Polyrhodanine sensitizers for organic photoconductors
United States Patent 11 1 Keller et al.  
 [ Apr. 15, 1975 POLYRHODANINE SENSITIZERS FOR ORGANIC PHOTOCONDUCTORS [75] Inventors: .luergen H. H. Keller, Chelmsford;  
 Robert H. Sprague, Carlisle, both 21 Appl. No.2 366,906  
 [52] US. Cl 96/1.6; 260/67.5; 260/302 R; 260/302 H [51] Int. Cl G03g 5/06 [58] Field of Search 96/1.6, 1.7, 127, 128  
 [56] References Cited UNITED STATES PATENTS 2,475,163 7/1949 Thompson 96/127 X 3,152,905 10/1964 Caspar 96/128 X 3,335,010 8/1967 Taber et al. 96/l28 X OTHER PUBLICATIONS Meier et al., Doping Organic Photoconductors,&#34; Zeit. Phys. Chem, Neue Folge, 39, pp. 249-261, 1963.  
 Yao 96/1.6 X Shiba et al. 96/128 X Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or Firm-Homer 0. Blair; Robert L. Nathans; David E. Brook 57 ABSTRACT Polyrhodanine compounds are disclosed which act as dopants for pigment-type, organic, photoconductor materials. Very small amounts of these polyrhodanines co-precipitated with the organic photoconductors produce dramatic increases in the sensitivity of the photoconductors.  
 8 Claims, No Drawings POLYRHODANINE SENSITIZERS FOR ORGANIC PHOTOCONDUCTORS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to dopants for pigment-type organic photoconductors to increase the speed of such photoconductors.  
 2. Description of the Prior Art Various photoconductive insulating materials have been used in electrophotographic processes. Recently, many new organic photoconductors have been described in the general scientific and patent literature. Some of these organic photoconductors can be described as pigment-type organic photoconductors.  
  Although it has been known to use spectral sensitizers with organic photoconductors, it is not believed that dopants have been used. Doping refers to the controlled introduction of trace impurities into pure crystal lattices in order to obtain desired physical properties, whereas spectral sensitization refers to admixing larger quantities of dyes with photosensitive materials to extend their sensitivity to other wavelengths of light.  
 to the purified photoconductors in a controlled manner.  
 DESCRIPTION OF THE INVENTION wherein:  
  n equals 1, 2, 3 or 4; and,  
  R represents hydrogen; C,C alkyl including unsubstituted, substituted and/or unsaturated alkyls; aryl; or aralkyl.  
 More specifically. the polyrhodanine dopants are represented as follows: SUMMARY OF AN EMBODIMENT OF THE INVENTION In one embodiment, the invention comprises the use R of small amounts of polyrhodanine dyes as dopants for H C cox pigment type organic photoconductors. These polyr- 2 C C E Iii hodanine compounds can be co-precipitated from solu- 5 5 c 0 tion with suitable pigment-type organic photoconduc- S tors, such as merocyanine dyes, to form highly sensitive P 1 h d i I organic photoconductive compositions. These photo- 0 y r 0 an conductive compositions can be dispersed in insulating binder materials, such as solutions of polystyrene, and ,C0 R  
  I then coated on suitable electrophotographlc supports. N /C0\ /R Pigment-type organic photoconductors are conve- 2 c r CO\N/ :0 ,R niently prepared in large batches and subsequently pu- 40 s/C=C a c/ l&#39;l rified by crystallization from appropriate solvents. This S c 0 usually removes trace amounts of impurities which 5/ tend to act as sensitivity enhancing dopants, resulting p l r ne 11 H 2 C l N x c=c l r\ co K S/ S/ it 0 [i1 /C 0\ S S C C t Polyrhodani ne III CO\ to /R H c N R Polyrhodanine IV Compound I above,&#39;where R is ethyl, may be named 3-ethyl-2-[ 3-ethyI-2( 3ethyl-2,4-dioxo-5- thiazolylidene )-4-oxo-5-thiazolidylidene ]rhodanine.  
 &#39; in relatively insensitive materials. Addition of these polyrhodanine dopants as described herein, by techniques such as co-precipitation, restores high sensitivity The other polyrhodanines can be named following this example.  
 The polyrhodanines described herein are also described in my copending application, Ser. No. 366,910, I  
 filed June 4, 1973, the teachings of which are hereby incorporated by reference.  
  One or more of the polyrhodanine dopants can be used to greatly increase the sensitivity of pigment-type photoconductors. They are used in extremely minute quantities, such as trace amounts. For example, amounts as low as 0.01 mg./l00 mg. of pigment-type photoconductors have been found to be effective.  
 It appears to be important, from the experimental evfollow.  
