Soluble titaniumoxophthalocyanines R_mPcTiO with m = 4,8 and R = n-alkyl (C₃-C₇), trifluoromethyl, trifluoroethoxy, and the corresponding (t-butyl)₄-naphthalocyanine are synthesized. The compounds are characterized spectroscopically, photochemically, and photoelectrically in solution and in vapor deposited or spin cast films. The results are compared to the insoluble parent compound PcTiO. According to positions of the electronic absorption bands, the films form amorphous and several crystalline phases depending on deposition and annealing conditions. All compounds are very weak electrical conductors in the dark, but much better ones upon irradiation. Conductivity increases also extremely upon oxygen doping. This effect is kinetically investigated and assigned to light assisted formation and dissociation of stable chemical complexes between Pc and 02. Complex formation is reversible to a high degree, but also irreversibly oxidized photoproducts are formed. Photoconductivity increases as function of chemical substituents in the series alkyl < BuNcTiO < PcTiO. The same series is also valid for stability against photooxidation in solution (as far as soluble) and in the films. For unannealed films of BuNcTiO and PcTiO the quantum yields for photooxidation in ambient atmosphere are in the order of φ_ph = 10^-6. After annealing, the yields of decomposition decrease by additional 1 -2 orders of magnitude.

The morphology ofphthalocyanine pigments used as charge generation materials can dramatically affect their photosensitivity. Titanyl phthalocyanine [O=Ti(pc)} is an example of this type of material, where the type IV polymorph possesses high photosensitivity (E_1/2, < 0.2 μJ/cm²) when formulated with appropriate charge transport materials. In this study, we have examined the known crystal structures (refined either from single-crystal or powder diffraction data) of titanyl phthalocyanine in detail, and have simulated their X-ray powder diffraction patterns for comparison with experimental data. Powder diffraction patterns have also been simulated for slight modifications of the crystal structures, with major effects observed for rotation of the molecule about the O=Ti axis and for variation in the crystallite size. Many so-called "polymorphs" claimed in the patent literature for the phthalocyanine pigments may well be ascribed to this type of subtle change in molecular geometry or crystal size.

Hole photogeneration efficiencies have been measured in a series of dual layer photoreceptors containing generation layers of a fluorinated titanylphthalocyanine (TiO(F4-Pc)). The transport layers contained different arylamine (AA) derivatives in a polycarbonate. The AA molcules were selected for differences in oxidation potential. The efficiencies are independent of wavelength and strongly dependent on the field and the composition of the transport layer. At high fields and high AA concentrations, the efficiencies approach 0.80. The results are described by a surface-enhanced exciton dissociation argument. The argument is premised on the assumption that the absorption of a photon creates a bound electron-hole pair, which diffuses to the surface of the pigment particle, where it either recombines or dissociates into a free electron hole pair through an interaction of the donor component of the transport layer. The dissociation efficiency increases with increasing oxidation potential of the donor component. The field dependence of the dissociation process is attributed to geminate recombination and described by a theory due to Onsager. Keywords: dual layer photoreceptors, geminate recombination, free carrier photogeneration

Charge-carrier transport and charge-carrier injection in mono-layers and two-layer photoreceptors with both "conventional", i. e., molecularly doped polymeric (MDP) transport layers and novel liquid-crystalline (LC) transport systems have been investigated by time-of-flight (TOF) experiments. As compared to the MDP materials, the LC model compounds showed a considerable potential towards high-speed xerographic application due to a charge-carrier mobility as high as 0,1 cm2/V s for the hexa(hexyltho)triphenylene (HHTT). In two-layer systems with MDP CTL, the formation of a sharp and well-defined interface between CGL and TL is impossible, a bulky intermediate layer is inevitable due to the wet-coating process. This results in a delayed charge-carrier injection due to space-charge effects originating from the intermediate layer. Two-layer systems with a LC CTL allow to investigate two novel aspects: (i) Since preparation of a LC CU is feasible in a solvent-free process, the formation of an ideal, i.e., sharp interface between CU and CGL is possible. Hence, charge-carrier injection from a well-defined interface can be studied. The results can be explained (i) by taking into account the different extrinsic charge-generation mechanisms for azo pigments (Azo) and phthalocyanine pigments and (ii) the different HOMO levels of Azo and phthalocyanine as compared to the HOMO-level of HHTT (ii) Due to the high charge carrier mobility, the LC CU is a "fast enough probe" to monitor time resolved injection phenomena.

