Photochemical generation of dyes

Abstract: A photosensitive element for the production of near infrared absorbing images having a photosensitive medium comprising a trialkyl orthoester, a photochemical source of a strong acid and a compound of the formula (I): in which: n is 0 or 1, R¹ and R² independently represent hydrogen or an alkyl group of up to 5 carbon atoms and Ar₁ and Ar₂ represents aryl groups bearing an electron-donating substituent in the 4-position.

Description: This invention relates to photosensitive elements capable of generating a near infrared (NIR) absorbing dye. Near infrared absorbing dyes find widespread use in imaging and information storage, particularly when information is to be written or read by means of a laser diode or similar source. As well as the conventional photographic applications such as sensitisation, acutance and antihalation, NIR dyes find increasing use in the conversion of radiant energy to thermal energy in systems such as optical data storage (by ablation of dyed polymer layers) and laser-addressed thermal transfer imaging. Tetra-arylpolymethine dyes have found use in many of these systems, as disclosed, for example, in U.S. Patent Nos. 4,547,444, 5,135,842 and 4,950,639. DE-A-3704263 discloses tetra-arylpolymethine dyes which are prepared by reaction of an appropriate diaryl precursor with a trialkyl orthoester in the presence of a strong acid, such as methane-sulphonic acid. In certain systems, notably in optical data storage, it is desirable to have the dye present in a coated layer in accordance with a predetermined pattern. For example, in optical discs it is necessary to provide servo tracking information to enable the read/write head to find any specified location on the disc with precision. This may be achieved by forming the storage media as a series of narrowly spaced concentric rings. Recently, the need has arisen to provide similar laser-readable information as an additional layer on otherwise conventional magnetic data storage media as disclosed, for example, in our copending application EP-A-0598503, which discloses a method of recording information, such as optical servo tracking information, on a magnetic recording element, e.g., magnetic tape, disk, diskette, drum, card, data cartridge, which method comprises: providing the element with a photosensitive layer in which the generation or destruction of a radiation-absorbing dye e.g. an NIR dye, is either promoted or inhibited by exposure to actinic radiation, said layer being separate from or integral with any magnetic recording layer, and imagewise exposing the photosensitive/recording layer to actinic radiation to record said information. A potentially attractive route for forming such a predetermined pattern of NIR dye is to create the dye by photochemical reaction of its precursors in response to image-wise light exposure. Unfortunately, very few suitable systems are known which allow such image-wise formation of NIR dye. European Patent No. 68876 discloses photoimagable oxoinodolizine dyes which are NIR absorbing. The dyes are formed by a thermal reaction between cyclopropenone derivatives and certain pyridine derivatives. Selective photolysis of the cyclopropenone prior to thermal processing inhibits dye formation in light struck areas. The present invention provides a photosensitive element comprising an alternative photoimagable dye class, with the formation of dye enabled, rather than disabled, in the light struck areas. According to the present invention there is provided a photosensitive element for the production of near infrared absorbing images having a photosensitive medium comprising a trialkyl orthoester, a photochemical source of a strong acid having a pKa<3 and a compound of the formula (I): in which: n is 0 or 1, R 1 and R 2 independently represent hydrogen or an alkyl group of up to 5 carbon atoms and Ar 1 and Ar 2 represents aryl groups bearing an electron-releasing substituent in the 4-position, generally containing up to 15 skeletal atoms. The invention also provides a method of generating an image comprising a near infrared absorbing dye which comprises image-wise exposing a photosensitive element of the invention to actinic radiation and optionally heating the exposed elements to develop the image. The invention finds particular utility in forming optical servo tracking systems on optical discs and magnetic discs and is also useful in other systems requiring the provision of laser readable information e.g. bar coding, security coding etc. Trialkyl orthoesters suitable for use in the invention have the general formula: (RO) 3 C-R 5 in which: each R independently represents a lower alkyl group of up to 5 carbon atoms, preferably each R is ethyl, and R 5 represents a hydrogen atom, a lower alkyl