David R. Kanis's research while affiliated with Hebrew University of Jerusalem and other places

A direct relation between the first hyperpolarizability, beta, of noncentrosymmetric molecular structures and the metric centrosymmetricity content, S(i), of such structures is shown for the first time. For a series of systematic, in-plane distortions (stretch, pull, shift, and squish deformations) of the model NLO chromophore benzene, we find a convincing monotonic relationship between calculated values of beta and S(i). These results suggest that the dominant variation in beta for these structures arises from the change in oscillator strength. More striking, these comparisons demonstrate the utility of the S(i) metric in correlating observable behavior with symmetry content.

The surface coordination chemistry of the cuprate superconductor YBa2Cu3O sub 7 - gamma was extensively surveyed for the purpose of understanding the factors that govern the formation of robust, highly-ordered monolayers on these superconductor surfaces. Of the many functionalities investigated, long-chain primary amines formed the densest packed monolayer structure on both ceramic pellets and thin film substrates of YBa2Cu3O sub 7 - gamma Evidence for copper-amine interactions was obtained through SERS, and a computational model to accurately determine the local structure of the monolayer material was developed and implemented. Results of the computational study agree with experimentally determined properties of the materials. The computational studies also suggest that harmful water molecules are not able to penetrate the monolayer structure to damage the superconductor. This has been confirmed experimentally. Experimental evidence for two plausible mechanisms of monolayer formation was obtained in these studies. In addition, all surface modifications examined in this work did not damage the inherent superconducting properties of the bulk material.

Methods capable of forming highly organized monolayers on top of YBa2Cu3O7 (YBCO), a high-Tc superconductor, have been identified and are described for the first time. Here, grazing reflectance infrared fourier transform spectroscopy (GRIFTS) is employed to evaluate the degree of order for these monolayer structures. Through these investigations, it is found that while octadecylamine forms a well-ordered, crystalline-like monolayer on the surface of c-axis-oriented YBa2Cu3O7 thin films, the same reagent adsorbed onto polycrystalline YBa2Cu3O7 pellets affords disordered, liquid-like monolayers. Computational studies of alkylamine packing, using a molecular mechanics methodology, reveal two plausible structures for the crystalline-like monolayer. A GRIFTS comparison of primary, secondary, and tertiary alkylamine reagents also has been completed, and the substitution pattern dependence of the monolayer order has been assessed experimentally. Moreover, comparisons between amine monolayers on top of YBa2Cu3O7 and alkyl thiol reagents on gold surfaces are made using GRIFTS and thermal desorption experiments. This work documents the initial report of the assembly and characterization of organized monolayers supported on high-Tc superconductor surfaces, the most complex substrate yet reported capable of fostering ordered adsorbate layers.

This contribution describes the use of the computationally efficient, chemically-oriented INDO-SOS electronic structure model (ZINDO) to elucidate the electronic origins of the second-order nonlinear optical (NLO) response in molecules with extended pi-architectures weakly coupled to transition-metal fragments. ZINDO-derived quadratic hyperpolarizabilities are found to be in excellent agreement with experiment for a variety of group 6 pyridine pentacarbonyl complexes in which coordination to the low-valent metal fragments enhances the NLO response of the free ligands. The metal-pyridine chromophores are found to obey the classical two-level model. However, the beta-dictating MLCT transitions possess significantly larger Delta mu(ge) values and markedly lower oscillator strengths relative to the traditional organic chromophore pi-donor-acceptor architectures by virtue of weak coupling between the metal and the ligand pi-network. The computed quadratic hyperpolarizabilities of group 6 stilbazole pentacarbonyl derivatives are in good agreement with experiment. In contrast to conventional organic chromophores, an increase in pi-conjugation length of the stilbazole ligands does not result in a dramatic increase in the second-order response or a decrease in the HOMO --> LUMO transition energy. The molecular orbital analysis indicates that the metal pentacarbonyl fragment acts as sigma-acceptor, forcing the adjacent pyridine ring to become the molecular LUMO. As a consequence, the seemingly innocent pyridine ring becomes a primary charge acceptor in these structures, regardless of the derivatization or conjugation length. The synthesis and characterization of the donor-functionalized chromophore (4-(dimethylamino)4'-stilbazole)W(CO)(5) is also reported. The large observed beta(vec) value supports the proposed NLO response model.

The dispersion of the second‐order optical nonlinearity in chromophoric self‐assembled stilbazolium films has been characterized in detail as a function of fundamental wavelength from 800 to 1600 nm using an optical parametric amplifier‐based measurement system. The second‐harmonic generation (SHG) spectrum exhibits a distinctive two‐photon resonance at ℏω=1.3 eV (960 nm). The maximum in the second‐order susceptibility coincides with a low‐energy chromophore‐centered charge‐transfer one‐photon excitation at 480 nm (2.6 eV). The experimental SHG dispersion values compare favorably with theoretical results computed using a semiempirical sum‐over‐states formalism. These indicate that the χ⁽²⁾ response is dominated by an excitation along the long axis of the stilbazolium chromophore from the highest occupied to the lowest unoccupied molecular orbital.

To understand the second-order nonlinear optical response of molecular chromophores, we have developed and applied a formalism based on the use of perturbation theoretic (sum-over-states) methods using an uncorrelated (single determinant) ground state. When this method is used with a semi-empirical INDO/S Hamiltonian, it affords calculational results that are in excellent systematic agreement with experimental observations on a wide variety of organic and organometallic chromophores. Perhaps more importantly, the sum-over-states formulation permits understanding of the nonlinear response in terms of the nature of the excited states, interstate transitions, and changes in localization and dipole moment character. This paper presents calculations on a series of substituted organics, revealing several important regularities. (1) The nonlinear response increases substantially with the increasing size of conjugating groups between donor and acceptor ends. (2) Generally, increased polarity in the ground state results in increased nonlinear response. (3) Two-level models offer qualitative guidance in many, but not all, situations. Three-level corrections to two-level terms generally scale straightforwardly, being roughly one-half the value the two-level terms. (4) For most donor-acceptor π organics, major variations arise in second-order properties from the nature of the acceptor moiety, since donor characteristics are largely dominated by the π electron centre to which the electron donating group is attached. Insights are obtained into why highly-polarizable second-row substituents are generally less effective than first-row substituents in β enhancement and into suggested design characteristics for improving the β response of molecular chromophores. Hammett substituents parameters are at best limited qualitative aids in chromophore design.

An attractive and challenging approach to the construction of robust, thin film materials with large second-order optical nonlinearities is the covalent self-assembly of aligned arrays of high-β molecular chromophores into multilayer superlattices. In this paper, we describe the dispersion of second harmonic generation (SHG) in a self-assembled (SA) monolayer containing a stilbazolium chromophore. The frequency-dependent measurements were performed on 25 Å thick monolayers on glass using a tunable (0.4–2 μm) light source based on optical parametric amplification (OPA). The SHG spectrum contains a clear two-photon resonance at hω = 1.3eV. The maximum in the second-order susceptibility coincides with a low energy chromophore-centered charge-transfer excitation at 480 nm. The experimental SHG dispersion values compare favorably with theoretical results computed using a sum-over-states (SOS) formalism. However, the measured values exhibit a somewhat broader band response than the theoretical curve, and the origin of this behavior is discussed.