CA1255309A - Dye-bleach imaging system using iodonium salts - Google Patents

Abstract

ABSTRACT
OXIDATIVE IMAGING

A radiation-sensitive element capable of recording an image upon image-wise exposure to radiation of selected wavelength, the element comprising, as the image-forming components, an effective amount of a bleachable dye in reactive association with an iodonium ion. Suitable dyes include polymethine dyes having an oxidation potential between 0 and +1 volt.

Description

5~

OXIl)ATIV~ IMAGING

Field of the Invention This invention relates to radiation~sensitive elernents which are capable of recording a positive image upon image-wise exposure to radiation, e~g.
visible light, and to their preparation and use. In particular, the invention relates to radiation-sensitive elements having a bleachable dye and an iodonium salt in reactive association.
Back~round of the Invention Positive working imaging systems in which an originally coloured species is decolourised in an ; image-wise manner axe known. These systems have the advantage o~ giving a positive copy of an original.
one of the earliest forms of positive working imaging systems was developed utilising the properties of photographic silver, e.g. as disclosed in British Patent Specification No~ 17773 (188g), Austrian Patent Specification No. OE42478 and B. Gaspar, Zeitschrift Wiss. Phot. 34, 119 (1935). Since then many fo~ms of colour silver halide photography have been developed.
Silverless dye bleaching processes are also known, but in spite of ~he apparent simplicity of these systems, they have encountered a number of problems~ The inadequate photosen~itivity of such systems consisting of colour layers, the lack of purity and stability of the white in the final print and difficulty of finding dyes which form a neutral grey and bleaching at equal rates, are some o~ the , .

.:

..

:~;25~

problems. Early systems are disclosed in Smith, Photogr. J., April 1910, page 141. More recently, cyanines with borate anions are dis-closed as a dye bleach system in British Patent Specification Nos.
1 370 058, 1 370 059 and 1 370 060. A dye bleach process involving tetra(alkyl)borate is disclosed in United States Patent Specifica-tion No. 4 307 182 and fixing methods are disclosed in European Patent Specification No. 0040978. United States Patent Specifica-tion No. 3 595 655 discloses a silverless dye bleach system consis-ting essentially of a polymethine dye and an activator which is a carbonyl, azo, diazo, organic-sulphur containing or peroxide com-pound.
It is an object of the present invention to provide new radiation-sensitive elements capable of recording a positive image.
Summary of the Invention Therefore according to one aspect of the invention there is provided a radiation-sensitive element capable of recording an image upon image-wise exposure to radiation of selected wavelength, the element comprising, as the image-forming components in a binder, an effective amount of a bleachable dye in reactive association with an iodonium ion.
According to another aspect of the invention there is provided a compound of either of the formulae:

CN

~ ~ M~
CO2C2H5 ~ ,,CH3 O
C~I3 , - 2 -: . :
~. :

,',';' ~ ' ~

53~
- 2a - 60557-2730 and CO Et ~ 0~) Et CN

in which M~ represents a cation.
The elements o-f the invention are capable of recording a positive image simply upon exposure to radiation of selected wave-length; the radiation absorbed by the dye which is in reactive as-sociation with an iodonium ion causes the dye to bleach. The dyes are believed to sensitise spectrally the reduction of the iodonium 10 ion through the radiation absorbed by the dyes associated with -the iodonium ion. Thereafter the element may be stabilised to fix :' - 2a -:

~ ~ ' , ;~ .

9-2~5~4.-~
=~ =

the image by destruction o~ the iodonium ion or by separation of the dye relative to the iodonlum ion.
The dyes used in the invention may be of any colour and any chemical class which is capable of bleaching upon exposure to radiation of selected wavelength in the presence of an iodonium ion.
By a suitable selection of dye an element of the invention may be prepared which is sensitive to radiation of a selected wavelength band within the general range 300 to 1000 nm, the particular wavelength and the width of the band depending upon the absorption characteristics of the dye. In general, where a dye has more than one absorption peak it is the wavelength corresponding to the longest wavelength peak at which one would choose to irradiate the element.
Elements intended for the production of images from radiation in the visible regio~ (400 to 700 nm~
will contain dyes which will bleach from a coloured to a substantially colourless or very pale state. In practice, such bleachable dyes will undergo a change such that the transmission optical density at the ~max will drop from 1.0 or more to less than 0.09, preferably less than 0.05. The dyes will generally be coated on the support to provide an optical density of about 3.0 or more.
In the case of elements sensitive to ultra-violet radiation (300 to 400 nm) the dyes will not normally be coloured to the eye and there may be no vi~ible change upon exposure to ultraviolet radiation and bleaching. The image-wise exposed elements may be used as masks for further ultraviolet exposure after fixing.
Infrared sensitive elements of the invention contain dyes have an absorption peak in the wavelength ' :: ~

, . . .
'` :: ,, ::

~ ' :, ranye 7~ to 1100 nm. ~ e~e ~yes may also have absorption pealcs in the visible region before and/or after bleaching. ThUs, as well as providing a means for obtaining masks for subsequent infrared exposure in a similar manner to the ultravlolet masks, infrared sensitive elements of the invention may record a visible image upon image-wise exposure to infrare~
radiation.
Certain of the elements o~ the invention, e.g.
those containing oxonol or cyanine dyes, will bleach upon heating and may be used as heat bleachable antihalation layers or to record thermal images. The heat bleaching effect of dye/iodonium ion combination may also be utilised as a method of fixing a visual image obtained with a different dye by reacting the excess iodonium ion upon heating.
The dyes used in the invention may be anionic, cationic or neutral. Preferred dye~ are anionic since they give very good photosensitisation which is believed to be due to an intimate reactive association between the negatively charged dye and the positively charged iodonium ion. Also anionic dyes may readily be mordanted to cationic polymer binders and it is relatively simple to remove surplus iodonium ions in an a~ueous bath in a fixing step if the mordanting polymer is cationic, However, neutral dyes also give good results and are preferred over cationic dyes for overall photosensitivity. Cationic dyes are least preferred since it is more difficult to achieve intimate reactive association between the positively charged dye and iodonium ion, and selective removal of iodonium ion after imaging is more difficult.
The bleachable dyes may be generically re~erred to as polymethine dyes which term characterises dyes having at least one electron donor and one electron acceptor~ group linked by me~hine 3~`~
=~ =

groups or aza analogues. The ~yes have an oxidation potential between O and -~1 volt, preferably be~weer ~0.2 and +0.6 volt. The bleachable dyes may be selected from a wide range of known classes of dyes ineluding allopolar cyanine ~ye bases, ~omplex cyanine, hemicyanine, merocyanine, azine, oxonol, streptocyanine and styryl.
Three species of dye are of particular significance for use in the invention. These species are dyes which include within their structure one of the following systems:

(a) ~N - C (=C C) = N~

(b) ~N = C (-C C)n Amidinium-ion system (a) 0 = C (-C = C)n -,0:
.

