Receiver for thermally-transferable fluorescent europium complexes.

0001This invention relates to a receiving element which is used with a donor element containing a 6-coordinate europium(III) complex to form a higher coordinate complex.

0002In 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 and is heated up sequentially in response to the cyan, magenta and yellow signals. 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. Patent No. 4,621,271 by Brownstein entitled "Apparatus and Method For Controlling A Thermal Printer Apparatus," issued November 4, 1986.

0003The system described above has been used to obtain visible dye images. However, for security purposes, to inhibit forgeries or duplication, or to encode confidential information, it would be advantageous to create non-visual ultraviolet absorbing images that fluoresce with visible emission when illuminated with ultraviolet light.

0004U.S. Patents 4,876,237, 4,871,714, 4,876,234, 4,866,025, 4,860,027, 4,891,351, and 4,891,352 all relate to thermally-transferable fluorescent materials used in a continuous tone system. However, none of those materials fluoresce a visible red color when illuminated with ultraviolet light, and none of them describe ligands for use in the receiving element. A red color is desirable for many security applications. EP-A-211754 discloses a system wherein a solution of a ligand is applied to a paper impregnated with a Europium (III) salt or vice versa.

0005U.S. Patent 4,627,997 discloses a fluorescent thermal transfer recording medium comprising a thermally-meltable, wax ink layer. It is an object of this invention to provide a receiving element which contains ligands to react with fluorescent materials transferred from a donor element.

0006These and other objects are achieved in accordance with this invention which comprises a receiving element for thermal transfer comprising a support having thereon a polymeric image-receiving layer, wherein the image-receiving layer also contains a monodentate or bidentate ligand capable of reacting with a 6-coordinate europium(III) complex to form a higher coordinate complex, the higher coordinate complex which is formed in situ in the receiving layer having the following formula: <chemistry id="chem0001" num="0001"><img file="EP0446834B1_D0001.tif" /></chemistry><dl id="dl0001"><dt>wherein:</dt><dd>D is substituted or unsubstituted aromatic, 5- or 6-membered carbocyclic or heterocyclic moiety; J is -CF₃, -CH₃ -CH₂F or -CHF₂; and B represents at least one monodentate ligand with an electron-donating oxygen or nitrogen atom, e.g., tri-n-octylphosphine oxide, pyridine-N-oxide or triphenylphosphine oxide; or at least one bidentate ligand with two electron-donating oxygen, nitrogen or sulfur atoms atoms capable of forming a 5-or 6-membered ring with the europium atom, e.g., 2,2′-bipyridine, 1,10-phenanthroline, ethylene diamine or 1,2-diaminobutane.</dd></dl>

0007In a preferred embodiment of the invention, the 6-coordinate europium(III) complex, which is supplied from a donor element, has the formula: <chemistry id="chem0002" num="0002"><img file="EP0446834B1_D0002.tif" /></chemistry><dl id="dl0002"><dt>wherein:</dt><dd>D is a substituted or unsubstituted, aromatic, 5- or 6-membered carbocyclic or heterocyclic moiety, e.g., phenyl, 2-thienyl, 2-furyl, 3-pyridyl, etc.; and J is -CF₃, -CH₃, -CH₂F or -CHF₂.</dd></dl>

0008The above fluorescent europium complexes are essentially non-visible, but emit with a unique red hue in the region of 610 to 625 nm when irradiated with 360 nm ultraviolet light. This red hue is highly desirable for security-badging applications.

0009Europium(III) is the only rare-earth known to be suitable for the practice of the invention. Rare earth metals, including europium, are described in the literature such as S, Nakamura and N. Suzuki, Polyhedron, <u style="single">5</u>, 1805 (1986); T. Taketatsu, Talanta, <u style="single">29</u>, 397 (1982); and H. Brittain, J.C.S. Dalton, 1187 (1979).

