Malcolm R. McLean

Optical, magnetic resonance and conductivity studies of the ladder polymer BBL

Abstract We report on the optical, magnetic and electric properties of the ladder polymer BBL. Our optical studies indicate a gap of approximately 1.8eV in the neutral undoped state and the presence of charged defects which increase in number upon doping. Electron spin resonance studies suggest that these defects are polarons. The resistivity is very anisotropic and could be fit to Motts model of variable range hopping. The anisotropy is attributed to anisotropic localization lengths (ξ‖, ξ⊥) of the localized states.

An X-ray and neutron diffraction structure analysis of a triply-bridged binuclear iridium complex, [[(C5(CH3)5Ir)2(μ-H)3]+ [ClO4]−• 2C6H6]

Abstract The structure of [[(C5(CH3)5Ir)2(μ-H)3]+ [ClO4]−· 2C6H6] has been studied with X-ray and neutron diffraction techniques at room temperature and 21 K, respectively. Crystal data from the X-ray diffraction analysis of the title compound at room temperature: space group pnnm; a = 13.358(4), b = 14.083(4), c = 8.846(3) A; V = 1664.3(9) A3;Z = 2. Final agreement factors are R(F) = 0.026 and R(wF) = 0.034 for 1007 independent X-ray reflections with I > 3σ(I) and 130 parameters varied. Crystal data for the neutron diffraction analysis at 21 K: space group P21212; a = 13.261(3), b = 13.625(3), c = 8.612(2) A; V = 1556.0(8) A3; Z = 2. Final agreement factors are R(F) = 0.038 and R(wF) = 0.042 for 2315 independent neutron reflections and 393 parameters varied. The neutron diffraction study represents the first accurate structure determination of a symmetric dimer with a metalmetal bond bridged by three hydride ligands. Average molecular parameters are: IrH = 1.78(1), IrIr = 2.465(3), H···H = 2.22(2) A, IrHIr = 87.8(4)°, HIrH = 77.2(7)°, HHH = 60.0(7)°.

Development of materials with enhanced optical non-linearity by control of ultrastructure

Abstract Copolymers incorporating rigid electroactive moieties and flexible segments have been synthesized in an effort to develop non-linear optical materials which can be processed into optical quality films. A variety of coupling linkages have been employed including ether, ester, amide, imine and azo linkages. Among the most attractive materials examined in this survey are heteroaromatic ladder and tetraazaannulene copolymers which yield optical non-linearities (ratioed to optical loss), χ (3) α of 10 −12 −10 −13 esu cm −1 and have desirable auxiliary properties of high laser damage threshold and fast optical switching (t ps for three-ring copolymer). Electron donating and withdrawing groups are conveniently incorporated and these have been observed to influence optical non-linearity. The copolymer approach yields linear optical spectra characterized by sharp band edges thus optimizing the window of transparency. Finally, it is noted that this approach yields excellent control of solubility and solution viscosity necessary for spin casting in the fabrication of thin films of uniform thickness.

Stabilization Of Bipolaronic States In Ladder Polymer Models: Implications For Polymers Having Enhanced Nonlinear Optical Properties

The chemistry and physics of highly delocalized electroactive polymers has been an area of great interest over the past decade, but it has recently become apparent that the unique nonlinear optical (NLO) properties of these polymers may be their most important characteristic. Ladder polymers have excellent thermal and chemical stability, and have recently been shown to exhibit high third order nonlinear properties. In this study we would like to report how bipolaronic states can be induced in model ladder structures related to PTL and POL systems, and how these results might indicate future polymer design criteria to control and maximize third order NLO behavior and absorption characteristics.

Photogeneration of spins and charges in a derivat polyquinolaxine (PQL-type) ladder polymer

Abstract ESR studies of photogenerated, thermally generated and native spins in a polyquinoxaline (PQL-type) ladder polymer and monomer are reported. Spins were photogenerated using different light excitation energies, smaller or larger than the bandgap. The results imply long-lived localized photogenerated polarons; their lifetime is described by a two-component exponential decay function. The lifetime dramatically increases with decreasing temperature and strongly depends on the wavelength of the incident light. The two different decay times may be explained by interchain and intrachain recombination of the localized polarons. The decay times are compared with those of the photogenerated charges in a cw photoconductivity study. The similarity of the ESR properties for the photogenerated, thermally generated and native spins in the polymer indicates that these spin species all exhibit polaronic character.

Multifunctional materials: new mechanisms for NLO effects

Multifunctional properties of nonlinear optical chromophores are discussed both in terms of a given chromophore exhibiting more than one type or mechanism of optical nonlinearity and in terms of a chromophore exhibiting useful auxiliary properties. For materials exhibiting more than one type of mechanism of optical nonlinearity, the concept of pulse-controlled optical nonlinearity is introduced and discussed. An analogy is drawn to multidimensional nuclear magnetic resonance studies which are useful in systematically elucidating excited state dynamics. Practically, pulsed control of optical nonlinearity provides a means of enhancing and modulating nonlinear optical phenomena. The photochemical reactivity of nonlinear optical chromophores is discussed in terms of fabricating ordered lattices appropriate for the development of integrated circuits and the realization of specific effects such as quasi-phase matching in second harmonic generation.

Transport properties of heat treated and doped leader type polymers BBB and BBL

Abstract We provide a short summary of some of the properties of heat treated and doped ladder-type polymers BBB and BBL with emphasis on the transport properties: electrical conductivity and photoconductivity.

New routes to large optical nonlinearities (Invited Paper)

Optical nonlinearities can arise as the result of intense electromagnetic radiation fields inducing either changes in electron or nuclear configurations. Indeed, as is discussed in this paper, several mechanisms, including mechanisms depending upon electron-phonon coupling, may be elicited from the same material. The precise contribution that a given mechanism makes to observed optical nonlinearity is often dependent upon pulse conditions employed in transient nonlinear optical experiments. The ability to control optical nonlinearity by pulse conditions is demonstrated and analyzed for a high symmetry ladder polymer where contributions from coherent parametric mixing, excitons, and bipolarons are observed. The different timescales associated with various mechanisms for index of refraction and absorption changes are discussed. The utilization of photo-induced changes occurring on widely different timescales is demonstrated in the realization of efficient second harmonic generation by quasi- phase matching. The role of chemical synthesis in engineering multi-functional materials is discussed.

Static and Dynamic Photo-Induced Index of Refraction Changes

This article deals upon three aspects of the interaction of light with matter. First of all, we consider exploitation of photo-induced chemical and structural changes for fabrication of integrated optical circuits and realization of phase matching in second harmonic generation. The systematic generation of spatial variations in index of refraction, which we demonstrate, can be a crucial factor in the development of nonlinear optical devices for commercial application. We then examine development of dynamic second order optical nonlinearities and the exploitation of these for frequency generation, electro-optic modulation, and beam steering. Our particular focus in this article is the stabilization of poling-induced order and second order nonlinear optical activity. The final discussion focuses upon third order optical nonlinearities. This area of research is less advanced and attention is directed toward definition of mechanisms of optical nonlinearity and upon identifying research avenues for significantly improving optical nonlinearities for device applications such as sensor protection, dynamic holography, and optical computing.