The temperature dependence of the picosecond photocurrent with below gap excitation (1.06 μm) has been measured for a highly oriented form of trans-polyacetylene. The 1-d picosecond photocurrent is independent of temperature. The 3-d picosecond photo-current is temperature dependent with an activation energy of 63 meV. These results demonstrate the photoproduction of nonlinear charged carriers ( solitons and polarons) at energies that are below the principal interband absorption edge.

It has been demonstrated that the oxidative doping of a.oo-diphenylpolyenes generates polaronic and/or bipolaronic intermediates which can be directly observed by optical spectroscopy. Furthermore, the absorption frequencies have been shown to correlate with conjugation length and the nature of the substituents. Species are stabilized by donor substituents and destabilized by acceptor substituents. We now report conductivity and ESR studies on iodine doped α,ω-diphenylpolyenes, containing donor p,p’-disubstituents. The implications of these studies can provide insight into electrical conductivity and optical nonlinearity of doped organic materials.

Time of flight (TOF) and thermally stimulated current (TSC) measurements were performed on poly(methylphenylsilane) (PMPS), poly(ethyl-p-methoxyphenylsilane) (PMeOPhEtS) and poly(ethylphenylsilane) (PEtPhS), to study the mechanism of interchain charge hopping transport. The value of the hole mobility at a given electric field and temperature varied with the substituent side groups. The activation energies, which were assigned to the hopping transport, ranged between 0.1-0.3eV and were dependent on the applied electric field. The nature of this dependence also varied with the substituent side groups. In the TSC spectra, a well defined current peak was observed in the temperature range between 100K-160K. The peak temperature was dependent both on the electric field and the heating rate. For a constant electric field and heating rate it was also dependent on the side groups, in a manner which was consistent with the results obtained by TOF. When the samples were exposed to high doses of UV radiation, a secondary peak at ∽180K was observed, which was attributed to surface defect sites produced by the UV exposure.

We have measured the quantum efficiency for carrier generation as a function of photon energy and the electroabsorption spectrum in poly (phenyl methyl silane). The experimental data are consistent with a band gap of about 4.6 eV. A strong absorption peak which is observed at 3.7 eV is shown to be an exciton band.

It is shown that the spectra of poly-p-phenylene sulfide doped at low doping level with different doping agents exhibit features similar to those already observed in polyacetylene and can be qualitatively interpreted in terms of the Amplitude Mode Theory.

In polymer structures, highly correlated virtual excitations of the π-electrons are responsible for the exceptionally large nonresonant nonlinear optical responses observed. Extremely large resonant nonlinear optical responses are also achievable in certain π-electron systems, which can be treated as optical Bloch systems. In addition to their obvious scientific importance, these large optical nonlinearities potentially make possible the implementation of powerful, new nonlinear optical devices and systems. After a description of nonlinear optical processes in polymers, two examples are presented. First, saturable absorption and optical bistability in ultrathin organic polymer films are described, illustrative of resonant third order processes. Saturable absorption studies of glassy polymer films consisting of quasi two-dimensional conjugated disc-like structures of silicon naphthalocyanine demonstrate that on-resonance the system behaves as an optical Bloch system with a linear absorptivity coefficient α0 of 1 × 105 cm−1 and an intensity dependent refractive index n2 of 1 × 104 cm2/kW in the wavelength range of standard laser diodes. A resonant nonlinear optical response of π-electron excitations provides the nonlinear interaction essential to the onset of bistability. Electronic absorptive optical bistability is observed on a nanosecond time scale in a nonlinear Fabry-Perot interferometer employing the saturably absorbing naphthalocyanine film as the nonlinear optical medium. As a second example, the nonresonant second order process of linear electrooptic effects in poled polymer films, is discussed. For such a second order nonlinear optical process, the broken global centrosymmetry is achieved by electric field poling of a thin polymer film. With high electrooptic coefficients of 10–50 pm/V and low dielectric constants of 3–4, poled polymers have potentially great advantages over inorganic crystals as electrooptic materials. As one device illustration, the application of poled polymers in electrooptic waveguide operations is presented.

The design of molecular crystals with specific optical properties, which are thought to arise from constituent molecules’ polarizability properties, is a desirable but currently unachievable goal. One can partially achieve this goal by choosing compounds with specific molecular attributes and empirically determining the manner in which these are translated into crystal properties. Besides the fact that there are no certain rules for prediction of crystal packing arrangements, there is also a problem in specifying molecular properties from what are today incomplete polarizability structure-property relationships. We have, realizing these limitations, identified new molecular crystals by a nonlinear optical (powder-SHG) scouting-screening program from lists of compounds chosen because of desirable molecular properties. Examination of successful materials has revealed interesting, new alignment motifs. Some of these materials, a set of halogen and cyano derivatives of aromatic compounds, are described relating properties and structures of molecules and crystals. In particular, the orientation directing influence of intermolecular halogen-cyano interactions and the use of heterocyclic compounds to improve transparency in the near infrared and in the blue and near ultraviolet spectral regions are demonstrated.

