Geminate recombination in the presence of an inhomogeneous electric field is analyzed theoretically. The model fields employed exhibit cylindrical and spherical symmetry. It is shown that when the spatial variation of the geminate pair concentration and that of the field intensity are comparable, there are significant differences from the response observed in homogeneous electric fields.

Hypothetical experiments are described in which excitons diffuse in systems that exhibit cylindrical or spherical symmetry. A line source of excitation will be assumed at the axis of the cylinder, and a point source at the center of the sphere. Diffusion equations that are appropriate for these geometries are set up and solved in closed form for the boundary conditions of complete quenching of excitons or complete reflection of excitons. Steady state and transient conditions are treated. Based upon assumed values of the diffusion lengths, plots are constructed of the spatial distribution of excitons. For a wide range of assumed values of the diffusion lengths, these values can be recovered from an analysis of the resultant diffusion currents and their absorption coefficient dependence to within a few percent.

The possibility of observing a CT exciton echo in organic molecular crystals and polymers is explored theoretically. Reference is made to the appearance in the literature of two possible examples of this phenomenon.

A prototype autocorrelator device for measuring the pulse width of ultrashort laser pulses in the picosecond and femtosecond regimes has been constructed. The device is based on multiphoton induced ionization of electrons from a charged metal surface mounted on a vibrating silica fiber, which is an entirely different physical principle from those employed by existing methods for ultrashort pulse characterization, e.g., streak cameras or second harmonic generation. The new method distinguishes itself from existing techniques in that it is, in principle, applicable to a very wide range of wavelengths, from the far ultraviolet to the infrared, requires no special optical orientation of a nonlinear crystal, and can be used for both femtosecond and picosecond pulses. The prototype device has been successfully applied to 532 nm laser pulses with a ∼20 ps pulse width, and preliminary work shows it is applicable to femtosecond pulses as well

ADVERTISEMENT RETURN TO ISSUEPREVBook ReviewOrganic Photoreceptors in Xerography. Vol. 49 Optical Engineering Series By Paul M. Borsenberger and David S. Weiss. Marcel Dekker, Inc.: New York. 1998. xxiv + 768 pp. $195.00. ISBN 0-8247-0173-9.Martin PopeView Author Information New York University Some useful sites with molecular modeling programs can be obtained by typing in the following URLs: http://www.molsci.ucla.edu; http://newtraditions.chem.wisc.edu/; http://chemlinks.beloit.edu/; http://wunmr.wustl.edu/EduDev/index_org.html; http://www.molecules.org/VSEPR_ list.html. This latter molecular modeling for chemical education Web site was developed at Lebanon Valley College. Two other excellent molecular modeling Web sites for viewing molecules using CHIME were developed at Cabrillo College and Washington State University: http://c4.cabrillo.cc.ca.us/projects/library/index.html and http://www/wsu.edu/∼wherland/wwwlist99.html. See also Dr. Ronald Rusay's extensive Web site on molecular modeling (software) and links within http://ep.llnl.gov:80/msds/orgchem/Chem226/Mol-Modl-II.html. Dr. Rusay is a chemistry instructor at Diablo Valley College, Pleasant Hill, CA.Cite this: J. Am. Chem. Soc. 2000, 122, 16, 3986Publication Date (Web):March 9, 2000Publication History Published online9 March 2000Published inissue 1 April 2000https://pubs.acs.org/doi/10.1021/ja995764xhttps://doi.org/10.1021/ja995764xbook-reviewACS PublicationsCopyright © 2000 American Chemical SocietyRequest reuse permissionsArticle Views269Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Dyes and pigments,Excitons,Photogeneration,Polymers,Recombination Get e-Alerts

Abstract In addition to the conjugated aromatic Jr-bonded organic compounds that constitute almost the entire group that possesses semiconducting properties, there is a sparsely populated group that depends on delocalized a-bonding. The dominant members of this group are the polysilanes, in which the backbone is composed of catenated silicon atoms that form a linear chain.

Abstract The first edition of Pope and Swenberg’s Electronic Processes of Organic Crystals, published in 1982, became the classic reference in the field. It provides a tutorial on the experimental and related theoretical properties of aromatic hydrocarbon crystals and includes emerging work on polymers and superconductivity. This new edition has been expanded to cover the major theoretical and experimental advances over the last fifteen years. It contains a unified description of what is known in almost every aspect of the field. The basic phenomena covered in the first edition included fluorescence, exciton and charge carrier generation, transport, recombination, and photoemission; the new edition adds solitons, polarons, bipolarons, spin waves, and charge density waves. It provides in-depth coverage of such model polymers such as polyacetylene, polydiacetylene, poly (phenylene-vinylene), polyanilines, polysilanes, and fullerenes. It also provides detailed treatments of the expanding areas of electroluminescence, non-linear optics, organic magnets, organic superconductors, and Langmuir-Blodgett films. In addition, it contains a chapter on major applications, including LED’s, photocopiers, photoconductors, batteries, transistors, liquid crystals, photorefractive devices, and sensors. As in the first volume, the authors take informed positions in controversial areas. This book will be an essential reference for organic material scientists, whether they are experienced researchers or just entering the field. It will also be a reliable guide to anyone interested in this rapidly growing field

Abstract Magnetic materials are technologically indispensable, with annual sales larger than that of semiconductors. Applications of magnetics find their way into telecommunications technology, energy storage, safety equipment, information storage devices, and a variety of medical instruments. At present organic magnets do not rival the inorganic variety in magnetization. However, there are potentialities in organic magnets that are absent in the inorganic systems, such as flexibility, transparency, thin-film-forming ability, and low density that will insure the organic materials some important niche in technology.

