US 3844792 A
Photoimaging method and sheet which is intensity dependent, whereby the sheet is not affected by ambient light conditions, but is activated by intense ultra-violet containing light.
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Description (OCR text may contain errors)
United States Patent Zweig et al.
[ 3,844,792 Oct. 29, 1974 A PHOTOSENSITIVE COMPOSITION CONTAINING A PHOTOCHROMIC BENZOYLCHROMONE OR DIBENZOFURAN AND A STRONG ORGANIC AMINEBASE Inventors: Arnold Zweig, Westport; Kenneth Robert Huffman, Stamford, both of Conn.
Assignee: American Cyanamid Company,
Filed: Dec. 7, 1972 Appl. No.: 313,027
US. Cl. 96/90 PC Int. Cl G03c 1/52 Field of Search 96/90 PC, 48 QP; 260/335,
Primary ExaminerRonald H. Smith Assistant ExaminerWOn H. Louie, Jr. Attorney, Agent, or FirmCharles J. Fickey [5 7 ABSTRACT Photoimaging method and sheet which is intensity dependent, whereby the sheet is not affected by ambient light conditions, but is activated by intense ultra-violet containing light.
5 Claims, No Drawings A PHOTOSENSITIVE COMPOSITION CONTAINING A PHOTOCHROMIC BENZOYLCHROMONE OR DIBENZOFURAN AND A STRONG ORGANIC AMINE BASE This invention relates to a photoimaging system. The invention more specifically relates to a photoimaging method and photosensitive sheet which is intensity dependent, i.e., it is not affected by ambient light conditions, but is activated by intense light.
Most conventional photoimaging systems, even those presently in widespread use, such as silver halide, diazo, Kalvar, etc., are encumbered by several inherent drawbacks which limit their capabilities, speed of processing, shelf life, or ease of handling. Thus, most such systems (a) require carefully controlled storage and handling conditions prior to use so as to avoid fogging, (b) require a separate developing and/or fixing step after exposure so as to preserve the image and-(c) have no add-on capability. If the image forming reaction were strongly intensity dependent, however, so that for all practical purposes the system would be insensitive to ambient room light yet could be efficiently activated by irradiation with intense light, the above difficulties would be obviated. Such an intensity dependent imaging system would require no developing or fixing, could be handled in ambient light before, during, or after exposure, and more information could be added at any time. The practical advantages would thus be considerable compared to conventional materials.
lntensity dependent photosensitive materials can have combinations of characteristics of considerable practical advantage over conventional imaging materials. One area of application is reflex copying. Another area is contact copying without fixing or developing to yield stable images. In principle a relatively simple intensity dependent process should allow efficient contact imaging with a high intensity long wavelength ultra-violet source, and should not fog under ambient intensities of the same wavelength.
Most known photoprocesses are not intensity dependent in that a photon has a given probability 1 of inducing a particular transformation independent of the photon flux. However, if two or more parallel or consecutive photoinduced transformations are required for the completion of the photoprocess, and if the first step (or steps) is thermally reversible, the materials reverting to their original state, intensity dependence of product formation is found.
In the parallel case, molecules A* and 8* (they may be the same compound) must both absorb light and then interact thermally to give the colored product I if?) The rate of formation of X is dependent on the square of the intensity. It is also very sensitive to the combination of lifetimes of A and B and their rates of I 2 The overall rate of consecutive photoinduced transformatlons is also dependent on the square of the imaging intensity but does not have a diffusion requirement and hence, is relatively insensitive to the medium. This system is indicated in eq. 4. A given concentration of A on irradiation 7 10] 7 low A i B C is converted to B at a rate dependent on the intensity of irradiation (10) and the efficiency of conversion (6, ln strong illumination, the steady state concentration of B is high, its probability of absorbing a photon is good and the efficiency of its irreversible conversion to C depends on I0 and its photoconversion efficiency (0 In weak illumination, the steady state concentration of B is low, thus it does not absorb much of the light and its photoconversion to C is inefficient compared to its thermal reversion to A, the rate of which is k Using a material that can strongly absorb the 3,660A. radiation available from a high pressure mercury lamp source, and that where A, B, and C have the same extinction coefficients at this wavelength, and for a low intensity of radiation, a small steady state concentration of B is set up, only a small fraction of the radiation will be absorbed by B and thermal reconversion to A will predominate. Under these conditions, it is found that C (I0 0 62/k 1) I O=Ao [1 l wi h s e 5 me In room light at desk level under GE Cool White bulbs the 3,660A. flux level is 10" einsteins/cm sec. With a high pressure mercury lamp the 3,660A. flux level that can be focussed on a 28 X 20 cm page is 10 einsteins/cm sec.