Bilayer xerographic photoreceptors in which (pi)-conjugated polymers and binary conjugated polymer blends are used as the charge generation layer have been fabricated, evaluated, and shown to be highly efficient. Devices incorporating poly(1,4-phenylene benzobisthiazole), poly(2,5-pyridylene benzobisthiazole), poly(l ,4-phenylenebisvinylene benzobisthiazole), poly(1 ,4-(2-hydroxy)phenylene benzobisthiazole), and poly(benzimidazobenzophenanthroline ladder) as the charge generation layer and a layer of tris(p-tolyl)amine (TTA) dispersed in polycarbonate as the charge-transport layer showed good photosensitivities (6-18 ergs/cm²), good dark decay characteristics (2-10 V/s at surface potentials of 400- 600 V), and high charge generation quantum efficiencies (20-50% at ~10⁶ V/cm). Photoreceptors using binary blends of conjugated polymers had enhanced spectral range of photosensitivity (300-700 nm) and significantly enhanced quantum yield for charge photogeneration relative to the component conjugated polymers. Photocarrier generation in these bilayer photoreceptors is extrinsic in nature and is mediated by exciplex formation between the conjugated polymer and TTA at the bilayer interface. Observed nanoscale size effects, in which the photoreceptor performance (quantum efficiency, photosensitivity) is enhanced with decreasing size of the charge generation layer, provide a means of tuning the photoconductive properties of optoelectronic devices based on conjugated polymers. A model derived from Onsager's 1934 theory was used to estimate the primary quantum yield of ion-pairs and ion-pair separation distance in the bilayer photoreceptors to be 0.21-0.62, and 4-6 angstroms, respectively.

We describe high electron mobility in organic solids in the form of bipolar molecular composites of N,N'-bis(1,2-dimethylpropyl)-1,4,5,8-naphthalenetetracarboxylic diimide (NTDI) and tri-p-tolylaniine (TTA). The electron mobility in the NTDI/TTA composites is ~2 x 10 cm2/Vs, which is a factor of 4 to 6 higher than in pure NTDI and isone of the highest values reported for disordered organic solids. The field and temperature dependencies of the charge mobility can be described using the disorder formalism due to Bassler and co-workers, which provides an estimation of the energy width σ of the hopping site manifold. Analysis of the data gave σ=0.081 and 0.060 eV for the electron and hole mobilities in a NTDI/TTA composite of 0.5510.45 molar ratio. The energetic disorder for electron transport in the bipolar composites is substantially lower than for pure NTDI, which is 0.093 eV. The results suggest that the observed enhancement arises from a substantial reduction of energetic disorder in the electron transport manifold of the bipolar composites. The reduction of energetic disorder may be due to intermolecular charge transfer between NTDI and TTA. Such a charge transfer could stabilize the electron transport manifold by better charge delocalization, and consequently, less energetic disorder. Another possible reason for the observed enhanced electron mobility is the reduction of NTDI dimers that can act as carrier traps by the presence of TTA molecules in the bipolar composites. These results also suggest that bipolar composites represent a promising new class of high electron mobility organic solids.

For homological arrays of main classes of molecular CT complexes used for xerographic media, it is shown that for arrays having a constant charge transfer (CT) distance, a parameter of photoprocesses (such as an optical CT degree, dipole moment, quantum gains of thermalized coulomb connected pairs and of free charge carrier photogeneration) are connected uniquely with an energy and space parameter of structure are investigated. For them, the existence of general regularities of the changing of photoprocesses' parameters from molecular structure are established. The experimental results are compared with a model calculation based on the structure-sensitive migration Onsager photogeneration. Using the results of the comparison values of initial CT distances are estimated. Established regularities allow to predict a limit value of photoprocesses gains and photosensitivity of media based on photoprocesses mentioned above.

