Thermal dye transfer system containing a N-arylimidoethylidene-benz[C,D]indole dye precursor

A thermal dye transfer assemblage comprising: PA1 (a) a dye-donor element comprising a support having thereon a dye layer comprising a dye dispersed in a polymeric binder, the dye comprising an N-arylimido-ethylidene-benz[c,d]indole dye precursor, and PA1 (b) a dye-receiving element comprising a support having thereon a polymeric dye image-receiving layer, the dye-receiving element being in a superposed relationship with the dye-donor element so that the dye layer is in contact with the dye image-receiving layer, the dye image-receiving layer containing an organic acid which is capable of converting the dye precursor into a cationic magenta anilinovinyl-benz[c,d]indolium dye.

This invention relates to a thermal dye transfer system and, more 
particularly, to an electrically neutral 
N-arylimidoethylidenebenz[c,d]indole dye precursor useful in thermal dye 
transfer imaging systems in which the receiver layer contains an acid 
moiety which is capable of converting the dye precursor into a cationic 
magenta anilinovinyl-benz[c,d]indolium dye. 
In recent years, thermal transfer systems have been developed to obtain 
prints from pictures which have been generated electronically from a color 
video camera. According to one way of obtaining such prints, an electronic 
picture is first subjected to color separation by color filters. The 
respective color-separated images are then converted into electrical 
signals. These signals are then operated on to produce cyan, magenta and 
yellow electrical signals. These signals are then transmitted to a thermal 
printer. To obtain the print, a cyan, magenta or yellow dye-donor element 
is placed face-to-face with a dye-receiving element. The two are then 
inserted between a thermal printing head and a platen roller. A line-type 
thermal printing head is used to apply heat from the back of the dye-donor 
sheet. The thermal printing head has many heating elements an is heated up 
sequentially in response to one of the cyan, magenta or yellow signals, 
and the process is then repeated for the other two colors. A color hard 
copy is thus obtained which corresponds to the original picture viewed on 
a screen. Further details of this process and an apparatus for carrying it 
out are contained in U.S. Pat. No. 4,621,271, the disclosure of which is 
hereby incorporated by reference. 
Dyes for thermal dye transfer imaging should have bright hue, good 
solubility in coating solvents, good transfer efficiency and good light 
stability. A dye receiver polymer should have good affinity for the dye 
and provide a stable (to heat and light) environment for the dye after 
transfer. In particular, the transferred dye image should be resistant to 
damage caused by handling, or contact with chemicals or other surfaces 
such as the back of other thermal prints, adhesive tape, and plastic 
folders, generally referred to as "retransfer". 
Commonly-used dyes are nonionic in character because of the easy thermal 
transfer achievable with this type of compound. The dye-receiver layer 
usually comprises an organic polymer with polar groups to act as a mordant 
for the dyes transferred to it. A disadvantage of such a system is that 
since the dyes are designed to be mobile within the receiver polymer 
matrix, the prints generated can suffer from dye migration over time. 
A number of attempts have been made to overcome the dye migration problem 
which usually involves creating some kind of bond between the transferred 
dye and the polymer of the dye image-receiving layer. One such approach 
involves the transfer of a cationic dye to an anionic dye-receiving layer, 
thereby forming an electrostatic bond between the two. However, this 
technique involves the transfer of a cationic species which, in general, 
is less efficient than the transfer of a nonionic species. 
U.S. Pat. No. 4,880,769 describes the thermal transfer of a neutral, 
deprotonated form of a cationic dye (dye precursor) to a receiver element, 
followed by protonation to the cationic dye and U.S. Pat. No. 4,137,042 
relates to transfer printing onto fabrics using dye precursors. 
There is a problem with using the dye precursors of the prior art in that 
the transfer efficiency for dye precursors which form a magenta cationic 
dye is low. 
It is an object of this invention to provide a thermal dye transfer system 
employing a dye-receiver having an acidic dye image-receiving layer which 
upon transfer of the dye forms a dye/counterion complex which is 
substantially immobile, which would reduce the tendency to retransfer to 
unwanted surfaces. It is another object of this invention to provide dye 
precursors which are more efficient, i.e., yield higher transferred dye 
densities. 