0010Diketone ligands from which the 6-coordinate complexes are derived include the following within the scope of the invention: <chemistry id="chem0003" num="0003"><img file="EP0446834B1_D0003.tif" /></chemistry>

0011Suitable monodentate and bidentate ligands within the scope of the invention for incorporation in the receiving element include: <chemistry id="chem0004" num="0004"><img file="EP0446834B1_D0004.tif" /></chemistry><chemistry id="chem0005" num="0005"><img file="EP0446834B1_D0005.tif" /></chemistry><dl id="dl0003"><dt>H₂NCH₂CH₂NH₂</dt><dd>Ethylene diamine (Kodak Lab. Chemicals No. 1915</dd><dt>(n-C₈H₁₇)₃PO</dt><dd>Trioctylphosphine Oxide (Kodak Lab. Chemicals No. 7440)</dd></dl>

0012These emission enhancing ligands are incorporated in the receiver at up to 70 weight percent, preferably 10 to 25 weight percent of the receiving layer polymer. This corresponds to from 0.1 to 10 g/m².

0013A visible dye can also be used in a separate or the same area of the donor element used with the receiving element of the invention provided it is transferable to the dye-receiving layer by the action of heat. Especially good results have been obtained with sublimable dyes such as: <chemistry id="chem0006" num="0006"><img file="EP0446834B1_D0006.tif" /></chemistry><chemistry id="chem0007" num="0007"><img file="EP0446834B1_D0007.tif" /></chemistry><chemistry id="chem0008" num="0008"><img file="EP0446834B1_D0008.tif" /></chemistry> or any of the dyes disclosed in U.S. Patent 4,541,830. The above dyes may be employed singly or in combination to obtain a monochrome. The above image dyes and fluorescent dye may be used at a coverage of from 0.01 to 1 g/m², preferably 0.1 to 0.5 g/m².

0014The fluorescent material in the above donor element is 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; a polycarbonate; poly(styrene-co-acrylonitrile), a poly(sulfone) or a poly(phenylene oxide). The binder may be used at a coverage of from 0.1 to 5 g/m².

0015Any material can be used as the support for the donor element used with the receiver of the invention provided it is dimensionally stable and can withstand the heat of the thermal printing heads. Such materials include polyesters such as poly(ethylene terephthalate); polyamides; polycarbonates; glassine paper; condenser paper; cellulose esters such as cellulose acetate; fluorine polymers such as polyvinylidene fluoride or poly(tetrafluoroethylene-co-hexafluoropropylene); polyethers such as polyoxymethylene; polyacetals; polyolefins such as polystyrene, polyethylene, polypropylene or methylpentane polymers; and polyimides such as polyimide-amides and polyether-imides. The support generally has a thickness of from 2 to 30 »m. It may also be coated with a subbing layer, if desired.

0016When using the donor element of the invention with a resistive head, the reverse side of the donor element is coated with a slipping layer to prevent the printing head from sticking to the donor element. Such a slipping layer would comprise a lubricating material such as a surface active agent, a liquid lubricant, a solid lubricant or mixtures thereof, with or without a polymeric binder.

0017The receiving element of the invention comprises a support having thereon an image-receiving layer and the ligand described above. The support may be a transparent film such as poly(ethylene terephthalate) or reflective.

0018The image-receiving layer may comprise, for example, a polycarbonate, a polyurethane, a polyester, polyvinyl chloride, poly(styrene-<u style="single">co</u>-acrylonitrile), poly(caprolactone) or mixtures thereof.

0019As noted above, the donor elements employed in the invention are used to form a transfer image. Such a process comprises a) imagewise-heating a donor element comprising a support having on one side thereof a layer comprising a material dispersed in a polymeric binder, and on the other side thereof a slipping layer comprising a lubricant, and b) transferring an image to a receiving element comprising a support having thereon an image-receiving layer to form the transfer image, and wherein the material is a 6-coordinate europium(III) complex and the image-receiving layer also contains an uncharged monodentate or bidentate ligand capable of reacting with the 6-coordinate europium(III) complex to form a higher coordinate complex as described above.

0020The donor element employed in the invention may be used in sheet form or in a continuous roll or ribbon. If a continuous roll or ribbon is employed, it may have only the fluorescent europium complex thereon as described above, with or without an image dye, or may have alternating areas of different dyes, such as sublimable magenta and/or yellow and/or cyan and/or black or other dyes.

0021If a laser is used to transfer dye from the dye-donor employed in the invention to the receiver, then an absorptive material is used in the dye-donor. Any material that absorbs the laser energy may be used such as carbon black or non-volatile infrared-absorbing dyes or pigments which are well known to those skilled in the art.

