio 
Compound No. 
(%) (dB) 
(dB) (%) (dB) 
__________________________________________________________________________ 
Ex. 6 
(5), (32) 
21.5 53 50 26.9 48 
Ex. 7 
(5), (32) 
20.4 54 51 24.0 50 
Comp. 
(32) 22.1 50 45 34.2 39 
Ex. 1 
Comp. 
(33) 19.8 54 49 25.0 47 
Ex. 2 
__________________________________________________________________________ 
Example 8 
A solution was prepared by mixing 2 parts by weight of compound No.(2 ) and 
1 part by weight of a nitrocellulose resin (Ohareslacker manufactured by 
Daiseru Kagaku Kabushiki-Kaisha) with 97 parts by weight of diacetone 
alcohol. This solution was applied, by spinner coating, to a PC substrate 
having a diameter of 130 mm and a thickness of 1.2 mm and equipped with 
pregrooves formed by injection molding, thereby obtaining an organic 
thin-film recording layer having a thickness of 1000 .ANG. after drying. 
The optical recording medium, prepared in this way, was measured in the 
same manner as in Example 1. The measurement results are given in Table 3. 
Example 9 
An optical recording medium was prepared in the same manner as in Example 
8, using compound No.(23 ) instead of the compound No.(2 ) used in Example 
8. 
The optical recording medium of Example 9, prepared in this way, was 
measured in the same manner as in Example 1. The measurement results are 
given in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Reproduction 
Environmental 
repeated 
preservation stability 
Initial stage 
10.sup.5 times 
65.degree. C., 85% RH approx. 2000 hrs. 
Trans- 
C/N 
C/N Trans- C/N 
mittance 
ratio 
ratio mittance 
ratio 
Compound No. 
(%) (dB) 
(dB) (%) (dB) 
__________________________________________________________________________ 
Ex. 8 
(2) 
23.2 54 51 25.6 51 
Ex. 9 
(23) 
22.5 54 52 24.7 52 
__________________________________________________________________________ 
Example 10 
Pregrooves were formed, by the heat press method, on a PC substrate of a 
wallet size having a thickness of 0.4 mm. and a solution prepared by 
mixing 3 parts by weight of the polymethine-type dye compound No.(5 ) with 
97 parts by weight of diacetone alcohol was applied to the surface of the 
substrate by the bar coating method. After drying, an organic thin-film 
recording layer having a thickness of 1000 .ANG. was obtained. Further, a 
PC substrate of a wallet size having a thickness of 0.3 mm was brought 
into close contact with the above substrate by the heat roll method using 
an ethylene-vinyl acetate film, thereby preparing an optical card of a 
close-contact structure. 
The optical recording medium, prepared in this way, was attached to a stage 
adapted to be driven in the X-Y direction, and a laser beam of a 
semiconductor laser having an oscillation wavelength of 830 nm was applied 
to the optical card from the side of the 0.4 mm thick PC substrate, in a 
spot size of 3.0 m, with a recording power of 3.5 mW, and in response to 
recording pulses of 50 .mu.sec, thereby writing information onto the card 
in the Y-direction. The information, recorded in this way, was reproduced 
with a reading power of 0.3 mW to measure the contrast ratio: 
##EQU1## 
A recording medium prepared under the above conditions was tested for 
environmental preservation stability under the same measurement conditions 
as in Example 1, and the transmittance and contrast ratio were measured. 
Further, a recording medium prepared under the above conditions was 
subjected to fade-meter light stability test (for 10 hours) to measure the 
transmittance thereof. The measurement results are given in Table 4. 
Example 11 
A recording medium was prepared in the same manner as in Example 10 except 
that compound No.(2 ) was used instead of the compound No.(5 ) used in 
Example 10. The optical recording medium of Example 11, thus prepared, was 
measured in the same manner as in Example 10. The measurement results are 
given in Table 4. 
Comparative Examples 3 and 4 
Recording mediums were prepared in the same manner as in Example 10 except 
that compounds No.(32 ) and (33 ) were used instead of the compound No.(5 
) used in Example 10. The optical recording mediums thus prepared were 
measured in the same manner as in Example 10. The measurement results are 
given in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Environmental 
preservation 
stability 65.degree. C., 
Fade-meter 
Initial stage 
85% RH after 2000 hrs. 
light stability test 
Trans- 
Contrast 
Trans- 
Contrast 
after 10 hrs. 
mittance 
ratio 
mittance 
ratio Transmittance 
Compound No. 