This and other objects are achieved in accordance with this invention which 
relates to a thermal dye transfer assemblage comprising: 
(a) a dye-donor element comprising a support having thereon a dye layer 
comprising a dye dispersed in a polymeric binder, the dye comprising an 
N-arylimido-ethylidene-benz[c,d]indole dye precursor, and 
(b) a dye-receiving element comprising a support having thereon a polymeric 
dye image-receiving layer, the dye-receiving element being in a superposed 
relationship with the dye-donor element so that the dye layer is in 
contact with the dye image-receiving layer, the dye image-receiving layer 
containing an organic acid which is capable of converting the dye 
precursor into a cationic magenta anilinovinyl-benz[c,d]indolium dye. 
In accordance with the invention, it has been found that 
N-arylimido-ethylidene-benz[c,d]indole dye precursors give much higher 
transferred densities upon transfer to an acidic receiver than do 
previously described dye precursors. 
In a preferred embodiment of the invention, the dye precursors have the 
general formula: 
##STR1## 
wherein: R.sub.1 represents a substituted or unsubstituted alkyl group of 
1-10 carbon atoms, a substituted or unsubstituted cycloalkyl group of 5-8 
carbon atoms, a substituted or unsubstituted aryl group of 6-10 carbon 
atoms, a substituted or unsubstituted hetaryl group of 5-10 atoms or a 
substituted or unsubstituted allyl group; 
R.sub.2 represents a substituted or unsubstituted aryl group of 6-10 carbon 
atoms or a substituted or unsubstituted hetaryl group of 5-10 atoms; and 
X and Y each independently represents hydrogen or one or more groups 
selected from halogen, cyano, alkyl, aryl, hetaryl, nitro, carboxy, 
alkoxy, aryloxy, alkoxycarbonyl, aryloxycarbonyl, acyloxy, aryloxy, amino, 
acylamino, arylsulfonamido, alkylsulfonamido, hydroxy, alkylcarbamoyl, 
dialkylcarbamoyl, arylcarbamoyl, diarylcarbamoyl, arylalkylcarbamoyl, 
alkylureido, arylureido, alkylthio, arylthio, etc. 
In a preferred embodiment of the invention, in the above formula, R.sub.1 
is CH.sub.3, R.sub.2 is phenyl, 2,4-dimethoxyphenyl, 2-methoxy-phenyl, 
4-methoxyphenyl or 2,5-dichlorophenyl, and X and Y are both hydrogen. 
The above dye precursors can be readily prepared by neutralization with 
base (see Example of the corresponding delocalized cationic dyes which 
have been described as intermediates in the production of cyanine and 
merocyanine photographic sensitizing dyes [see Helv. Chim. Acta.,70, 
1583(1987), and Khim Geterotsikl. Soedin., 340(1973) {see Chem. Abstr. 79, 
39629}]. The delocalized cationic dyes may be prepared as described in 
these references or they may be prepared by an adaptation of the procedure 
described for Basic Yellow 11 on page 194 in "The Chemistry and 
Application of Dyes", D. R. Waring and G. Hallas (ed.), 1990, Plenum 
Press, New York. 
The structures of the dye precursors of the invention and proposed cationic 
dye formed upon thermal transfer to a receiver containing an acidic moiety 
are illustrated below. 
##STR2## 
Following are examples of the dye precursors within the scope of the 
invention: 
______________________________________ 
##STR3## 
.lambda.-max 
.lambda.-max 
(.epsilon.)* 
Dye 
Dye (.epsilon.)* 
[ethanol + 
Molecular 
Precursor 
R [ethanol] 
HCl] Weight 
______________________________________ 
1 H 479 513 284 
(30,500) (40,400) 
2 2,4-(CH.sub.3 O).sub.2 
488 534 332 
(28,200) (32,000) 
3 2,5-(Cl).sub.2 
479 502 353 
(28,600) (33,000) 
4 2-CH.sub.3 O 
481 521 314 
(29,000) (36,400) 
5 4-CH.sub.3 O 
487 531 314 
(26,500) (31,700) 
______________________________________ 
*(.epsilon.) is the molar absorptivity or extinction coefficient 
The polymeric dye image-receiving layer employed in the invention contains 
an organic acid, such as a sulfonic acid, a carboxylic acid, a phosphonic 
acid, a phosphoric acid or a phenol as part of the polymer chain, or 
contains a separately added organic acid. The polymeric dye 
image-receiving layer acts as a matrix for the magenta dye and the acid 
functionality within the dye image-receiving layer will convert the dye 
precursor to a magenta cationic dye. 