0022A thermal transfer assemblage of the invention comprises <ul id="ul0001" list-style="none"><li>a) a donor element as described above, and</li><li>b) a receiving element as described above, the receiving element being in a superposed relationship with the donor element so that the fluorescent material layer of the donor element is in contact with the image-receiving layer of the receiving element.</li></ul>

0023The following example is provided to illustrate the invention.

Example 1

0024This example shows the enhanced fluorescence obtained by transferring 6-coordinate europium complexes from a donor to a receiver containing an auxiliary ligand.

0025A donor element was prepared by coating the following layers in the order recited on a 6 »m poly(ethylene terephthalate) support: <ul id="ul0002" list-style="none"><li>1) a subbing layer of duPont Tyzor TBT® titanium tetra-<u style="single">n</u>-butoxide (0.12 g/m²) from 1-butanol; and</li><li>2) a layer containing the 6-coordinate europium fluorescent complex with the diketone ligand, as identified above (0.38 g/m²) or comparison material identified below (0.16 g/m²) in a cellulose acetate butyrate 17% acetyl and 28% butyryl binder (0.43 g/m² or control at 0.32 g/m²) coated from a cyclopentanone, toluene and methanol solvent mixture.</li></ul> On the back side of the donor-element was coated: <ul id="ul0003" list-style="none"><li>1) a subbing layer of duPont Tyzor TBT® titanium tetra-<u style="single">n</u>-butoxide (0.12 g/m²) from 1-butanol; and</li><li>2) a slipping layer of Emralon 329® polytetrafluoroethylene dry film lubricant (Acheson Colloids) (0.54 g/m²) and S-Nauba 5021 Carnauba Wax (Shamrock Technology) (0.003 g/m²) coated from a n-propyl acetate, toluene, 2-propanol and 1-butanol solvent mixture.</li></ul>

0026A receiving element was prepared by coating a solution of Makrolon 5700® (Bayer A.G. Corporation) a bisphenol-A polycarbonate resin (2.9 g/m²), the auxiliary ligand indicated above (0.38 g/m²) or control material (0.38 g/m²) indicated below, and FC-431® surfactant (3M Corporation) (0.16 g/m²) in a methylene chloride and trichloroethylene solvent mixture on a transparent 175 »m polyethylene terephthalate support subbed with a layer of poly(acrylonitrile-co-vinylidene chloride-co-acrylic acid) (14:79:7 wt ratio) (0.005 g/m²).

0027The following control material, lacking coordinating atoms, which was coated in a receiver, is available commercially from Kodak Laboratory Products and Chemicals Division. <chemistry id="chem0009" num="0009"><img file="EP0446834B1_D0009.tif" /></chemistry>

0028The fluorescent material layer side of the donor element strip approximately 9 cm x 12 cm in area was placed in contact with the image-receiving layer of a receiver element of the same area. The assemblage was fastened in the jaws of a stepper motor driven pulling device. The assemblage was laid on top of a 14 mm diameter rubber roller and a TDK Thermal Head L-133 (No. 6-2R16-1) was pressed with a spring at a force of 36 N against the donor element side of the contacted pair pushing it against the rubber roller.

0029The imaging electronics were activated causing the pulling device to draw the assemblage between the printing head and roller at 3.1 mm/sec. Coincidentally the resistive elements in the thermal print head were pulsed at a per pixel pulse width of 8 msec to generate a maximum density image. The voltage supplied to the print-head was approximately 25 v representing approximately 1.6 watts/dot (13. mjoules/dot).

0030The receiving element was separated from the donor element and the relative emission was evaluated with a spectrofluorimeter using a fixed intensity 360 nm excitation beam and measuring the relative area under the emission spectrum from 375 to 700 nm. The following results were obtained (all transferred materials emitted between 610 and 625 nm.): <tables id="tabl0001" num="0001"><img file="EP0446834B1_D0010.tif" /></tables><tables id="tabl0002" num="0002"><img file="EP0446834B1_D0011.tif" /></tables><chemistry id="chem0010" num="0010"><img file="EP0446834B1_D0012.tif" /></chemistry> This compound is the subject of U.S. Patent 4,876,237.

0031The above results show that using an auxiliary ligand in the receiver in accordance with the invention to coordinate with the fluorescent materials supplied by a donor has much more fluorescence than the control or comparison compounds.