The performance of amorphous organic photorefractive (PR) materials in applications such as optical data storage is generally limited by the concentration of active molecules (chromophores) that can be incorporated into the host without forming a crystalline material with poor optical quality. In polymeric PR systems described previously, performance has been limited by the necessity of devoting a large fraction of the material to inert polymer and plasticizing components in order to ensure compositional stability. A new class of organic PR materials composed of multifunctional glass-forming organic chromophores is described that have long-term stability and greatly improved PR properties.
It is demonstrated that the microscopic mechanism of the photorefractive (PR) effect in organic composites with low glass transition temperatures involves the formation of refractive index gratings through a space-charge field-modulated Kerr effect. A tensorial formulation of the macroscopic aspects of the PR Kerr effect and its microscopic interpretation is presented. The second-order dipole orientation term containing the anisotropy of the first-order optical polarizability α(−ω;ω) is shown to yield the dominant contribution to the Kerr susceptibility χ(3)(−ω;ω,0,0). A class of special chromophores having negligible second-order polarizabilities β(−ω;ω,0) and large dipole moments μ has been identified in order to optimize this term. These chromophores are not subject to the efficiency-transparency tradeoff typically encountered with second-order nonlinear optical (NLO) chromophores, providing highly transparent materials with large PR Kerr response. Contrary to previous approaches in this field, the best-performing PR polymers are then expected to employ chromophores that would be useless for second- order applications (negligible β). We report PR of the material 30% 2,6-di-n-propyl-4H-pyran-4-ylidenemalononitrile (DPDCP): 15% N,N′-bis(3-methylphenyl)- N,N′-bis(phenyl)benzidine (TPD):55% poly(methyl methacrylate) (PMMA):0.3% C60 as an illustration of this principle. A 100 μm thick film of this material exhibits a steady-state diffraction efficiency of η=25% and net two-beam coupling of Γ=50 cm−1 at a bias field of 100 V/μm and a wavelength of 676 nm. The macroscopic Kerr susceptibility of the material is related to molecular electronic properties of the chromophore DPDCP which were independently determined by experiments in solution and by quantum chemical calculations.
Phenyllithium was labeled with 6Li and I3C at the ipso carbon atom, and the tetramer was measured by solid-state NMR. The chemical shift tensor data were obtained by a moment analysis of the spinning side bands and were compared with the results obtained by calculations with the IGLO method. Although no splitting by dipolar spin coupling to 6Li was found the very good agreement between IGLO predictions and experimental results allowed alignment of the tensor axis to the molecular frame and interpretation of the data. The large deshielding of the isotropic chemical shift is mainly due to a decrease of the AE term in GP.Phenyllithium plays a role in organometallic chemistry, both as an important reagent for preparative chemistry ['] and as a model compound for quantum mechanical calculations[*]. Numerous papers were published on its preparation ["], and UV spectra were reportedlb1, ebullioscopic measurements have been performedL71, and several crystal structures of complexes with different ligands forming tetramers, dimers, and monomers are known[s-lOl. Similarly, many NMR studies were reported both in the liquid ["-"] and the solid ~t a t e [ *~-~~I . Astonishingly, however, is the lack of understanding of the 13C-chemical shift data with respect of the @so carbon atom and the C-Li bond. Normally, if in an aliphatic compound H is replaced by Li, the corresponding carbon signal is shifted to lower frequencies. This is usually interpreted in terms of the higher electron density at the anionic carbon atom. On replacement of one hydrogen atom by lithium in benzene, however, in phenyllithium the signal of ipso carbon atom is shifted to higher frequencies (A6 = 58 ppm). Several interpretations have been forwarded for this effect. Grant and Fraenkel[21] were the first to argue that a lower averaged excitation energy would be responsible for a larger op term and therefore yielding the high frequency shift. Seebach and cow~rkers[~~l distinguished on a qualitative basis between a CT and a x electron density at the metalated carbon atom and proposed enhanced r s density vs lowered x density. Similar arguments were put forward by Schleyer and coworker~ [~~].Understanding 13C-chemical shift data on a molecular level is only possible, when the chemical shift tensor and its alignment with respect to the molecular frame are known. This approach has been amply forwarded by the research group of Grant, who succeeded in measuring the chemical shift tensors of many basic molecules, such as acetylene, benzene, ethylene, and o t h e r~ [~~-~~] .We have therefore set up a project to measure the chemical shift tensor of the @so carbon in phenyllithium in order to see whether a congruence with the theoretical predictions by the IGLO method can be obtained. This should lead to a more profound interpretation of the chemical shift in solution. Initially it was hoped that alignment of the principal axis of the chemical shift tensor with respect to the molecular frame would be achievable by an observation of the dipolar 6Li,13C spin coupl...
The course of the Wittig reaction was investigated by rapid injection NMR spectroscopy. Rate constants for the formation of oxaphosphetanes were determined. A new dynamic equilibrium of oxaphosphetanes was observed for the first time. The solvent and substituent dependence of the new effect was investigated. By labeling various oxaphosphetanes with I3C and " 0 the lithium salt dependence of the new equilibrium was shown. A lithium adduct of oxaphosphetanes under these conditions is proposed.The Wittig reaction[" is still one of the most significant methods for the generation of carbon-carbon double bonds, and many publications have delt with the mechanism and stereochemistry of this reaction12]. In practice, the reaction was performed either under lithium salt-containing or lithium salt-free conditions, resulting in different stereochemical outcomes[31. Furthermore, a distinction was made between stabilized, half-stabilized, and unstabilized ylidesL2]. On the basis of early results of the Wittig reaction[41 betaines 3 were proposed as intermediates formed by the reaction of ylides 1 with aldehydes 2 (see Scheme 1). However
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