Thermally stimulated currents (TSC) were measured in molecularly doped polymers consisting of the hole transport molecule p-diethylaminobenzaldehyde diphenyihydrazone (DEH) and the polymer binder bisphenol A polycarbonate (PC) at two different doping concentrations. The TSC spectrum, which consisted of a single, well resolved peak, was found to be dependent on the applied electric field, the heating rate and the dopant concentration. The peak maxima were located between 170K and 250K. The spectra were analyzed within the theoretical framework of Zielinski and Samoc which provided a procedure to extract the hopping activation energy for each concentration. The principle observations of this study are: (1) the TSC peak is unambiguously associated with charge transport, (2) the magnitude of the activation energies were found to be larger than values obtained from isothermal transient photocurrent measurements and (3) the activation energies obtained from analysis of the TSC spectra were found to be dependent on the doping concentration. This last observation is inconsistent with previous isothermal transient photocurrent measurements of doped polymer systems containing DEH.

Correlation functions for energetic disorder arising from charge-charge, charge-dipole, and charge-induced dipole interactions are calculated. These are integrated to determine the mobility for transport in one dimension. Compatibility with the Poole-Frenkel law is examined as a function of temperature and concentration.

Recent attempts to explain the observed field dependence of photoexcited carrier mobilities in molecularly-doped polymers have focused on the role played by local correlations in the energetic landscape characterizing the transport site manifold. Site energy correlations that fall off algebraically with distance have been shown to arise when the primary source of energetic disorder in the material is the interaction of carriers with random electric dipoles of the dopant and host molecules of the medium. In one dimension, such correlations lead to a Poole-Frenkel field dependence that is in qualitative agreement with experiment. We have recently investigated the stability of this one dimensional Poole-Frenkel prediction in the presence of other sources of disorder (e.g., charge-induced dipole interactions) that might lead to strong local energy fluctuations that fall off rapidly with distance. Our analysis suggests that local fluctuations of this type need not change the predicted field-dependence provided that they are themselves uncorrelated with the permanent dipole component of disorder.

We present results of the Monte Carlo simulation of charge carrier transport in a simple model of randomly oriented dipoles which orientations are locally correlated and compare our results with those obtained for the previously studied model of totally non-correlated dipoles. Also we have Studied the influence of the anisotropic transfer rate on the mobility field dependence. The major result is that Poole -Frenkel field dependence is a very stable phenomenon - it was observed in simulation results for locally correlated media as well as for non-correlated one. The most significant difference is the shift of Poole-Frenkel region to higher fields in the lattice with micro-ferroelectric local ordering. We also study anisotropic charge carrier mobility in the globally sterically ordered dipolar lattice. At last, we made comparison of the simulation results with the analytical solution of 1D model.

Two novel photorefractive polymers are presented, based on the charge transport molecule N,N'-diphenyl-N,N'- bis(3-methylphenyl)-{1,1'-biphenyl]-4,4'-diamine (TPD). In one polymer the TPD unit is chemically modified so that it can function both as charge transport and as electro-optic molecule. In the other polymer the TPD is incorporated into the polymeric backbone and provides the charge transport and functions as a host for the dispersed electro-optic molecules. In both types of polymers the trap density is very low, which causing a 900 phase shift between the refractive index grating and the illumination pattern and a rather small photorefractive performance. The trap density can in the case of the bifunctional molecule based polymer be increased by adding small amounts of N,N,N',N'-tetramethyl-paraphenylene diamine (TMPD) which has a lower ionization energy than the bifunctional molecule. This results in a strong increase of photorefractive performance and simultaneously in a lowering of the phase-shift. The ionization energy of some molecules used in photorefractive polymers is determined using gas-phase ultraviolet photoelectron spectroscopy and cyclovoltammetry. Using these ionization energies the process of the space-charge field formation can be understood on the basis of the components of the photorefractive polymer.

