The basic steps in the xerographic process are depicted in Figure 1. The process involves charging (steps 1 and 2) and imagewise photodischarging (step 3) of the photoreceptor. The photogenerated electrostatic images are then developed by toner (or dry ink) electrostatically (steps 4 and 5). After transferring the toned images to paper (step 6), the images are then fused to produce a photocopy.
Figure 1: Basic steps in the xerographic process.
A single layer of amorphous selenium was used as the photoconductor in earlier copier products. For cost and performance reasons, recent research has been focused on the use of organic materials in a bilayer device configuration. In bilayer photoreceptor devices, the charge generation and the charge transporting functions are separated into two discrete layers, the CGL for generating the charges and the CTL for transporting the charges.3 Since the technology of hole transporting molecules is more advanced than that of electron transporting molecules, most practical photoreceptors today are negatively charged at the surface. The configuration and the imaging mechanism of a bilayer photoreceptor device is given in Figure 2.
Figure 2: Schematics of the configuration and the photodischarge process of a bilayer photoreceptor (CGL: charge generation layer; CTL : charge transporting layer)
Illumination of the bilayer device results in the formation of electron-hole (e-h) pairs in the CGL. The photogenerated holes inject to the CTL, wherein these holes migrate across the device to form the intended electrostatic images. In the last decade, many classes of organic photoconductors, such as phthalocyanines, squaraines, azo pigments, perylenes, etc. have been successfully developed for photoreceptor application. The structures and the photoconductivity of these materials are currently being reviewed by Kock-Yee Law (Xerox) in an article prepared for Chemical Reviews. Partly due to experimental difficulties and partly due to the rapid pace of technology development in industry, research efforts have been concentrated on the development of better or new materials and devices. Not enough attention has been paid to the basic sciences of the photodischarge process. Interesting research areas, such as molecular and architectural effects on photogeneration, and factors that affect the charge recombination of organic photoconductors and the subsequent interfacial electron transfer processes, remain to be explored.
In one of the Center projects, Kock-Yee Law and David G. Whitten (UR) attempt to use molecular aggregates assembled by Langmuir-Blodgett (LB) film techniques to investigate the fundamental processes of photogeneration and electron transfer in organic photoconductors. Surfactant squaraines 1-3, which were designed to orient the "brick-like" squaraine chromophore in three different orientations when organized as monolayers, have been synthesized.4
Figure 3: Structures and orientations (at air-water interface) of surfactant squaraines.
Surface pressure-area isotherms show that the squaraine chromophores in 1-3 orient on water as designed. The monolayers of 1 and 2 have been transferred to glass substrates. The absorption spectra of 1 and 2 in LB films, which are different from their solution spectra, are also different from each other (Figure 4). The absorption spectra suggest that the squaraine chromophores in the LB films of 1 and 2 form different aggregates and that there is an orientation effect on the aggregation. The stacking arrangements of the aggregates are given in the insert of Figure 4 based on absorption and structural data obtained from microcrystalline squaraine compounds.
Figure 4: Absorption spectra of (a) LB film of squaraine 1 and (b) LB film of squaraine 2 on glass (inset: solution absorptions in chloroform and schematics of intermolecular interactions of the squaraine chromophores in LB films).
Now, a photoelectrochemical apparatus has been set up in the center by Young-Soon Kim (CPCT) to examine the photoconductivity of these aggregates. Preliminary experiments using a monolayer of 1 on SnO2-glass substrates indicate that both cathodic and anodic photocurrents can be observed and that the spectral response parallels the absorption of the aggregate in Figure 4a. These initial findings offer numerous possibilities for further experimentation, where fundamental issues related to the stacking arrangement, orientation and inter site distance of the chromophore on the photogeneration efficiency and the subsequent electron transfer process can be addressed.