Organic acids which can be separately added to the polymer to provide its 
acidic nature generally comprise ballasted organic acids, e.g., carboxylic 
acids such as palmitic acid, 2-(2,4-di-tert-amylphenoxy)butyric acid, 
etc.; phosphonic/phosphoric acids such as monolauryl ester of phosphoric 
acid, dioctyl ester of phosphoric acid, dodecyl-phosphonic acid, etc.; 
sulfonic acids such as hexadecanesulfonic acid, p-octyloxybenzenesulfonic 
acid; a phenol such as 3,5-di-tert-butyl-salicylic acid, etc. 
Any type of polymer may be employed in the receiver e.g., condensation 
polymers such as polyesters, polyurethanes, polycarbonates, etc.; addition 
polymers such as polystyrenes, vinyl polymers, etc.; block copolymers 
containing large segments of more than one type of polymer covalently 
linked together; provided such polymeric material contains acid groups 
either as part of the polymer chain or as a separately added organic acid. 
In a preferred embodiment of the invention, the dye image-receiving layer 
comprises a polyester, an acrylic polymer, a styrene polymer or a phenolic 
resin. In another preferred embodiment of the invention, the dye 
image-receiving layer comprises a polyester ionomer as described in 
copending application Ser. No. 08/469,132, filed of even date herewith, by 
Bowman, Shuttleworth and Weber, and entitled "Thermal Dye Transfer System 
With Polyester Ionomer Receiver". 
The following receiver polymers may be used in accordance with the 
invention: 
______________________________________ 
Receiver 1 
poly(butyl acrylate-co-2-acrylamido-2- 
methyl-propanesulfonic acid) 75:25 
Receiver 2 
poly(2-ethylhexyl acrylate-co-2- 
acrylamido-2-methyl-propanesulfonic 
acid) 75:25 
Receiver 3 
poly(2-ethylhexyl methacrylate-co-2- 
acrylamido-2-methyl-propanesulfonic 
acid) 75:25 
Reciever 4 
poly(2-hexyl methacrylate-co-2- 
acylamido-2-methyl-propanesulfonic 
acid) 75:25 
Receiver 5 
poly(butyl acrylate-co-methylacrylic 
acid) 75:25 
Receiver 6 
poly(butyl acrylate-co-2-acrylamido-2- 
methyl-propanesulfonic acid-co-methyl 2- 
acrylamido-2-methoxyacetate) 65:25:10 
Receiver 7 
poly(hexyl methacrylate-co-2-sulfoethyl 
methacrylate-co-2-acrylamido-2- 
methoxyacetate) 65:25:10 
Receiver 8 
polystyrenesulfonic acid 
Receiver 9 
poly(ethyl methacrylate-co-2-sulfoethyl 
methacrylate) 75:25 
Receiver 10 
poly(methyl methacrylate-co-2-sulfoethyl 
methacrylate) 75:25 
Receiver 11 
N-15 Novolak (a phenolic resin, Eastman 
Chemical Co.) 
Receiver 12 
3.23 g/m.sup.2 Poly(2-phenylethyl 
methacrylate) (Scientific Polymer 
Products Inc.) containing 0.54 g/m.sup.2 of 
3,5-di-t-butylsalicyclic acid 
Receiver 13 
##STR4## 
##STR5## 
The polymer in the dye image-receiving layer may be present in any 
amount which is effective for its intended purpose. In general, good 
results have been obtained at a concentration of from about 0.5 to about 
10 g/m.sup.2. The polymers may be coated from organic solvents or water, 
The support for the dye-receiving element employed in the invention may be 
transparent or reflective, and may comprise a polymeric, a synthetic 
paper, or a cellulosic paper support, or laminates thereof. Examples of 
transparent supports include films of poly(ether sulfone)s, poly(ethylene 
naphthalate), polyimides, cellulose esters such as cellulose acetate, 
poly(vinyl alcohol-co-acetal)s, and poly(ethylene terephthalate). The 
support may be employed at any desired thickness, usually from about 10 
.mu.m to 1000 .mu.m. Additional polymeric layers may be present between 
the support and the dye image-receiving layer. For example, there may be 
employed a polyolefin such as polyethylene or polypropylene. White 
pigments such as titanium dioxide, zinc oxide, etc., may be added to the 
polymeric layer to provide reflectivity. In addition, a subbing layer may 
be used over this polymeric layer in order to improve adhesion to the dye 
image-receiving layer. Such subbing layers are disclosed in U.S. Pat. Nos. 
