Process for Generation of Squaric Acid and for Imaging, and Imaging Medium for Use Therein

Attention is directed to copending application Ser. No. 07/965,172 of even date herewith and assigned to the same assignee as the present application; this copending application describes and claims a process and imaging medium generally similar to those of the present invention, but in which the breakdown of a squaric acid derivative is initiated thermally.

Attention is also directed to copending application Ser. No. 07/965,161 of even date herewith and assigned to the same assignee as the present application; this copending application describes and claims a process and imaging medium generally similar to those of the present invention but in which acid is generated using a mixture of an infra-red dye, a superacid precursor and an acid-sensitive acid generator. This mixture is exposed to an imagewise exposure to infra-red radiation, followed by a blanket exposure to ultra-violet radiation.

This invention relates to a process for generation of acid and for imaging, and to an imaging medium for use in this imaging process.

Some conventional non-silver halide photosensitive compositions, for example photoresists, contain molecules which are inherently photosensitive, so that absorption of a single quantum brings about decomposition of only the single molecule which absorbs the quantum. However, a dramatic increase in the sensitivity of such photosensitive compositions can be achieved if the photosensitive molecule initiates a secondary reaction which is not radiation-dependent and which effects conversion of a plurality of molecules for each quantum absorbed. For example, photoresist systems are known in which the primary photochemical reaction produces an acid, and this acid is employed to eliminate acid-labile groups in a secondary, radiation-independent reaction: the compound containing such acid-labile groups may hereinafter be referred to as a "secondary acid generator". See, for example, U.S. Pat. Nos. 3,932,514 and 3,915,706; Reichmanis et al., Chemical Amplification Mechanism for Microlithography, Chem. Mater., 3(3), 394 (1991) and Berry et al., Chemically Amplified Resists for I-line and G-line Applications, SPIE, 1262, 575 (1990). Also, U.S. Pat. No. 5,084,371 describes a radiation-sensitive mixture which contains a water-insoluble binder which comprises a mixture of phenolic and novolak polymers and which is soluble or dispersible in aqueous alkali, and an organic compound whose solubility in alkaline developer is increased by acid, and which also contains at least one acid-cleavable group, and in addition a further group which produces a strong acid upon exposure to radiation.

U.S. Pat. No. 4,916,046 describes a positive radiation-sensitive mixture using a monomeric silylenol ether, and a recording medium produced therefrom. This patent also contains an extensive discussion of radiation-sensitive compositions which form or eliminate an acid on irradiation. According to this patent, such radiation-sensitive compositions include diazonium, phosphonium, sulfonium and iodonium salts, generally employed in the form of their organic solvent-soluble salts, usually as deposition products with complex acids such as tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroantimonic acid and hexafluoroarsenic acid; halogen compounds, in particular triazine derivatives; oxazoles, oxadiazoles, thiazoles or 2-pyrones which contain trichloromethyl or tribromomethyl groups; aromatic compounds which contain ring-bound halogen, preferably bromine; a combination of a thiazole with 2-benzoylmethylenenaphthol; a mixture of a trihalomethyl compound with N-phenylacridone; .alpha.-halocarboxamides; and tribromomethyl phenyl sulfones.

A secondary acid generator needs to fulfil several differing requirements. It is desirable that the material generate a strong acid, since generation of a weak acid, such as the carboxylic acids generated by some prior art processes, may limit the types of acid-sensitive compound which can be used. The secondary acid generator is desirably of low molecular weight in order to reduce the amount of material required to generate a specific amount of acid. Finally, the secondary acid generator must be compatible with all the other components of the imaging medium in which it is to be used, and should not pose environmental problems, such as offensive smell or severe toxicity.

The aforementioned copending application Ser. No. 07/965,172 discloses that certain squaric acid derivatives are effective thermal acid generators which, upon heating, liberate squaric acid or an acid derivative thereof; thus, these squaric acid derivatives can be used in thermochemical processes for the generation of acid, and for imaging.

It has now been found that breakdown of the squaric acid derivatives described in the aforementioned copending application Ser. No. 07/965,172 can be catalyzed by acids which can protonate these derivatives, the efficiency of such protonation being dependent on the strength of the acid and thus greatest for very strong acids (superacids); the catalyzed breakdown of the squaric acid derivatives by superacids occurs rapidly at temperatures significantly lower than those required for uncatalyzed thermal breakdown of the squaric acid derivatives. Since superacid precursors are known which generate superacids on exposure to actinic (usually ultra-violet) radiation, a combination of a superacid precursor and one of the aforementioned squaric acid derivatives allows radiation-induced generation of relatively strong acid. Thus, this combination is useful for generation of acid and for imaging.

Accordingly, this invention provides a process for generation of squaric acid or an acidic derivative thereof, which process comprises:

exposing a superacid precursor to actinic radiation effective to generate superacid from the superacid precursor; and

heating the superacid while the superacid is admixed with a 3,4-disubstituted-cyclobut-3-ene-1,2-dione in which at least one of the 3- and 4-substituents consists of an oxygen atom bonded to the squaric acid ring, and an alkyl or alkylene group, a partially hydrogenated aryl or arylene group, or an aralkyl group, bonded to said oxygen atom, said 3,4-disubstituted-cyclobut-3-ene-1,2-dione being capable of decomposing so as to cause replacement of the or each original alkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkyloxy group of the derivative with a hydroxyl group, thereby producing squaric acid or an acidic squaric acid derivative having one hydroxyl group, the heating being continued for a temperature and time sufficient to cause the superacid to produce, from the 3,4-disubstituted-cyclobut-3-ene-1,2-dione, squaric acid or said acidic derivative thereof.

For simplicity, the 3,4-disubstituted-cyclobut-3-ene-l,2-dione used in the process and medium of the present invention may hereinafter be referred to as a "squaric acid derivative."

This invention also provides an imaging medium comprising a superacid precursor capable of generating a superacid on exposure to actinic radiation, and a squaric acid derivative as defined in the preceding paragraph, this derivative being capable, when heated in the presence of the superacid, of decomposing to form squaric acid or an acidic derivative thereof.

FIG. 1 of the accompanying drawings shows a synthesis of a squaric acid derivative of Formula I below; and

FIG. 2 is a schematic cross-section through an imaging medium of the present invention as it is being passed between a pair of hot rollers during the imaging process of the present invention.