(2) ~ I ~l I
(b) :0 - C (=C - C)n =
Carboxyl-ion system .

., (a) )N.- C ~=C - C)n = .

(b) ~N = C ( C = C) - 0:
Amidic system .

':
'~

=~=

It will be appreciated that the two structures a) and b) for each system differ only in the way the electrons are disposed, not in the location of atoms.
One or more carbon atoms in the chains may be replaced by nitrogen providing the conjugated structure is not disrupted. In actual dye examples the valencies shown unsatisfied in the skeletal formulae are completed as will be described and illustrated hereinafter.
In generall bleachable dyes for use in the invention will be of the general formula:

R~-~ R 4 in which:
n is an integer of 1 to 5, and Rl to R4 are selected to provide an electron donor moiety at one end of the conjugated chain and an electron acceptor moiety at the other, and represent halogen, alkyl, aryl groups or he~erocyclic rings any of which may be substituted, said groupsgenerally containing up to 14 atoms selected from C, N, O and S;
or Rl and R2 and/or R3 and R4 may represent the necessary atoms to complete optionally substituted aryl groups or heterocyclic rings, generally containing up to 14 atoms selected from C~ N, O and S.
The conjugated chain is preferably composed of carbon atoms but may include one or more nitrogen atoms providing the conjugation is not disrupted. The free valencies on the chain may be satisfied by hydrogen or any substituent of the type used in the cyanine dye art including fused ring systems.

.

~, 2~i53 Irhe particular selection of substituents Rl to R effects the light absorbance properties of the dye which may be varied to provid0 absorption peaks ranging from the ultraviolet (300 to 400 nm), near visible (400 to 500 nm), far visible (500 to 700 nm) and infrared (700 to 1100 nm). The absorption charact~ristics of ~he dyes do not significantly effect the sensitivity of the composition of the invention, which is governed by the particular selection of mesoionic compound.
Within the above general structure o~ dyes are various classes of dyes including:
1) Cyanine dyes of the general formula:

~ ~ R6 in which:
p is an integer of 0 to 5, R5 and R6 are independently hydrogen or substitLIents which may be present in conventional cyanine dyes, e.g. alkyl (preferably of 1 to 4 carbon atomsl, etc., X represents an anion, and A and B independently represent alkyl, aryl or heterocyclic groups or the necessary atoms to complete heterocyclic rings which may be the same or differentO The groups A and B generally contain up to 14 atoms selected from C, N, O and S.
This class of dyes is very well known particularly in the silver halide photographic art and are the subject of numerous patents. General references to these dyes include The Chemistr~ of Synthetic Dyes, K. Venkataraman ed , Academic Press, Vol. 4 ~1971) and The Theory of the Photo~raphic Process, T.H. James, ed., MacMillan, Editions 3 and 4 :J~

~ .

=~ =

2) Merocyanine dyes of the general formula.
o in which:
q is an integer of 0 to 5, R5 and A are as defined above, and B is as defined above or may complete a carbocyclic ring.
These dyes are also well known in the silver halide photoyraphic art and are described in The Theor~__f the Photographic ~rocess, referred to above.

3) Oxonols o~ the general ~ormula.
A~ B~
~5 `_ ~ ~ o O
in which:
q is an integer of 0 to 5~
A and B may be the same or different and are as defined above in relation to cyanine and merocyanine dyes, and represents a cation.
Oxonol dyes are similarly well known in the silver halide photographic art and are disclosed in the above mentioned reference, 5r~ ~n~orV ~
Photoyraphic Process and, for example, United States Patent Specification No. 2 611 696.
It is to be understood that these cyanine~
merocyanine and oxonol dyes may bear substituents along the polymethine chain composed of C, N, O and S, ; and that these substituents may themselves join to form 5 6 sr 7 membered rings/ or may bond with rings ' ~ .

" ~., . ::

., ~ .

=~=

A and ~ to forM further rings, possibly with aromatic character. Rings A and B may also be substitute~ by C, N~ H, O and S containing groups.
Other known classes of dyes useful in the invention which possess an activated methylene chain include bisquinones, bisnaphthoquinones, hemicyanine, streptocyanine/ anthraquinone, indamine, indoaniline and indophenol.
Preferred dyes for use in the invention are merocyanlne and oxonol dyes. Examples of oxonol dyes include:

CN
~ e \ Me (yellow dye) NHEt3 ~ S~

n O ~
O ~ (n = 1, magenta dye ;~ NHEt3 n = 2, cyan dye) 1 '~ .

.

=1~=

rl~he cation of tlle oxonol dye need not be the iodonium ion and can be any cation including li~, Na~
and K~ or quaternary ammonium cations, e.g. as represented by the formula:
S R~

in which R6 to R9 may be selected ~rom a wide range of groups including hydrogen, alkyl, preferably of 1 to 4 10 carbon atoms, aryl, e.g. phenyl, aralkyl of up to 12 carbon atoms. Preferably at least one of R6 to X9 is hydrogen and the rest are alkyl or aralkyl since such amines are readily available and allow easy synthesis of the dyes.
In some aspects o~ the invention, it is essential to immobilise the oxonol dye in the binder during the fixing process. ThiS can be achieved by incorporation of a mordant in the form of the oxonol dye cation. ThUS ~ any cationic polyelectrolyte may be 20 used, e.g. those of the formula:

