In PEP imaging processes, an imaging layer comprising an electrically photosensitive material is placed between two electrodes, subjected to the influence of an electric field and exposed to an image pattern of electromagnetic radiation to which the electrically photosensitive material is sensi¬ tive. This causes electrically photosensitive compo¬ nents in the material to migrate imagewise in the layer to form a record of the imaging electromagnetic radiation.

Regardless of the particular PEP imaging process employed, an essential component of any such process is the electrically photosensitive material. Such materials are composed of an electrically insu¬ lating carrier containing electrically photosensitive particles which serve as the migrating components in a PEP imaging process. The particles, in turn,. com¬ prise pigments or dyes (herein referred to as color- ants) which themselves are electrically photosensi¬ tive, or form electrically photosensitive colored

particles in association with other photoactive mate¬ rials.

Certain PEP applications require the use of a colorant which is neutral in hue. Such a neutral colorant must exhibit several properties to give optimum results. In particular, it must exhibit sig¬ nificant absorption across the entire visible region of the spectrum to give it a good neutral hue. It must also exhibit high electrical photosensitivity and possess good light stability.

There exists a continuing effort to find materials which possess useful levels of electrical photosensitivity and exhibit good colorant properties including neutral ^" or near neutral coloration, as well as other desired hues.

The present invention provides electrically photosensitive materials which are useful in PEP pro¬ cesses. The materials comprise particles containing an electrically photosensitive merocyanine-cyanine- merocyanine (MCM) compound comprising two merocyanine moieties joined by a cyanine moiety. These compounds are colorants and contain the following structure:

I

wherein:

and n , which are the same or different , are 0 ,

1 or 2 ; t and u, which are the same or different, are 0 or 1; p is 0, 1 or 2;

R ¹ and R ¹¹ , which are the same or different, represent an alkyl, aryl, aralkyl, alkaryl, carbocy¬ clic or heterocyclic group or, when taken together and p is zero, are a one- or two-carbon alkylene bridge;

R ² and R ²² , which are the same or different, represent an alkyl, aryl, aralkyl, alkaryl, carbocy¬ clic or heterocyclic group;

R ³ and R ³³ , which are the same or different, represent hydrogen or an alkyl group; .

R* and R^*, which are the same or different, represent hydrogen or an alkyl group;

R , when taken together with R ² and t is 0, and R ¹ * ^*1 , when taken together with R ²² and u is 0, represent the atoms necessary to complete an alkylene or heteroalkylene bridge,

R ³ and R ¹ *, and R ³³ and R* when taken together, represent the atoms necessary to complete an alkylene or heteroalkylene bridge, R ⁵ , R ⁶ and R ⁷ , which are the same or dif¬ ferent, represent hydrogen or an alkyl group or, when individually taken together with R ¹ or R ¹¹ , repre¬ sent the atoms necessary to complete an alkylene or heteroalkylene bridge; and Y and Y', which are the same or different, repre¬ sent the atoms necessary to complete a basic hetero¬ cyclic cyanine dye nucleus; and

A and A ^x , which are the same or different, rep¬ resent oxygen, sulfur, selenium or -NR* wherein R ^β represents an alkyl, aryl, aralkyl, alkaryl, carbocy¬ clic or heterocyclic group.

Each of the foregoing subεtituents on Struc¬ ture I, moreover, is optionally further substituted to provide other properties as desired. For example, any alkyl or aryl group is either unsubstituted or substituted by cyano, halogen, hydroxy, sulfo, car- boxy, alkoxy, or carboxyalkyl groups as desired.

The present MCM compounds can contain dis- sociatable counterions such as £-toluenesulfonate, chloro or iodo negatively charged counterions. Alternatively, the compounds can be zwitterionic, in which case the positively charged nitrogen is elec¬ trically balanced with a negatively charged substitu- ent such as a -SO, ^" , -PO- ^" or -COO ^" substituent on one of the R groups of the MCM compound. Prefera- bly, the negatively charged εubstituent is part of the R ² or R ²² subεtituent.

As noted above, each of Y and Y ¹ repreεentε the atoms necesεary to complete a baεic heterocyclic cyanine colorant nucleuε. Preferred nuclei contain from 5 to 6 atomε in the heterocyclic ring, which may be further condenεed to another ring εyεtem εuch aε an aromatic or heterocyclic ring εyεtem. Representa¬ tive preferred nuclei include: (a) thiazole nuclei such as: (i) thiazoles (e.g., thiazole, 4-methylthia- zole, 4-phenylthiazole, 5-methylthiazole, 5-phenylthiazole, 4,5-dimethylthiazole, 4,5-diphenylthiazole, 4-(2-thienyl)thia¬ zole, etc.); (ϋ) benzothiazoleε (e.g., benzothiazole, 5- aminobenzothiazole, 4-chlorobenzothiazole, 5-chlorobenzothiazole, 6-chlorobenzothia- zole, 7-chlorobenzothiazole, 4-methylbenzo- thiazole, 5-methylbenzothiazole, 6-methyl- benzothiazole, 5-bromobenzothiazole, 6-bro- mobenzothiazole, 4-phenylbenzothiazole, 5-

