Synthetic Principles for Bandgap Control in Linear pi-Conjugated Systems.

Molecular scale charge motion in disordered organic materials at ambient temperature occurs via a hopping-type mechanism with rates dictated both by the charge transfer integral and by the reorganization energy due to geometric relaxation This contribution presents a systematic theoretical analysis of cation internal reorganization energies for a broad family of organic oligoheterocycles-variation of reorganization energy with oligomer chain length, heteroatom identity, and a range of heterocycle substituents provides key information on important structural properties governing internal reorganization energies At room temperature, the range in reorganization energies induced by substituent variations corresponds to a >10(2)-fold variation in intrinsic hole transfer rate, suggesting that changes in reorganization energy dominate variations in charge-transfer rates for many semiconducting/conducting oligomers

Hopping transport in conductive heterocyclic oligomers: reorganization energies and substituent effects.

This paper deals with recent and important developments in the field of organic materials for second-order nonlinear optics. Attention is drawn to current trends in chromophore design with a discussion of current progress and problems in this field. A number of important classes of chromophores, such as one-dimensional charge-transfer molecules, octopolar compounds, ionic materials, multichromophore systems and organometallics, are discussed.

Second-order nonlinear optical materials: recent advances in chromophore design

Predicting the electronic structure of extended organic molecules constitutes an important fundamental task of modern chemistry. Studies of electronic excitations, charge-transfer, energy-transfer, and isomerization of conjugated systems form the basis for our understanding of the photophysics and photochemistry of complex molecules1-3 as well as organic nanostructures and supramolecular assemblies.4,5 Photosynthesis and other photochemical biological processes that constitute the basis of life on Earth involve assemblies of conjugated chromophores such as porphyrins, chlorophylls, and carotenoids.6-8 Apart from the fundamental interest, these studies are also closely connected to numerous important technological applications.9 Conjugated polymers are primary candidates for new organic optical materials with large nonlinear polarizabilities.10-19 Potential applications include electroluminescence, light emitting diodes, ultrafast switches, photodetectors, biosensors, and optical limiting materials.20-27 Optical spectroscopy which allows chemists and physicists to probe the dynamics of vibrations and electronic excitations of molecules and solids is a powerful tool for the study of molecular electronic structure. The theoretical techniques used for describing spectra of isolated small molecules are usually quite different from those of molecular crystals, and many intermediate size systems, such as clusters and polymers, are not readily described by the methods developed for either of these limiting cases.28

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Density Matrix Analysis and Simulation of Electronic Excitations in Conjugated and Aggregated Molecules

We report multilayer nanocrystal quantum dot light-emitting diodes (QD-LEDs) fabricated by spin-coating a monolayer of colloidal CdSe/CdS nanocrystals on top of thermally polymerized solvent-resistant hole-transport layers (HTLs). We obtain high-quality QD layers of controlled thickness (down to submonolayer) simply by spin-coating QD solutions directly onto the HTL. The resulting QD-LEDs exhibit narrow ( approximately 30 nm, fwhm) electroluminescence from the QDs with virtually no emission from the organic matrix at any voltage. Using multiple spin-on HTLs improves the external quantum efficiency of the QD-LEDs to approximately 0.8% at a brightness of 100 cd/m(2) (with a maximum brightness over 1,000 cd/m(2)). We conclude that QD-LEDs could be made more efficient by further optimization of the organic semiconductors.

Efficient CdSe/CdS quantum dot light-emitting diodes using a thermally polymerized hole transport layer.

There has been tremendous interest in asymmetric cyanine and merocyanine compounds because of their applications as photographic sensitizers, membrane potential probe, and photochromic dyes for all-optical memory. On the basis of UV-visible spectroscopic data, Brooker suggested that merocyanine molecules could be described by a superposition of neutral and charge-separated canonical resonance forms and that, by changing the basicity of the endgroups and/or the solvent polarity, one could tune the molecular structure from neutral and polyene-like through polar and cyanine-like (with equal contributions from neutral and charge-separated resonance forms) to highly polar, charge-separated polyene-like.

Direct observation of reduced bond-length alternation in donor/acceptor polyenes

In summary, the compounds reported here demonstrate that it is possible to design chromophores that simultaneously exhibit large nonlinearity and good thermal stability. Furthermore, with the synthesis of bis(4-hydroxyphenyl)thiobarbituric acid, we believe that it should be possible to covalently incorporate these and other rather nonlinear chromophores into poled polymers at high concentrations. Experiments to test this hypothesis are in progress.

Synthesis of Diarylthiobarbituric acid Chromophores with Enhanced Second-order Optical Nonlinearities and Thermal Stability

Large second-order optical nonlinearities and enhanced thermal stabilities in extended thiophene-containing compounds†

Ultrasonic, piezoelectric and dielectric measurements on single crystals of (–)-2-a-α-methylbenzylamino-5-nitropyridine (MBANP) are reported. Longitudinal and transverse velocity measurements in six crystal directions were used to calculate the 13 elastic constants. Dielectric measurements were carried out in four directions and compared with refractive index measurements carried out at optical frequencies. MBANP exhibited piezoelectric coefficients of similar magnitude to those found in lithium niobate. These data are discussed in terms of the intermolecular interactions occurring in the crystal structure.

Ultrasonic, piezoelectric and dielectric investigations of (−)-2-α-methylbenzylamino-5-nitropyridine

Ultrasonic, piezoelectric and thermal expansivity measurements have been carried out using single-crystal samples of 3-nitroaniline (meta-nitroaniline mNA). Longitudinal and transverse velocity measurements in six crystal directions were used to determine the nine elastic coefficients. These data are discussed in relation to the intermolecular interactions existing within the crystal structure of mNA. Piezoelectric measurements show that this organic material exhibits coefficients which are of similar magnitude to those observed for inorganic materials such as lithium niobate.

Sandra Gilmour

Papers

Direct observation of reduced bond-length alternation in donor/acceptor polyenes

Synthesis of Diarylthiobarbituric acid Chromophores with Enhanced Second-order Optical Nonlinearities and Thermal Stability

Large second-order optical nonlinearities and enhanced thermal stabilities in extended thiophene-containing compounds†

Ultrasonic, piezoelectric and dielectric investigations of (−)-2-α-methylbenzylamino-5-nitropyridine

Ultrasonic and piezoelectric investigations on single crystals of 3-nitroaniline