group of up to 5 carbon atoms or an aryl group. Exemplary trialkyl orthoesters include triethyl orthoformate, triethylorthoacetate, trimethyl orthobenzoate etc. The photochemical source of a strong acid is a compound which releases a strong acid (pKa<3, preferably <1) on exposure to actinic radiation. Such photoacid generators are well known and include photosensitive onium salts where the anion is PF 6 , AsF 6 , SbF 6 , BF 4 , C10 4 , CF 3 SO 3 , etc., such as sulphonium salts, iodonium salts and iron onium salts. Preferred onium salts include diaryliodonium salts. A sensitiser may be optionally included to extend the spectral sensitivity as is well known in the art provided it does not scavenge the photoacid. Polycyclic hydrocarbons (e.g. anthracene derivatives) are suitable sensitisers for use with onium salts. In the compounds of formula (I) n is preferably 0 and when n is 1 R 1 and R 2 are preferably hydrogen. Ar 1 and Ar 2 are preferably: para-R 3 R 4 N-C 6 H 4 - in which: R 3 and R 4 independently represent alkyl groups of up to 5 carbon atoms, aryl groups or together complete a 5, 6 or 7 membered ring e.g. pyrrolidine, morpholine, piperidine, or R 3 and R 4 complete a julolidine ring system. Ar 1 and Ar 2 may include other electron-releasing substituents in the 4- position, e.g., alkoxy groups and alkythio groups in which the alkyl group generally contain up to 5 carbon atoms. As is well understood in this technical area, a large degree of substitution is not only tolerated, but is often advisable. As a means of simplifying the discussion and recitation of these groups, the terms "group" and "moiety" are used to differentiate between chemical species that allow for substitution or which may be substituted and those which do not or may not be so substituted. For example, the phrase "alkyl group" is intended to include not only pure hydrocarbon alkyl chains, such as methyl, ethyl, octyl, cyclo-hexyl, iso-octyl, tertbutyl and the like, but also alkyl chains bearing conventional substituents known in the art, such as hydroxyl, alkoxy, phenyl, halogen atoms (F, C1, Br and I), cyano, nitro, amino etc. The phrase "alkyl moiety" on the other hand is limited to the inclusion of only pure hydrocarbon alkyl chains, such as methyl, ethyl, propyl, cyclohexyl, iso-octyl, t-butyl and the like. The components of the photosensitive medium may be present in a single layer with or without binder or distributed between 2 or more contiguous layers. A single layer is preferred. Any of the commonly used film forming binders may be suitable provided there is adequate solubility in the coating solvent. The coating thickness will vary with the end use, but for optical servo tracking, very thin (<1µm dry thickness) coatings with a minimum of binder are preferred. The orthoester and acid source are normally each present to the extent of 50 to 60 mol% of the compound of formula (I). The photosensitive medium is exposed image-wise to a suitable source e.g. a UV lamp via a photographic mask. Exposure may be sufficient to complete the reaction and generate the NIR dye. However, a heating step, such as several minutes at about 70°C or above may be required to develop the image. The compound of formula (I) reacts with orthoester and is believed to yield a tetra-arylpolymethine dye of the following formula: in which: R 5 is hydrogen, lower alkyl or aryl. To stabilise the image completely, orthoesters and photoacid generator may be removed by washing in dilute acid, although this may not be necessary in many applications. Suitable acids include acetic acid. The invention will now be illustrated by the following Examples in which the NIR dye generated was believed to have the formula: where R 5 = C 6 H 5 (Example 1) CH 3 (Example 2) H (Examples 3 to 5) Example 1 A mixture of 1,1-bis(4-Dimethylamino)phenylethylene (Compound A) (0.5g, 1.9mmol), trimethyl orthobenzoate (0.18g, 1mmol) and a triarylsulphonium salt photoinitiator commercially available from Minnesota Mining and Manufacturing Company under the trade designation FX512 (0.5g) was dissolved in methylethyl ketone (MEK) (30ml). The mixture was then hand coated onto polyester and dried. Two techniques were employed: a) a sample was exposed to a 4kW Parker Graphic UV light source for 100 seconds. There was an immediate formation of a near infrared absorbing dye in the exposed regions. b) a sample was subjected to 10 seconds exposure with a handheld laboratory UV lamp followed by heating at 70°C for 5 minutes. The latter process of thermal cure completes formation of a near infrared absorbing dye in the exposed regions. Example 2 A mixture of Compound A (0.5g, 1.9mmol), triethyl orthoacetate, (0.16g, 1mmol) and FX512 (0.5g) was dissolved in MEK (30ml). The mixture was handcoated onto polyester. Two exposure