Hyperbranched polymers: from synthesis to applications

Developing networks follow common rules to shift from silent cells to coactive networks that operate via thousands of synapses. This review deals with some of these rules and in particular those concerning the crucial role of the neurotransmitter gamma-aminobuytric acid (GABA), which operates primarily via chloride-permeable GABA(A) receptor channels. In all developing animal species and brain structures investigated, neurons have a higher intracellular chloride concentration at an early stage leading to an efflux of chloride and excitatory actions of GABA in immature neurons. This triggers sodium spikes, activates voltage-gated calcium channels, and acts in synergy with NMDA channels by removing the voltage-dependent magnesium block. GABA signaling is also established before glutamatergic transmission, suggesting that GABA is the principal excitatory transmitter during early development. In fact, even before synapse formation, GABA signaling can modulate the cell cycle and migration. The consequence of these rules is that developing networks generate primitive patterns of network activity, notably the giant depolarizing potentials (GDPs), largely through the excitatory actions of GABA and its synergistic interactions with glutamate signaling. These early types of network activity are likely required for neurons to fire together and thus to "wire together" so that functional units within cortical networks are formed. In addition, depolarizing GABA has a strong impact on synaptic plasticity and pathological insults, notably seizures of the immature brain. In conclusion, it is suggested that an evolutionary preserved role for excitatory GABA in immature cells provides an important mechanism in the formation of synapses and activity in neuronal networks.

GABA: A Pioneer Transmitter That Excites Immature Neurons and Generates Primitive Oscillations

The importance of nonlinear optical phenomena has been known for some time, however, since the mid-1980s, there has been an explosion of interest in the search for and development of non-linear optical materials that possess commercial device applications. To date, the systems have been utilized in information processing, optical switching, optical frequency conversion, and telecommunications, and with the advancing development of optotechnology, burgeoning demands for suitable materials are becoming apparent. Of the many systems studied for example inorganic crystals and semiconductors, organic crystalline monomers, and long-chain polymers with delocalized π electrons, no one species has proved to be all-encompassing, with advantages for one application being negated by disadvantages for another. This is likely to remain the case for the foreseeable future with the criteria of the application governing the type of material used. However, chemists are gradually elucidating what actually governs second- and third-order nonlinear activity and thereby, tailoring species to show greater effects. In recent years, organometallic compounds, through their unique characteristics such as diversity of metals, oxidation states, ligands, and geometries, have found success and brought a new dimension to the area. This review attempts, with critical appraisal, to bring the discussion of organometallic nonlinear optical systems up to date, as well as giving a brief but general introduction to the field of nonlinear optics.

For corrigendum see DOI:10.1002/anie.199508261

Organometallic Compounds for Nonlinear Optics—The Search for En‐light‐enment!

Linear and Nonlinear Optical Properties of Photochromic Molecules and Materials.

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjug...

Organic Optoelectronic Materials: Mechanisms and Applications

Structural studies of semifluorinated n-alkanes. 1. Synthesis and characterization of F(CF2)n(CH2)mH in the solid state

A simple experimental method for determining optical second-order polarizabilities of organic molecules for second harmonic generation (SHG) is developed by using a two-level quantum mechanical model. Required values of excited-state dipole moments are obtained from a solvatochromic method based on the theoretical treatment of McRae. Second-order polarizabilities obtained by this method for a series of compounds compare well with those obtained by the conventional EFISH technique. The solvatochromic method has the important advantage that measurements can be made rapidly with simple equipment available in most chemistry laboratories.

A solvatochromic method for determining second-order polarizabilities of organic molecules

A series of symmetric derivatives of 4-(dicyanomethylene)-2-methyl-6-(p-(dimethylamino)styryl)-4H-pyran (DCM laser dye) has been synthesized, and the nonlinear optical properties and thermal decomposition temperatures of each of the chromophores has been measured. The symmetric chromophores all have larger hyperpolarizabilities than one would expect based on their absorption maxima. Estimations of their oscillator strengths based upon UV/vis spectra are all well above 1.0. It is proposed that these compounds possess two excited electronic states close to each other in energy, both contributing to the hyperpolarizability in an additive fashion. Electro-optical absorption spectroscopy data and semiempirical calculations on a model compound support this hypothesis. The implications of these results with regard to the tradeoff between nonlinearity and transparency are discussed. The large dipole moments and high decomposition temperatures of these chromophores make them attractive candidates for use in electr...

(dicyanomethylene)pyran derivatives with c2v symmetry : an unusual class of nonlinear optical chromophores

Two photorefractive polymer composites are presented that exhibit the fastest response times reported to date by an order of magnitude (τg≈5 ms at 1 W/cm2), while maintaining large gain coefficients (Γ≈230 and 130 cm−1). These materials show promise for video-rate optical processing applications. The factors limiting the photorefractive speed in these materials are investigated.

High-speed photorefractive polymer composites

Une serie de composes modeles de n-alcanes semifluores F(CF 2 ) n (CH 2 ) m H a ete etudiee, quant a la transition de phase solide-solide: structure de ces composes au-dessus et au-dessous des temperatures de transition

Structural characterization of semifluorinated n-alkanes. 2. Solid-solid transition behavior

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