phenylbenzothiazole, 4-methoxybenzothia- zole, 5-methoxybenzothiazole, 6-methoxyben- zothiazole, 5-iodobenzothiazole, 6-iodoben- zothiazole, 4-ethoxybenzothiazole, 5- ethoxybenzothiazole, tetrahydrobenzothia- zole, 5,6-dimethoxybenzothiazole, 5,6-meth- ylenedioxybenzothiazole, 5-hydroxybenzothi¬ azole, 6-hydroxybenzothiazole, etc.); (iii) naphthothiazoles (e.g., naphtho[l,2-d]thia- zole, naρhtho[2,1-d]thiazole, naρhtho[2,3- b]thiazole, 5-methoxynaphtho[2,1-d thia¬ zole, 5-ethoxynaphtho[2,l-d]thiazole, 8- methoxynaphtho[1,2-d thiazole, 7-methoxy- naphth t2,1-d3thiazole, etc.); ( iv) thianaphtheno[7,6-d]thiazole (e.g., 4'- methoxythianaphtheno[7,6,d thiazole, etc.); (b) oxazole nuclei such as:

(i) oxazoleε (e.g., 4-methyloxazole, 5-methyl- oxazole, 4-phenyloxazole, 4,5-diphenyloxa- zole, 4-ethyloxazole, 4,5-dimethyloxazole,

5-phenyloxazole, etc.); (ii) benzoxazoles (e.g., benzoxazole, 5-chloro- benzoxazole, 5-methylbenzoxazole, 5-phenyl- benzoxazole, 6-methylbenzoxazole, 5,6- dimethylbenzoxazole, 4,6-dimethylbenzoxa- zole, 5-methoxybenzoxazole, 5-ethoxybenz- oxazole, 6-chlorobenzoxazole, 6-methoxy- benzoxazole, 5-hydroxybenzoxazole, 6- hydroxybenzoxazole, etc.); (iϋ) naphthoxazoles (e.g., naphtho[l,2-d3oxa¬ zole, naphtho[2,l-d3oxazole, etc.); (c) selenazole nuclei such as:

(i) εelenazoleε (e.g., 4-methylεelenazole*, 4- phenylselenazole, etc.); ( ii) benzoselenazoles (e.g., benzoselenazole, 5- chlorobenzoselenazole, 5-methoxybenzo-

εelenazole, 5-hydroxybenzoselenazole, tet- rahydrobenzoselenazole, etc.) ; (iii) naphthoεelenazoleε (e.g., naphtho[l,2-d - εelenazole, naphtho[2,l-d3εelenazole, etc.) ; (d) thiazoline nuclei εuch aε thiazoline, 4-methyl- thiazoline, etc. ;

(e) quinoline nuclei εuch aε

( i) 2-quinolines (e.g., quinoline, 3-methyl- quinoline, 5-methylquinoline, 7-methyl- quinoline, 8-methylquinoline, 6-chloro- quinoline, 8-chloroquinoline, 6-methoxy- quinoline, 6-ethoxyquinoline, 6-hydroxy- quinoline, 8-hydroxyquinoline, etc.); ( ii) 4-quinolines (e.g., quinoline, 6-methoxy- quinoline, 7-methylquinoline, 8-methyl- quinoline, etc.); (iii) iεoquinolineε (e.g., 1-iεoquinoline, 3-iεo- quinoline, 3,4-dihydroiεoquinoline, etc.) ;

(f) imidazole nuclei εuch aε: (i) imidazoleε (e.g., 4-phenylimidazole, 1,3,4- triphenylimidazole, 1,4-diphenylimidazole, etc.); ( ii) benzimidazoleε (e.g., 1,3-diethylbenzimida- zole, 5-chloro-l,3-diethylbenzimidazole, 5,6-dichloro-l,3-diethylbenzimidazole, 1- ethyl-3-phenylbenzimidazole, etc.); (iii) naphthimidazoleε (e.g., 1,3-diethylnaphth- [2,3-d3imidazole, 6-chloro-l,3-diethyl- naphth[2,3-d3imidazole, etc.); ( iv) imidazo[4,5-b3pyridines (e.g., 1,3-diethyl- i idazo[4,5-b pyridine, 1,3-diethyl-5,6- dimethylimidazo[4,5-b3pyridine, etc. (v) imidazo(4,5-b3quinolineε (e.g., 1,3-dieth- ylimidazo(4,5-b quinoline, l-ethyl-3-phen- ylimidazo[4,5-b quinoline, etc.;

(g) indole nuclei εuch aε:

(i) 3H-indoleε (e _? g., 3,3-dimethyl-3H-indole, 3,3,5-trimethyl-3H-indole, 3,3,7-trimethyl- 3H-indole, etc.); (ii) benzindoleε; (h) pyridine nuclei such aε 2-pyridine, 4-pyridine, 5-methylρyridine, etc; (i) pyrazole nuclei εuch aε 4,4,5-trimethylρyrazole; and (j) diazole nuclei εuch aε 1,3,4-oxa- or -thiadia- zole, or 5-methyl-l,3,4-thia- or -oxathiazole. Additional nuclei include pyrrole and pyran nuclei and otherε known by thoεe εkilled in the art for uεe as cyanine dye nuclei.