As already mentioned, in the present process a superacid precursor is exposed to actinic (typically ultra-violet) radiation, thereby generating a superacid from the precursor. The resultant superacid is heated while in admixture with a squaric acid derivative in which there is bonded to the squaric acid ring, via an oxygen atom, an alkyl or alkylene group, a partially hydrogenated aryl or arylene group, or an aralkyl group. This heating is continued for a temperature and time sufficient to cause the superacid to produce, from the squaric acid derivative, squaric acid or an acidic derivative thereof (hereinafter sometimes referred to as the "second acid"). The superacid catalyzes the breakdown of the squaric acid derivative, so that the final quantity of second acid present is substantially larger than the quantity of superacid produced directly by the actinic radiation acting on the superacid precursor, although of course the second acid is typically a weaker acid than the superacid itself. This "chemical amplification" of the superacid by the squaric acid derivative increases the number of moles of acid generated per einstein of radiation absorbed, and thus increases the contrast of the image produced by the present process as compared with simple generation of acid by a superacid precursor.

The term "superacid" is used herein in its conventional sense, that is to say an acid with a pK.sub.a less than about 0. Any of the known superacid precursors, for example diazonium, phosphonium, sulfonium and iodonium compounds, may be used in this invention, but iodonium compounds are preferred. Especially preferred superacid precursors are diphenyliodonium salts, specifically (4-octyloxyphenyl)phenyliodonium hexafluorophosphate and hexafluoroantimonate and bis(N-dodecylphenyl)iodonium hexafluoroantimonate.

The exact mechanism by which squaric acid or an acidic derivative thereof is formed in the present process may vary depending upon the type of squaric acid derivative employed. In some cases, for example di-t-butyl squarate, one or both groups attached via oxygen atoms to the squaric acid ring may decompose to yield an alkene or arene, thereby converting an alkoxy or aryloxy group to a hydroxyl group and forming the squaric acid or acidic derivative thereof. In other cases, for example 3-amino-4-(p-vinylbenzyloxy)cyclobut-3-ene-1,2-dione, there is no obvious mechanism for formation of a corresponding alkene or arene, and it appears that the mechanism of acid formation is migration of the vinylbenzyl or similar group to a different position within the molecule (probably to the amino group), and protonation of the remaining oxygen atom to form a hydroxyl group at the position from which the group migrates. In other cases, neither of these pathways is possible. However, in all cases the net effect is the replacement of the alkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkoxy group present in the original derivative with a hydroxyl group to form squaric acid or an acidic derivative thereof.

There are four preferred groups of squaric acid derivatives for use in the present process:

(a) those of the formula: ##STR1## in which R.sup.1 is an alkyl group, a partially hydrogenated aromatic group, or an aralkyl group, and R.sup.2 is a hydrogen atom or an alkyl, cycloalkyl, aralkyl, aryl, amino, alkylamino, dialkylamino, alkylthio, alkylseleno, dialkylphosphino, dialkylphosphoxy or trialkylsilyl group, subject to the proviso that either or both of the groups R.sup.1 and R.sup.2 may be attached to a polymer. Among the derivatives of Formula I, especially preferred groups are those in which (a) R.sup.1 is an unsubstituted or phenyl substituted alkyl group containing a total of not more than about 20 carbon atoms in which the carbon atom directly bonded to the oxygen atom has not more than one hydrogen atom attached thereto, and R.sup.2 is an alkyl group containing not more than about 20 carbon atoms, or a phenyl group (which may be substituted or unsubstituted); and (b) R.sup.1 is a benzyloxy group and R.sup.2 is an amino group. Specific preferred compounds of Formula I are 3-amino-4-(t-butoxy)cyclobut-3-ene-1,2-dione, 4-hexyl-3-(p-vinylbenzyloxy)cyclobut-3-ene-1,2-dione and 3 -amino-4-(p-vinylbenzyloxy )-cyclobut-3-ene-1,2-dione.

(b) those of the formula: ##STR2## in which R.sup.1 and R.sup.3 independently are each an alkyl group, a partially hydrogenated aryl group or an aralkyl group, subject to the proviso that either or both of the groups R.sup.1 and R.sup.3 may be attached to a polymer. Among the derivatives of Formula II, an especially preferred group are those in which R.sup.1 and R.sup.3 are each independently an unsubstituted or phenyl substituted alkyl group containing a total of not more than about 20 carbon atoms in which the carbon atom directly bonded to the oxygen atom has not more than one hydrogen atom attached thereto. Specific preferred compounds of Formula II are 3,4-bis(t-butoxy)cyclobut-3-ene-1,2-dione, 3,4-bis(benzyloxy)cyclobut-3-ene-1,2-dione, 3,4-bis(.alpha.-methylbenzyloxy)cyclobut-3-ene-1,2-dione, 3,4-bis(p-methylbenzyloxy)cyclobut-3-ene-1,2-dione and 3,4-bis(cyclohexyloxy)cyclobut-3-ene-1,2-dione,

(c) those of the formula: ##STR3## in which n is 0 or 1, and R.sup.4 is an alkylene group or a partially hydrogenated arylene group. Among the derivatives of Formula III, an especially preferred group are those in which n is 1 and R.sup.4 is an alkylene group containing not more than about 12 carbon atoms, in which each of the carbon atoms directly bonded to the oxygen atoms has not more than one hydrogen atom attached thereto.

(d) those having at least one unit of the formula: ##STR4## in which n is 0 or 1, R.sup.5 is an alkylene or partially hydrogenated arylene group. In addition to the fragmentable groups R.sup.5, the compounds may also contain one or more units in which a non-fragmentable group is attached to a squarate ring, directly or via an oxygen atom.

The squaric acid derivatives of Formula IV include not only high polymers, but also dimers, trimers, tetramers, etc. including at least one of the specified units. The terminating groups on the derivatives of Formula IV may be any of groups OR.sup.1 or R.sup.2 discussed above with reference to Formula I. Thus, for example, Formula IV includes the squaric acid dimer derivative of the formula: ##STR5##

The squaric acid derivatives of Formulae I and II are usually monomeric. However, these derivatives of Formulae I and II can be incorporated into polymers by having at least one of the groups R.sup.1, R.sup.2 and R.sup.3 attached to a polymer. Attachment of the squaric acid derivatives to a polymer in this manner may be advantageous in that it may avoid incompatibility and/or phase separation which might occur between a monomeric squaric acid derivative of Formula I or II and a polymeric binder needed in an imaging medium.

The attachment of the groups R.sup.1, R.sup.2 and R.sup.3 to a polymer may be effected in various ways, which will be familiar to those skilled in the art of polymer synthesis. The squaric acid derivatives may be incorporated into the backbone of a polymer; for example, the groups may contain unsaturated linkages which enable the squaric acid derivatives to be polymerized, either alone or in admixture with other unsaturated monomers. Alternatively, the squaric acid derivatives may be added as sidechains to a polymer; for example, one of the groups R.sup.1, R.sup.2 and R.sup.3 could contain an amino group able to react with a polymer containing a carboxyl groups or derivatives thereof to form an amide linkage which would link the squaric acid derivative as a sidechain on to the polymer.