Abstract The polymer poly(p-phenylenevinylene) (PPV for short) is an important polymer from theoretical and practical viewpoints and is one of the polymers to be considered as a model. PPV and its derivatives can be prepared in the form of thin films, many of which have been found to be functional as the luminescent layer in an electroluminescent light emitting diode (LED) configuration. By suitable chemical modification of the basic polymer, it is possible to prepare films that will luminesce in colors that range from red to blue (Burroughes et al. 1990). The polymer is also relatively stable in air, unlike polyacetylene; however, it too suffers by exposure to light in the presence of moisture and oxygen (Greenham et al. 1995), and therefore must be rigorously protected from these reagents for extended use. The commercial potential for such films is obvious. It follows that improvements in the efficiency of LEDs based on the PPVs will depend to a considerable extent on a knowledge of the nature of the electronically excited states, the dynamics of their motion and decay, the physical and chemical structure and state of aggregation of the polymer, and the relationship of all of the above to the state that fluoresces. In addition, other factors are critical to the development of successful LEDs, such as the requirement that the holes and electrons be present in high concentration (space-chargelimited currents) and that they recombine rapidly and with high efficiency, preferably in a manner to maximize the formation of luminescent excitons. Finally, and of major practical importance, is the requirement of chemical stability in the operating ambiance. These will be discussed in Chapter XX dealing with applications.

Abstract The treatment of polydiacetylenes (PDA) in Chapter VI is extensive. Not mentioned there are the triplet exciton and possible polaron states. The relation between the optical band gap and the valence to conduction band transition remained to be clarified. In addition, there is the question of the carrier mobility, which has remained controversial. The energy-level structure of PDA-TS (PTS in Chapter VI) is also not fully outlined. The following discussion will deal with these and other features, starting with the singlet exciton, which, unlike the exciton in anthracene, is short-lived and nonfluorescent.

Abstract Up to now, consideration has been given to carrier production in the organic crystal by uni and bipolar injection. That is, carriers of either sign were produced as a result of the dissociation of an exciton at a surface or at an impurity molecule, or by thermal injection from an electrode into the crystal. The process of intrinsic carrier production or ionization, in which holes and electrons are formed simultaneously and in equal numbers by the deposition of at least the band gap energy, will now be discussed. There are several ways in which this ionization may be effected.

Abstract A remarkable aspect of organic systems that in their pure state have such a high resistivity is that some of them show enormous increases in conductivity when mixed with each other. As may be seen in Table V.l, the dark conductivities of some of the organic compounds resemble those of metals. This is particularly true of tetracyanoquinodimethane{TCNQ} salts, to be described at greater longth shortly.

Abstract Among the most important polymeric conductors from the technological standpoint (as distinct from molecularly doped polymer films, which constitute the much more commercially important materials used in electrophotography) are the polyanilines. Polyanilines are a family of polymers of the general formula shown in Fig. X. l. These compounds have been known for more than 100 years as aniline blacks and are important dyestuffs. The great interest in these polymers at present is due to their ability to behave either as semiconductors or as metals, depending on the degree and type of doping used. The dopants are typical electron acceptors, such as AsF5 and electron donors, such as Li. Polyanilines have been used as active electrodes in lightweight rechargeable batteries, as hole-injecting electrodes in electroluminescent flexible LEDs, as a conductive adhesive, and in many other applications.

Abstract a. Autoionization. In molecular crystals, it is well established that autoionization is the dominant process for the intrinsic generation of free carrier pairs (see Chapter Ill). In this process, the ground state of the crystal is electronically excited to a bound electron-hole state in which the electron-hole distance can vary from that of closest approach (electron and hole on the same molecule, or Frenkel exciton), to that just short of complete separation (electron and hole on different molecules but still Coulombically bound to each other, or CT exciton). Additional refinements to this mechanism have been revealed due mainly to electroabsorption studies and make possible detailed pictures of the photogeneration process. As an example, the studies on anthracene (Sebastian et al. 1983) reveal a series of peaks that may be assigned to different values of what is the equivalent of the principal quantum number in the Bohr atom (p. 71).

Abstract The electrons that are ejected from a solid subsequent to the absorption of highenergy light contain considerable information about the static energy level structure of the solid and the density of states at these energies. In addition, photoemission that is produced as a result of bimolecular reactions between two excited states (of which at least one is mobile) inside the solid constitutes a new and powerful method not only for elucidating static energy levels but also for studying the dynamic properties of excitons, free and trapped carriers, and other transient species. Kinetically bimolecular photoemission was discovered by Pope, Kallmannn, and Giachino (1965) and is referred to as the double-quantum external photoelectric effect (DQEPE). The energy released in the bimolecular reaction can be determined from the kinetic energy of the emitted electron, and the details of the kinetics can be determined from the light-intensity dependence of the process.