As noted above, the described MCM compounds are symmetric or asymmetric with respect to the core portion:

R ⁵ R ⁶ I I

of Structure I; that is, in the case of symmetric compounds, the following substituentε are equal: R^R ¹¹ , R ² «R ²² , R ³ «R ³³ , R^ ^*1 *, R ^5«

R ⁷ , Y-Y ¹ , A ^« A', t-u and n ^« m. From a synthetic εtandpoint, the εymmetric MCM compoundε are pre¬ ferred. However, from a hue εtandpoint, an aεymmet- ric MCM compound may exhibit either an increaεed num- ber of absorption peaks, or broadened peaks acroεε the entire viεible region of the εpectrum, thereby providing enhanced neutral hue.

Of the groups set forth above, moεt pre¬ ferred baεic heterocyclic cyanine colorant nuclei comprise 3,3-dialkyl-3H-indole, for example, 3,3- dimethyl- and 3,3-diethyl-3H-indole.

A preferred MCM compound containε the fol¬ lowing εtrueture: _/ -^UR-

-8-

wherein R ¹ , Rx 1 22 , R\ R 33 A A*

Y and Y' are as defined with respect to Structure I, and R ⁵ representε hydrogen or alkyl.

The MCM compounds are structurally similar to compounds reported in British Patent 487,051 accepted June 14, 1938. The MCM compounds diεcloεed In British Patent '051, however, are employed as spectral εenεitizerε for εilver halide photographic compoεitionε within which the phenomenon of photo- electrophoreεiε playε no role. The present invention is made posεible by the discovery that particles com¬ posed of the MCM compounds defined herein are photo- electrophoretic; i.e., charged particles composed of these compounds are capable of undergoing charge polarity reversal and migrating within an electri¬ cally inεulating carrier to one of two electrodeε when the particleε are illuminated in the preεence of an electric field.

The MCM compoundε have uεeful photosenεi- tivity and in many cases exhibit exceptionally good light stability. Furthermore, many of these com¬ pounds, particularly compounds containing the Struc- ture II color-conferring group, abεorb radiation relatively uniformly in the range of from about 400 nm to about 700 nm and therefore exhibit a neutral or nearly neutral denεity coloration.

The electrically photoεensitive material of this Invention compriεeε an electrically insulating

-9- carrier, and particleε comprising at least one elec¬ trically photosenεitive MCM compound, as described above, disperεed in the carrier. The carrier is either liquid or liquefiable under use so aε to facilitate migration of the particleε during use. Other componentε are optionally present such as any one or more of the following: charge control agent, disperεing polymer, binder polymer, chemical ^" sensi- tizer, spectral εensitizer and additional colorants which are optionally electrically photosensitive. The present invention also provides a PEP image-recording method comprising the stepε of:

(a) εubjecting a layer of an electrically photoεenεi- tive imaging material compriεing an electrically inεulating carrier and an MCM compound aε described above to an electric field; and simul¬ taneously, or thereafter, _

(b) exposing said layer to an image pattern of elec¬ tromagnetic radiation to which said material is εensitive, to form a record of the image pattern of electromagnetic radiation in εaid layer.

If the layer iε εolid, it iε at leaεt par¬ tially liquefied before, during or after εtepε (a) and (b) to facilitate migration of' the MCM compound in the layer. Meanε for achieving such liquefication will be deεcribed hereinafter.

Representative MCM com- pounds are included In Table I. The symbol " ^• " in the Table represents a carbon atom having sufficient hydrogen atoms to satisfy carbon valence requirements.

-*£\JIiE

OMP

V 1

VJ1

-13-

The described MCM compounds are useful in all electrically photosensitive materials, imaging layers and photoelectrophoretic imaging processes which require the combined action of an electric field and exposure to an image pattern of electromag¬ netic radiation to obtain an image. These compounds are also useful in imaging processes such as those described in U.S. Patents 3,520,681, 3,770,430, 3,975,195, 4,013,462, 3,707,368, 3,692,516 and 3,756,812, all relating to manifold imaging or photo- electrosolography.

In one PEP imaging process, an element com- prising a conductive support, or a support having a conductive layer, in electrical contact with a lique¬ fied or partially liquefied imaging layer of electri¬ cally photosenεitive material is employed. The layer is uniformly electrostatically charged and then exposed to an image pattern of activating electromag¬ netic radiation. The electrically photosensitive particles in the imaging layer which have been exposed to radiation migrate through the imaging layer, leaving an undeveloped image record of the charge pattern on the conductive substrate. This image is developed by submerging the element in a solvent which removes or dissolves the exposed or the unexposed portions of the imaging layer.

In another such process, a liquid, or at least a partially liquid, electrically photoεensitive imaging layer is poεitioned between two spaced elec¬ trodes, hile so positioned between the spaced elec- trodes, the imaging layer iε subjected to an electric field and exposed to an image pattern of activating radiation. As a consequence, the charge-bearing, electrically photosenεitive particleε in the imaging layer migrate to one or the other of the electrode surfaces to form on at least one of the electrodes an image record representing a positive-εenεe or negative-εenεe image of the original image pattern. The image record iε developed by separation of the electrodeε. In this process the layer of electri- cally photoεenεitive material may be sandwiched between two support εheetε to form an imaging ele¬ ment. After application of the field and expoεure, a visual record of the image pattern is developed on at least one of the two sheets by separation of the εheetε. The εupport εheetε may be electrodeε, or electrodeε may be directly attached to the back εur- faceε of the εupport εheetε. Alternatively, one or both of the support sheets may be made of a conduc¬ tive material. In some embodiments, at least one of the εheetε is transparent or translucent so as to permit exposure of the imaging layer.