In the present process, it is generally undesirable to form substantial quantities of gas during the superacid-catalyzed decomposition of the squaric acid derivative, since such gas may distort the medium containing the squaric acid derivative or form vesicles therein, and such distortion or vesicle formation may interfere with proper image formation. Accordingly, if the decomposition of the squaric acid derivative yields an alkene, it is desirable that the groups R.sup.1, R.sup.3, R.sup.4 and R.sup.5 be chosen so that this alkene is a liquid at 20.degree. C., and preferably higher, since some heating of the alkene will inevitably occur during the superacid-catalyzed decomposition. In some cases, however, the alkene liberated may be sufficiently soluble in the medium containing the squaric acid derivative that liberation of a highly volatile alkene will not result in distortion of, or vesicle formation in, the medium.

Although the present process may be used for other purposes, such as triggering an acid-catalyzed chemical reaction (for example, polymerization or depolymerization reactions), it is primarily intended for use in image formation processes, and thus, simultaneously with or subsequent to the heating of the squaric acid derivative and the superacid, the second acid liberated is desirably contacted with an acid-sensitive material which changes color in the presence of the second acid. (It will be appreciated that the "color change" involved in such an imaging process need not be a visible color change. If, for example, the present process is used to provide security markings intended to be machine-readable, the "color change" could be a change in absorption from one non-visible wavelength to another, such that it can be detected by the appropriate machine-reading device.) Also, desirably the exposure of the superacid precursor to the actinic radiation is effected in an imagewise manner so that the color change of the acid-sensitive material occurs only in areas which have been exposed to actinic radiation, thereby forming an image.

The acid-sensitive material used in the process of the present invention may be any material which undergoes a color change in the presence of acid. Thus any conventional indicator dye may be used as the acid-sensitive material, as may the leuco dyes disclosed in the aforementioned U.S. Pat. Nos. 4,602,263; 4,720,449 and 4,826,976, which are also sensitive to acid.

To prevent premature color formation in an imaging process of the present invention prior to the exposure step, and thus avoid the increase in D.sub.min which may occur when some prior art imaging media are stored for long periods before use, advantageously, prior to the exposure step, the acid-sensitive material is in admixture with an amount of a basic material insufficient to neutralize all the acid capable of being liberated by the superacid precursor. The provision of this basic material serves to "soak up" minor amounts of acid which may be generated by decomposition of the superacid precursor by, for example, accidental exposure of the medium to ultraviolet light during transportation and storage. Upon exposure, the large amount of acid generated by the superacid precursor and squaric acid derivative overwhelms the amount of basic material, leaving excess acid which effects the color change in the acid-sensitive material.

The exposure of the superacid precursor to the actinic (typically ultraviolet) radiation can be effected in any of the ways conventionally used for exposing media to the same type of radiation. Thus, for example, the present medium can be exposed using the G-line from a mercury arc lamp. In some cases, it may be convenient to employ an ultra-violet laser. The use of a laser is a convenient way to record data as an image pattern in response to transmitted signals, such as digitized information.

Some imaging media of the present invention (for example those intended for use as photoresists and containing polymerizable monomers or oligomers or depolymerizable polymers) may comprise only a single layer containing all the components of the imaging medium. However, media containing an acid-sensitive material desirably comprise two separate layers or phases, so that, prior to the heating, the acid-sensitive material is present in a layer or phase separate from the layer or phase containing the superacid precursor and the squaric acid derivative, and following the generation of the second acid from the squaric acid derivative, the two layers or phases are mixed, thereby effecting the color or other change in the acid-sensitive material.

It should be noted that, although in principle the mixing of the acid-sensitive material with the superacid precursor and squaric acid derivative should be effected after the generation of the second acid from the squaric acid derivative, in practice if the superacid precursor and squaric acid derivative are present in one layer of a two-layer imaging medium, and the acid-sensitive material in the other layer of the medium, these two layers being such that they mix on heating, both the generation of the second acid and the mixing of the two layers may be effected in a single heating step, since the superacid-catalyzed decomposition of the squaric acid derivative will typically be essentially complete before mixing of the two layers becomes significant.

When a two-layer structure is used, it is not necessary that the two layers be affixed to one another before imaging. The production of second acid in exposed regions effected by the present process is a "permanent" chemical change, and hence it is possible to delay contacting the exposed medium with an acid-sensitive material for a substantial time. (Obviously, excessive delay may reduce the quality of an image produced by allowing second acid to diffuse from exposed into unexposed areas of the medium.) Accordingly, the two layers of the imaging medium may be laminated together after the second irradiation. However, in general it is most convenient to form the two layers by coating one on the other, or laminating the two layers together before imaging, since in this way only a single sheet of material has to handled during the imaging process. Since it is important that the two layers not mix prematurely, if the two layers are to be coated successively on to a support, it is usually desirable to coat one layer from an aqueous medium and the other from a non-aqueous medium. Typically, the layer containing the superacid precursor is coated from an organic solution and the layer containing an acid-sensitive leuco dye or other material is coated from an aqueous dispersion.

As already mentioned, prior to the heating step, the acid-sensitive material may be in admixture with an amount of a basic material insufficient to neutralize all the second acid liberated by the squaric acid derivative during the heating, so that the second acid liberated by the squaric acid derivative during the heating neutralizes all of the basic material and leaves excess second acid sufficient to effect the change in the acid-sensitive material. The provision of this basic material serves to "soak up" minor amounts of acid which may be generated in unexposed areas after exposure due, for example, to slow decomposition of the superacid precursor during protracted storage. Since obviously the basic material cannot be allowed to contact the superacid prior to the heating step, desirably the acid-sensitive material is present in a layer or phase separate from the layer or phase containing the superacid precursor and the squaric acid derivative and, following the generation of the second acid, the two layers or phases are mixed, thereby effecting the change in the acid-sensitive material.

In addition to the two aforementioned layers containing the superacid precursor, squaric acid derivative and acid-sensitive material, the imaging media of the present invention may comprise a support and additional layers, for example, a subbing layer to improve adhesion to the support, interlayers for thermally insulating multiple imaging layers from one another, an anti-abrasive topcoat layer, and other auxiliary layers.

The support employed may be transparent or opaque and may be any material that retains its dimensional stability at the temperature used during the heating step. Suitable supports include paper, paper coated with a resin or pigment, such as, calcium carbonate or calcined clay, synthetic papers or plastic films, such as polyethylene, polypropylene, polycarbonate, cellulose acetate and polystyrene. The preferred material for the support is a polyester, desirably poly(ethylene terephthalate).