New noncentrosymmetric compounds having both electron-accepting and electron-donating substituents on a conjugated, aromatic heterocyclic structure were synthesized and their spectroscopic, thermal, and solubility properties were determined. D.C. electric field-induced second harmonic generation (EFISH) experiments were performed to provide values of the second-order nonlinear optical susceptibility/dipole moment product βμ. Long alkyl substituents were required in heterocycle-containing compounds for sufficient solubility to conduct EFISH analysis. 2,4-Dinitro-substitution on these imine-linked materials gave the highest βμ in the series.

Z-β-(N-alkyl-4-pyridinium)-α-cyano-4-styryldicyanomethanide (R-P3CNQ) and its quinolinium analogue (R-Q3CNQ) are photochromic. Their LB films show sharp charge transfer bands at 495 nm (C16H33-P3CNQ) , 565 nm (C16H33-Q3CNQ) and 614 nm (C8H17-Q3CNQ) with half widths at half maximum of 27, 22 and 37 nm respectively. They show interesting nonlinear optical properties and have the potential to store three bits per pixel when addressed at these wavelengths.

We report a new organic nonlinear optical crystal, 2, 6-dibromo-N-methyl-4-nitroaniline (DBNMNA). The refractive indices of DBNMNA were measured at several wavelengths between 500 and 1100 in and fit to a single oscillator Sellmeier equation. Second harmonic Maker fringe data yielded d33 values through comparison with y-cut crystalline quartz. We also report measurements of the electro-optic coefficients, (na3r13-nc3r33) r42, and r51 at several wavelengths. We find good agreement between the observed dispersion and that predicted by the simple two level dispersion model. However, the second order susceptibility is not well described by a single tensor component hyperpolarizability.

New organic nonlinear optical crystal : dicyanovinyl anisole (DIVA) has been grown from vapor by low pressure sublimation and from saturated solution by solvent evaporation. Crystallographic structure and quadratic nonlinear optical properties were investigated. Molecular orientation in DIVA crystal with space group P21, is favorable for the highest possible value of bulk phase-matchable coefficient. Both type I and type II phase matched second harmonic generation (PMSHG) were observed from naturally grown faces of single crystals. Efficient PMSHG was achieved at a fundamental wavelength of 812 and 1064 nm. The efficient type I PMSHG coefficient was determined as deff = 4.9 × 10−8 esu = 40 × (d11 of α-quartz) at a wave.LH length of 1064 nm. Blue light generation was demonstrated by both PMSHG and sum-frequency mixing of the 812 nm laser with 1064 nm.

Measurements of the complex electric permitttivity for the Sm C* and for three highly ordered smectic phases: Sm I*, Sm J* and Sm K* of 4- (2 - methylbutyl)-phenyl-4′-(octyloxy)-(1,1′)-biphenyl-4-carboxylate (8 OSI) have been done in the frequency range from 5 Hz to 13 MHz. For the first three chiral phases the Goldstone mode was found with temperature independent critical frequencies, νc = 1.5kHz and the relaxation time τG = 106 μs - for the Sm C* phase, and νc = 1 kHz and τG - 160 μs - for the Sm I* one. In two highly ordered smectics, i.e. Sm J* and Sm K* with inter-layer correlations, the Goldstone mode seems to be suppressed. By applying a D.C. bias field of 1.4 kV/cm the Goldstone mode has been suppressed and shifted towards higher frequencies in both the Sm C* and Sm I* phase. For the Sm J* phase the Goldstone mode is very weak and practically insensitive to the bias field used. In the Sm K* phase there is no Goldstone mode like relaxation, and the only contribution to electric permittivity comes from the fast molecular motion connected with reorientation of molecules about their long axis.

The progress of the Organic Rectifier Project (ORP) is reviewed. Several molecules of the type D-σ-A (D=organic one-electron donor, σ = covalent saturated bridge, A = organic one-electron acceptor) have been synthesized, to test the Aviram-Ratner Ansatz that they could be one-molecule thick rectifiers of electrical current. Many of these molecules (σ= urethane) self-assemble as Langmuir-Blodgett films; the cyclic voltammetry peaks confirm that these molecules preserve the good D and A characteristics. Calculations confirm that the zwitterionic state D+-σ-A- is much below the state D-σ-A+, as expected. Preliminary tests of electrical rectification have failed. With the installation of a new scanning electron microscope, the Aviram-Ratner Ansatz will soon be put to a final and definitive test