As an example, take the initial concentration of A as A0 5 X l0"M/cm If this is 20 percent converted to C and if C has an e of l0,000 in the visible, an effectively opaque material with an CD. of 1.0 by transmission and essentially 2.0 by reflection is achieved. The 0, and 0 are chosen as 0.25 and 0.50 initially. These are of the order found in quite efficient photochemical reactions. The rate of thermal return of B to A (k,) was taken as 10"sec (a return rate of per hour, a moderate figure for a first order reaction).
The ratio of the amount of B reverting to A in equations 5 and 6 to that of B going to C is k B/l00 .-B/A0. This ratio is 10:] for the ambient intensity of 3,660A. radiation and l0:l for the intensity achievable with high pressure mercury irradiation.
lnserting the chosen parameters into the high intensity equation 6 shows that a l sec. exposure converts 16 percent of A to C, enough to produce percent of the desired O.D. of l. The overall quantum yield D6) is a function of time, with a theoretical maximum (0,0,) of 0.125. At the intensity employed under the above conditions. De 0.08 at 1 sec. With the same molecular parameters using the ambient intensity and eq. 5, the same CD. will require 2 X l yrs. to be reached.
With this result as a bench mark the effects of varying the parameters individually is examined. In the cases discussed below eq.s and 6 are found to be valid in that the ratio k BI00 :B/A0 is g :1 for ambient radiation and S l0' :l for high intensity radiation.
If the initial concentration of A (A0) is reduced from 5 X 10 M/cm to 2 X lO M/cm"- then C 0.49 A0 after 1 sec. at high intensity, thus giving the desired O.D. of l. The time to achieve the same CD. at ambient radiation levels remains essentially the same (-2 X 10yrs.).
If the high intensity light source is weakened by an order of magnitude, then the time to image will increase an order of magnitude. Thus a'very powerful radiation source is essential.
If the values of 0 and 6 are made 0.5 and 0.25 respectively (the reverse of the original case), then the efficiency of imaging at high intensity will be reduced -5 percent, while the rate of color-up at ambient intensity will remain the same.
The importance of high values of 6, and 0 can be seen from the following. If these processes are less efficient, say 0,=0.05 and 0 =0.l, then with the other parameters the same as in the bench mark calculations, C=0.05 A0 after 25 sec. of high intensity irradiation, a very poor result. The time to fog with ambient radiation will be even more drastically lengthened to 10 years, but this is not a valuable accomplishment in a practical sense.
The rate of the return reaction (k.,) may be increased one mor more orders of magnitude without significantly affecting the imaging with high intensity light but will make the low intensity time to color-up proportionally slower. Conversely, decreasing k will increase the fogging rate proportionally. However, two orders of magnitude may safely be spared in this direction.
In any real system it is unlikely that 6,, s and i at the exciting wavelength will have the same values. From the results with 0, and 0 it can be anticipated that as long as e,. and a are within 50 percent of each other the system will function satisfactorily. If 60 should be much greater than 6, and 6 then self-quenching of the imaging process would reduce the optical density of the final image. Energy transfer between the absorbing products and reactants would also affect the outcome of this type of system. However, in rigid media, this is not expected to be of consequence.