Usually the layer or layers containing the superacid precursor, squaric acid derivative and acid-sensitive material will also contain a binder; typically this layer or these layers are formed by combining the active materials and the binder in a common solvent, applying a layer of the coating composition to the support and then drying. Rather than a solution coating, the layer may be applied as a dispersion or an emulsion. The coating composition also may contain dispersing agents, plasticizers, defoaming agents, coating aids and materials such as waxes to prevent sticking.

The binder used for the layer(s) in which superacid is to be generated must of course be non-basic, such that the superacid is not buffered by the binder. Examples of binders that may be used include methyl cellulose, cellulose acetate butyrate, styrene-acrylonitrile copolymers, polystyrene, poly(.alpha.-methylstyrene), copolymers of styrene and butadiene, poly(methyl methacrylate), copolymers of methyl and ethyl acrylate, poly(vinyl acetate), poly(vinyl butyral), polycarbonate poly(vinylidene chloride) and poly(vinyl chloride). It will be appreciated that the binder selected should not have any adverse effect on the superacid precursor, squaric acid derivative or the acid-sensitive material incorporated therein. Also, the binder should be heat-stable at the temperatures encountered during the heating step and should be transparent so that it does not interfere with viewing of the image. The binder must of course transmit the actinic radiation used to expose the medium.

In forming the layer containing the squaric acid derivative, temperatures should be maintained below levels that will initiate thermal decomposition of the squaric acid derivative so that the acid-sensitive material will not be prematurely colored or bleached.

The squaric acid derivatives used in the process of the present invention can be prepared by known methods, such as those described in U.S. Pat. No. 4,092,146 and Tetrahedron Letters (1977), 4437-38, and 23, 361-4, and Chem. Ber. 121, 569-71 (1988) and 113, 1-8 (1980). In general, the diesters of Formula II can be prepared by reacting disilver squarate with the appropriate alkyl halide(s), preferably the alkyl bromides. The ester groupings may be varied by routine transesterification reactions, or by reacting the diacid chloride of squaric acid with an appropriate alkoxide.

The derivatives of Formula I in which R.sup.2 is an alkyl, cycloalkyl, aralkyl or aryl group can be prepared from derivatives of Formula II by the synthesis shown in FIG. 1. The diester of Formula II is first condensed with a compound containing a negatively charged species R.sup.2 ; this compound is normally an organometallic compound, and preferably an organolithium compound. The reaction adds the -R.sup.2 group to one of the oxo groups of the diester to produce the squaric acid derivative of Formula VI; to avoid disubstitution into both oxo groups, not more than the stoichiometric amount of the organometallic reagent should be used.

After being separated from unreacted starting material and other by-products, the squaric acid derivative VI is treated with an acid, for example hydrochloric acid, to convert it to the desired squaric acid derivative I. Although it is possible to simply add acid to the reaction mixture resulting from the treatment of the diester with the organometallic reagent, this course is not recommended, since the squaric acid derivative I produced may be contaminated with unreacted diester, and the diester and squaric acid derivative I are so similar that it is extremely difficult to separate them, even by chromatography.

It will be appreciated that the synthesis shown in FIG. 1 may be modified in various ways. If, for example, the nature of the group R.sup.1 desired in the final compound of Formula I is such that it would react with the organometallic reagent, the reactions shown in FIG. 1 may be carried out with a diester in which the ester groupings do not contain the group R.sup.1, and the final product of Formula I may be subjected to transesterification or other reactions to introduce the group R.sup.1.

The derivatives of Formula I in which R.sup.2 is an amino, alkylamino or dialkylamino group can be prepared by similar methods from squaric acid diesters. For example, as illustrated in the Examples below, reaction of bis(4-vinylbenzyl) squarate with methylamine gives 3-amino-4-(p-vinylbenzyloxy)cyclobut-3-ene-1,2-dione. Analogous methods for the synthesis of the other compounds of Formula I will readily be apparent to those skilled in the art of organic synthesis.

The forms of the squaric acid derivative of Formulae I and II in which at least one of R.sup.1, R.sup.2 and R.sup.3 is attached to a polymer may be prepared by reactions analogous to those used to prepare the monomeric derivatives of Formulae I and II, for example by treating a polymer containing appropriate alkoxide groups with the diacid chloride or a monoester monoacid chloride of squaric acid. Alternatively, these polymer-attached derivatives may be prepared by transesterification, for example by treating a polymer containing esterified hydroxyl groups with a monomeric squaric acid derivative of Formula I or II. Other methods for attachment of these derivatives to polymers, or inclusion of these derivatives into polymer backbones, have already been discussed above.

The derivatives of Formula III may be prepared by transesterification from derivatives of Formula II, or another squaric acid diester, and the appropriate diol.

A preferred embodiment of the invention will now be described, though by way of illustration only, with reference to FIG. 2 of the accompanying drawings, which shows a schematic cross-section through an imaging medium (generally designated 10) of the invention as the image therein is being fixed by being passed between a pair of hot rollers 12.

The imaging medium 10 comprises a support 14 formed from a plastic film. Typically the support 14 will comprise a polyethylene terephthalate film 3 to 10 mils (76 to 254 m.mu.) in thickness, and its upper surface (in FIG. 2) may be treated with a sub-coat, such as is well-known to those skilled in the preparation of imaging media, to improve adhesion of the other layers to the support.

On the support 14 is disposed an imaging layer 16 comprising an acid-sensitive material, which changes color in the presence of squaric acid or an acidic derivative thereof, and a small amount of a base, which serves to neutralize any acid produced by breakdown of the superacid precursor in the acid-forming layer 18 (see below). On the opposed side of the imaging layer 16 from the support 14 is disposed an acid-forming layer 18 comprising a superacid precursor and a squaric acid derivative from which squaric acid or an acidic derivative thereof can be liberated by a superacid. The acid-forming layer 18 may also contain a small amount of a basic material to neutralize any acid produced by breakdown of the superacid precursor or squaric acid derivative prior to exposure of the medium. The imaging layer 16 and the acid-forming layer 18 both contain a binder having a glass transition temperature substantially above room temperature.

Finally, the imaging medium comprises an abrasion-resistant topcoat 20.

The imaging medium 10 may be formed by coating the layers 16, 18 and 20 on to the support 14. Alternatively, for example, the layers 16 and 18 may be coated on to the support 14, and the topcoat 20 laminated on to the resultant structure.