4,748,150, 4,965,238, 4,965,239, and 4,965,241, the disclosures of which 
are incorporated by reference. The receiver element may also include a 
backing layer such as those disclosed in U.S. Pat. Nos. 5,011,814 and 
5,096,875, the disclosures of which are incorporated by reference. In a 
preferred embodiment of the invention, the support comprises a microvoided 
thermoplastic core layer coated with thermoplastic surface layers as 
described in U.S. Pat. No. 5,244,861, the disclosure of which is hereby 
incorporated by reference. 
Resistance to sticking during thermal printing may be enhanced by the 
addition of release agents to the dye-receiving layer or to an overcoat 
layer, such as silicone-based compounds, as is conventional in the art. 
Dye-donor elements that are used with the dye-receiving element of the 
invention conventionally comprise a support having thereon a dye layer 
containing the dyes as described above dispersed in a polymeric binder 
such as a cellulose derivative, e.g., cellulose acetate hydrogen 
phthalate, cellulose acetate, cellulose acetate propionate, cellulose 
acetate butyrate, cellulose triacetate, or any of the materials described 
in U.S. Pat. No. 4,700,207; or a poly(vinyl acetal) such as poly(vinyl 
alcohol-co-butyral). The binder may be used at a coverage of from about 
0.1 to about 5 g/m.sup.2. 
As noted above, dye-donor elements are used to form a dye transfer image. 
Such a process comprises imagewise-heating a dye-donor element and 
transferring a dye image to a dye-receiving element as described above to 
form the dye transfer image. 
In a preferred embodiment of the invention, a dye-donor element is employed 
which comprises a poly(ethylene terephthalate) support coated with 
sequential repeating areas of a dye precursor as described above capable 
of generating a magenta dye, a cyan and a yellow dye, and the dye transfer 
steps are sequentially performed for each color to obtain a three-color 
dye transfer image. Of course, when the process is only performed for a 
single color, then a monochrome dye transfer image is obtained. 
Thermal print heads which can be used to transfer dye from dye-donor 
elements to the receiving elements of the invention are available 
commercially. There can be employed, for example, a Fujitsu Thermal Head 
(FTP-040 MCS001), a TDK Thermal Head F415 HH7-1089 or a Rohm Thermal Head 
KE 2008-F3. Alternatively, other known sources of energy for thermal dye 
transfer may be used, such as lasers. 
When a three-color image is to be obtained, the assemblage described above 
is formed on three occasions during the time when heat is applied by the 
thermal printing head. After the first dye is transferred, the elements 
are peeled apart. A second dye-donor element (or another area of the donor 
element with a different dye area) is then brought in register with the 
dye-receiving element and the process repeated. The third color is 
obtained in the same manner. After thermal dye transfer, the dye 
image-receiving layer contains a thermally-transferred dye image.

The following examples are provided to further illustrate the invention. 
EXAMPLE 1 
Preparation of N-arylimidoethyliene-benz[c,d]indole magenta dye precusor 
A solution of 1.4 g (0.00436 mole) of 
1-methyl-2-(2-anilinovinyl)-benz[c,d]indolium chloride in 15 mL of 
methanol is added slowly to a mixture of 50 mL ethyl acetate, 20 mL 10% 
aqueous sodium carbonate and 10 mL 10% aqueous sodium hydroxide. The ethyl 
acetate layer is separated, washed with water and saturated sodium 
chloride and evaporated to dryness. Recrystallization of the residue from 
15 mL methanol yields 0.9 g (72%) of Dye Precursor 1 as a brown solid. 
Other dye precursors of the invention can be prepared in an analogous 
manner. 
EXAMPLE 2 
Dye-donor elements were prepared by coating on a 6 .mu.m poly(ethylene 
terephthalate) support: 
1) a subbing layer of Tyzor TBT.RTM., a titanium tetrabutoxide, (DuPont 
Company) (0.16 g/m.sup.2) coated from 1-butanol; and 
2) a dye layer containing dye precursors 1-5 of the invention and Control 
Dye C-1 and Control Dye C-2 shown below, and FC-431.RTM. fluorocarbon 
surfactant (3M Company) (0.01 g/m.sup.2) in a Butvar.RTM. 76 poly(vinyl 
butyral) binder, (Monsanto Company) coated from a tetrahydrofuran and 
cyclopentanone solvent mixture (95:5). 
