The synthesis of a family of symmetrical derivatives of tetrathiafulvalene bearing long chain ether groups is discussed. A comparison will be made between this family of compounds and those obtained with symmetrically substituted zinc octakis(alkoxyethyl)porphyrins, which have been shown to exhibit unusual photoeffects when dispersed as liquid crystals in symmetrical cells consisting of indium tin oxide electrodes as windows.

Recent developments in ring-opening metathesis polymerization (ROMP) have enabled the synthesis of poly-cyclooctatetraene (poly-COT), a material which is isostructural to polyacetylene. This liquid-phase polymerization method allows facile construction of interfaces, films, and devices with polyacetylene-like materials. The ROMP method also allows the preparation of soluble, yet highly conjugated polyacetylene analogs from substituted cyclooctatetraenes (R-COT). The redox characteristics of R-COT polymers were investigated at electrodes modified with thin polymer films. Voltammetric methods were used to characterize the redox response, band gap, electrochemical doping, and cis-trans isomerization properties of these polyenes. We have applied poly-COT technology to the fabrication of Schottky diodes and photoelectrochemical cells, by forming poly-COT films on semiconductor surfaces. The resultant semiconductor/organic-metal interfaces behave more ideally than semiconductor contacts with conventional metals, in that changes in the work function of the conducting polymer exert a large and predictable effect on the electrical properties of the resulting Schottky diodes. Transparent films of the solution-processible polymer poly- trimethylsilyl-cyclooctatetraene (poly-TMS-COT) have been cast onto n-silicon substrates and doped with iodine to form surface barrier solar cells. These devices produce photovoltages that are much larger than can be obtained from n-silicon contacts with conventional metals.

The behavior of dimethylcadmium (DMCd) molecules chemisorbed on n-type Si(100) with native oxide in ultrahigh vacuum conditions at room temperature and the photoprocesses in the adlayer induced by resonant excitation of DMCd by KrCl laser pulses have been studied using desorption mass spectroscopy. 222 nm irradiation resulted in both the efficient complete dissociation of the initially adsorbed organometallic molecules and the photoionization of fragments desorbed from the substrate surface. One-photon release of methyl radicals from chemisorbed DMCd species was observed. The nature of adsorbed species and the mechanisms of photoprocesses are discussed.

The capacity of optical memories can be enhanced by using extra dimensions to store the signals besides the spatial dimension. We have investigated the photochemical persistent hole burning (PHB) reaction in a series of quinone derivatives as the guest molecules. In wavelength dimension, we showed that 600 ultra-high multiple spectral holes can be recorded in a double layered system. To increase the multiplicity of spectral holes, we intended to use electric field as another dimension. We have investigated the influence of electric field effect (Stark effect) in the PHB reaction characteristics. We observed for the first time with the quinone derivative systems that the burnt holes were broadened and then diminished when the applied electric field was changed, and that they reappeared when the applied electric field was changed again to the initial value. Multiple hole formation in the electric field dimension and optical modulation by electric field were also achieved.

The crystalline structure of chloroaluminum phthalocyanine (ClAlPc) is modified upon contact with an acidic solution (pH equals 3) containing 0.1 M of salts like KCl, KBr, KClO3, KBrO3 or NaHSO3. This modification is labelled 'H type' on the basis of the film Q band shape. Even though the absorbances in the Q band region are the same for all the salts used in this work, the photoactivity of the transformed films varies greatly from one salt to another. Indeed, under 35 mW cm-2 white light illumination, short circuit photocurrents range from 0.25 mA cm-2 (NaHSO3) to 1.0 mA cm-2 (KCl) depending upon the salt used during the modification of ClAlPc. It is shown by electron diffraction, that transformations of the H type induce the same changes for all salts in the interplanar distances of the ClAlPc crystallites. It is probably the result of anion penetration into the film to compensate for the protonation of the macrocycles. This anion uptake in the ClAlPc structure is demonstrated by X-ray absorption in the case of KBr for which Br...C distances of 2.6 angstrom and Br...N distances of 4.2 angstrom are measured. The most photoactive H type transformations of ClAlPc are obtained in the presence of KCl, KBr and KClO3. They have a common point: a similar film morphology. For these salts, the long axis of the crystallites lies along a direction parallel to the substrate surface while it was perpendicular before the transformation. After the H-type modification with NaHSO3, the film morphology is similar to the one of the as-sublimed ClAlPc. NaHSO3 transformed films display the lowest photoactivity even if the crystallinity of those films is comparable, if not better, than the crystallinity of ClAlPc transformed with KCl.