The imaging medium 10 is exposed by writing on selected areas of the medium with an ultra-violet laser or using incoherent ultra-violet radiation with exposure being effected through a mask; the exposure of the medium 10 is effected through the topcoat 20, as indicated by the arrow 22 in the drawing. Within the exposed regions of the acid-forming layer 18, the exposure to ultra-violet radiation causes breakdown of the superacid precursor with the formation of the corresponding superacid. After exposure, the imaging medium 10 is passed between the heated rollers 12. The heat and pressure applied by the rollers 12 causes the superacid present in the exposed regions of the acid-forming layer 18 to bring about catalytic breakdown of the squaric acid derivative therein, thereby causing formation of a quantity of second acid substantially larger than the quantity of superacid originally generated by the ultra-violet radiation. The heat and pressure applied by the rollers 12 also heats the color-forming layer 16 and the acid-forming layer 18 above their glass transition temperatures, thereby causing these two layers to become intermixed so that, in exposed regions, the acid produced in the acid-forming layer 18 effects the color change of the acid-sensitive material, thereby forming an image.

The following Examples are now given, though by way of illustration only, to show details of preferred reagents, conditions and techniques used in the process and imaging medium of the present invention.

3,4-Bis(t-butoxy)cyclobut-3-ene-1,2-dione ("bis t-butyl squarate"; hereinafter referred to as "Compound A") used in certain Examples below was prepared as described in E. V. Dehmlow et al., Chem. Bet. 113, 1-8 (1980). 3,4-Bis(benzyloxy)cyclobut-3-ene-1,2-dione ("dibenzyl squarate"; hereinafter referred to as "Compound B") used in certain Examples below was prepared as described in (N.) Islam et al, Tetrahedron 43, 959-970 (1987).

This Example illustrates the preparation of 3,4-bis(3-bromo-2,3-dimethylbut-2-oxy)-cyclobut-3-ene-1,2-dione ("bis(3-bromo-2,3-dimethylbut-2-yl) squarate"), the compound of Formula II in which R.sup.1 and R.sup.3 are each a 3-bromo-2,3-dimethylbut-2-yl group.

Silver squarate (1.0 g, 3.0 mmol) was added to a solution of 2,3-dibromo-2,3-dimethylbutane (1.0 g, 4.0 mmol) in dry ether (3 mL) at room temperature. The suspension became warm, and was cooled by a water bath at room temperature. After six hours' stirring, the precipitate remaining was removed by filtration, and washed with ether. The combined ether extracts were concentrated, and the crude product obtained therefrom was purified by flash chromatography on silica gel with 1:3 ether/hexanes as eluent to give the diester (140 mg, 11% yield) as a white powder which decomposed at 131.degree.-132.degree. C. The structure of the compound was confirmed by mass spectroscopy and by .sup.1 H and .sup.13 C NMR spectroscopy.

This Example illustrates the preparation of 3-t-butoxy-4-phenyl-cyclobut-3-ene-l,2-dione, the compound of Formula I in which R.sup.1 is tertiary butyl group and R.sup.2 is a phenyl group.

Phenyl magnesium bromide (4.6 mL of a 1.0M solution in THF, 4.6 mmol) was added dropwise over a period of 5 minutes to a solution of di-t-butyl squarate (1.0 g, 4.42 mmol) in dry ether (10 mL) at -78.degree. C. under nitrogen. After 30 minutes, the reaction mixture was warmed to 0.degree. C., and stirred at this temperature for an additional one hour. Water (10 mL) and ether (10 mL) were then added to the reaction mixture and the layers were separated. The aqueous layer was extracted twice with dichloromethane. The combined organic layers were dried over magnesium sulfate and concentrated, to give a yellow oil (1.43 g), which crystallized. The resultant material was dissolved in dichloromethane (25 mL) and concentrated hydrochloric acid (4 drops) was added, with stirring, to this solution at room temperature. After 30 minutes, a further four drops of concentrated hydrochloric acid were added. Dichloromethane (25 mL) was added, and the resultant solution was washed with a saturated solution of sodium bicarbonate and then with brine, dried over magnesium sulfate, and concentrated. The crude product thus obtained was purified by flash chromatography on silica gel with toluene as eluent. The chromatographed material was further purified by recrystallization from toluene/hexanes to give the desired monoester as yellow crystals (142 mg, 14% yield) which decomposed at 105.degree.-110.degree. C. The structure of this compound was confirmed by mass spectroscopy and by .sup.1 H and .sup.13 C NMR spectroscopy.

This Example illustrates the preparation of 3,4-bis(.alpha.-methyl-benzyloxy)-cyclobut-3-ene-1,2-dione ("bis(.alpha.-methylbenzyl) squarate"; hereinafter referred to as "Compound C"), the compound of Formula II in which R.sup.1 and R.sup.3 are each an .alpha.-methylbenzyl group.

1-Bromo-1-phenylethane (3.1 g, 16.8 mmol) was added dropwise to a suspension of silver squarate (2.5 g, 7.62 mmol, prepared as described in S. Cohen et al., J. Am. Chem. Soc., 88, 5433 (1966)) in dry ether (40 mL) at 0.degree. C. After the addition was complete, the reaction mixture was allowed to warm to room temperature and was stirred for four hours in the dark. The solid remaining after this time (silver bromide) was removed by filtration and washed with more ether. The combined ether solutions were washed with a saturated solution of sodium bicarbonate and dried over sodium sulfate. Evaporation of the solvent was followed by purification by flash chromatography on silica gel with 0-60% ether/hexanes as eluant to give the desired diester (394 mg, 16% yield) as a colorless oil. The diester was obtained as a mixture of diastereoisomers which were not separable by this type of chromatography. The structure of the diester was confirmed by mass spectroscopy and by .sup.1 H and .sup.13 C NMR spectroscopy.

This Example illustrates the preparation of 3,4-bis(p-methyl-benzyloxy)-cyclobut-3-ene-1,2-dione ("bis(p-methylbenzyl) squarate"; hereinafter referred to as "Compound D"), the compound of Formula II in which R.sup.1 and R.sup.3 are each a p-methylbenzyl group.

Triethylamine (0.93 g, 9.2 mmol) was added to a stirred suspension of squaric acid (0.5 g, 4.38 mmol) in chloroform (10 mL) and the resultant solution was cooled with an ice/water bath. A solution of .alpha.-bromo-p-xylene (2.03 g, 11.0 mmol) in chloroform (10 mL) was then added dropwise over a period of 30 minutes. After this time, the cooling bath was removed and the solution was held at room temperature for 4.5 hours. The reaction mixture was then diluted with chloroform (20 mL), washed successively with a saturated aqueous solution of sodium bicarbonate (2.times.20 mL) and saturated brine (20 mL), dried over magnesium sulfate and concentrated under reduced pressure. The resultant oil was further purified by partition between ether (50 mL) and saturated aqueous sodium bicarbonate (20 mL) and separation of the organic layer. The organic layer was washed successively with a saturated aqueous solution of sodium bicarbonate (20 mL) and saturated brine (20 mL), dried over magnesium sulfate and concentrated under reduced pressure. The oil which resulted was crystallized from hot hexanes (20 mL) to give the desired compound (300 mg, 21.3% yield) as off-white crystals. The structure of this compound was confirmed by mass spectroscopy and by .sup.1 H and .sup.13 C NMR spectroscopy.