techniques were employed: a) a sample was exposed to a Parker Graphics 4kW UV light source for 100 seconds. Formation of a NIR absorbing dye was observed in the exposed region. b) a sample was subjected to a 10 seconds exposure with a laboratory handheld UV light and then a thermal cure at 70°C for 5 minutes resulting in formation of a near infrared absorbing dye in the exposed region. Example 3 A mixture of Compound A (0.25g, 1mmol) diphenyl iodonium hexafluorophosphate (0.22g, 0.5mmol) and triethyl orthoformate (0.08g, 0.5mmol) was dissolved in MEK (15ml). The mixture was hand coated onto polyester and exposed using the handheld laboratory UV light for 15 seconds. The spectrum of the exposed coating had no absorption at 820nm. The sample was then heated at 70°C for 5 minutes and a near infrared absorbing dye was produced in the exposed region. Example 4 A mixture of Compound A (0.5g, 1.9mmol), Photoinitiator CG 24-61. commercially available from Ciba Geigy, (0.35g) and triethylorthoformate (0.15g, 1mmol) was dissolved together in ethanol (30ml) and hand coated onto polyester. The sample was exposed for 500 seconds to the 4kW Parker Graphic UV lamp to give a coating which absorbed near infrared light. 4 micron line resolution could be achieved by exposing a sample through an Ugra Plate Control Wedge. Example 5 A solution comprising: was spin coated onto two 8.75cm (3.5 inch) magnetic diskettes at speeds of 1000rpm and 500rpm. The coated discs were each exposed to UV radiation through a suitable mask and heat treated at 95°C for 5 minutes to produce NIR dye in the exposed regions. The sample was then washed in aqueous acetic acid (10% solution), followed by water, to remove any unreacted photoinitiator. A TECHWAY 8.75cm (3.5 inches) external drive modified to provide an oscilloscope trigger signal was used to determine storage capacity. The disc coated at 1000rpm could be formatted to 100% capacity (720kbytes) and completed 22 x 10 3 repeat read cycles without failure. The disc coated at 500rpm could be formatted to >95% capacity and completed >34 x 10 3 repeat read cycles without failure.

Claims: A photosensitive element for the production of near infrared absorbing images having a photosensitive medium comprising a trialkyl orthoester, a photochemical source of a strong acid having a pKa<3 and a compound of the formula (I): in which: n is 0 or 1, R 1 and R 2 independently represent hydrogen or an alkyl group of up to 5 carbon atoms and Ar 1 and Ar 2 represents aryl groups bearing an electron-donating substituent in the 4-position. A photosensitive element as claimed in Claim 1 in which R 1 and R 2 are hydrogen. A photosensitive element as claimed in Claim 1 in which n is 0. A photosensitive element as claimed in any preceding Claim in which Ar 1 and/or Ar 2 have the formula: para-R 3 R 4 N-C 6 H 4 in which: R 3 and R 4 independently represent alkyl groups of up to 5 carbon atoms, aryl groups or together complete a 5, 6 or 7 membered ring, or R 3 and R 4 complete a julolidine ring system. A photosensitive element as claimed in Claim 1 in which the compound of formula (I) is 1,1,-bis(4-dimethylamino)phenylethylene. A photosensitive element as claimed in any preceding Claim in which the trialkyl orthoester has the formula: (RO) 3 C-R 5 in which: each R independently represents a lower alkyl group of up to 5 carbon atoms, and R 5 represents a hydrogen atom, a lower alkyl group of up to 5 carbon atoms or an aryl group. A photosensitive element as claimed in Claim 6 in which the trialkyl orthoester is selected from triethyl orthoformate, triethyl orthoacetate and trimethyl orthobenzoate. A photosensitive element as claimed in any preceding Claim in which the photochemical source of a strong acid is an onium salt. A photosensitive element as claimed in Claim 8 in which the anion is selected from PF 6 , AsF 6 , SbF 6 , BF 4 , ClO 4 , CF 3 SO 3 . A photosensitive element as claimed in Claim 8 or Claim 9 in which the photochemical source of a strong acid is a diaryliodonium or triarylsulphonium salt. A photosensitive element as claimed in any preceding Claim in which the orthoester and photochemical source of a strong acid are each present in amounts of from 50 to 60 mol % of the compound of formula (I). A photosensitive element as claimed in any preceding Claim in which the photosensitive medium comprises two or more contiguous layers. A method of generating an image comprising a near infrared absorbing dye which comprises imagewise exposing a photosensitive element as defined in any preceding Claim to actinic radiation and optionally heating the exposed element.

Inventors: ELLIS RICHARD J, ELLIS, RICHARD J
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1993
Main classification: G03C5
IPC codes: G11B5, G11B13, G03C1, G03C5, G11B7