Scientists have sought for over two decades to incorporate the necessary attributes of transparency, stability, and high nonlinear susceptibilities into optimized organic or organometallic chromophores for third-order nonlinear optical (NLO) applications. These investigations have provided an ever-expanding understanding of structure−function relationships for the second hyperpolarizability γ and the bulk third-order nonlinear optical susceptibility χ(3) in organic materials, which are reviewed herein. Contributing to this understanding are the studies of the third-order NLO properties displayed by an array of structurally related organic chromophores based on the conjugated carbon skeletons of hex-3-ene-1,5-diynes (1,2-diethynylethenes, DEEs) and 3,4-diethynylhex-3-ene-1,5-diynes (tetraethynylethenes, TEEs). A comprehensive series of donor (D) and/or acceptor (A) substituted derivatives of DEEs and TEEs has been measured by third-harmonic generation (THG) experiments, and the investigations on these one- and two-dimensionally conjugated chromophores have provided fundamental insight into routes leading to enhanced optical nonlinearities. The molecular characteristics identified to impact the second hyperpolarizability γ include conjugation length, heteroaromatic conjugation, degree of D/A substitution, cis, trans, and geminal D/A conjugation, and molecular asymmetry. A comparison of NLO properties for small molecular systems to those of a number of larger oligomers based on the DEE and TEE framework is also presented.

Abstract:Poly( triacety1ene)s [PTAs,are a new class of linearly conjugated polymers with a nonaromatic all-carbon backbone. To explore structure-property relationships in PTAs, we prepared a series of monodisperse oligomers ranging from monomer to hexamer by oxidative Glaser-Hay coupling of a bifunctional ("chain-forming") (El-hex-3-ene-I ,5-diyne in the presence of an eridcapping unit. All six oligomers are amazingly stable towards exposure to light, air, and temperatures beyond their melting points.

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The photoconductive, orientational, and photorefractive properties of monolithic glasses based on new nonlinear optical chromophores containing a 2-dicyanomethylene-3-cyano-2,5-dihydrofuran (DCDHF) acceptor group are presented. Large net gain coefficients are observed in both red and infrared wavelength regions. The physical and optical properties of glasses based on various DCDHF-containing derivatives are compared and analyzed, and the factors limiting steady-state and dynamical photorefractive performance are discussed.

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