References
1. Carlson, C.F. U.S. Patent 2, 221,776, 1940.
2. Mort, J. The Anatomy of Xerography. Its Invention and Evolution; McFarland & Co. Inc., London, 1989.
3. Melz, P.J.; Champ, R.B.; Chang, L.S.; Chiou, C.; Keller, G.S.; Liclican, L.C.; Neiman, R.B.; Shattuck, M.D.; Weiche, W.J. Photogr. Sci. Eng. 1977, 21, 73
4. Law, K.Y.; Chen, C.C. J. Phys. Chem. 1989, 93, 2533.
Professor A.K.M Hoque, Lane College, located in Jackson, TN, spent Summer '92 doing research with Professor J.P. Dinnocenzo
Professor M. Kojima, Shinshu University, Japan, collaborated on a Center Project involving Professors Dinnocenzo and Goodman of the University of Rochester and Drs. Farid and Gould of the Eastman Kodak Co.
Dr. T. Takagahara, NTT, Japan, visited the Center on October 15, 1992.
Dr. Shobatake, Institute for Molecular Science, visited the Center on March 2, 1993.
The NSF Science & Technology Center's Summer Research Program for High School and Community College Teachers once again demonstrated its mutually beneficial interaction between high school teachers and research groups during the Summer 1992. This year's constituent was made up of nine high school teachers and one community college teacher. Six came from the Rochester area (Gordon Dorway, Franklin High School; Richard Hendricks, Joseph Wilson Magnet High School; Linda Hobart, Finger Lakes Community College; Donald Horner, Fairport High School; Claude Meyers, Greece Arcadia High School; and Carl Thielking, Fairport High School) and four from a national pool (Ernest Harrington, Staples High School in Westport, CT; John Lilga, Thomas S. Wooton High School in Rockville, MD; David Rader, The Indiana Academy in Muncie, IN; and M. Gwen Siebert, The Governor's School in Roanoke, VA). In addition, one of the 1991 teachers, Ed Hull, returned to the project in 1992 with special supplementary funding from NSF.
Their primary focus was hands-on scientific research. Our view is that this is the best possible continuing education for a professional teacher of science; the responses of the teachers confirm the exceptional value of the experience. Teachers also participate in a short course and a weekly group meeting at which they present and discuss their research plans and accomplishments for each other. Several of these group meetings turned into discussions about teaching. One of the meetings brought our university teachers of freshman chemistry together with the visiting teachers. The quality of the 1992 teachers was dazzling. We have received additional funding from the National Science Foundation and will host eleven teachers in Summer '93. Anyone interested in more information about this program should contact Professor Jack Kampmeier (716) 275-4441 or Debbie Shannon (716) 275-8286.
A photo of the 1992 group is shown below.
Left to right: C. Meyers, K. Thielking, E. Hull, D. Shannon, J. Lilga, R. Hendricks, G. Dorway, J. Kampmeier, D. Horner, L. Hobart, K. Loos, E. Harrington, M.G. Siebert, D. Rader
The Scanning Tunneling Microscope loaner library started its circulation in December '92. The Center was able to purchase a 486 computer to circulate with the high school loaner STM with supplemental funding from the Xerox Corp. The instrument has been circulated to nine high schools locally and two in the Washington, DC area. Two other STMs have also been circulated to eight colleges in the DANA program for use in undergraduate research and education.
Photoinduced Charge Transfer Course
CHM 552: Topics in Chemical Physics: Photoinduced Charge Transfer was offered during the Spring '93 semester. If you would like copies of the notes or have questions concerning this course, contact Professor George McLendon (275-5751) or Debbie Shannon at the Science and Technology Center (275-8286).
The course provides an overview of the fundamentals of photoinduced charge transfer in solution and the solid state, focusing on key technologies associated with the NSF Science and Technology Center.
A list of the topics presented follows:
Professor Anne Myers received a 1992 Dreyfus Teacher Scholar Award
We have sent out the announcement for the '93 proposal renewals and "Call for Proposals" with a due date of May 21, 1993. The External Review Committee will review these proposals on Saturday, July 24. If you would like more information, you may contact Debbie Shannon at (716) 275-8286.
If you would like a copy of this report please contact the Center at (716) 275-8286.