In each of the foregoing processes, the imaging layer of electrically photoεenεitive material iε, or can be rendered, at least partially liquid. The phrase "partially liquid" iε uεed herein to mean that the cohesive forces of the materials forming the layer are εufficiently weak, or weakened, to permit some imagewise migration of the defined MCM com¬ pounds, under the combined influence of expoεure to activating electromagnetic radiation and an electric field, in the layer of electrically photoεenεitive material.

OM

-15- In general, imaging layers which are not at leaεt partially liquid may be gendered at leaεt par¬ tially liquid by treatment with, for example, heat, a εolvent and/or εolvent vaporε before, during or after the exposure to an image pattern of electromagnetic radiation and application of an electric field. It will be clear to those skilled in the PEP imaging art, that at least partial liquefaction of the imag¬ ing layer before or during the application of the field and exposure will achieve resultε substantially identical with those obtained with an imaging layer which is at least partially liquid to begin with. Good resultε are obtained if the layer iε liquefied subsequent to the-exposure and field-application steps. In the latter situation, the imaging layer iε liquefied in the preεence of a εecond electric field and the image is developed according to one of the techniques previously mentioned herein.

The extent to which the MCM compounds migrate in those imaging layers which must be lique¬ fied can be controlled by varying the strength and duration of the electric field, the intenεity and duration of the expoεure, and the time which the imaging layer iε expoεed to a particular liquefying medium εuch aε heat and/or solvent. For example, if the imaging layer is only slightly liquefied, the compounds will migrate only slightly, thus forming an underdeveloped image record. This image layer, con¬ taining the underdeveloped image record, can be stored and developed more fully at a later date.

This delayed development can be carried out εimply by placing the underdeveloped image layer in an electric field and then liquefying the layer sufficiently to allow the expoεed electrically photosensitive ate- rial to resume migration. Development of the visual record of the image pattern iε then carried out according to one of the above-mentioned techniques.

The electrically photosensitive material of this invention compriseε the MCM compounds diεpersed in an electrically insulating carrier material such as an electrically insulating liquid, or an electri- cally insulating, liquefiable carrier, such as a heat- and/or solvent-liquefiable polymer or a thixo- tropic polymer.

The electrically photosensitive material of this invention will comprise from about 0.05 part to about 2.0 parts of electrophoretic compound for each 10 parts by weight of electrically insulating mate¬ rial.

Useful liquefiable electrically insulating carriers are disclosed in aforementioned US Patents 3,520,681, 3,975,195, 4,013,462, 3,707,368, 3,692,516 and 3,756,812. The carrier can comprise an electri¬ cally insulating liquid such as decane, paraffin, Sohio Odorlesε Solvent 3440" (a kerosene fraction marketed by the Standard Oil Company, Ohio), variouε iεoparaffinic hydrocarbon liquidε, such aε thoεe sold under the trademark Isopar G" by Exxon Corporation and having a boiling point in the range of 145° C to 186° C, various halogenated hydrocarbons such as car¬ bon tetrachloride or trichloromonofluoromethane, various alkylated aromatic hydrocarbon liquids such as the alkylated benzenes, for example, xylenes, and other alkylated aromatic hydrocarbonε εuch aε are deεcribed in US Patent 2,899,335. An example of one εuch uεeful alkylated aromatic hydrocarbon liquid iε Solvesso 100" εold by Exxon Corporation. Solvesεo 100" haε a boiling point in the range of about 157° C to about 177° C. Typically, whether solid or liquid at normal room temperatures, I.e., about 22° C, the electrically insulating carrier used in the present invention has a resistivity greater than about 10 ⁹ ohm-cm, preferably greater than about

The electrically photoεenεitive material according to this invention compriεeε PEP particleε having an average particle size within the range of from about .01 micron to about 20 microns, preferably from about .01 to about 5 microns. Generally, these particles are composed of one or more colorants and/or PEP compounds, including the compounds of the invention.

The MCM compounds can also be combined with polymers containing organic photoconductive repeating units to form electrically photosenεitive compoεite particleε. Uεeful polymerε are disclosed in Research Diεcloεure, Vol. 190, February, 1980, Item 1914 (pub¬ lished by Industrial Opportunitieε Ltd., Homewell, Havant, Hampεhire, P091EF, United Kingdom), entitled "Compoεite Electrically Photosensitive Particles".

Charge control agents may be Incorporated to improve the uniformity of charge polarity of the electrically photosenεitive materialε of the present Invention. Charge control agents preferably are polymers and are incorporated in the electrically photosenεitive materialε by admixture with the car¬ rier. In addition to enhancement of uniform charge polarity, the charge control agentε often provide more ε able suspensions, i.e., suspensions which exhibit εubεtantially leεε settling out of the dis- perεed photoεensitive particles.

Illustrative charge control agents Include those discloεed in US Patentε 4*,219,614 and 4,273,849. The polymeric charge control agentε diε- cloεed in US '614 compriεe a copolymer having at leaεt two different repeating unitε, (a) one of εaid unitε being preεent in an amount of at leaεt about 0.5 x lO ^* * mole/gram of said copolymer and being derived from monomers selected from the group consisting of metal salts

-18- of sulfoalkyl acrylates and methacrylateε and metal εaltε of acrylic and,methacrylic acidε, and (b) one of said repeating units being derived from monomers soluble in the carrier and present in an amount sufficient to render said copolymer solu¬ ble in the carrier material.