  The identity and proof of structure for Compound I where R=ethyl can be established by converting it to an analagous merocyanine dye by a condensation reaction with a methylmercaptobenzothiazole quaternary salt,  
 idence available, to form an intimate mixture of dopant 15 as f ll and photoconductor. One suitable way of achieving this is to coprecipitate the photoconductor and dopant from solution. For example, 0.01 mg. of Compound I can be added to a solution of 100 mg. of highly purified 3-ethyl-5-(3-ethyl-2-3(H)benzoselenazolylidene)-2-(3- ethyl-5-oxo-2-thiono-5-thiazolylidene )-4-thiazolidone,  
 35 known The merocyanine dye was also synthesized from a starting dye 5-( 3-methyl- 2( 3 H )benzothiazolylidene )-3-ethyl-2[2-( 3-ethyl-4- oxo-2-thiono-5-thiazolylidene )-3-ethyl-4-oxo-5- thiazolylidene]-4-thiazolidone, by quarternization with methyl-p-toluene sulfonate followed by boiling in pyria merocyanine pigment-type photoconductor, in cre- 40 dine to hydrolyze the salt, as follows:  
 sol, and the two can be c o pre ci pitated by adding meth- 6 5 The absorption maxima of the merocyanine dyes preanol. This has resulted in.;the merocyanine&#39;photoconductor having a sensitivity more than double that of the undoped form. V v 7 One method of preparing the polyrhodanine dopants pared as above were found to be identical.  
  Elemental analyses run on Compound I and the merocyanine dye synthesized from it produced the following results:  
 Merocyaninc Dye ANALYSIS Calc&#39;d. Found 50.614 49.86 H 4.056 3.80 N, 10.248 10.22 11.706 (12.64) by difference 5. 23.460 23.48 100084 100.00  
 Compound 1 45.095 45.25 H 4.289 4.36 N, 10518 10.30 0, 16.018 (15.72) by difference zi 24.077 24.37 99.997 100.00  
  The dopants described herein can be used with pigment-type type organic photoconductors. These photoconductors are dispersed in insulating binders and coated onto suitable substrates. Examples of pigmenttype photoconductors include merocyanine photoconductors as described in my co-pending application, ser. No. 366,907, filed June 4, 1973, and phthalocyaninetype photoconductors as described in US. Pat. No. 3,594,163 to Radler and the references mentioned therein. Phthalocyanines are more insoluble, however, than merocyanines, and it might be desirable to dissolve these in a strong acid such as sulfuric and to coprecipitate them with the polyrhodanine by adding water.  
  Suitable binders for these photoconductors are insulating binder materials such as polystyrene. Suitable supports are conducting supports such as aluminum plates. Other binders and supports will be known to those skilled in the art, and many have been described in the photoconductor literature.  
  The following examples further illustrate the invention. All parts and percentages are by weight unless otherwise indicated.  
 EXAMPLE 1 Preparation of Polyrhodanine 1 Ten grams of N-ethylrhodanine were mixed with 20 grams of methyl-p-toluenesulfonate and heated to 150C. After reaching 150C, the reaction mixture turned darker rapidly and the temperature rose to 200C. without further application of heat. The reaction mixture was left standing for 5 minutes, during which time the temperature fell to 160C. The mixture was further cooled to 20C., stirred with ether, decanted and treated with several further portions of ether until a heavy, sticky residue remained. This residue was stirred with 350 ccs. of absolute ethanol, heated and left standing overnight at room temperature. The next morning the solution was filtered with suction and the solid product was washed with ethanol. 4.2 grams of a solid product were obtained; this product was a mixture of polyrhodanine compounds I, 11, Ill, and IV.  
  The ethanol filtrate was concentrated to 150 ccs. at which time crystals started to form in the boiling solution. The mixture was cooled and the product filtered off to yield 2.1 grams of yellow crystals, with an absorption maximum in Cresol at 450 nm. The crystals were extracted with 7 200 ccs. of boiling methanol. The extracts were combined and chilled. Crystals formed and were filtered off. A small amount of an impurity, having an absorption maximum at 494 nm., was removed by dissolving in 300 ccs. in boiling acetone and treatment with four consecutive portions of Norite A. The acetone solution was concentrated to 50 cc. and cooled. On filtration, Polyrhodanine l was obtained as yelloworange crystals. M.P. 269,2 7lC. D-max: 1.24 in pyridine, l/l 10,000 at 424 nm. (430 in Cresol).  
 EXAMPLE 2 Preparation of Polyrhodanine 11 EXAMPLE 3 Preparation of Polyrhodanine Ill The solid material remaining after extraction with boiling pyridine in Example 2 was further extracted with 4X50 ccs. of boiling pyridine. Only a very small amount went into solution. After standing overnight in the refrigerator, the solution was filtered, and 0.015 grams of a light purple dye, polyrhodanine [11, was obtained having an absorption maximum at 550 nm. in Cresol.  