~ CH~CH2~q ~ 0 ~ (CH-fH~
10110 z l~

in which:
25 q is an integer, R10 and Rll independently represent hydrogen, alkyl, preferably containing l to 4 carbon atoms, groups~ e.g. methyl~ ethyl, or a group having a =11=

yuaternary ammonium group at the end of an alkyl chain, e.~. CH~-CH2-cH2- ~(Me)3Z~; preferably hydrogen or alkyl ammonium~ and 2~ represents an anion, e.g. acetate, chloride with proper selection of the quaternary ammonium or pyridinium cations, such polymeric materials may also serve as the binder for the system.
It may be desirable to have a selec~ion of R10 and Rll groups in the polymer. Preferably up to 80~ of R10 and Rll groups are hydrogen to ensure compatibility with gelatln binders.
The dye/iodonium system has its greatest sensitivity at the ~nax f the longest wavelength absorbance peakO Generally it is necessary to irradiate the system with radiation of wavelength in the vicinity of this ~max for bleaching to occur.
Thus, a combination of coloured dyes may be used, e.y.
yellow, magenta and cyan, in the same or different layers in an element and these can be selectively bleached by appropriate visible radiation to form a full colour image. Monochromatic or polychromatic images may be produced using the photose~sitive materials of this invention with relatively shsrt exposure times in daylight or sunlight or even ar~ificial sources of light (e.g. fluorescent lamps or laser beams). The exposure time, for adequate results, for example when using a 0.5 kW tungsten lamp at a distance of 0.7 m, may be between 1 second to 10 minutes r The iodonium salts used in the invention are compounds consisting of a cation wherein a positively charged iodine atom bears two covalently bonded carbon atoms, and any anion. Preferably the acid from which the anion is derived has a pKa ~5. The preferred compounds are diar~l, aryl~heteroaryl or diheteroaryl ,,~

=1~=

iodonlum salts in whic~l the carbon-to-iodine bond~ are from aryl or heteroaryl groups. Aliphatic iodonium salts are not normally ~hermally stabls at temperatures above 0C. However, stahilised alkyl phenyl iodonium salts such as those disclosed in Cheln.
Lett. l9R2 t 65-6 are stable at ambient temperatures and may be used in the invention.
Suitable iodonium salts may be represented by the formula:

Ar I~ A~
Ar2 in which:
Arl and Ar2 independently represent c~rbocyclic or heterocyclic aromatic-type groups generally havin~ from ~ to 20 carbon atoms, or together with the iodine atom complete a heterocyclic aromatic ring.
These groups include substituted and unsubstituted aromatic hydrocarbon rings, e.g. phenyl or naphthyl, which may be substituted with alkyl groups, e.g.
methyl, alkoxy groups, e.g. methoxy, chlorine, bromine, iodine, fluorine, carboxy, cyano or nitro groups or any combination thereof. Examples of hetero-aromatic groups include thienyl, furanyl and pyrazolyl which may be substituted with similar substituents as described above. Condensed aromatic/hetero-aromatic groups, e.g. 3-indolinyl, may also be p~esent.
A represents an anion which may be incorporated into Arl or Ar2.
Preferably Arl and Ar2 do not have more ~han two substituents at the ~ positions o~ the aryl groups. Most preferably Ar and Ar are both phenyl ~roups containing no ~ substituents.
.

~ I~he ~ positions ol the aryl groups may ~
linked together to include the iodine atom within a ring structure, e.g.

~ I
A ~

in which z is an oxygen or sulphur atom. An example of such an iodonium salt is:

~ Q

Other suitable iodonium salts include polymers containing the unit:

/ -CH-CH~-\

0 I ~-Ph ./
/

in which Ph rep~esents phenyl.
~ Examples of such polyme~s are disclosed in Yamada and ;~ Okowara, Makromol Chemie, 1972, 152, 61-G.
' , , .~

g =14=

Any anion may be used as the counter-ion A~
provided that the anion does not react with the iodonium salt. Suitable inorganic anions include halide anions, HSO~, and halogen-containing complex anions, e.g. tetrafluoroborate, hexafluorophosphate, hexafluoroarsenate and hexafluoroantimonate. Suitable organic anions include those of the formulae:
RCOO~ or RSO3~
in which R is an alkyl or aryl group of up to 20 carbon atoms, e~g. a phenyl group, either of which may be sub~tituted~ ~xamples of such anions include CH3COO~ and CF3COO~.
~ may be present in ~rl or Ar2, e.g.

1 5 ~A~

in which A~ represents COO~, etc.
Furthermore, A~ may be present in a molecule containing two or more anions, e.g. dicarboxylates containing more than 4 carbon atoms.
The most significant contribution of the anion is its effect upon the solubility of the iodonium salt in different solvents or binders. This criterion is also important for systems fixed by removal of the unreac~ed iodonium ion in an aqueous processing step where good solubility of the iodonium salt in water is essential.
Most of the iodonium salts are known, they may be readily prepared and some are commercially available The synthesis of suitable iodonium salts is disclosed in F.~. Beringer et al, Journal of the American chemical Society, 80, 4279 (195~).
Previously, these salts have been used in cationically 3~

induced epoxy polymeri~ation or radically induced monomer polymerization as disclosed, for example, in United States Patent Specification Nos. 3 741 769, 3 729 313, 3 808 006, 4 026 705, 4 228 232 and

4 250 053. Such polymerization systems may form the basis of imaging systems of the type utili~ing a coloured to~er which will selectively adhere only to the tacky unexposea areas ~hich have not undergone polymeriæation.
The iodonium salts disclosed in the above referenced Patents have been sensitised with a wide range of dyes to increase speed and/or broaden spectral response and have been used as components in image forming systems in the absence of polymerizable monomers. However, heretofore there has been no disclosure nor indication in the prior art of a dye-bleach system suitable for image recording employing a bleachable dye and iodonium salt as the image recording medium.
The bleachable ~ye and iodonium salt are in reactive association on the supportO Reactive association is defined as such physical proximlty between the compounds as to enable a chemical reaction to take place between them upon exposure to light. In practice, the dye and iodonium salt are in the same layer or in adjacent layers on the support.
In general, the weight ratio of bleachable dye to iodonium salt in the element of the invention is in the range from 1:1 to 1:50, preferably in the range from 1:2 to 1:10.
T~le bleachable dye and iodonium salt may be applied to the support in a binder~ Suitable binders are transparent or translucent, are generally colourless and include natural polymers, e.g. gelatin, gum arabic, syn~hetic resins~ polymers and copolymers, e.g. polyvinyl acetals, cellulose esters, polyamides, .