The electrical and optical properties of the junction between undoped semiconductor-like or highly doped metallic-like films of PMeT [PMeT equals poly(3-methylthiophene)] and n-CdS are described. The junctions were prepared by photoelectrochemical deposition of the polymer onto the semiconductor surface from a solution of the monomer. Dark J-V measurements of the PMeT(undoped):n-CdS contact indicate rectifying behavior characteristic of a p-n junction diode, thus confirming that the undoped polymer is p-type. The photocurrent- photovoltage characteristics of the heterojunction under illumination appear stable, showing little sign of deterioration over a five-day period of observation. The presence of charge recombination centers at the p-PMeT:n-CdS interface and in the bulk is indicated. The photoresponse spectrum of the cell suggests that the observed photocurrent is due to both the polymer and n-CdS. The dark J-V characteristics of the PMeT(doped):n-CdS junction also signify a rectifying barrier. The rectification ratio of the junction is typical of a good metal- semiconductor contact. The electrical and photovoltaic properties of the doped polymer-based cell compare favorably with those of metal:n-CdS Schottky barrier devices. The parameters of the hybrid organic-inorganic heterojunction are compared with those of the Schottky barrier.

Pulsed photoconduction study of Al/Chlorophyll (alpha) /Al sandwich cells has been carried out in order to investigate the charge transport in chlorophyll (alpha) (CHl (alpha) ). The charge carriers are generated close to the electrode by laser pulse (800 ps) of strongly absorbed wavelength 750 nm by Chl (alpha) . The results suggest that the trapping of photogenerated charge carriers is quite significant. The electron transient current is practically absent, and the hole current signal rapidly decreases due to deep trapping. The signal shows a biexponential behavior in time period of approximately 50 microsecond(s) which could be related to the presence of two trapping species. In millisecond time range, the hole current shows a long tail in addition to the fast component. The analysis of the long tail seems to suggest the addition to the fast component. The analysis of the long tail seems to suggest the dispersive hole transport phenomenon in microcrystalline Chl (alpha) .

The interface between two immiscible liquids with immobilized photosynthetic pigments can serve as the simplest model of a biological membrane convenient for the investigation of photoprocesses that are accompanied by spatial separation of charges. Oxygen evolution with a quantum efficiency of 10 - 20% can be seen by illuminating the octane/water interface having hydrated oligomer of chlorophyll (alpha) with proton acceptors in octane and electron acceptors in water. Considering the importance of such a model of photosynthetic membranes, in the present communication we have investigated the optical and absorption properties of wet chlorophyll (alpha) . The absorption, circular dichroism, and fluorescence properties of chlorophyll (alpha) in a non-polar solvent and in thin films were studied. The adsorption isotherms of dry and wet chlorophyll (alpha) at octane/water interface were also measured. The difference in adsorption behavior of chlorophyll (alpha) dissolved in dry and wet octane is discussed.

A number of surfactant trans-stilbene derivatives have been synthesized and investigated in supported Langmuir-Blodgett assemblies. Layers of pure stilbene surfactants or mixtures with fatty acids show 'H' aggregate characteristics including relatively low oscillator strengths for the lowest excited singlet state. Consequently these exhibit relatively low ability, individually, to donate excitation energy or transfer electrons over long ranges. Mixtures of these stilbenes, layered together to form mixed films, also show 'H' aggregate characteristics but give evidence for delocalization which can effectively span the width of an entire monolayer and result in efficient energy and electron transfer processes over distances spanning several monolayers. The extension to 'H' aggregated and dilute (monomer) diphenylbutadiene and diphenylhexatriene layers results in assemblies in which vectorial energy transfer and redox processes can occur. One of the molecules used as an energy transfer and electron transfer quencher is the cobalt (III) complex of 8S1A. Although this molecule undergoes photochemical intramolecular-electron transfer-induced fragmentation in solution and microheterogeneous media, it show net photostability in the films but serves a highly effective trap.