Examples of such copolymers are poly(vlnyl- toluene-co-lauryl methacrylate-co-lithium methacry- late-co-methacrylic acid), ρoly(εtyrene-co-lauryl methacrylate-co-lithium sulfoethyl methacrylate), poly(vinyltoluene-co-lauryl methacrylate-co-lithium methacrylate), poly( -butylstyrene-co-lauryl methac¬ rylate-co-lithium methacrylate-co-methacrylic acid) or poly(t butylεtyrene-co-lithium methacrylate). Other uεeful charge control agents include phosphonate materialε deεcribed in US Patent 4,170,563 and quaternary ammonium polymerε described in US Patent No. 4,229,513.

Various polymeric binder materials such as various natural, semiεynthetic or synthetic resins may be diεperεed or diεεolved in the electrically inεulating carrier portion of the electrically photo¬ εenεitive material to serve aε a fixing material for the final PEP image. The use of εuch fixing addenda iε well-known in the art of liquid electrographic developer compoεitions.

Further addenda include polysiloxane fluids, oils or elastomerε to prevent clumping of the parti¬ cleε in the electrically photoεenεitive material. Preferred oilε compriεe dimethyl polyεiloxaneε or siloxanes in which a εmall portion of the methyl εub- εtituents are replaced by phenyl. Such polyεiloxaneε are available commercially from the Dow-Corning- Com¬ pany under their DC* silicon disperεant εerieε, for example, DC*510, DC ^# 233A and DC ^# 200. Uεeful reεultε are obtained with polyεiloxane oil concentra- tionε of from about 1/4 to about 10 weight percent

baεed on the amount of electrically photoεenεitive particleε.

Imaging elementε compriεing layers of the electrically photosenεitive material of this inven- tion are made according to well-known techniques. The elements may be formed simply by dispersing the electrically photosenεitive material in an electri¬ cally insulating liquid or liquefied carrier and coating the resulting suspenεion or diεperεion on a support according to well-known coating techniques. The support can be insulating or conductive depending on the desired use. Useful supports and coating techniques are described throughout the literature of electrophotography and photoelectrophoretic imaging. The utility of the electrically photosenεi¬ tive materialε of the invention in a PEP imaging pro- ceεε will be deεcribed in more detail with reference to the accompanying drawing, FIG. 1, which illus¬ trates a typical apparatus for carrying out PEP imag- ing processes.

FIG. 1 shows a transparent electrode 1 sup¬ ported by two rubber drive rollers 10 capable of imparting a translating motion via original image 11 to electrode 1 in the direction of the arrow. Elec- trode 1 may be composed of a layer of optically transparent material, such as glass or an electri¬ cally inεulating, transparent polymeric εupport such aε polyethylene terephthalate, covered with a thin, optically tranεparent, conductive layer εuch aε tin oxide, indium oxide, nickel and the like. Option¬ ally, depending upon the particular type of PEP imaging process desired, the surface of electrode 1 may bear a "dark charge exchange" material, such as a solid solution of an electrically insulating polymer and 2,4,7-trinitro-9-fluorenone as described by

Groner in US Patent 3,976,485 iεεued Auguεt 24, 1976.

Spaced oppoεite electrode 1 and in pressure contact therewith iε a second electrode 5, an idler roller which serves as a counter electrode to elec¬ trode 1 for producing the electric field used in the exemplified PEP Imaging procesε. Typically, elec¬ trode 5 haε on the surface thereof a thin, electri¬ cally insulating layer 6. Electrode 5 is connected to one side of a power source 15 by switch 7. The opposite side of the power source 15 is connected to electrode 1 εo that when an expoεure takes place, switch 7 can be cloεed and an electric field applied to the electrically photoεenεitive material 4 which iε poεitioned between electrodeε 1 and 5. Electri¬ cally photoεenεitive material 4 comprises an electri- cally insulating carrier material εuch aε deεcribed hereinabove.

The electrically photoεenεitive material 4 iε formed into a layer between electrodeε 1 and 5 by applying the material 4 containing a Table I compound tb either or both of the εurfaceε of electrodeε 1 and 5 prior to the imaging proceεε or by placing the diε- perεion between electrodeε 1 and 5 during the PEP imaging procesε.

Aε shown in FIG. 1, exposure of layer 4 takes place by use of an exposure system consisting of light source 8, an original image 11 to be repro¬ duced, such as a photographic transparency, a lens syεtem 12, and any necessary or desirable radiation filters 13, εuch aε color filterε, whereby electri- cally photoεenεitive material 4 iε irradiated with a pattern of activating radiation corresponding to original image 11. Although the PEP imaging εyεtem repreεented in FIG. 1 εhowε electrode 1 to be tranε- parent to activating radiation from light source 8, It is poεεible to Irradiate electrically photoεenεi¬ tive material 4 in the nip 21 between electrodeε 1 and 5 without either of electrodeε 1 or 5 being

tranεparent. In εuch a εystem, although not shown in FIG. 1, the exposure source 8 and lens system 12 is arranged so that electrically photosenεitive material 4 iε expoεed in the nip or gap 21 between electrodeε 1 and 5.