Molecular electronics requires materials of molecular size exhibiting switching or memory properties. The Langmuir-Blodgett method (L.B.) and its variants provide controlled fabrication of molecular films one molecule thick and their transfer onto solid supports. In these 2D monolayers, molecules are highly organized, so that complex organizates aimed at specific properties can be built. One of these, a semi-amphiphilic complex made of two NCS and two amphiphilic phenantrolines in a cis position, exhibits a spin transition around - 20 °C. It switches from high spin (S = 2) at room temperature to low spin (S = 0) at low temperature. The transition does not follow the expected thermal Boltzman statistics, but is rather steep, pointing out molecular cooperation. The L.B. film exhibits a hysteresis about 4 °C in width, while the parent 3D compound exhibits a hysteresis of less than 0.2 °C, but a much sharper transition. This apparently contradictory behaviour is ascribed to the high impurity content and the multicristallinity of the L.B. film in association with high intermolecular coupling. Because of its intrisic properties this new material is a breakthrough on the way to magnetic memories at the molecular scale for molecular electronics.

The relationship between the supramolecular conformational structure of assembled chromophores and their susceptibility to electric field poling is of interest for maximizing the bulk alignment achievable in an electro-optic material. We have employed solution phase dielectric constant measurements to investigate possible enhancements in dipolar susceptibility as a function of connectivity and state of aggregation in rationally synthesized chromophore assemblies, including conformationally defined head-to-tail oligomers. On the other hand, conformationally unrestricted, highly dipolar azo dyes behave as relatively isolated molecules even when present in supersaturated solutions and in close proximity on polymer chains.

In order to discern the source of the third order nonlinear optical (NLO) phenomenon both morphological and electronic considerations are required. A series of styrene copolymers with varying amounts of p-aminostyrene or p-chloromethylstyrene were prepared as anchoring sites for chemically reactive electrophilic and nucleophilic dyes. For comparison, the same dyes were incorporated into composites of polycarbonate. Preliminary degenerate four wave mixing results for all materials indicate that the copolymeric support allowed incorporation of greater amounts of electroactive units, versus composites, and hence allowed high third order nonlinear responses (X(3)/α of approximately 10−13 esu cm). Third order NLO responses were absent in some composite systems due to limited solubility and/or phase separation difficulties.

Recent results indicate that certain organic molecules whose electronic structures are characterized by extended pi-molecular orbitals can exhibit significant second and third order nonlinear optical (NLO) effects [1]. Unfortunately, this same arrangement which leads to the NLO effects, can also result in essentially one-dimensional bonding coordination. This in turn means that crystals grown from these materials do not readily form good three-dimensional optical-quality crystals, but rather tend to form needles. In addition, pure organic crystals are usually bonded by weak van der Waals forces, often resulting in poor mechanical properties. Indeed, organic impurities are frequently incorporated into these systems during crystallization resulting in poor crystallinity, spurious absorptions, and low damage thresholds. This is particularly true in the case of polymeric NLO materials, where impurities result from the polymerization steps and/or starting materials.

A series of fused three-ring compounds have been synthesized and characterized for their possible use as organic crystals for second-harmonic generation. Polymer films doped with these compounds were aligned via corona poling and their second order non-linear optical properties compared with layers doped with stilbene like molecules. Values of ¼β0 have been obtained and the effects of different heteroatoms and substituents on λ max and on ¼β0 were examined. An X-ray diffraction crystal structure on one member of the series, 1,2,4-Trichloro-8-nitro-phenoxazin-3-one (Compound 1), revealed a centrosymmetric crystal habit in a P21/n space group, with a = 9.829(3), b = 8.300(2) and c = 15.542(4) Å. Final R(F) = 0.0939 for 754 reflections where I σ; 3σ(I).

Similarities in the molecular energy levels of alkylcarbazoles and aliphatic nitroxides motivate the detailed study of 4,4′-(butadiyne-l,4-diyl)-bis-(2,2,6,6-tetramethyl-4-hydroxypiperidin-l-oxyl) () as a precursor to a ferromagnetic polymer. A more convenient synthetic route to is reported, and an X-ray-induced transformation of the β-phase of to the α-phase has been observed. The magnetic properties of and derived polymers have been studied using a SQUID system. The observed level of ferromagnetism found in samples of polymerized is comparable to the amount of trace iron materials detected in these samples.

In-situ visible dichroism/tensile elongation studies of segmented polyurethanes containing conjugated polydiacetylene chains within their hard segment domains have revealed a new reversible mechanochromic transition that manifests itself as a color change from blue to yellow during tensile elongation. This unusual strain induced transition, found in an elastomer whose hard segment structure is derived from hexamethylene diisocyanate and 5,7-dodecadiyne-l,12 diol, closely resembles the thermochromic transition also exhibited by this same material. The color change and the associated shift in the position of the polydiacetylene absorption band to higher energies is not simply an order-disorder transition but reflects a strain induced reorganization of the hard domain structure. It has been found that this blue to yellow transition is reversible as long as the material is not strained beyond about 300%. This represents the first reversible, mechanically induced chromic transition in a cross-polymerized diacetylene containing polymer.