Contributions of Conformational and Structural Defects to the Hyperpolarizabilities of Polymers.
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CONTRIBUTIONS OF CONFORMATIONAL AND STRUCTURAL DEFECTS TO THE HYPERPOLARIZABILITIES OF POLYMERS. Steven M. Risser and Kim F. Ferris, Pacific Northwest Laboratory Richland, WA 99352. The extent of the delocalized it electron network is of prime importance in determining hyperpolarizabilities of conjugated molecules. Thus, for conjugated polymers, disruptions to this continuous order such as structural and/or conformational defects may have a large influence on the hyperpolarizabilities. We have performed semiempirical electronic structure calculations for a series of model compounds such as polyenes and polyphosphazenes to determine the effects of structural and conformational defects on the hyperpolarizabilities. Briefly, we find that structural and conjugational defects enhance the hyperpolarizabilities of polyenes, but have little influence on polyphosphazenes due to their limited it delocalization. INTRODUCTION Much of the current research into nonlinear optical materials has focussed on small organic molecules, where the large hyperpolarizabilities have been attributed to delocalized nt electrons. Such bonding conditions are not limited to carbon based systems, and recent work has suggested that small conjugated inorganic molecules, with alternating first and second row elements along the backbone, may possess equally large or larger nonlinearities than their organic equivalents[l]. The icelectron bonding in these inorganic molecules differs from their organic counterparts due to electronegativity differences among the atomic constituents and lone pair interactions. Large hyperpolarizabilities observed for organic molecules also exist for conjugated organic polymers, such as polyacetylene. For these polymers, the extent of the nt electron delocalization may not be the only important feature for determining the optical properties[2]. Both neutral and charged defects such as solitons and polarons are well known in these polymers, causing large changes in the electronic and transport properties. Recent work, both theoretical[3] and experimental[4], has suggested that these defects may be principally responsible for the large macroscopic nonlinearities of polyacetylene. The existence of such defects in the inorganic polymers may not be as significant in determining hyperpolarizabilities, as the -t bonding tends to be already localized by the electronegativity differences along the backbone. In this work we examine the effect of conjugational and conformational defects on the hyperpolarizabilities of conjugated organic and inorganic polymers. METHODS Electronic structure calculations were carried out within the NDDO approxirnation[5] to the Hartree-Fock method using the MNDO Hamiltonian in MOPAC 5.0[6]. The MNDO Hamiltonian is a valence electron method that is extensively parameterized from thermodynamic, geometric and spectroscopic data of molecules. Polyene geometries were formed using 120" bond angles and 1.35A and 1.45A bond lengths for double and single bonds, respectively. Structural parameters for the polypho
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