Professor Keith Oldham, Trent University, "Extracting Kinetic Information from Voltammetric Data", (March, 1992)
Professor Dwight Sweigart, Brown University, "Applications of Voltammetry to Organometallic Chemistry" (March, 1992)
Professor Michael Kuzmin, Moscow University, "Proton Transfer Photoreactions in Organized Molecular Systems: Exciplex Mechanism of Fluorescence Quenching in Polar Media" (March, 1992)
Professor Dennis Evans, University of Delaware, "Mechanisms of Organic Electrochemical Reactions" (March, 1992)
Professor Fred Lewis, Northwestern University, "Applications of Photoinduced Charge Transfer to the Synthesis of Nitrogen Heterocycles" (April, 1992)
Professor Hiroshi Masuhara, Osaka University, "Laser Manipulation, Spectroscopy, and Photochemistry of Polymer Microspheres in Solution" (April, 1992 at Kodak)
Professor Chris Foote, University of California at Los Angeles, "Photoinduced Electron Transfer in Fullerenes and other Photochemical Reactions" (April 1992)
Professor A. Vlcek, Jr., J. Heyrovsky Institut of Physical Chemistry and Electrochemistry, Prague, Czechoslovakia, "Redox Chemistry and Spectroelectrochemistry of Mn and Re Dioxolene Complexes" (April, 1992)
Professor Michael Kasha, Florida State University, "Excited State Electron Transfer and Proton Transfer in Molecular Probes", and "From Flower Pigments to the Proton-Transfer Laser (at Kodak, May 1992)
Professor Anthony F. Garito, University of Pennsylvania, "Excited State Enhancement Mechanism for Nonresonant Nonlinear Optical Responses" (May, 1992)
Dr. Klaas Zachariasse, Max Planck Institut für Biophysikalische Chemie, "Intramolecular Charge Transfer in Aminobenzonitriles" (August, 1992)
Professor Philipp Gütlich, Institut für Anorganische Chemie, Johannes-Gutenberg-Universität, "Thermal- and Light-Induced Switching of Iron (II) Coordination Compounds" (September 1992)
Dr. Urs Wild, ETH-Zurich "From Single Molecule Spectroscopy to the Molecular Computer" (October 1992)
Professor Daniel Wayner, National Research Council of Canada, "Reactions of Ketyl and 1,3-Dioxolanyl Radicals with Electron Deficient Bromides: Electron Transfer or Atom Transfer" (October, 1992)
Dr. Victor Kanarov, State University of Tashkent, USSR, "Influence of Electron Structure on a Electron Transfer on Atom-Solid Collisions" (December, 1992)
Professor Andre Braun, University Karlsruhe, "Application of Xe-Excimer Lamps in the Oxidation Degradation of Organic Water Pollutants" (December, 1992)
Professor Roland Zimmerman, Humboldt-Universitätat zu Berlin, "Excitons in Semiconductor Quantum Structures" (January, 1993)
Professor Gertz Likhtenshtein, Ben Gurion University, Israel, "Factors Affected Electron Transfer in Proteins Modified by a Donor-Acceptor Pair: Dynamics, Micropolarity, and Orbital Overlap" (March, 1993)
Professor Nenad M. Kostic, Iowa State University, "Electron Transfer Between Cytochrome c and Plastocyanine: Evidence for Protein Dynamics" (March, 1993)
Professor E. Guy Wilson, Queen Mary and Westfield College, London, "Control of Electron Transfer in Nanostructure Assembled from Organic Molecules" (April, 1993)
Dr. David Citrin, Max-Planck-Institute, "Theory of Excitons in Low-Dimensional Semiconductor Structures: Polaritons and Radiative Decay" (April, 1993)
David G. Whitten, Director
Samir Farid, Associate Director
Jack Kampmeier, Associate Director
Esther Conwell, Associate Director
Kenneth Simolo, Assistant Director
Debbie Shannon, Administrative Assistant
TEL. (716) 275-8286
FAX (716) 473-6889
May 1993, Vol. 5, No. 1
University of Rochester
NSF Center for Photoinduced Charge Transfer
Department of Chemistry
200 Hutchison Hall
Rochester, NY 14627