Aε shown in FIG.l, electrode 5 is a roller electrode having a conductive core 14 connected to power source 15. The core Is In turn covered with a layer of insulating material 6, for example, baryta- coated paper. Insulating material 6 serves to pre¬ vent or at least substantially reduce the capability of electrically photosenεitive material 4 to undergo a charge alteration upon interaction with electrode 5. Hence, the term "blocking electrode" may be uεed, aε iε conventional in the art of PEP imaging, to refer to electrode 5.

Although electrode 5 iε shown aε a roller electrode and electrode 1 iε shown as essentially a translatable, flat transparent plate electrode in FIG. 1, either or both of these electrodes may asεume a variety of alternative εhapes such as a web elec¬ trode, rotating-dru electrode or opaque-plate elec¬ trode, as iε well-known in the field of PEP imaging. When the electrically inεulating carrier of the elec- trically photoεenεitive material 4 is a liquid, elec¬ trodes 1 and 5 are spaced such that they are in pres¬ sure contact or very close to one another during the PEP imaging process, e.g., less than 50 microns apart. However, when the electrically photosensitive material is εimply diεpoεed in the gap between elec¬ trodes 1 and 5 as a separate layer on electrodeε 1 and/or 5, the electrodeε can be εpaced more than 50 micronε apart during the imaging process.

The strength of the electric field imposed between electrodes 1 and 5 during the PEP imaging process varies considerably; however, It has gener¬ ally been found that optimum image density and reso-

"

lution are obtained by increasing the field strength to as high a level as possible without causing elec¬ trical breakdown of the carrier medium in the elec¬ trode gap. For example, when electrically insulating liquids such as isoparaffinic hydrocarbons are uεed aε the carrier in the imaging apparatuε of FIG.l, the applied voltage acroεε electrodeε 1 and 5 typically iε within the range of from about 100 volts to about 4 kilovolts or higher. Aε explained above, image formation occur in PEP imaging processeε aε the reεult of the com¬ bined action of activating radiation and electric field on the electrically photoεenεitive material diεpoεed between .the electrodeε. For beεt results, field application and exposure to activating radia¬ tion occur concurrently. However, by appropriate selection of parameters such as field strength, acti¬ vating radiation intensity, and Incorporation of suitable light-senεitive addenda with the electri- cally photosensitive particles, it iε possible to use sequential rather than concurrent field-application and exposure.

When diεpoεed between imaging electrodeε 1 and 5 of FIG. 1, electrically photoεenεitive material 4 exhibitε an electroεtatic charge polarity. Such charge results from either the triboelectrlc Inter¬ action of the particleε or as a reεult of the parti¬ cleε interacting with the carrier material In which they are diεpersed, for example, an electrically Insulating liquid. Such charging iε εimilar to that occurring in conventional liquid electrographic developing compoεitions composed of toner particles which acquire a charge upon being disperεed In an electrically inεulating carrier liquid. In a typical imaging operation, upon appli¬ cation of an electric field between electrodes 1 and 5, the charge-bearing particleε within electrically

photoεenεitive material 4 are attracted in the dark to either electrode 1 or 5, depending upon which of theεe electrodeε has a polarity opposite to that of the original charge polarity acquired by the electri- cally photosenεitive particles. It iε theorized that, upon exposing electrically photosenεitive mate¬ rial 4 to activating electromagnetic radiation, the charge polarity associated of either the expoεed or unexpoεed particles reverses. In PEP imaging syεtems wherein electrode 1 bears a conductive surface, the exposed charged particleε within electrically photo¬ εenεitive material 4, upon coming Into electrical contact with εuch conductive surface, undergo a reversal of their ^" original charge polarity as a result of the combined application of electric field and activating radiation. Alternatively, In the case of photoimmobilized PEP recording (PIER), wherein the surface of electrode 1 bears a dark charge exchange material as described by Groner in aforementioned US Patent 3, 976,485, one obtainε reverεal of the charge polarity of the unexpoεed particles, while maintain¬ ing the original charge polarity of the exposed elec¬ trically photosenεitive particleε, aε theεe particleε come into electrical contact with the dark charge exchange εurface of electrode 1. In any case, upon the application of electric field and activating radiation to electrically photosensitive material 4 dispoεed between electrodeε 1 and 5 of the apparatuε shown In FIG. 1, one effectively obtains Image dis- crimination εo that an image pattern iε formed by the electrically photoεenεitive particle layer which cor- reεpondε to the original pattern of activating radia¬ tion. Uεing the apparatus shown in FIG. 1, one obtainε a visible image on the εurface of electrode 1 and the complementary image pattern on the εurface of electrode 5.

Subsequent to the application of the elec¬ tric field and exposure to activating radiation, the images which are formed on the surface of electrodes 1 and/or 5 of the apparatus shown In FIG. 1 may be temporarily or permanently fixed to these electrodes or may be transferred to a final Image-receiving ele¬ ment. Fixing of the final Image can be effected by various techniques, for example, by applying a resin- ouε coating over the εurface of the Image-bearing substrate. For example, if electrically photosenεi¬ tive material 4 includeε a liquid carrier between electrodeε 1 and 5, one may fix the image or Imageε formed on the εύrface of electrodes 1 and/or 5 by incorporating a particulate polymeric binder material in the carrier liquid. Many εuch binders are well- known for use in electrophotographic liquid develop¬ ers. They are known to acquire a charge polarity upon being admixed in a carrier liquid. Therefore, they will, themselves, electrophoretically migrate to the surface of one or the other of the electrodes. Alternatively, a coating of resinous binder (which has been disεolved in the carrier liquid) may be formed on the εurfaceε of electrodeε 1 and/or 5 upon evaporation of the liquid carrier. The electrically photoεenεitive materialε of thiε invention are preferably uεed to form monochrome imageε. Alternatively, the electrically photoεenεi¬ tive materialε are uεed to form polychrome imageε. In εuch instance, the material compriεeε, in addition to the MCM compoundε, εpecific electrically photoεen- sitive cyan, magenta or yellow particles necessary to form the deεired polychrome image.