This Example illustrates the preparation of 3,4-bis(cyclohexyloxy)-cyclobut-3-ene-1,2-dione "dicyclohexyl squarate"; hereinafter referred to as "Compound E"), the compound of Formula II in which R.sup.1 and R.sup.3 are each a cyclohexyl group.

Cyclohexyl bromide (9.95 g, 61 mmol) was added dropwise over a period of 20 minutes to a stirred suspension of silver squarate (4.0 g, 12.2 mmol, prepared as described in S. Cohen et al., J. Am. Chem. Soc., 88, 5433 (1966)) in ether (80 mL) in the dark with ice/water cooling. The ice bath was then removed and the reaction mixture was stirred overnight at room temperature, then filtered to remove silver bromide, and the residue was washed with ether (2.times.20 mL). The ether solutions were combined and washed successively with a saturated aqueous solution of sodium bicarbonate (50 mL) and saturated brine (50 mL), dried over magnesium sulfate and concentrated under reduced pressure to give the desired compound as a viscous oil which solidified upon storage in a refrigerator to give an off-white solid (0.55 g, 16% yield). The structure of this compound was confirmed by mass spectroscopy and by .sup.1 H and .sup.13 C NMR spectroscopy.

This Example illustrates the preparation of 3-amino-4-(t-butoxy)-cyclobut-3-ene-1,2-dione (hereinafter referred to as "Compound F"), the compound of Formula I in which R.sup.1 is a tertiary butyl group and R.sup.2 is an amino group.

A stream of ammonia gas was passed into a stirred solution of Compound A (0.7 g, 3.07 mmol) in methanol (40 mL) for 2 minutes. The solution was then allowed to stand at room temperature for 1 hour, during which time a small amount of insoluble material was precipitated. The sediment was removed by filtration, and the solvent was removed under reduced pressure to yield a yellow solid, which was washed with ether (2.times.50 mL) to remove starting material and butanol (0.16 g of impurities were collected, after solvent evaporation). The solid which remained was dissolved in dichloromethane (150 mL) and the solution was filtered. Removal of the solvent under reduced pressure yielded the desired compound as white crystals (0.25 g, 48% yield) which melted at 220.degree.-225.degree. C. The structure of this compound was confirmed by .sup.1 H NMR spectroscopy.

This Example illustrates the preparation of 4-hexyl-3-(p-vinylbenzyl-oxy)-cyclobut-3-ene-1,2-dione (hereinafter referred to as "Compound G") compound of Formula I in which R.sup.2 is a hexyl group and R.sup.1 is an p-vinylbenzyl group.

Part A: Preparation of 2,3-dibutoxy-4hexyl-4-hydrocycyclobut-2-en-1-one

Hexyl magnesium bromide (40 mL of a 2M solution in ether, 80.0 mmol) was added dropwise over a period of 45 minutes to a solution of di-n-butyl squarate in dry THF (150 mL) at -78.degree. C. under nitrogen, and the reaction mixture was held at that temperature for 1 hour. The reaction mixture was then allowed to warm to room temperature are stirred for an additional 3 hours, after which time it was cooled using an ice/water bath, and quenched by the addition of water (25 mL) added dropwise over a period of 5 minutes. Saturated brine (300 mL) and ether (300 mL) were then added, the layers were separated, and the aqueous layer was extracted with additional ether (300 mL). The ether extracts were combined and dried over magnesium sulfate, and the solvents were removed to give a golden oil (15.64 g) containing the desired product; this oil was used without further purification in Part B below.

Part B: Preparation of 3-hexyl-4-hydroxy-cyclobut-3-en-1,2-one

6N Hydrochloric acid (150 mL) was added in one portion to a stirred solution of crude 2,3-dibutoxy-4-hexyl-4-hydroxycyclobut-2-en- 1-one (15.1 g, prepared in Part A above) in THF (150 mL), and the resultant solution was stirred at room temperature for 3 hours. The reaction mixture was then concentrated under reduced pressure to give a yellow solid. To this solid was added water (100 mL), which was then removed under reduced pressure. Toluene (100 mL) was similarly added and removed under reduced pressure, and then dichloromethane (200 mL) was added to the residue and the resultant solution was filtered and concentrated to produce a yellow oil. Hexanes (200 mL) were added and the resultant solution was cooled to induce crystallization. After recrystallization from hexanes, the desired compound was isolated as tan crystals (4.28 g, 33% yield over Parts A and B). The structure of this compound was confirmed by mass spectroscopy and by .sup.1 H and .sup.13 C NMR spectroscopy.

Part C : Preparation of 4hexyl-3-(p-vinybenzyloxy)-cyclobut-3-en-1,2-one

Triethylamine (1.75 g, 17.3 mmol), 2,6-di-t-butyl-4-methylphenol (a radical inhibitor, 0.7 mg, 3.41 .mu.mol) and 4-vinylbenzyl chloride (5.04 g, 33 mmol) were added, in that order, to a solution of 3-hexyl-4-hydroxy-cyclobut-3-en-l,2-one (3.0 g, 16.5 mmol, prepared in Part B above) in chloroform (90 mL), and the resultant solution was heated at reflux for 7 hours. The solution was then cooled and allowed to stand overnight at room temperature, after which it was heated at reflux for a further 7 hours, then cooled and allowed to stand overnight a second time. The reaction mixture was then concentrated under reduced pressure, the residue dissolved in dichloromethane (150 mL), and the resultant solution washed with water (2.times.75 mL), dried over magnesium sulfate and concentrated under reduced pressure to yield a yellow oil, which was purified by short-path distillation (to remove excess 4-vinylbenzyl chloride) at 72.degree.-74.degree. C. and 1.7 mm Hg pressure. The residue from the distillation was purified by flash chromatography on silica gel with dichloromethane as eluant to give the desired compound (1.23 g, 25% yield) as a golden oil. The structure of this compound was confirmed by mass spectroscopy and by .sup.1 H and .sup.13 C NMR spectroscopy.

This Example illustrates the preparation of 3-methylamino-4-(p-vinylbenzyloxy)-cyclobut-3-ene-1,2-dione (hereinafter referred to as "Compound H"), the compound of Formula I in which R.sup.2 is a amino group and R.sup.1 is an p-vinylbenzyl group.