The photoelectrochemical behavior of chlorophyll (alpha) and pheophytin (alpha) monolayers, deposited on SnO2 optically transparent electrodes by means of the Langmuir-Blodgett technique, has been investigated. Spectra of photocurrents coincided with the absorption spectra of photosynthetic pigment monolayers at the SnO2-solution interfaces. The anodic and cathodic photocurrents were measured at different electrode potentials. The effects of solution pH, electrode potentials and redox reagents on the conversion of solar energy in photocurrent by photosynthetic pigments in monolayers on optical transparent electrodes are discussed.

Various methods have been used to investigate photocurrent generation and charge transport in a system composed of zinc meso-tetraphenylporphyrin (ZnTPP) incorporated in an ion- exchange polymer which is a blend of polyvinylpyridine-costyrene and a random ternary copolymer with cationic sites. Thin films of this mixture were cast onto SnO2 optically transparent electrodes and the conductivity cells were then completed by the evaporation of gold contacts on top of the films. Measurements of the dark current as a function of applied voltage, performed on polymer films with or without ZnTPP, show the existence of traps at a density of approximately 4 X 1014 cm-3 which is left unchanged by the presence of the dye molecules. This also has little effect on the charge mobility which was found to vary from 3 to 4 X 10-9 cm2V-1s-1. The temperature dependence of the dark conductivity, however, shows that the activation energy for the conduction process changes from a (Delta) E of approximately 4 eV for the polymer alone, to a (Delta) E of approximately 2 eV for the dye/polymer system, in the same temperature range. Photoconductivity measurements performed in the visible on the dye/polymer system clearly indicate that light absorption by ZnTPP is responsible for the observed photocurrents. The response times for the photoinduced currents, during a light- on/light-off sequence, support a trap limited conduction process. The isothermal time dependence of photocurrent decay indicates the existence of a narrow trap distribution with an Et of approximately 0.8 eV, which is assumed to lie below the first excited singlet state of ZnTPP. A model based essentially on the energetic relation between the system's individual components, which invokes the distributions of donor and acceptor states associated to the polymer blend's pendant groups, is proposed to explain these observations.

Thin polymeric porphyrin films have been prepared by interfacial polymerization of a pair of reactive comonomers, one or both of which are derivatized porphyrins. Typical monomer pairs are tetra(p-hydroxyphenyl)porphyrin (THPP) in aqueous base with tetra(p- chlorocarbonylphenyl)porphyrin (TCCPP) in chloroform or dichloromethane, tetra(p- aminophenyl)porphyrin (TAPP) in DMSO with TCCPP in ethyl acetate, and various aliphatic diamines in water with TCCPP in chloroform. Typical film thicknesses are in the range 0.01 - 10 micrometers . The films display a unique chemical asymmetry, in the sense that opposite surfaces of the films show distinctive differences in the concentration and type of functional groups that are present. When placed between semitransparent electrodes and irradiated with either steady-state broad-band light or a pulsed laser, these films develop directional photopotentials, whereby the film surface that was prepared in contact with the TCCPP solution (the acid surface) develops a more negative potential than the opposite (amine or hydroxyl) surface. We consider the directional photopotentials to be a manifestation of the chemical asymmetry of these interfacial films -- that is, photoinduced charge separation involves electron transfer towards the acid surface of the film, which corresponds to the predicted trends of oxidation and reduction potentials of the various porphyrins within the polymer film.

