Polymorphism and C?N?N?C Conformational Isomers of Azines: X-ray Crystal and Ab Initio Structures of Two Rotational Isomers of Methyl (para-Tolyl) Ketone Azine

Azines: synthesis, structure, electronic structure and their applications

Azines (2,3-diaza-1,3-butadienes): structure, electronic structure, tautomerism, and their applications in organic synthesis, medicinal chemistry and materials chemistry.

Spectral and conformational studies on 3-pyridinealdazine by DFT approach

3-Pyridinealdazine was synthesized and characterized by FT-IR, (1)H, (13)C NMR and mass spectroscopy. The conformations of azine was determined theoretically besides selected geometrical parameters, HOMO-LUMO energies, polarizability, hyperpolarizability, natural bond orbital (NBO), atomic charges, Mulliken charges and atom in molecule (AIM) analysis were also calculated. The optimized geometry of the symmetrical azine, HOMO-LUMO and molecular electrostatic potential (MEP) surface were also evaluated using B3LYP/6-31G(d,p) basis set. (13)C NMR data were also computed using Gaussian-03 package and compared with the observed values according to density functional theory (DFT) method and analyzed.

A combined experimental and DFT study of a novel unsymmetrical azine 2-(4-methoxybenzylidene)-1-(1-(4-isobutylphenyl) ethylidene) hydrazine

A novel unsymmetrical azine 2-(4-methoxybenzylidene)-1-(1-(4-isobutylphenyl) ethylidene) hydrazine (UA) was prepared and characterized by IR, (1)H and (13)C NMR spectral studies. A 2D - potential energy scan (PES) of p-isobutylacetophenone (IBAP) was the portal to the conformational analysis of UA by density functional theory (DFT) methods using 6-31G(d,p) basis set by Gaussian 03 program. The theoretical IR frequencies were found to be in good agreement with the experimental values. The IR frequencies of UA were analyzed by means of Potential energy Distribution (PED %) calculation using Vibrational Energy Distribution Analysis (VEDA 4) program. The experimental NMR chemical shift values of UA were compared with the theoretical values obtained by DFT method. Nonlinear optical behavior of the unsymmetrical azine is also examined by the theoretically predicted values of dipole moment (μ), polarizability (α0) and first hyperpolarizability (βtot). Stability of the UA molecule has been analyzed using NBO analysis. The electrochemistry of UA studied experimentally by cyclic voltammetry is complemented by the computational analysis of the anionic form of the molecule UA. The determination of various global and local reactivity descriptors in the context of chemical reactivity is also performed and the electrophilicity at the vital atomic sites in UA is revealed. Bader's Atoms in molecules (AIM) theory of UA indicated the presence of intramolecular hydrogen bonding in the molecule. The molecular electrostatic potential (MEP) and HOMO-LUMO orbital analysis are also performed for the molecule UA.

Crystal structure analysis of some mesogenic cyanoazines

The crystal structure analysis of the following three mesogenic cyanoazines is reported: 4-methoxybenzaldehyde-(4-cyanobenzylidene)hydrazone (E, E) (azine 1), 4-methoxyacetophenone-(4-cyanobenzylidene)hydrazone (E, E) (azine 2), 4-methoxybenzaldehyde-[4-cyanophenyl)-ethylidene]hydrazone (E, E) (azine 3). These azines are simple models of monomeric precursors for the synthesis of comb-like polymers with potential nonlinear optical properties of the second order. The crystal structures have been determined at room temperature by direct methods and refined by full matrix least squares method. Some differences in the solid state conformation of the azines are found as a result of the introduction of a methyl group onto the azine system. The relation between molecular conformation and mesogenic behavior of the three azines is also discussed.

Donor-acceptor triazenes: synthesis, characterization, and study of their electronic and thermal properties

A new class of 1,3-disubstituted-triazenes were synthesized by coupling functionalized benzimidazol-2-ylidenes, as their free N-heterocyclic carbenes or generated in situ from their respective benzimidazolium precursors, to various aryl azides in modest to excellent isolated yields (36-99%). Electron delocalization between the two coupled components was studied using UV-vis spectroscopy, NMR spectroscopy, and X-ray crystallography. Depending on the complementarity of the functional groups on the N-heterocyclic carbenes and the organic azides, the respective triazenes were found to exhibit lambda(max) values ranging between 364 and 450 nm. X-ray crystallography revealed bond alteration patterns in a series of triazenes characteristic of donor-acceptor compounds. Triazene thermal stabilities were studied using thermogravimetric analysis and found to be strongly dependent on the sterics of the benzimidazol-2-ylidene component and the electronics of the azide component. Triazenes possessing bulky N-substituents (e.g., neo-pentyl, tert-butyl, etc.) were stable in the solid-state to temperatures exceeding 150 degrees C, whereas analogues with small N-substituents (e.g., methyl) were found to slowly decompose at room temperature. Triazenes featuring electron-rich phenyl azide components decomposed at higher temperatures than their electron-deficient analogues. Products of the thermally induced triazene decomposition reaction were identified as molecular nitrogen and the respective guanidine. Using an isotopically labeled triazene, the mechanism of the decomposition reaction was found to be analogous to the Staudinger reaction.