The following exampleε illuεtrate the ^' utility of the MCM compoundε in PEP imaging proceεεeε. SYNTHESIS OF SYMMETRIC MCM COMPOUNDS

The εyntheεls of symmetric mono ethine MCM compounds Is conveniently carried out by forming a

merocyanine, followed by coupling two molecules of the merocyanine with malonic acid to form the mono- methine bridge of a central cyanine. For illustra¬ tion, the syntheεiε of Compound A (5,5 ^, -biε[(3,3- dimethyl-1-ethyl-indolinylidene)ethylidene]- 3,3'-diethyl-4,4 ¹ -dioxothiazollnocyanine £-toluene- εulfonate) of Table I iε set forth below. The reac¬ tion scheme is as follows:

1)

where Me Is methyl, Et iε ethyl, φ iε phenyl and Ac iε acetyl.

Et

A*

2)

- ^■ gϊXR OM ^■ -. Wϊt

_B * . HOOC. H .COOH Pyrldina _t _ _{momd A}

B* + ^\ _c ^/ -άθ ₂ , -MeSH Compound A

H

A* is 5- [ (3 , 3-dimethyl-l-ethyl-2-Indolinylidene) ^■ ethylidene 3 -3-ethylrhodanine

B* is 5[ (3, 3-dImethyl-l-ethyl-2-Indolinylidene) - ethylidene3-3-ethyl-2-methylthio-4-oxo-2- thlazolinium p_-toluenesulfonate

Step 1;

A 1-liter, round-bottom flaεk containing a magnetic εtir bar was charged with 63 g (0.2 mole) of l-ethyl-2,3,3-trimethyl-3H-indolium iodide, 61.5 g (0.2 mole) of 5-acetanilidomethylene-3-ethylrhoda- nine, 30.8 ml (0.22 mole) of triethylamine and 500 ml of anhydrous ethanol. The mixture was refluxed with stirring for 1/2 hr in a 110° C oil bath. After cooling to 0° C for several hours, the red crystal¬ line dye A* was filtered, washed with ethanol and air-dried.

Yield: 68.0 g (94.8X). Step 2: in a 300-ml, round-bottom flask, 5-[(3,3- dimethyl-l-ethyl-2-indolInylidene)ethylidene]-3-ethyl- rhodanine (10.7 g, 0.03 mole) and methyl p_-toluene- εulfonate (18.6 ml, 0.120 mole) were combined and heated over a εteam bath for 5 hr. The dark red syrup was diluted to 200 ml with acetone, then refrigerated (0 ^β C) overnight. The black prismε of B* were filtered, waεhed with acetone and air-dried.

Yield: 12.07 g (741). Step 3: In a 1-liter, round-bottom flaεk were placed a magnetic εtir bar, 11.97 g (0.022 mole) of 5-[(3,3-

O

dimethyl-l-ethyl-2-Indolinylidene)-ethylidene -3-ethyl-

2-methylthio-4-oxo-2-thiazolinium £-toluenesulfonate, 22.8 g (0.22 mole) of malonic acid and 200 ml of anhydrous pyridine. The mixture was heated strongly until carbon dioxide evolution began, then maintained at a temperature of 100-105° C for 1 hr. After cool¬ ing to room temperature, the black solution was poured into 2 literε of ether with εtirring. The mixture waε refrigerated at 0° C overnight, then fil- tered. The εolid waε waεhed generouεly with water to remove pyridinium p_-tolueneεulfonate, then air- dried. The Compound A dye (4.53 g) waε recryεtal- lized from 350 ml (75 ml per gram) of ethanol. Final Yield: . ^" 3.64 g (39.6X). The εyntheεiε of symmetric trimethine or pentamethine MCM-containing compoundε involveε the coupling of two molecules of a compound having the structure:

N

12

R

with any one of such materials as diethoxymethyl ace¬ tate, ethyl orthoformate, ethyl orthoacetate or ethyl orthopropionate to give trimethine MCM-containing compounds (p«l In Structure I), or with 1,3,3-trieth- oxypropene to give a pentamethine MCM-containing com- pound (p«2 in Structure I).

SYNTHESIS OF ASYMMETRIC MCM COMPOUNDS Aεym etric MCM compoundε were prepared by coupling two dissimilar merocyahine-containing com¬ pounds. The synthesiε of compound H, Table I, iε shown for illustration:^

In 7 ml of pyridine, 1.65 g (0.0027 mole) of 2-(2-anilinovinyl)-3-ethyl-4-oxo-l-phenyl-5-[(1,3,3- tri-methyl-2-indollnylldene)ethylidene -2-imldazoll- nium iodide were suspended with 1.34 g (0.0026 mole) of 3-ethyl-5-[(3-ethyl-2-benzoxazolinylIdene)ethyli¬ dene3-2-methyl-4-oxo-l-phenyl-2-imidazolinium iodide. (These two compoundε were prepared by the procedure described in US Patent 3,576,639.) To the εuspenεion were added 0.75 ml of triethylamine and 0.38 ml of acetic anhydride, and the reεulting mixture waε heated with stirring at refluxing temperature for 10 min. A tarry but filterable product separated when the reaction mixture was chilled and diluted to about 50-ml volume with Ice and water. Recryεtallization of the gummy product from ethyl alcohol yielded bronze crystalε. Additional amountε were obtained from the filtrates by seeding and chilling the solu¬ tions. Total crude yield was 0.87 g (36%).