We present a microscopic mechanism of optical nonlinearity in quasi-one-dimensional semiconductors within the context of rigid band Peierls-extended Hubbard models. A detailed configuration space analysis is done to predict the dominant excitation paths. We show that only two channels contribute to the bulk of the optical nonlinearity, even though an infinite number of channels are possible in principle. Most importantly, these channels involve a virtual two photon excited state whose relative energy should be nearly parameter independent in the infinite chain limit. This would imply that the mechanism of optical nonlinearity, as well as the frequency dependence of the third order optical susceptibility, are also largely parameter independent. This universality is a consequence of the one dimensionality alone and remains valid for arbitrary convex Coulomb interactions. These conjectures are confirmed by exact numerical calculations on finite chains that do very careful analysis of finite size effects.

Bacteriorhodopsin (BR) is a protein found in the cell membrane wall of Halobacterium halobium and serves as a light-activated proton pump (i.e., the protein converts light energy into chemical energy). The chromophore (retinal) responsible for light absorption is located within a pocket of the opsin and is bound via a Schiff base to a lysine residue in the amino acid sequence. When BR is illuminated by a laser light flash, transient changes occur in the visible absorption spectrum of the protein -- i.e., the material is photochromic. The optical absorption changes are characterized by a series of photointermediates, with characteristic rise and fall times that range from less than a picosecond to more then 10 milliseconds. This photochromic property of BR makes it a useful material for optical devices. With an aim toward developing an optical switch for the Army, we are studying the transient absorption of nanosecond light pulses from a dye laser in the spectral region in which the absorbance of BR increases as light intensity increases. This nonlinear effect is wavelength-dependent and becomes a bleach in a different spectral window. The nonlinear absorption change is reversible.

Doped polymers are highly transparent materials that are rendered nonlinear with dye molecule dopants. The second-order nonlinear-optical properties of doped polymers have been extensively studied over the last five years with several laboratories reporting the demonstration of second harmonic and electrooptic switching devices. The third-order nonlinear-optical properties of these materials are also large. In this contribution, we report on progress in understanding and optimizing the nonlinear response of such materials. Also, we discuss a process for making polymer fibers with doped cores for all-optical device applications.

We have prepared Langmuir-Blodgett monolayer and multilayer films of a polymeric amphiphile with a side chain containing a chromophore of high molecular hyperpolarizability for second order nonlinear optics. Using optical techniques including polarized transmission and attenuated total reflectance spectroscopies, second harmonic generation, and ellipsometry, we have characterized the orientational order of the chromophore and other polymer groups in the films. The high degree of chromophoric order seen in these films indicates that such polymers are good candidates as materials to fabricate Langmuir-Blodgett films for nonlinear optical applications.

We report here the numerically exact Pockels' and Kerr's coefficients of polyenes within the Pariser-Parr Pople Model. We have computed the model exact polarizability of polyenes ot chain length, N=6,8 and 10 at various static electric fields and excitation frequencies. From a 3rd degree polynomial fit of the model exact polarizability 1(Q;Q,O) (linear term) and t(Q;Q,O,O) (2nd order term) are obtained. The Pockels and Kerr coefficients are calculated from J and t. In polyenes the Pockels coefficients are small and the Kerr coefficients dominate and are the largest when the field is applied along the chain axis. The coefficients, a(Q;Q), 1(Q;Q,O) and t(Q;Q,O,O) all increase with chain length. a(Q;Q)and t(Q;Q,O,O) have the usual power law dependence and have exponent values as 1.54 and 3.78 respectively which are in agreement with the values reported in the literature. The coefficients a(Q;Q), .(Q;Q,O) and t(Q;Q,O,O) increases with increase in the excitation frequency, for frequencies below the two photon resonance value.

The photophysics of 1,3,5-triphenyl-2-pyrazoline (I) and 1,3-diphenyl-5-(p-nitrophenyl)-2- pyrazoline (II) have been probed using steady-state and time-resolved fluorescence spectroscopy, laser flash transient absorption spectroscopy, and photochemical observations. II (and to a lesser extent, I) undergoes efficient H-D exchange at the 5-position on the pyrazoline ring when irradiated at 365 nm in CD3OD. These observations are interpreted in terms of initial photoexcitation to S2, which partitions between S1 and a non-emissive intramolecular charge-transfer state. This viewpoint helps rationalise some of the differences between solution and solid state photophysics of the pyrazolines.