Part A: Preparation of bis(4-vinylbenzyl) squarate

4-Vinylbenzyl chloride (13 g, 85 mmol) was added to a suspension of silver squarate (freshly prepared from squaric acid (5.5 g, 48 mmol) by the method described in S. Cohen et al., J. Am. Chem. Soc., 88, 5433 (1966)) in dry ether (100 mL), and the resultant mixture was stirred in the dark for 3 days. The reaction mixture was then filtered and the solvent removed under reduced pressure. The residue was taken up in dichloromethane and filtered through a short column of silica gel, then concentrated under reduced pressure, to yield the desired compound in a crude form, which was used in Part B below without further purification.

Part B: Preparation of 3-methylamino-4-(p-vinylbenzyloxy)-cyclobut-3-en-1,2-dione

The crude product from Part A above was dissolved in ether (300 mL) and gaseous methylamine was bubbled through this ether solution for 1 minute. The resultant mixture was allowed to stand for 5 minutes, then the precipitate which had formed was removed by filtration, redissolved in chloroform and filtered through Celite (manufactured by Johns-Manville Corporation, Denver, Colo. 80217). The solvent was removed under reduced pressure to give Compound H as a colorless oil (3.5 g, 30% yield over Parts A and B). The structure of this compound was confirmed by .sup.1 H NMR spectroscopy.

This Example illustrates the preparation of a 1:1 w/w copolymer of Compound H prepared in Example 8 above with lauryl methacrylate.

Compound H (1 g) and lauryl methacrylate (1 g) were dissolved in a mixture of 2-propanol (30 mL) and ethanol (20 mL), and the resultant solution was purged with nitrogen. Azoisobutyronitrile (0.01 g) was then added, and the solution was held at 65.degree. C. overnight, during which time a precipitate (250 mg) formed. This precipitate was collected and shown by infra-red spectroscopy to contain squarate esters.

Pentamethylenebis(magnesium bromide) (25 mL of a 0.5M solution in THF, 12.5 mmol) was added dropwise over a period of 15 minutes to a solution of dibutyl squarate (5.66 g, 25 mmol) in dry THF (50 mL) at -78.degree. C. under a stream of nitrogen. The resulting suspension was stirred at -78.degree. C. for 1 hour, then allowed to warm to room temperature and stirred for a further 2 hours. The homogeneous yellow solution which resulted was cooled to 0.degree. C., and water (10 mL) was added dropwise over a period of 2 minutes. After standing for 5 minutes, the solution was diluted with THF (50 mL) and washed with saturated sodium chloride solution (150 mL). An emulsion was formed, which was separated by evaporative removal of THF and addition of dichloromethane (200 mL). The organic layer was separated and the aqueous layer was extracted with more dichloromethane (100 mL). The combined dichloromethane layers were dried over magnesium sulfate and concentrated under reduced pressure to yield a golden oil which was shown by thin layer chromatography, on silica gel with 1:1 ether/hexanes as eluent, to consist of five components.

This mixture was separated by flash chromatography on silica gel with 1:1 ether/hexanes, followed by pure ether, as eluents. Each of the five components was examined by .sup.1 H NMR spectroscopy. The third and fourth components (in order of elution from the column) were tentatively assigned as 4-[5-[1,2-dioxo-3-butoxy-cyclobut-3-en-4-yl]pent-1-yl]-3-butoxycyclobut-3- ene-1,2-dione (0.69 g) and 2,3-dibutoxy-4-[5-[1,2-dioxo-3-butoxycyclobut-3-en-4-yl]pent-1-yl]-4-hydro xycyclo-but-2-ene-1-one (2.14 g).

A portion of the isolated fourth component (2.01 g) was dissolved in THF (20 mL), and the resultant solution was treated with 6M hydrochloric acid (20 mL). The two-phase mixture became warm, and after 15 minutes stirring was observed to have become homogeneous. After a further two hours stirring, the solution was concentrated to dryness under reduced pressure. Water (20 mL) was added, and removed evaporatively, in order to drive off excess hydrogen chloride. The remaining water was removed by azeotropic distillation under reduced pressure with dichloromethane/acetone, to yield an off-white solid. This material was purified by recrystallization from THF/ether to yield the desired compound as a tan powder (542 mg, 18% yield over two steps). The structure of this compound was confirmed by .sup.1 H and .sup.13 C NMR spectroscopy.

This Example illustrates the preparation of a dimeric squaric acid derivative in which two 4-methylbenzyloxy]cyclobut-3-ene-1,2-dione groups are linked via a pentamethylene chain.

Triethylamine (423 mg, 4.18 mmol) and p-methylbenzyl bromide (1.47 g, 7.96 mmol) were added sequentially to a suspension of 4-[5-[1,2-dioxo-3-hydroxycyclobut-3-en-4-yl]pent-1-yl]-3-hydroxy-cyclobut- 3-ene-1,2dione (526 mg, 2.0 mmol, prepared in Example 10 above) in chloroform (15 mL) at room temperature, and the mixture was then heated at reflux for 9 hours. The solvent was removed under reduced pressure, and the resultant oil was purified by flash chromatography on silica gel with dichloromethane, followed by ether, as eluents. The product eluted with ether, and was obtained as a yellow oil (591 mg, 63% yield). The structure of this compound was confirmed by .sup.1 H and .sup.13 C NMR spectroscopy.

This Example demonstrates that, in the presence of 1 mole percent methanesulfonic acid (a strong acid), various squaric acid derivatives used in the processes and media of the present invention decompose at substantially lower temperatures than the same derivatives do in the absence of this acid, and thus that the thermal breakdown of these squaric acid derivatives is catalyzed by strong acids.

Compounds A, B, D, E and F described above were doped with 1 percent by weight methanesulfonic acid (MSA) by addition of the appropriate amount of a 2 mM solution of the acid in dichloromethane, followed by evaporation of the solvent. The acid-treated materials were compared with the pure compounds using thermal gravimetric analysis (TGA) under a nitrogen atmosphere with a rate of temperature increase of 10.degree. C. per minute. Compounds B and D, whose decompositions did not result in loss of an obvious gaseous by-product, were also examined by differential scanning calorimetry (DSC) under the same conditions of temperature and inert atmosphere. Calculated onset temperatures of decomposition (COTD's), and percent weight loss (for TGA) or heat evolved (for DSC) are shown in Table 1 below.

From the data in Table 1, it will be seen that, in the presence of a catalytic amount of the strong acid, Compounds A, B, D, E and F decomposed at temperatures lower (by about 15.degree.-30.degree. C.) than the same compounds decomposed in the absence of the strong acid.