In this paper we report on the synthesis of a number of photorefractive materials with high glass transition temperatures. We have developed a series of fully-functionalized polymethacrylates with both photoconducting and NLO-active moieties. Secondly, low molecular weight glasses based on triphenylamine and carbazole units together with different NLO-chromophores have been designed which show no tendency towards crystallization. We now report on the synthetic approach to these novel materials and their properties with regard to photorefractive applications.

We investigated polymeric materials based on polysiloxane (PSX), polymethylmethacrylate (PMMA), polyurethane (PU), as well as a triphenylamin-based glass (DRDCTA) with respect to their photorefractive properties. Electric-field dependencies of the two-beam coupling gain, diffraction efficiencies, refractive index amplitudes and holographic rise-times could be obtained by means of two-wave mixing and degenerate four-wave mixing measurements. The examined PSX polymer systems were composed of a photoconducting polysiloxane host doped with trinitrofluorenone (TNF) as a sensitizing moiety and various chromophores, namely, an azo derivative, a stilben derivative and a tolan derivative. Due to their comparatively low glass-transition temperatures T_g, an orientational enhancement of the photorefractive properties was observed. Furthermore, the influence of photoisomerisation (based on trans-cis-trans cycles) on the holographical properties could be determined for the different chromophores. In addition, a class of fully functionalized polymers with azo chromophores and carbazole-units covalently attached to PMMA- and PU-backbones was synthesized. These systems show comparatively high glass transition temperatures of more than 80 degrees C. The third type of materials investigated is a glass of triphenylamin with attached carbazole and NLO-chromophore moieties. It has a glass transition temperature of 120 degrees C. For the high-T_g materials, poling procedures--essential for the photorefractive properties--could be monitored in-situ by second-harmonic generation. Absolute values for the nonlinear Pockets coefficients χ⁽²⁾ (-ω; ω, 0) have been obtained by electro-optical measurements.

Nonlinear optical (NLO) chromophores are essential components in photorefractive materials. Several stilbene derivatives, functionalized with an electron-donating substituent in the 4-position and electron-withdrawing substitutent in the 41-position, are known to display second-order NLO properties. Commonly used electron-donating groups include ether and amino, while electron-withdrawing groups include nitrile, nitro, and sulfone. Many of the NLO chomophores have absorptions that extend well into the visible wavelength range, potentially interfering with charge generation or sensitization. Additionally, crystallization of NLO chromophores in doped polymer systems often occurs and is undesirable. We wish to report the synthesis and characterization of a new class of electron donor-acceptor substituted low molar mass and polymeric stilbenes. The phosphonate ester functionality is employed as the electron-withdrawing moiety, affording excellent solubility. These materials are single component, i.e., the charge transport functionality is covalently incorporated into the NLO chromophore.

We present the electro-optic, photoconductive and photorefractive characterization of a novel organic photorefractive structure. It is composed of a bifunctional dimeric molecule combining both photoconduction and electro-optic properties. This low molecular weight material is a glass, showing a glass transition temperature (Tg) at 27 degrees C. Doped with (2,4,7-trinitro-9-fluorenylidene)malononitrile (TNFDM) as photosensitizer (1% wt), this dimer exhibits 200 cm^-1 photorefractive gain at 633 urn for 50 V μm^-1 electric field applied. Compared to usual doped polymers which generally show inhomogeneities in the mixtures, this material presents good optical quality thanks to its monophasic character. A serie of functionalized copolymer has also been synthesized, in which the ratio of the chromophore moieties over the charge carrier moieties was varied. These copolymers show photoconductivity but no photorefractivity.

The rapid improvement of the performance of photorefractive polymers over the past few years has generated a strong technological interest for these new materials. The areas of application include holographic storage, image processing, optical correlation, and phase conjugation. This paper reviews some of our recent advances in the field of organic photorefractive materials. We will first present several new polymeric composites that combine high efficiency and long shelf lifetimes. Then we will discuss the performance of a new class of organic photorefractive materials: photorefractive polymer dispersed liquid crystals. Finally, we will present two examples of applications based on photorefractive polymers: (i) optical correlators for security verification, and (ii) imaging through scattering media in the near infra-red.