~5;5;3~3~`~

=16=

polyacrylates, polymethacrylates, polyurethanes, polyepoxides, polycarbonates, polyvinylacetate, polyvinyl butyral, polyvinyl alcohol, polyvinyl pyrrolidone, polyvinylidene chloride, poly(4-vinyl-N-alkylpyridinium salt), and other film forming media. The binders may range from thermoplastic to highly crosslinked, and may be coatea from aqueous or organic solvents or emulsion.
It is also possible for the binder to form part of the dye molecule as described above with reference to oxonol dyes. In practice, when separate binders are used the binder comprises from 50 to 98~
by weight based on the total dry weight of binder, dye and iodonium salt.
Suitable supports for use in the invention are any stable substrate, including transparent film, e.g.
polyester, paper e.g. baryta-coated photographic paper, and metallised film. Opaque vesicular polyester films are also useful.
It is not essential for the elements of the invention to comprise a separate support since a binder, e.g. a synthetic polymer, together wi~h the dye and iodonium salts may be cast to form a sel~-supporting film.
The fixing of the radiation sensitive elements of the invention may be effected by destruction of the iodonium ion by disrupting at least one of the carbon-to~iodine bsnds since the resulting monoaryl iodine compound is no longer sensitive to the dye.
The conversion of the iodonium salt ~o its non-radiation sensitive form can be e~fected in a variety of fashionsO Introduction of ammonia and amines in reactive association with the iodonium ion, or a reaction caused on heatin~, or UV irradiation of a nucleophilic anion such as I , Br~, Cl~, ~Ar (tetra-arylboronide), ArO~ (e.g.

~ 3 =17=

ph~nX~ r 4-N~2~6ll4~2~ with the iodonium ion, will effect the conversion.
~n alternative method of achieving post imaging stabilisation or fixing is to remove the iodonium ion from reactive association with the dye by washing with an appropriate solvent. ~or example, in the case of elements using mor~anted oxonols an~ water soluble iodonium salts formulated in gelatin, after imaging, the iodonium salt is simply removed by an aqueous wash, which leaves the immobilised dye in the binder. The dye stability to light is then e~uivalent to that of the dye alone. An element in which the dye and iodonium salt is formulated in polyvinylpyridine may be treated with aliphatic ketones to remove the iodonium salt and leave the dye in the binder.
The elements of the invention have excellent ageing properties. Tests over a period of several months have shown that there is a minimal variation of maximum density, DmaX, and photosensitivity when elements are stored in the dark in a refrigerator (3 to 5C) and under ambient conditions [18 to 20~C, relative humidity 50 to 70~).
A variety of conventional additives such as surfactants, antioxidantsl stabilisers, plasticisers, ultraviolet absorbers, coating aids, may be used to prepare the elements of the invention to achleve benefit of their known properties.
The elements o~ the invention may be used for transparencies or overhead visuals, making enlarged copies of colour slides and related graphics applications, such as pre-press colour proof materials.
The thermally bleachable elements of the invention can be used to give transparency copies from a black on white original, e.gO printed or typed matter and more particularly a photocopy. For example, ';~ ;.~' ~.

~5~3~
=1~=

the elelllent~, when ~la~ed fillll lac~ ~wn on cl photocopy and passed through a 3M Thermofax machine set at the lightest control, are bleached in the areas corresponding to the black areas of the photocopy.
Thus, a negative ~clear on colour) of the black on white original is obtained which after fixing is ready fQr overhead projection. With suitable photographic negatives, this method could be used to ass~mble colour overlaps rapidly and conveniently. A water wash fixing step may be used to stabilise the element.
The invention will now be illustrated by the following ~xamples.
The oxidation potentials referred to in the Examples were measured with an Ag/AgCl~saturated KCl reference electrode.

Examples 1 to 9 Effect of iodonium ion tvPe on the reaction with a magenta dye ~0 C~, ; NH(C2H5)3 Magenta Dye (1) ~maX = 550 nm Eox = ~0-60V

3~

=19--In all the ~xamples, ~.U20 g of the magenta oxonol dye was coated as a solution in 10 ml of 10~ by weigh~ ~utva~ ~B76) in butan~2-one (Butvar is a registered trade mark of Monsanto Company for polyvinylbutyral polymers~. The dye solution was prepared in yellow light and the iodonium compounds tested were added in their respective proportions in red light. The photosensitive solution was then coated in red light at 100~ m wet thickness on a polyester base (75 ~m~. The sheets were air dried at 20C for 1 hour. A 2.5 crn square piece of each sample was then exposed over an area of 2.5 mm2 with focussed light filtered, using a Kodak narrow band filter (551.4 nm:power output - 2.36 x 10 W/cm ) and the change in the transmission optical density with time was monitored using a Joyce Loebl Ltd, microdensitometer. A plot o~ transmission optical density versus time was made and the exposure time (t) for the optical density to fall from DmaX to (D
was determined. The energy required ~E) was max calculated as the exposure time (t) x power output (=
2.36 x 10 3 W/cm2): this gives an indication of the sensitivity of the elements.
The iodonium compounds used and the results obtained are reported in Table 1. In Examples 6 and l~ 1 ml of dimethylfo~mamide was added to the coating : solution to solubilise the iodonium salt.

~55~
=20=

~e` u~ c=o . ~ O ~.

. _ O N Ln ~
~ O O O

I

U O

~0 ~. . . __ , . _ lY __._ '_. .. _ .... _____ ., = 2 1=

,~,~ .. ._, __ .

. U~ O OD
X ~ ,~
U _ . _.

~a~ ~ r _ _ e __ _ ~ ~ ~
_ _~ ___ _ _ .__ _ _.__ .______ ___.

~ ~ O O
'~

_~_ _ __ _ _._ .__ .__ __ _ ___ __ ~' ~ ____~ ~-:~ ____ 4 ~ ' _ . ~
' ' ~ ` ' ` ' =22=
. ~ ~

~ ~ ~ ~r O
mo X
_ ....... ..... ... ...
~a~ co I_ ~ N
, _ . __ _ ~ . ~ O
~ . ~ r~ ~
_ _ . . ... _.

~0~ ~ O
O O O
.,1~ ~1 ~1 _.
~ ' W ~ W ~ C~ 3 N
~ 1~ H C ~ ~3 H U

H
.

_ _ ~ ., . X
_ , .~f , . . . .
~, .