The MCM compound was recryεtallized twice from methanol. Each time, additional amounts were recovered from the filtrates. Yield 0.53 g (221) m.p. 247-248 ^β C.

Imaging Apparatus An imaging apparatus waε uεed in each of the following exampleε to carry out the PEP imaging pro¬ cess described herein. This apparatus was a device of the type illustrated in FIG. 1. In this appara¬ tus, a translating film base having a conductive coating of 0.1 optical density cermet (Cr ^β Si0) εeryed aε electrode 1 and waε in preεsure contact with a 10-cm-diameter aluminum roller 14 covered with dielectric paper coated with poly(vinyl butyral)

reεin which served as electrode 5. Electrode 1 was supported by rubber drive rollers 10 positioned beneath and spaced sufficiently apart from electrode 1 to allow exposure of electrically photosenεitive material 4 to activating radiation. The light source consiεted of an Ektagraphic AV 434 A ^* Projector with a 1000-watt xenon lamp. The light waε modulated with a Kodak" No. 5 flexible M-carbon, 0.3 neutral denεity, 11 εtep tablet taped to the backside of electrode 1. The residence time in the action or exposure zone was 10 milliseconds. The voltage between the electrode 5 and electrode 1 waε about 2 kv and the current was about 10 microamps. Electrode 1 was of negative ^' polarity in the case where electri- cally photosenεitive material 4 carried a poεitive electrostatic charge, and electrode 1 waε poεitive in the caεe where electrically photosensitive electro¬ statically charged particles were negatively charged. The translational speed of electrode 1 was about 25 cm/second. In the following exampleε, image formation occurε on the surfaces of electrode 1 and electrode 5 after si ultaneouε application of light exposure and electric field to electrically photoεen¬ εitive material 4 formed from the diεperεion of elec- trically photoεenεitive material containing an MCM compound in a liquid carrier. The liquid imaging diεperεion waε placed in nip 21 between the elec¬ trodeε 1 and 5. If the material being evaluated for uεe possessed a useful level of electrical photoεen- sitivity, one obtained a negative-appearing image reproduction of original 11 on electrode 5 and a poεitive image on electrode 1.

Imaging Diεperεion Preparation Imaging diεperεions were prepared to evalu- ate each of the compounds In Table I. The disper- εlons were prepared by first making a εtock εolution

of the following components. The εtock solution was prepared simply by combining the componentε.

Iεopar G" 2.2 partε by weight

Solveεεo 100" 1.3 parts by weight Piccotex 100 ^* 1.4 parts by weight

PVT 0.1 parts by weight

PVT is poly(vinyltoluene-co-lauryl methacry¬ late-co-lithium methacrylate-comethacryllc acid) 56/40/3.6/0.4. Piccotex 100" is a mixture of styrene- vlnyl toluene copolymers available from Pennεylvania Induεtrial Chemical Corp.

Iεopar G" iε an iεoparaffinic aliphatic hydrocarbon available from Exxon Corporation. Solveεεo 100" iε an alkylated aromatic hydrocarbon liquid available from Exxon Cor¬ poration.

A 5-g aliquot of the εtock εolution waε combined in a cloεed container with 0.045 g of the Table I compound to be tested and 12 g of Pioneer 440" stainleεε- εteel balls. The mixture was then milled for 3 hr on a paint shaker. EXAMPLES 1-6:

Table I compounds A-F were tested according to the above procedures and found to be electropho¬ retic as evidenced by obtaining a negative-appearing image of the original on one electrode and a positive image on the other electrode. Compounds A and D pro¬ vided the best Image quality. Image quality waε determined visually on the basis of minimum and maxi¬ mum densitieε.

Relative εenεitivity waε determined by nor¬ malizing to 100 the reciprocal of the clear expoεure In ergε/cm ² needed by the diεperεion containing Compound 1 to produce a denεity 0.10 above D . on the negative image. The expoεure necessary to pro¬ duce the same density for the other disperεionε waε

-31- determined relative to the 100 value of the diεper¬ εion of Compound A aε a control. The results are reported in Table II below.

The light stability of Compounds A and D-F was determined by the following fade test: Imageε compoεed of the compoundε were expoεed to high- intenεity daylight (HID) (50,0001ux) for 24 hr. The loεε In density at D BlfiX was measured spectroscopi- cally and determined as a percentage loεs from the unexposed image. The reεultε are reported in Table II.

TABLE II

Example/ Relative X Fade Compound max ^D mln Senεitivity (24 hr HID)

1/A 0.64 0.01 100 111

2/B 0.66 0.08 61 *

3/C 0.52 0 35 *

4/D 0.75 0.01 100 4.1

5/E 0.60 0.01 41 11.4

6/F 0.32 0.01 6 40.6

* not evaluated

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.

-^TR OM ^"