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.

Abstract As with most of the subjects dealt with in this chapter, the theoretical and experimental activity in this area has been stimulated by the potential practical applications of superconductivity in organic materials. Superconductivity represents one phase among several that are possible in certain special compounds under special circumstances. The only theory that has been successful in explaining superconductivity applies to metals; this due to Bardeen-CooperSchrieffer (1957) (BCS) theory (see p. 586), and it is based on an electronlattice phonon-electron attraction that creates a correlated free electron pair that obeys Bose-Einsten statistics. There have been many attempts to rationalize superconductivity in organic systems in terms of the BCS theory, but at present this is still an open question. In part, this is due to defects and impurities in the materials, making reproducibility a problem, and in part to the difficulty of making the required definitive experiments. Since the BCS theory involves the coupling of electrons to lattice phonons, there should be a change in Tc if deuterium is substituted for hydrogen.

Abstract As with most of the materials discussed in this book, the potential for significant technological discoveries is a powerful driving force for research in this area. However, there is much of extreme theoretical importance in these studies. The study of nonlinear excited state resonances with even-parity states has potentially rich rewards both for spectroscopy and NLO. Then there is always the challenge to see how far one can get in designing and synthesizing organic molecules that can outperform inorganic materials. The relationship between molecular structure and bulk nonlinear properties is still in its infancy (Marder et al. 1997).

Abstract In 1985, a remarkable discovery was made of a new allotropic form of carbon with the molecular formula C60 (Kroto et al. 1985). It is called buckminsterfullerene or fullerene, named after the famous architect R. Buckminster Fuller, who designed structures that resembled C60, which looks like a soccer ball. Although C60 is not a hydrocarbon, it behaves as a typical molecular solid like anthracene, which is an insulator, but it also forms compounds that are metallic and even superconducting. These properties make C60 a fascinating and important subject for study.

Abstract This book deals with electronic processes, as distinct from ionic processes, in organic crystals. By organic, one customarily means a compound containing carbon. Of the almost two million compounds known, approximately 90% are organic, and it is obvious that severe limitations must be placed on the choice of subject matter. Thus, the organic compounds considered here will be chiefly conjugated aromatic hydrocarbons as exemplified by anthracene, the molecular formula of which is shown in Fig. I.A. I.

Abstract Since the publication of the first edition (Pope and Swenberg 1982), the main activity on the theoretical front in the polyacetylenes has been the following: a verification of some of the main features of the Su et al. (1982) electronphonon-based soliton theories; an integration of soliton dynamics with those of polarons; the measurement of the lifetimes and kinetics of these quasiparticles; the inclusion of electron-electron interactions as an important partner to the electron-phonon interactions as the basis for a more complete theory of the quasi-particles and energy levels in t-PA; and finally the resolution of the question of whether the lowest optically excited state in t-PA is an exciton or a valence band-to-conduction band transition. The predominant evidence favors the formation of an overall neutral state with charge-transfer character following optical excitation to the first allowed electronic state. This tendency is maintained in the other polymers to be discussed, and is extended to some degree to higher excited states as well.

Abstract Exciton states, as has been noted in Chapter I, can be viewed in some sense as correlated electron-hole pairs (see Fig. I.D.26). For a mean separation of less than a lattice constant between the hole and the electron, the neutral states were called Frenkel excitons, whereas when the negative and positive charges were separated by distances of one or two lattice constants, the neutral state was referred to as a charge-transfer exciton (see Fig. I.D.29). As the separation between the electron and hole increases, the coulombic interaction between the charged particles eventually becomes less than kT; at this point the thermal energy is sufficient to separate the hole and electron completely. However, the charged particles are still coupled to the molecular lattice by a local polarization field. With further increases in the kinetic energy of the charges, this local coupling is overcome and the electron and hole move in a more wavelike fashion. This chapter deals with the generation, transport, and energy spectrum of positive and negative charges in organic crystals. As in Chapter I, discussion will be restricted to crystals of conjugated aromatic hydrocarbons. A discussion of the mobility of carriers in charge-transfer salts and in polymers will be deferred until Chapters V and VI. The existence of free carrier energy levels in addition to the neutral Frenkel exciton bands allows for additional modes of exciton decay at higher energies of excitation, e.g., auto-ionization; this is discussed in Chapter IV. Before considering the behavior of excess charges in crystals, it is useful to examine the case of an excess charge in a molecule.