"

Comparison of the results, which are all acceptable for imaging systems, reveals:
(a) the anion of the iodonium salt helps to solubilise the onium ion (greater solubility leads to great~r bl~aching speeds), (b) substituents to the carbon-to-iodine bond on the iodonium ion inhibit the bleaching reaction, and (c) electron donating groups, e.g. S-alkyl, OMe, Me~
on the aryl groups of the iodonium ion decrease the photosensitivity.
Under the same conditions and using triphenyl sulphonium hexafluorophosphate in place of the iodonium salt, bleaching was only observed at high temperature (>100C). Addition of 2,4,6-triphenyl pyrylium trifluoromethane sulphonate or 1-(2,4-dinitro-phenyl)pyridinium chloride in place of the iodonium salt did not lead to bleaching of the oxonol.
Excellent ageing properties have been obtained with the elements. In Examples 1 and Examples 3~ to 36, hereinafterl the variation in the standard deviation of the maximum density, DmaX and the photosensitivity remained well within 5~ during storage assessments over a period of thirteen weeks.
Thus, samples were retained in the dark in a re~rigerator at 3 to 5 C, relative humidity (RH) 30%, in arl enclosure 18 to 20C, 50 to 70% RH, and under laboratory ambient conditions of 18 to 20C, 50 to 70~ RH: all exhibited minimal variation in the above properties indicating good dark shelf life.
~
The effect of iodonium ion concentration Me ~ O

SO3~ Me ~ ~

Magenta Dye (2) ~max = 545 nm (in Butvar (B76) layer~

:

3~
=~4=

4 ml of a 2% ethanolic solution of magenta dye ~2) was added, in r~om light, to a 6 ml solution of Butvar B76 ~lg~ in butan-2-one. In red light, varying proportions of the iodonium salts reported in Table 2 were added. The resulting lacquer was knife edge coated at 125 ~m wet thickness on a 75 m polyester base and the photosensitive sheets dried in air at 20C for 1 hour. From the optical density versus time plots using filtered light 551.4 nm (with output 2.35 x 10 3 W/cm2), exposure time (t) were calculated and the energy value (E) determined as in ~xamples 1 to 9. The results are reported in Table 2.

3~
=25 --~. _ ._ , . ~

a~ o O C
X
~ , , . _ . ., ~ ~n ~
u~ a~ N
. _.. __ _ . - . _~
X
~ ~ o Ln 0 1 ~ t~7 N ~) N ~ N N
._ ,......... __ .. _ - - . ._ ~0 ,~ _ _ ~: ~ 4~ 0 r~ ~ O ~ O
~ O E~ ~ LJ N ~D ~J N ~ ~
~1 ~ rl rl C
E- ~ ra ~-rlr-l N U) ~:) N L~`) a) ~ ~ ~O O O O O O
.. __ _ H ~

~ , , . ......... _ ... _ . I

O i~
æ ~ ,, _ .. _ ... . . .

.~.
;
:

.

53~

=~6=

The results indicate that increased addition of the iodonium salt leads to increa~ed photosensitivity. An oxonol iodonium salt where the iodonium is the gegenion of the oxonol will show the best photosensitivity.
Example 17 C~H5 o ( 2 5)3 ~ ~ 2 ~

Blue Dye (3) AmaX = 605 nm (measured in gelatin layer) Eox ~ ~ 0.47V

2 ml of 24 ethanolic blue dye (3) was added in room light to 8 ml aqueous solution at 55C of gelatin (19~ and poly[4-vinyl-1-methyl-pyridinium methylsulphate) (0.2 93. The latter polymer was 10~
molar methylated. 0.5 g 11:12 dye/onium w/w ratio) of phenyl(4-methoxyphenyl)iodonium trifluoroacetate was added in the dark and the mixture kni~e edge coated at 100 ~Im wet thickness onto polyester film (100 ~m) which was subbed with a conventional wetting coat.
~fter drying in the dark at 20C for 1 hour, a strip of the film was sub~ected to laser light of wavelength 632 nm. At the laser power density of 6.0 x 102 W~cm2, a 10 ~m diameter bleach spot required 1.5 seconds exposure. After exposure the film was fixed by washing (5 minutes) in water at 15C.
Examples 18 to 26 These Examples illustrate a range of dyes and the colour change upon exposure to light and reaction with dlphenyl-iodonium hexafluorophosphate when mixed in acetone. A mixture of the dye (0.005 9~ and :~, ~.
:,, ~:

~25~i3~3~
=2~=

iodonium salt ~Ool g) in 10 ml acetone was irradiated 1 foot from a 0.5 kW tungsten source. The results are reported in Table 3 whose AmaX figures are measured in acetone solution.

i3~

=2~=

.
~ ~ a) ~ h h ~ g g ~ ~ ~1 O O O
O ~ ~:

X_~ - _~ .

. _ __ _ I~ ~
X~ o o .~

D, ~Z

:~ ~ ~ ~ ~

_ ,_ , ~1 ~1 ~
~ ~O _ ~
~Z . , .

~' ~

'' ' ' ' ~5~

. ~
U) U~
U~ U) ~ o o o o ~, o~
. _ ~
X~ o o ,~ ~ q~
_ . _. ,. .,, _ . I
~D ~
X~ o o ~ l '~ ' Z

X _ _ 4j~5 _ _ ~ 3 0 ~

~: ~
.ra o o o __ . _ _ . . _ .. .. _ X~ Ir)~ X ~ X X

~ m _ R~ a Ul ~ ~ .... ~ ~
L

, ~ ' . .
~ . .;

~ ~.

- -~ .
s u o ~ o o o u ~ m . . ~
n o + U~ + +
X X X ~ o ~ m ... ~ .

o c: m . _ . __ . .

_ _ .

. .

3~D
=~2=

Examples 27 to 31 These Examples illustrate the use of various binders, 4 ml of 2% magenta dye (2) was added to a 6 ml solution of 10~ w/v binder in an appropriate solvent.
0.2 g of diphenyliodonium hexafluorophosphate was added in red light and the mixture kni~e edge coated at 125 ~Jn wet thickness. After drying in air at room temperature for 1 to 2 hours, optical density versus time plots on a Joyce Loebl microdensitometer using filtered light at 551.4 nm were determined. Exposure times (t) were calculated and thence the energy value (E) as in Examples 1 to 9. The results are reported in Table 4, :, , ''':
' `

5,~

-33=
_~ _ , . ~r u~ ~ ~1 ~n o . X
. _ _ I

X û a~ ~ r o ~r C~3 U~ r X ~ :~
~ o a~ ~ r a . ~ ~ r~
u7 ~ ,q ~ a~ ~ ~
~ ~ .~
o ~r P~ ' P P P
__ . I
R
E~ ~: ~ P~ K O - O
P ~ C'l ~
O . O
_.
~ h _ ~ ~U ' ' ~ ~

~1 Oh h h , tq ~ ~ o v~
a ,, o ~ s~
h ~ 1 h ~1 1~
P ~ o ~o ~ ~ ~n m u~
__ r GO a~
Z; ~ ~ ~ r~
~ _ _ .

"~
.

-.. . .