Perfect polar stacking of parallel beloamphiphile layers. Synthesis, structure and solid-state optical properties of the unsymmetrical acetophenone azine DCA

Extraordinary high degrees of polar order can be achieved by a rational design that involves the polar stacking of parallel beloamphiphile monolayers (PBAM). This strategy is exemplified by the acetophenone azines MCA (4-methoxy-4'-chloroacetophenone azine) and DCA (4-decoxy-4'-chloroacetophenone azine). The beloamphiphile design aims to achieve strong lateral interactions by way of arene-arene, azine-azine, arene-azine and halogen-bonding interactions. Dipole-induced interactions and halogen bonding dominate interlayer interactions and halogen bonding is shown to effect the layer stacking. Crystals of DCA contain PBAMs with perfect polar order and perfect polar layer stacking, while crystals of MCA features perfect polar order only in one of two layers and layer stacking is polar but not entirely perfect. We report the synthesis of the beloamphiphile DCA, its crystal structure, and we present a comparative discussion of the structures and intermolecular interactions of MCA and DCA. Absorbance and photoluminescence measurements have been carried out for solutions of DCA and for DCA crystals. DCA exhibits a broad emission centered at 2.5 eV when excited with UV radiation. The nonlinear optical response was studied by measuring second harmonic generation (SHG). Strong SHG signals have been observed due to the polar alignment and the DCA crystal's NLO response is 34 times larger than that of urea. Optimization of the beloamphiphile and systematic SAR studies of the polar organic crystals, which are now possible for the very first time, will further improve the performance of this new class of functional organic materials. The materials are organic semiconductors and show promise as blue emitters, as nonlinear optical materials and as OLED materials.

From the reactivity of N-heterocyclic carbenes to new chemistry in ionic liquids

N-Heterocyclic carbenes have numerous applications in synthetic chemistry. We detail the reactivity and chemistry of these molecules including investigations into their reactions with small reagents, their use for the preparation of polarised azines and their potential application as NLO materials. The chemistry of imidazolium salts, which are related to NHCs by the addition of a proton, is also discussed. New chemistry for ionic liquids is also revealed.

Azines possessing strong push–pull donors/acceptors

Azines (R(2)C[double bond, length as m-dash]N-N[double bond, length as m-dash]CR(2)) are 2,3-diaza analogues of 1,3-butadiene. In this report we show that strong polarisation of the azine imparts structural features consistent with delocalization within the azine fragment; NLO properties for the azines are also reported.

The azine bridge as a conjugation stopper: an NMR spectroscopic study of electron delocalization in acetophenone azines

Dipole parallel-alignment of organic molecular crystals of azines has been achieved with a design that was based on the hypothesis that the azine bridge is a conjugation stopper. This hypothesis has now been tested in detail, and (1)H and (13)C NMR spectroscopic data of symmetric and asymmetric acetophenone azines are presented in support of this design concept. Previous structural, ab initio, and electrochemical studies have shown that the azine bridge largely inhibits through-conjugation in molecules with the general structure DPhC(Me) [double bond] N [bond] N [double bond] C(Me)PhA, where D is a donor group and A is an acceptor group. NMR spectroscopy is an excellent tool to probe the degree of conjugation through the azine bridge. The NMR results reported here for nine symmetrical and 18 asymmetrical azines show in a compelling fashion that the hypothesis holds. Varying the donor group does not change the chemical shifts of the aromatic hydrogen and carbon atoms on the acceptor-substituted phenyl ring. Likewise, varying the acceptor group does not change the chemical shifts of the atoms in the donor-substituted phenyl ring.

The redox properties of ferrocenyl-substituted aryl azines

A series of ferrocenyl substituted azines (1-Fc/Ar, where Ar = 4-NO2C6H4, 4-CNC6H4, 4-OCH3C6H4, C5H4N, ferrocene, anthracene, and pyrene) were investigated by electrochemical and photochemical techniques. All the 1-Fc/Ar exhibited oxidation waves within 60 mV of each other, consistent with the expected oxidation of the ferrocene moiety. The reduction properties of 1-Fc/Ar is governed by the nature of the Ar substituent. The standard reduction potentials suggest that ferrocene has comparable electron donating abilities as a 4-methoxyphenyl and 4-dimethyl aminophenyl group. The anthracenyl azines exhibit one-electron reversible reduction followed by dimerization of the radical anion resulting in dimerization rate constants (kd) between 5.1 × 104 and 1.5 × 105 M–1 s–1. 1-Fc/Anth and related azines undergo photochemical E/Z isomerization of the C=N bonds to produce E/Z and Z/Z isomers from the thermodynamically most stable E/E form. Fluorescence at 77 K, was observed for these compounds only after long- wavelength irradiation to produce a mixture of E/E, E/Z, and Z/Z isomers. Fluorescence quantum yields of 0.042, 0.090, and 0.176 were determined for 2-Anth/H, 2-Anth/Anth, and 1-Fc/Anth, respectively. The electrochemical, photochemical, and X-ray data suggest that the azine unit is a conjugation "limiter" and may be a general characteristic of azine molecules.Key words: azines, electrochemistry, cyclic voltammetry, reduction, electron transfer.