This study describes and characterizes the interactions of nitrogen mustard mechlorethamine (HN2) with guanine and the radiation sensitivity of guanine in the presence of HN2. Briefly, in an equimolar solution (0.5 mmol dm-3) the pH-dependence (pH 3.0-12.0) and time-dependence (0-36 h) of alkylation of guanine at room temperature were determined using a reverse-phase high-performance liquid chromatography (hplc) column. Based on the hplc peak areas of the product and intact guanine, the optimal pH for alkylation was determined to be 8.0. Similarly, the optimal time required for alkylation was 10 h. Two products, i.e. alkylated guanines, were detected (10:1, peak areas measured at 260 nm) and purified. Structural studies of the products were performed by direct insertion probe-electron impact mass spectrometry. These products were identified as N-(2-chloroethyl)-N-[2-(7-guanyl)ethyl]-methylamine (product 2). At optimal conditions, samples of either guanine or an equimolar solution of guanine and HN2 were 60Co irradiated (gamma-ray) at 25 Gy min-1 at doses up to 400 Gy. Both sets of samples were analysed by hplc. In each case, the sole radiation product observed and characterized was 8-hydroxy-guanine. Dose-yield plots were linear and showed that HN2 enhanced the radiation sensitivity of guanine. This increase in radiation sensitivity is attributed to the differences in electrophilic properties between nitrogen mustard and guanine.

The relation between the site of energy deposition and the site of its biological action is an important question in radiobiology. Even at 77 degrees K, evidence is clear that these two sites must be separated since energy deposition is random but specific products are formed. Several processes that may contribute to this separation are: 1) hole migration and stabilization through deprotonation to give neutral oxidation product radicals; 2) electron trapping and transfer to form specific radical anions, possibly followed by protonation to give neutral reduction product radicals; and 3) recombination of spatially separated charges or radicals. These microscopic processes will be reviewed critically in an analysis using electron paramagnetic resonance spectroscopy (EPR) evidence for and against long-range transfer of energy and/or charge in frozen, hydrated DNA.

A reverse-phase high-performance liquid chromatography methodology was validated for rapid, sensitive and simultaneous analysis of all stereoisomers of thymidine glycol. The procedure involved direct injection of the samples on a microbore C-18 reverse-phase column with ultraviolet detection at 220 nm wavelength. The lower limits of detection for all thymidine glycol stereoisomers were close to 2.5 pmole under optimized conditions of detection and were linear up to at least 5000 pmole. The mobile phase consisted of 3% acetonitrile in water (v/v). The procedure allowed qualitative as well as quantitative measurements of pH and temperature -dependent interconversions of isomers of thymidine glycol directly (underivatized) from aqueous solution.

Energy transport by secondary electrons is a well known example of energy transfer on the naonometer scale, but intramolecular energy and charge transfer after collisional excitation and ionization of molecules have received much less attention. Recent findings have raised questions about the common assumption that radiation-induced DNA damage remains fixed on the nanometer scale during formation of biologically significant lesions. We have investigated mechanisms for the observation that DNA base radicals induced by neutrons are dependent on the orientation of DNA fibers relative to the neutron flux. We show radicals produced by direct irradiation of DNA with protons do not show dependence on DNA orientation. This may have been due to the high doses and dose rates of our experiments. Experiments with larger samples, higher proton energies, and improved detection sensitivity might reveal orientation effects not seen in our samples. 17 refs., 4 figs., 1 tab.