~55i3~

Example 32 Stabilisat_on by disru~tion of carbon-to-iodine bond This Example illustates the use of ammonia to stabilise the elements of the invention. The ammonia reacts with the light-sensitive iodonium salt and thus decreases the photosensitivity o~ the film.
4 ml of 2~ magenta dye (2) was added to a 6 ml solu~ion of 10~ w/v poly(methylacrylate/methyl methacrylate~ in butan-2 one. 0~5 g of diphenyl-iodonium hexafluorophosphate was added in red light and the resulting lacquer knife edge coated at 125 ~m wet thickness. After drying in air, the film was exposed through a black and white transparency for 10 sec on an overhead projector (0.5 kW ~uartz iodine lamp) to give a magenta copy. The resulting film was exposed to ammonia vapour in the dark for 12 hours.
Subseguent photosensitivity of the film was substantially reduced: determination of the energy values ~E) in accordance with ExamPles 1 t~ 9 revealed a 17-fold increase ~4.7 x 105 mJ/m2 to 8Q x 105 mJ/m2).

Stabilisation by removal of the iodonlum salt Blue dye (3) 0.04 g in ethanol ~4 ml) was added to a photoyraphic grade gelatin (1 g) solution in water ~6 ml) at 55C containing aqueous Tergitol TMN10 (Union Carbide Company) non-ionic surfactant ~10~, 0.3 ml), poly(4-vinyl-1-methylpyridinium methylsulphate~ as in Example 17 (0.2 g) and 0.5 ml acetic acid, In green light phenyl(2-thienyl)-iodonium trifluoroacetate (1.0 g~ was a~ded. The blue solution was knife edge coated at 100 ~m wet thickness on 100 ~um subbed polyester. After chilling at lnC
for 10 minutes~ the coated sheet was dried in air at 20C for 2 hours. The film was exposed through a black and white transparency on an overhead .

3~
=35-projector (0.5 kW quartz iodine lamp) usiny an exposure of 60 seconds. A blue copy of the oEiginal resulted. The imaged film was fixed by washing in water at 18C for 3 ~o 5 minutes. Af~er drying in air upon subsequent exposure to laboratory light nc further bleaching was noticeable. The compara~ive grey scale and resolution of the copy were excellent.
Exam~les 34 to 36 .~ .. _ . _ n These three Examples demonstrate the utility of the imaging system described herein in colour proofing materials for the graphic arts industry.
The dyes in the quantities reported in Table 5 in 4 ml of ethanol were added to a solution of gelatin (1 g) and poly(4~vinyl-1-methylpyridiniummethyl-sulpha~e) as in Example 17 (0.2 g) in 6 ml o~ water at 55C. 0.5 9 of phenyl(4-methoxyphenyl~iodonium trifluoroacetate was added in red lig~t to the solutions of yellow and magenta dyes and the same addition was made to the cyan solution in green light.
After the addition of aqueous Tergitol suxfactant (10~, 0.3 ml), the solutions were coated at 75 ~m thickness on subbed polyester, the coated sheets chilled tv 10C for 10 minutes and then dried in air for 1 hours. Density versus time plots were measured as in Examples 1 to 9 using Kodak ~ilters (output in brackets), respectively 461.6 nm (5.41 x 10 4 W/cm2), - 551.4 nm ~2.36 x 10 3 W/cm2) and 670.7 nm (4.75 x 10 3 W/cm ) for Examples 35, 36 and 37 respectively. The results are reported in Table 5.

,~i ;

~LZ~
=36=

._ . . . _ _ ,t o o ~X , . . ........ _ .
o o C) o O
X~ _ _ o ~ 0 r- ~ In _ ~ Lr~ ~D
.
" U~ .~ o ,, o ~, ~ _ _ _ _ j ~ æ ~ .

~.~
'~ ' ' -~5 ~3 =37=

Imaginy the samples wi~h the appropriate colour separation positive transparen~y was achieved by contacting the transparency with coated sheet (coated side up) on a vacuum frame and exposing at 0.5 m to an unfocussed 1 kW tungsten halide source. After imaging, the film was washed with agitation in a water bath at 15C for 5 minutes. Drying in air and arranging the three samples, yellow~magenta/cyan, one on top of the other gave a colour proof with a very good grey scale ttonal reproduction3 and resolution.
Identical samples were taped in the following order - magenta, yellow, cyan to a 35 mm colour transparency slide The composite was then placed into the slide comparment of a slide projector with the coated sheets farthest from the quartz iodine projector source (240 W). After an exposure of 60 seconds, a positive full colour reproduction of the original slide re~ulted. The individual sheets were then washed in water at 15C for 5 minutes, dried in air and reassembled to give a ^table copy of the slide.
Example 37 ~ ~ull-colour single sheet film element imageable by a tungsten visible source was constructéd by coating one side of a 100 ~m ~subbed on both sides) polyester film with a 75~m wet thickness cyan layer and on the other side of the film with a mixed magenta and yellow layex of the same wet thickness. The coating compositions comprised phenyl(2-thienyl~-iodonium trifluoroacetate and as the film-forming binder a mixture of gela~in and poly(4 vinyl-l-methyl-pyridinium methylsulphate) as in Example 17 (1:0.2 by weight).
The dyes used and the weight of the components are reported in Table 6.

' . ,~

:, :

ii3~3~
=3~1=

.
4,~
O ~ _~ U~ O
o _ O ~1 :~ O
.....

~H h ~
O ~ ~ ~ ~
. ~ ~ O O
~-~1 ta~
~ ~J ~

___ . . ~

O O ~ .
~_~ ~ ~I t'~l .
, ` ~ :

, ~ ;:
-,.,`
'' ~. ":
.:. ., ,, ,,: , : , , '',:: :, ' .
. .
, ' '' ~ '' :

~'~5~3 =39=

After drying in the dark for 4 hours at room temperature, the multicolour film element was placed in contact with a full colour transparency with the magenta/yellow coating next to the transparency and the composite exposed through the transparency in a slide projector having a 240 watt source bulb for ~5 to 50 seconds. A full co~our reproduction of t~e original was obtained. The copy was rendered stable to light by a wash in water for 3 to 5 minutes.
The yellow dye reported in Table 6 is a novel compound.
Example 38 A solution of the yellow dye in Example 18 (0.02 g) in ethanol (4 ml) was added to a solutlon of 1 g gelatin and 0.3 g poly(4-vinyl-1-methylpyridinium methylsulphate) as in Example 17 in 10 ml water and 0.5 ml acetic acid at 40C. 0.3 ml Tergitol-4 (10%
aqueous solutio~) was added to this yellow lac~uer.
0.9 g o~ 4-methoxyphenyl-phenyl iodonium trifluoro-acetate in 1 ml dimethyl-formamide was added in red light. The solution was then knife-edged coated at lOOIlm wet thickness onto a 1~5 ~m subbed polyester base and dried in air for 0.5 hours at ca 15 to 20~C
to give a yellow film, Amax 474 nm, VmaX = 2-1-An inch square piece was exposed with an Ar-ion laser operating at 488 nm onto a spot area of 8 ~m . Dwell times varied between 5 ms to 18 ~s; the minimum dwell time required to bleach a spot of diameter 2.5 ~m was 18 ~s. Thus, the energy/unit area re~ulrements fo~ this film were ~ x 106 mJ/m2 to bleach from DmaX of 2 to 0.10.