We have developed novel multifunctional materials for photorefractive applications. Two kinds of molecular systems were studied: acceptor-substituted carbazole main-chain polymers and oligomers. The photocarrier generation properties and second-order nonlinear optical responses in acceptor-substituted carbazoles were examined by xerographic discharge, second-harmonic generation and electro-optic measurements. The multifunctionalities of the main-chain polymers and oligomers with carbazole moieties were confirmed, which fulfill all the requirements for photorefraction. Photorefractive effects were confirmed by two-beam coupling and four-wave mixing measurements. These multifunctional carbazole main-chain polymers and oligomers enable us the development of monolithic photorefractive materials.

In this report, we demonstrate the effect of the polar non-linear chromophore and other polar additives on hole mobilities in photorefractive polymer systems. The hole mobility measurements are presented as a function of applied field and temperature in two model systems. The nonlinear optical chromophore 4' nitro-4'-aminostilbene (NAS), having a large dipole moment of 6.7 Debye, is doped or covalently attached into a polymer matrix containing 30% by weight of diethylamino-benzaldehyde diphenyl hydrazone (DEH), a hole transport agent. The results are described by the Gaussian disorder model based on hopping through a manifold of states with superimposed energetic and positional disorder. We conclude from the results that the main effect produced by the polar additives is the reduced mobility, in agreement with the dipolar disorder model. The dipolar chromophores required in photorefractive polymers significantly decrease carrier mobility and also the speed of response.

Writing a refractive index grating by anomalous carrier diffusion without applying an external electric field has been achieved in a photorefractive polymer whose D/μ value is larger than that derived from Einstein's law. A grating was written in a photorefractive polymer by anomalous diffusion.

We describe on-going research to develop photorefractive polymeric materials with improved speed, material stability, and high beam coupling gain. The demonstration of significantly improved two-beam coupling is shown to mark the entry into a gain regime which enables the observation of new effects for the first time, such as beam fanning and self-pumped phase-conjugation. These effects have previously been reserved to a few thick high gain inorganic photorefractive crystals. We discuss how the large beam coupling has forced the reinterpretation of such traditional characterization techniques such as the grating translation method for the determination of the spatial phase of the index grating. Our subsequent material study focuses on several compositional variations to investigate the effect that varying the chromophore and charge transporting polymer has on the photorefractive effect.

We report recent improvements in the photorefractive performance of liquid crystalline thin film composites containing electron donor and acceptor molecules. The improvements primarily result from optimization of the exothermicity of the intermolecular charge transfer reaction and improvement of the diffusion characteristics of the photogenerated ions. Intramolecular charge transfer dopants produce greater photorefractivity and a 10-fold decrease in the concentration of absorbing chromophores. The mechanism for the generation of mobile ions is discussed.

Hole mobilities of a series of enamine (ENA) derivatives doped into poly(styrene) (PS) have been measured over a wide concentration range. At high fields and high ENA concentrations, the room temperature mobilities are in excess of 10^-3 cm²/Vs. These are the highest of any doped polymers described in the literature. The results are described by a model based on disorder. According to the model, charge transport occurs by hopping through a manifold of localized states that are distributed in energy and distance. The key parameters of the model are a, the energy width of the hopping site distribution, the degree of positional disorder, and μ₀, a prefactor mobility. The width of the hopping site manifold is described by a model of dipolar disorder. The model is premised on the assumption that the total width is comprised of a dipolar component and a van der Waals component. For weakly polar molecules, the dipolar component vanishes and the total width is determined only by the van der Waals component. The values for ENA doped PS are betweeen 0.077 and 0.103 eV, increasing with increasing dilution. The prefactor mobilities are between 10^-6 and 10^-1 cm²/Vs, increasing with increasing concentration. Values of the positional disorder parameter are between 1.6 and 4.8. The high mobilities in these materials are due to the low values of the van der Waals components and the high prefactor mobilities.