Details of dye and binder laydowns are tabulated in Table 1 below. Dye 
levels were adjusted for differences in dye molecular weight and molar 
extinction coefficient to ensure a more accurate evaluation of transfer 
efficiency. The dye:binder ratios were held constant. 
On the back side of the dye-donor element was coated: 
1) a subbing layer of Tyzor TBT.RTM., a titanium tetrabutoxide, (DuPont 
Company) (0.16 g/m.sup.2) coated from 1-butanol; and 
2) a slipping layer of Emralon 329.RTM. (Acheson Colloids Co.), a dry film 
lubricant of poly(tetrafluoroethylene) particles in a cellulose nitrate 
resin binder (0.54 g/m.sup.2) and S-nauba micronized carnauba wax (0.016 
g/m.sup.2) coated from a n-propyl acetate, toluene, isopropyl alcohol and 
n-butyl alcohol solvent mixture. 
TABLE 1 
______________________________________ 
Magenta Dye Dye Laydown Binder Laydown 
Precursor (g/m.sup.2) (g/m.sup.2) 
______________________________________ 
1 0.22 0.24 
2 0.32 0.35 
3 0.33 0.36 
4 0.26 0.28 
5 0.28 0.30 
C-1 0.22 0.24 
C-2 0.26 0.28 
______________________________________ 
##STR6## 
Control Dye C-1 
.lambda.-max(ethanol) = 459 
.lambda.-max(ethanol + HCl) = 522 
(.epsilon. = 44,700) 
molecular weight = 336 
##STR7## 
Control Dye C-2 
Example 9 of U.S. Pat. No. 4,137,042 
.lambda.-max(ethanol) = 464 
.lambda.-max(ethanol + HCl) = 539 
(.epsilon. = 43,700) 
molecular weight = 368 
Dye-receiver elements according to the invention were prepared by first 
extrusion laminating a paper core with a 38.mu. thick microvoided 
composite film (OPPalyte 350TW.RTM., Mobil Chemical Co.) as disclosed in 
U.S. Pat. No. 5,244,861. The composite film side of the resulting laminate 
was then coated with the following layers in the order recited: 
1) a subbing layer of Polymin Waterfree.RTM. polyethyleneimine (BASF, 0.02 
g/m.sup.2), and 
2) a dye-receiving layer composed of the receiver polymer 13 above (5.38 
g/m.sup.2) and a fluorocarbon surfactant (Fluorad FC-170C.RTM., 3M 
Corporation, 0.022 g/m.sup.2) coated from water. 
Preparation and Evaluation of Thermal Dye Transfer Images 
Eleven-step sensitometric thermal dye transfer images were prepared from 
the above dye-donor and dye-receiver elements. The dye side of the 
dye-donor element approximately 10 cm.times.15 cm in area was placed in 
contact with the dye image-receiving layer side of a dye-receiving element 
of the same area. This assemblage was clamped to a stepper motor-driven, 
60 mm diameter rubber roller. A thermal head (TDK No. 8I0625, 
thermostatted at 31.degree. C.) was pressed with a force of 24.4 newtons 
(2.5 kg) against the dye-donor element side of the assemblage, pushing it 
against the rubber roller. 
The imaging electronics were activated causing the donor-receiver 
assemblage to be drawn through the printing head/roller nip at 11.1 mm/s. 
Coincidentally, the resistive elements in the thermal print head were 
pulsed (128 .mu.s/pulse) at 129 .mu.s intervals during a 16.9 .mu.s/dot 
printing cycle. A stepped image density was generated by incrementally 
increasing the number of pulses/dot from a minimum of 0 to a maximum of 
127 pulses/dot. The voltage supplied to the thermal head was approximately 
9.25 v resulting in an instantaneous peak power of 0.175 watts/dot and a 
maximum total energy of 2.84 mJ/dot. 
After printing, each dye-donor element was separated from the imaged 
receiving element and the Status A green reflection density of each of the 
eleven steps in the stepped-image was measured with a reflection 
densitometer. The maximum reflection density is listed in Table 2. 
TABLE 2 
______________________________________ 
Maximum Transferred 
Magenta Reflection Density 
Dye Precursor (Status A Green) 
______________________________________ 
1 1.9 
2 2.6 
3 2.3 
4 2.8 
5 3.1 
C-1 1.6 
C-2 1.7 
______________________________________ 
As the above results show, the N-arylimidoethylidene benz[c,d]indole 
magenta dye precursors of the invention provide higher maximum transferred 
densities (are more efficient) than the magenta dye precursors of the 
prior art. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.