~53~
=40=

Example 39 Preparation of:
C~
>~ P~ 6~
2 2 ~ N 3 0~1 0 To 5-acetanilino-allylidene-1,3 dimethyl-barbituric acid (6.4 g, 20 mmol) and excess ethyl cyanoacetate ~5.0 9) in 50 ml ethanol was a~ded triethylamine (5 ml3. The mixture was heated for 0.5 hour, by which time a red solution resulted. The UV-visible spectrum of this sclution in ethanol showed two hands: major Alnax 465 nm and minor ~max 490 nm.
on cooling, orange crystals of the minor product (1.0 g) were isolated: the minor product was the symmetrical bis-barbiturate trimethin oxonol. The mother liquors were diluted with diethyl ether (Z00 ml) and cooled to give yellow ~fluf~yR crystals of 5-(ethyl-cyanoacetyl-allylidene~-1,3-dimethyl-barbiturate triethylammonium salt, ~max ~tO~) ~60 nm, ~6.5 x 104. The yield was 3.2 g, 40~.EoX is : ~0.47V (~e~. Ag/A9Cl in sat. ~Cl).
Empirical formula: C20~30N45 ~ , ,. . _ a~
C~ ~% N~
~ ~ . e Calculated 59~00 7.44 13.78 : . Found 59.03 7.40 13.9a, .

:
:
.;'` :. ' ~' =~1=

Exam Preparation of CN
> = \ _ O NHEt3 5Co2C2~5 ~ OC2~5 K~
C~
Diethyl 2,6~dicyano-2,4,6-heptatriene-1,7-dicarboxylate triet~ylammonium or potassium salt.
A mixture of 3-anilinoacrolein anil (2 22 g, 10 mmol), ethyl cyanoacetate (4.8 g, 42 mmol) and triethylamine (3~3 mlJ in 30 ml ethanol was heated for

6 hours. The reaction was followed by UV-visible spectrometer monitoring for completion of the reaction which is observed by the formulation of a single band at 450 nm tEtO~). Evaporation o~ the ~olvent gave a red oil which was washed several times with ether to give a red viscous oil (blue reflecting), yield ca S g, ~max (EtOH) 445 nm, ~ 8 x 104.
A sample of the red oil 11 g) was dissolved in ethanol with potassium acetate (1 g). The mixture was evaporated and the potassium salt taken up in acetone and reprecipitated with ether to give ca 0.5 g of the potassium salt, ~max (~tOH) 445 nm~ ~ - loOl x 105 Eox is +D.62V (vs ~g/AgCl sat, KCl reference).
EmPiriCal ~ormula: C13~13N2O4K
~ ,_ - - _, C% H% N~
~ _ _ ..... , __ Calculated 52.0 4.36 9.32 Found ~ 4.61 9.95 _ Low carbon due to residual potassiulr acetate.

. ., ..
: ;:
. .

::.
......

3~

=42=

~E~
Bleachin~ of an I.R. Absorbing ~e _ I ~

N.I.R. Cyanine Dye Amax (acetone) 762 nm 1 mg of the above dye was dissolved in 5 ml o~
acetone and additioned with diphenyliodonium - 10 hexafluorophosphate (50 mg). The mixture was irradiated for 5 seconds at a distance of 1 foot from a 0.5 kW tungsten lamp. The following Table 7 shows the absorbances o~ the dye (a) alone, (b) with the iodonium salt in the dark, and (c) after irradiation with tun~sten light.
Table 7 Composition Absorbance A~sorbance at 700 nm at 750 nm -- -- . _."
Dye in acetone 0.84 2.36 20 Dye + iodonium salt in acetone in dark 0.95 2.50 : Dye + iodonium salt in acetone after irradiation 0.22 0.32 ' ,-,, : : -:, ~ ,, . .

..... : " . ~ .

-.93=

Thus, suitable I.~. dyes in combination with iodonium salts may be used to forJn I.R. sensitive elements useful, for example, as I.R. masks, I.R.
bleachable antihalation layers, and optical data storage.

a) Preparation of:

I_ O NHEt ~ 3 CN

Dye W-1 A mixture o~ dimethyl~ormaldehyde dimethoxy-acetal (2.0 9), ethyl cyanoacetate (5.0 g) and triethylamine (10 ml) in ethanol ~30 ml~ was heated at reflux ~or 12 hours. The solution was cooled, and diluted with diethyl ether (100 ml) and petroleum ether (40/60C 50 ml). The resulting ~opaque~
solution was cooled ~or 24 hours, yielding dense, white needles of the dye as the NHEt salt: 1.8 g;
~m~x (ethanol) 355 nm (~ = 4./5 x 103~; Eo~ ~0~45 Empirical formula: C17H27N3O4 : ~ c~ a~ N~
.. . .. ,. . .. . .. ..
Calculated 60.5 8.07 12.45 Found 59.9 7.80 12.37 . , ~,, .
.
'` ' -: ~ . '~

:

. .
, .
.

~L~553 =44=

b) ~ye bleach system A mixture of UV-l (1 mg) and diphenyliodonium hexafluorophosphate ~0.01 g~ in acetone (5 ml) was irradiated l foot from a 4 kW metal halide source for ~0 seconds, The UV spectrum was monitored before and after irradiation to show the ~bleaching~ of the UV
dye. The results are reported in the following Table 8.
Table 8 Composition Absorbance at 356 nm _ . , ",.. ", .
UV-l + iod~nium salt in acetone 3.
UV-l ~ iodonium salt in acetone - lO units exposure 3.12 - 40 units exposure U~2 Thus, elements comprising suitzble UV
absorbing dyes and iodonium salts may be used to form UV masks, UV-bleac~able antihalation layers, etc.
-Example 43 A mixture of Dye UV-l ~0.3 9), diphenyliodonium hexafluorophosphate tO.3 g) and Butvar B76 (lg) dissolved in butan-2-one ~15 ml) was coated in red light onto a 25 ~ polyeste~ film. The absorbance o~
this layer at 360 nm was approxilllately 3.8 which decreased to 3O3 after heating to 150C for 30 seconds.
Such an element or mixture may be used for heat-bleachable antihalation layers, UV masks, etc., or for a method o~ ~ixing a visible image by heat destruction of the excess iodonium ion.