Intensity dependent transformation is achieved with the photochromic, colorless, 2-benzhydryl-3- benzoylchromone (Ia) which undergoes two consecutive photochemical transformations, the first of which is thermally reversible, to ultimately photogenerate the yellow benzoxanthrone III which has an absorption maximum at 403 nm. The p-cyano derivative (lb), a new compound, shows the same effect.
In neutral methanol solution k for this system was too small to observe intensity dependence. The intensity dependence was achieved when it was discovered that k., could be increased in the presence of organic bases. In deoxygenated pyridine, k is increased to about 10 sec and conversion of a l0 M solution of la to III was almost twice (1.82X) as efficient with a high pressure mercury lamp as it was with about 10 percent of this light intensity. Two minutes of irradiation resulted in an OD of l at 403 nm, corresponding to approximately 15 percent conversion to 111.
Similar intensity dependence was measured in methanolic solutions 10 to l0 M in DABCO, a strong amine base, and also in polyurethane film coating on Mylar which was 0.5M in both DABCO and chromone la. The film coating was also imaged with a Xenon Flash lamp.
Stronger bases were found to be even more effective in increasing k moreover, the rate could be altered by varying the concentration of added base. Thus l in methanol at 30 was determined as 10', 10', and
1 sec as, the concentration of added 1,4- diazabicyclo [2,2,2] octane (DABCO) varied in the order 0, IO' M, and l0 M. The corresponding ratios of amounts of l 1 formed upon direct vs screened irradiation were 1.0, 1.3, and 2.2, respectively. (Table I) These results illustrate the importance of k in determining the degree of intensity dependence and show that in this system kcan be adjusted so as to obtain the optimum effect.
The behavior of I in polymer films was further investigated to use the system in an actual photoimaging situation. As a first step, avisual estimate of the fading rate of the orange photoenol II in various films was made. The following film substrates were tested: polyvinyl chloride, polystyrene, polyvinylpynollidone, polyethyl methacrylate, cellulose acetate, and polyurethane. As the fading rate of II was observed to be much faster in the polyurethane film, a series of different polyurethane including four Estanes (5701Fl, 5702, 570Fl, 5710Fl) from BF. Goodrich and four different compositions from Spencer-Kellogg (DV-l99l, XP 2085, XP 2108, XP 2109) were compared. Of these, S-K XP 2085, a soft aromatic polyurethane, permitted the fastest fading rate, thus most of the subsequent work was done in this medium. Films of 1 mil thickness were cast on a Mylar backing from a 10 percent solution of XP 2085 in methyl ethyl ketonetetrahydrofuran by a Gardner blade set at 10 mils. The concentration of l was adjusted so that the dried films were approximately 0.5M in 1. Although kwas estimated as approaching 10 sec in this film and the final product III, max 397 and 410 nm, was formed as expected, no measurable intensity dependence was found. A second film prepared as above, but containing 0.5M DABCO, sufficiently increased k so that intensity dependence was readily demonstrable by the previously described method. After exposing the film to the flux required to reach an OD of I in the high intensity run (7 mins.) corresponding to percent conversion, the ratio of optical densities at 397 nm. in the direct and screened runs was 1.65:1.
Examination of the changes in the spectral curves with time, however, showed that a side reaction was intruding in the lower intensity run. Since the intermediate II is known to react with oxygen (Chart I) to which polyurethanes are relatively permeable, it is likely that the longer time periods employed in the Iowerjntensity 6 this ambierit radiation reach attained by a I- minute exposure to the B-H6 lamp. In the most favorable case, the lO' M solution of I in methanol containing lO M added DABCO, no detectable amount of III was formed after 2-month exposure to ambient light, whereas subsequent l-minute irradiation with the B-H6 lamp gave III with an optical density of 0.35. By contrast, under non-basic conditions, where k is small and the formation of III is not intensity dependent, an exposure period of only 2 weeks to ambient light permitted attainment of the same optical density as I minute with the B-H6 lamp. 4
Table 1 Summary of Results of Irradiation of I in Various Media photopolymer time required to reach OD=I in high intensity run ratio of 00s at absorption maximum of III d= direct irradiation with unfiltered B-H6 lamp d same through l0% transmission screen for 10 X longer color which developed upon exposure to room light for period of two months,
W weak, M moderate, 0 none.
run allowed greater diffusion of oxygen resulting in more photo-oxidation. The added basic DABCO may also function as an antioxidantto minimize this undesirable side reaction.