(%) (dB) (%) (dB) (%) 
__________________________________________________________________________ 
Ex. 10 (5) 
19.7 0.62 21.5 0.60 30.4 
Ex. 11 (2) 
21.3 0.62 23.5 0.60 36.8 
Comp. Ex. 3 
(32) 
20.7 0.55 31.4 0.42 89.3 
Comp. Ex. 4 
(33) 
18.5 0.59 24.7 0.49 65.4 
__________________________________________________________________________ 
Example 12 
An Optical card was prepared in the same manner as that of Example 10 
except that the coating solution for forming the recording layer was 
prepared by mixing 2.4 parts by weight of the above polymethine compound 
No.(5 ) and 0.6 parts by weight of a stabilizer which can be expressed by 
the following formula (34 ) with 97 parts by weight of diacetone alcohol. 
The optical card, prepared in this way, was measured for the transmittance 
and contrast ratio both in the initial stage and after being allowed to 
stand for 2000 hours under the conditions of 65.degree. C. and 85% RH, and 
for the transmittance after a fade-meter light stability test (lasting 100 
hours). The results of the measurement are given in Table 5. 
##STR72## 
TABLE 5 
__________________________________________________________________________ 
Environmental 
preservation 
stability 65.degree. C., 
Fade-meter 
Initial stage 
85% RH after 2000 hrs. 
light stability test 
Trans- 
Contrast 
Trans- 
Contrast 
after 100 hrs. 
mittance 
ratio 
mittance 
ratio Transmittance 
Compound No. 
(%) (dB) (%) (dB) (%) 
__________________________________________________________________________ 
Ex. 12 
(5), (34) 
20.5 0.60 22.1 0.58 22.4 
__________________________________________________________________________ 
Examples 13 and 14 
PC substrates of a wallet size (85 mm.times.54 mm) having a thickness of 
0.4 mm and equipped with stripe-like pregrooves formed by the 2P method 
and having a width of 3 .mu.m, a pitch of 12 .mu.m and a depth of 3000 
.ANG., were prepared as substrates for optical cards. 
Further, a coating solution for forming recording layers was prepared by 
dissolving the polymethine dye compound No.(5 ) and (24 ) of the present 
invention in 2,2,3,3-tetrafluoropropanol. Then, this coating solution was 
applied to the sides of the above optical-card substrates having the 
pregrooves by the roll coating method, thereby forming recording layers. 
The process was repeated in succession on 100 optical-card substrates. 
Next, PC protective substrates having a thickness of 0.3 mm were glued to 
the recording layers using hot-melt adhesive sheets of an 
ethylene-vinyl-acetate-copolymer type, thereby producing optical cards. 
Reproduction was performed on the first and the 100-th of the optical 
cards, prepared in this way, by using an optical-card 
recording/reproducing apparatus (manufactured by Canon Inc.), and the 
signal waveforms thereof were observed to measure the noise levels of the 
optical cards at 70 KHz (video band width: 1 KHz). The measurement results 
are given in Table 6. 
Comparative Examples 5 and 6 
Optical cards were prepared in the same manner as in Examples 13 and 14 
except that compounds which can be expressed by the following formulas (35 
) and (36 ) were used instead of the polymethine compounds (5 ) and (24 ) 
of Examples 13 and 14, and were measured in the same manner. The results 
of the measurement are given in Table 6. 
##STR73## 
TABLE 6 
______________________________________ 
Noise level (dBm) 
Compound No. First card 
100-th card 
______________________________________ 
Ex. 13 (5) -80.7 -80.5 
Ex. 14 (24) -79.8 -79.6 
Comp. Ex. 5 
(35) -63.2 -49.8 
Comp. Ex. 6 
(36) -68.5 -56.1 
______________________________________ 
As can be seen from Table 6, the 100-th optical cards in Examples 13 and 14 
are little changed in noise level as compared to the first optical cards, 
whereas the noise levels of the 100-th optical cards in Comparative 
Examples 1 and 2 have been remarkably raised as compared to those of the 
first optical cards. 
It is assumed that this is attributable to the fact that the 
polymethine-type dye compounds used in Comparative Examples 5 and 6 did 
not have satisfactory solubility in solvent. That is, when recording 
layers were formed by applying the coating solution to a plurality of 
substrates, even a slight change in concentration of the solution was 
enough to cause crystallization of minute crystals of the pigment 
compounds in the solution. Such crystallization was transferred to the 
substrates, resulting in an increase in noise level. 
The polymethine-type compounds of Examples 13 and 14, in contrast, had high 
solubility in solvent, so that it is hard for crystallization to occur in 
the coating solution if there is some change in concentration of the 
solution. Accordingly, it is to be assumed that the recording layers are 
protected from being mixed with solid particles thus making it possible to 
always obtain optical cards having a low noise level.