This Example illustrates an imaging process of the present invention, in which superacid is generated by ultra-violet irradiation of a film containing a superacid precursor (a sulfonium salt), this superacid is used to catalyze the decomposition of a squaric acid derivative, thereby amplifying the quantity of acid present, and the acid forms a visible image upon lamination of the film to a second film containing an irreversible indicator dye.

Four coating fluids were prepared as follows:

Fluid A

Triphenylsulfonium hexafluoroarsenate (6 mg) and poly(vinyl butyral) (B-76 Butvar, supplied by Monsanto Chemical Corp., 25 mg), dissolved in methyl ethyl ketone (0.5 mL)

Fluid B

As Fluid A, with the addition of 10 mg of Compound A

Fluid C

As Fluid A, with the addition of 15 mg of Compound C

Fluid D

A leuco dye of the formula: ##STR6## (20 mg; this leuco dye may prepared by the procedure described in U.S. Pat. No. 4,345,017) and poly(vinyl butyral) (B-76 Butvar, supplied by Monsanto Chemical Corp., 25 mg), dissolved in methyl ethyl ketone (0.5 mL)

These four fluids were coated on to a 4 mil (101 .mu.m) poly(ethylene terephthalate) film using a number 8 coating rod to produce coatings A-D respectively.

Coatings A, B and C were then exposed to ultra-violet radiation of 254 nm wavelength from a Spectroline Model ENF-240C ultra-violet lamp (available from Spectronics Corporation, Westbury, N.Y.) for periods of 15, 30, 60 and 120 seconds; one portion of each coating was left unexposed as a control, One portion of each exposed coating was then heated to 90.degree. C. for 20 seconds, while a second portion of each exposed coating was kept at room temperature. Finally, both portions of each exposed coating were separately laminated, with the coated sides in contact, to portions of Coating D, this lamination being effected at a temperature of 180.degree. F. (88.degree. C.) and 60 psi (0.43 MPa). Lamination caused mixing of the coatings, so that any acid present in the exposed portions of Coating A, B or C protonated the indicator dye in Coating D and produced a magenta color, the density of which was proportional to the amount of protonated dye present.

Table 2 below shows the green optical densities produced in each portion of each coating. Coating A, which contains no squaric acid derivative to act as an acid amplifier, is used as a control. For each optical density measured for Coatings B and C, an acid amplification factor ("AAF", i.e., the number of moles of acid liberated from the squaric acid derivative for each mole of superacid which the ultra-violet irradiation liberates from the superacid precursor) was calculated by dividing the difference in optical density between exposed and unexposed portions of the same coating (otherwise treated identically) by the difference between exposed and unexposed portions of Coating A treated in the same manner. The AAF's shown in Table 2 refer to the line coating in the immediately preceding line of the Table.

From Table 2, it will be seen that the use of a squaric acid derivative provides very substantial amplification of the acid generated by irradiation of the superacid precursor, with acid amplification factors in excess of 40 being achieved in some instances.

This Example illustrates an imaging process of the present invention, in which superacid is generated by ultra-violet irradiation of a two-layer film, one layer of which contains a superacid precursor (an iodonium salt), this superacid is used to catalyze the decomposition of a squaric acid derivative, thereby amplifying the quantity of acid present, and the acid forms a visible image upon heating of the film, with consequent mixing of the acid produced with an indicator dye originally present in the second layer of the film.

Four coating fluids were prepared as follows:

Fluid A

t-Butyl anthracene (5 mg), (4-n-octyloxyphenyl)phenyliodonium hexafluoroantimonate (10 mg, prepared as per U.S. Pat. No. 4,992,571) and poly(methyl methacrylate) (Elvacite 2021, supplied by DuPont de Nemours, Wilmington, Del.; 30 mg), dissolved in methyl ethyl ketone (0.6 mL)

Fluid B

As Fluid A, with the addition of 20 mg of Compound A

Fluid C

As Fluid A, with the addition of 20 mg of Compound H

Fluid D

As Fluid A, with the addition of 30 mg of the polymer prepared in Example 9 above.

These four fluids were coated on to a 4 mil (101 .mu.gm) poly(ethylene terephthalate) film (ICI Type 3295, supplied by ICI Americas, Inc., Wilmington, Del.) using a number 18 coating rod to produce coatings A-D respectively.

To provide the indicator dye layer for the imaging media, an indicator dye coating fluid was prepared as follows. De-ionized water (60 mL) was added dropwise to a magnetically stirred solution of a surfactant (Aerosol TR-70, adjusted with potassium hydroxide to pH 6, 0.65 g), the leuco dye used in Example 13 above (2.5 g), a base (HALS-62, supplied by Fairmount Chemical Company, Inc., 117 Blanchard Street, Newark, N.J. 07105, 0.25 g) and a polymeric binder (Elvacite 2043, supplied by DuPont de Nemours, 2.75 g) in dichloromethane (46 mL). The resultant, very viscous mixture was sonicated, causing the viscosity to decrease, and then allowed to stir overnight at room temperature, during which the dichloromethane evaporated. A fluorinated surfactant (FC-120, supplied by Minnesota Mining and Manufacturing Corporation, St. Paul, Minn., 56 mg of a 25% aqueous solution) was then added. A sample of the resultant dispersion (1 mL) was diluted with an equal volume of a 5% aqueous solution of poly(vinyl alcohol) (Vinol 540). Coatings A-D were then overcoated with the resultant solution using a number 7 coating rod to form imaging Media A-D respectively.

These Media A-D were then imagewise exposed through their film bases to ultra-violet radiation from a Universal UV unit (nominally emitting at 375 nm) supplied by Gelman Instrument Company, then heated on a hot plate at 110.degree. C. for 45 seconds. This heating served to cause decomposition of the squaric acid derivatives in regions in which acid had been generated by ultra-violet irradiation of the sensitized iodonium salt, and to mix the photochemically active layer with the indicator dye layer, thereby producing a visible image whose density was proportional to the amount of protonated indicator dye present (and therefore to the amount of acid generated by the iodonium salt and decomposition of the squaric acid derivatives).

After this heating, the difference in green optical density between exposed and unexposed regions were measured for Media B, C and D using an X-Rite 310 photographic densitometer, supplied by X-Rite, Inc., Grandville, Mich., with the appropriate filter. The ratio of this difference to the difference measured for Medium A, which contained no squaric acid derivative, was the acid amplification factor (AAF) due to the squaric acid derivatives. The results are shown Table 3 below.

From the data in Table 3, it will be seen that the presence of the squaric acid derivative provided very substantial amplification of the acid produced by ultra-violet irradiation of the superacid precursor.

From the foregoing, it will be seen that the present invention provides a process for generation of an acid and for forming an image, and an imaging medium, which permit generation of a strong acid in substantially greater quantity (and thus with greater sensitivity) than by simple irradiation of a superacid precursor.