; , ~, . .
.

5~ ~g =45=

xample 44 In some applications, e.g. ~opies of 35 mm colour slides, it is necessary to attain DmaX values of 2.0 to 2.5. Oxonol dyes have a peak half-width o~
45 nmo thus to achieve neutral densities of 2.0, high dye densities are required.
The required density is achieved by the addition of two extra dyes termed ~blocking dyes~ at 500 and 600 nm. This Example illustrates a typical five-dye, single layer element, in which the five dyes are matched in sensitivity to the requirements of the exposure ~ouroe 7 To a ~olution at 50C of gelatin ~5.4 g) and poly~4-vinyl-1-methylpyridinium methylsulphate) (0.4 g) in acetic acid ~0.5 ml) and aqueous Tergitol No. 10 (2.0 ml, 10~) were added in ethanol ~10 ml3 and water (2 ml) the following dyes:
(A) ~ye of Example 19 0.03 9 (B) Dye of Example 21 0.02 g (C~ Dye of Example 23B 0.025 g, (D) Dye of Example 25 0~01 g, an~
(~) Dye of Example 26B 0.04 g.
To this resulting dark blue solution, in the dark, was added 4~methoxyphenyl-phenyliodonium trifluoroaçetate (2.5 9) in N,N-dimethyl-formamide (3.0 ml~ and chrome alum (0.05 9 in 1 ml H2O). The mixtur~ was placed in the loop-coater vessel and loop-~oated on subbed polyester to give 2 m x 0.15 m of coated film. The ~ilm was dried in an air cupboard at 21C for 2 hours.

~ ,.,:. : , '` " .

~5~
=46=
Table 9 records the ~max of each of the five dyes in the composite coating, measured by a transmission spectrometer. The transmission optical density of each dye at or close to its Amax is S recorded in Table 9 as DmaX. The energy, ~, required to reduce the optical density of each dye at its Amax by 1 optical density unit on irradiation with light of a wavelength corresponding to the AmaX is also recorded.
The five dye composite was found to have an optical density of at least 2, balanced to a good neutral, average~ across the spectrum from 430 to 700 nm.
~he film was placed in contact with a 35 mm colour slide in the focussed beam of a tin halide or Xenon source for 30 seconds. The resulting copy was fixed by a water wash ~5 minutes/20~C) and drying in air. Good separation of yellow, magenta, red and blue were obtained: cyan and green colours were weak.
Ta~le 9 Dye A B C D E

~ max 454 514 563 604 672 Dmax 3.4 2.1 3.4 ~.3 4.3 Energy ~E) (xlO~ mJ/m2 15 21 36 9 3 , -.'' ~ ' .. :

Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A radiation-sensitive element capable of recording an image upon image-wise exposure to radiation of selected wavelength, the element comprising, as the image-forming components in a binder, an effective amount of a bleachable dye in reactive association with an iodonium ion.

2. An element as claimed in Claim 1, in which the bleachable dye is a polymethine dye or aza analogue having an oxidation poten-tial between 0 and +1 volt.

3. An element as claimed in Claim 2, in which the dye has an oxidation potential between +0.2 and +0.6 volt.

4. An element as claimed in Claim 1, characterized in that the bleachable dye has the general formula:

in which n is an integer of 1 to 5, and R1 to R4 are selected to provide an electron donor moiety at one end of the conjugated chain and an electron acceptor moiety at the other, the free valences on the chain being satisfied by hydrogen or any substituent of the type used in cyanine dyes.

5. An element as claimed in Claim 4, characterized in that R1 to R4 independently represent halogen, alkyl, aryl groups or heterocyclic rings any of which may be substituted, said groups containing up to 14 atoms selected from C, N, O and S; or R1 and R2 and/or R3 and R4 may represent the necessary atoms to complete op-tionally substituted aryl groups or heterocyclic rings, containing up to 14 atoms selected from C, N, O and S.

6. An element as claimed in Claim 5, in which the bleachable dye is a cyanine, merocyanine or oxonol dye.

7. An element as claimed in Claim 1, 4 or 6 characterized in that the iodonium salt has the general formula:

in which Ar1 and Ar2 independently represent carbocyclic or hetero-cyclic aromatic-type groups having from 4 to 20 carbon atoms, or together with the iodine atom complete a heterocyclic aromatic ring, and A? represents any anion which does not react with the iodonium salt and may be present in Ar1 or Ar2.

8. An element as claimed in Claim 1, in which the weight ratio of bleachable dye to iodonium salt is from 1:1 to 1:50.

9. An element as claimed in Claim 8, in which the weight ratio of bleachable dye to iodonium salt is from 1:2 to 1:10.

10. An element as claimed in Claim 1, in which the image-forming components are present in one or more layers coated on the surface of a support.

11. An element as claimed in Claim 10, in which the bleachable dye and iodonium salt are present in one or more layers which com-prise a binder.

12. An element as claimed in Claim 11, in which the binder comprises from 50 to 98 percent by weight of the total weight of binder, dye and iodonium salt.

13. An element as claimed in Claim 1, in the form of a self-supporting film comprising the image-forming components and a binder.

14. An element as claimed in Claim 1, in which the dyes com-prise cyan, magenta and yellow dyes.

15. An element as claimed in Claim 14, which additionally com-prises two further dyes having a peak absorbance at about 500 and 600 nm respectively.

16. The combination of three light-sensitive imaging layers, each layer comprising an element according to Claim 1, a first element containing a bleachable cyan dye, a second element contain-ing a bleachable magenta dye and a third element containing a bleachable yellow dye.

17. A method of recording a positive image comprising image-wise exposure to visible light of an element as claimed in Claim 1.

18. A method as claimed in Claim 17, which additionally com-prises the step of stabilizing the exposed element by separation of the iodonium salt relative to the dye or destruction of the iodonium ion by disruption of at least one of the carbon-to-iodine bonds thereof.

19. A method as claimed in Claim 18, in which the iodonium salt is removed by washing with water or solvent thereof.

20. A method as claimed in Claim 18, in which the carbon-to-iodine bonds of the iodonium salt are disrupted by addition of ammonia or an amine.

21. A method as claimed in Claim 18, in which the carbon-to-iodine bonds are disrupted by subjecting the element to heat or ultraviolet irradiation in the presence of a nucleophilic anion.

22. A compound of either of the formulae:

M?
and in which M? represents a cation.