Samples of the film were also overcoated with aqueous emulsions of polymers which gave clear hard surfaces on drying which further inhibited photooxidation. Polyvinylidene chloride was examined for its known low oxygen permeability and the resulting film indeed produced identically shaped curves in the two runs at different intensities, with an optical density ratio of l.5l:l. However, this overcoating proved to. have poor light stability and caused an overall orange coloration to develop. A polyacrylate overcoating also produced undesirable coloration on exposure to light. Flexclear (an aqueous emulsion of nylon, silicone, and
other unidentified polymers) gave a photostable overcoating which exhibited intensity dependent imaging but which did not completely eliminate undesirable competing photooxidation.
A polymer containing basic groups, poly B-dimethylaminethyl methacrylate (photopolymerized Alcolac Sipomer 2 Ml-M), was found which gave a hard, clear colorless film in which intensity dependent conversion of I to III was demonstrated.
A summary of the results of irradiation of I in various media is presented in Table I. The p-cyano compound Ia with added DABCO was also shown to give Illa in an intensity dependent manner both in solution and film giving, ratios of up to 2.2:] in the amounts of product formed as the intensity was varied by a factor of 10. Under use conditions a difference in intensity of several orders of magnitude beween ambient light and a high intensity source would be employed thus greatly increasing the ratio.
Table I also contains the results of an exposure test over a 2-month period of the solutions and films containing I to fluorescent room lights at table level. In no case did the amount of color in the samples exposed to Anotherreaction which is intehsity dependent is the photochemical ring opening of the dihydrodibenzofuran IV l-Acetoxy-l ,2-dihydrol ,4-diphenyl-2,2,3- tricyano-dibenzofuran in the presence of a triplet sensitizer. Direct irradiation of a dilute benzene solution of the pale yellow IV, A max 380 nm, gives the purple triene V, )t max 553 nm, which reverts back to IV at a rapid rate (k 3 X 10' sec). Irradiation of IV in the presence of a large amounts of sensitizer gives no color, but if an amount of sensitizer insufficient to absorb all of the light is used, a blue-violet product, presumably YIJLEQQYOJQL is. form d.
Ph OAc P11 I X) 0A 0 hv h ON CN S8115 O c A k I N 0 l ON Ph Ph CN IV V Plt /\ WOAc i Ph o E-CN ...h....- WY! k lntensity dependent format of VI was demonstrated using a l0 M solution of IV containing 5 X l0 M benzophenone, irradiated with 240-400 nm. light from the B-H6 lamp. The ratio d/d' at 570 nm. was found to be l.73. Conversion of VII to VIII is also intensity dependent and this observation has been invoked as evidence for the presence of a transient intermediate [In] which thermally reverts to VII. This system is also useful from the standpoint of photoimaging.
7 (If fl) (I) planisyl p-anisyl h h H [In] O A VIII U Irradiation for Table l were performed with broad spectrum light from a General Electric B-H6 1000 watt mercury are equipped with a Vycor filter transmitting wavelengths 225 nm., unless otherwise specified. Films were prepared on a mil polyester backing by the Gardner blade method using 10 percent solutions of the appropriate polymer in tetrahydrofuran or in mixtures of tetrahydrofuran with toluene or methyl ethyl ketone. The dried films were cut into strips which were supported in standard 1 cm. UV cells for purposes of irradiation and spectroscopic measurements. The
4. A photosensitive composition containing a photochromic 2-p-anisyl-3-(2-furyl)chromone and a strong organic amine base.
5. A photosensitive sheet comprising a substrate having thereon a photosensitive composition as in claim 1.