 EXAMPLE 4 Preparation of Polyrhodanine IV 0.5 grams of the material still undissolved after Example 3 was extracted with 50 ccs. of Cresol at l 60C. The remaining residue was stirred with acetone and filtered, and then thoroughly washed with acetone to remove all the Cresol. 0.2 grams polyrhodanine IV were obtained as a dark purple material, having an absorption maximum at 610 nm. in Cresol.  
 EXAMPLE 5 Preparation of Electrophotographic Plate Containing Undoped Merocyanine Photoconductor An electrophotographic plate containing a merocyanine pigment-type photoconductor was prepared as described herein. The merocyanine photoconductor had the following structural formula:  
  C H 5 CO W N c0\.,/ 2 5 l c l:-c  
 l CH 3 The preparation of this merocyanine pigment-type photoconductor is disclosed in detail in Example 1 of my co-pending patent application, Ser. No. 366,907, filed concurrently herewith.  
  50 mg. of pigment was dispersed in one-half cc. of a 10 percent solution of polystyrene in tetrahydrofuran. This solution was coated onto a 0.006 inch grained aluminum plate by means of a Bird applicator to give a 0.003 inch wet thickness. The plate was dried for a minimum of 30 minutes at 100C. before testing for dark decay and sensitivity on an electrostatic testing apparatus. This apparatus rotated the plate under a corona charger until the surface potential (measured on an oscilloscope) reached 500 volts. The charging current was shut off and the percent drop in voltage after seconds was recorded (dark decay). The plate was then recharged to 500 volts and exposed to a 15 watt tungsten light source held 2 inches from the sample. Percent drop in voltage in 3 seconds was recorded as sensitivity. lf 100 percent discharge, or amounts approaching 100 percent occurred in less than 3 seconds, the charge exposure cycle was repeated using a 0.5, 1.0 or 2.0 neutral density filter between the sample and the light source.  
  The results of the dark decay and sensitivity testing are presented in the table which follows Examples EXAMPLES 6-l6 Co-precipitation of Merocyanine Photoconductors With Polyrhodanine Dopants These examples illustrate the co-precipitation technique using a highly purified pigment-type photoconductor and a doping polyrhodanine of this invention. The organic photoconductor is the merocyanine described in Example 5. The amounts of photoconductor, and the type and amount of polyrhodanine dopant are specified in the table which follows.  
 This photoconductor is made following the procedure of Example 1 of my co-pending application Ser. No. 366,907, filed concurrently herewith, except that the benzothiazole compound used as a starting material therein is replaced with an analagous benzoselenazole.  
  Co-precipitation of the merocyanine of this example is accomplished with the appropriate polyrhodanine dopant following the co-precipitation technique described in Examples 6-16. Electrophotographic plates are prepared as described in Example 5. The results are presented in the following table.  
 &#39;0.5 neutral density filter &#34;&#39;LO neutral density filter &#39;2.0 neutral density filter TABLE Merocyanine -Photoconductor Dopant Dark Ex. No. Amt.(mg) Type Amt.(mg) Decay Sensitivity 6 100 ll. 5 l6 l00 V. sec.  
 IOO&#34; 2 sec.  
 7 100 ll. 5 20 I00 1 sec. 90&#34; 2 sec 8 I00 Ill. 5 20 I00 A see.  
 100&#34; 2 sec. [6 It&#39;ll 9 I00 ll. 5 33 l00- I sec.  
 I0 100 IV. 5 20 l00- I sec.  
 ll I00 Ill. 5 20 100- I see.  
 12 I00 IV. 1 l6 l00 1 see.  
 l3 100 IV. 0.] l0 l00- I see.  
 I4 100 IV. 0.0] 10 I00- l /see.  
 15 I00 lll. 0.l l5 l00- I see.  
 I6 I00 lll. 0.! 20 I00 I sec.  
 &#34;&#39;05 neutral density filter *l .0 neutral density filter &#34;10 neutral density filter What is claimed is:  
  l. A process of improving the sensitivity of an organic, pigment-type photoconductor by forming an intimate mixture of said photoconductor and a small amount of a polyrhodanine compound, said polyrhodanine being represented by the following structural formula:  
  ll c R N I l c n-n&#39;O wherein:  
 n equals 1, 2. 3 or 4; and,  
 R represents hydrogen; C -C alkyl; aryl; or aralkyl.  
  6. A photoconductive composition comprising an organic, pigment-type. photoconductor and intimately dispersed therewith a small amount of a polyrhodanine compound having the following structural formula:  
 wherein:  
 n equals 1, 2, 3 or 4; and,  
 R represents hydrogen; C,C alkyl; aryl; or aralkyl.  
  7. A composition of claim 6 wherein said organic photoconductor comprises a merocyanine.  
  8. A composition of claim 7 wherein R equals a C -C alkyl.