Reactions of 4,5-difluoro-1,2-dinitrobenzene with amines in dimethylformamide or EtOH

Substitution reactions of 1,2-difluoro-4,5-dinitrobenzene were explored in dimethylformamide alone and with KOH/H2O, Hünig’s base or Et3N and with Me2NHCl/Et3N in EtOH. The fluorine atoms were always displaced in preference to the nitro groups. Three compounds were prepared from these studies and were characterised by X-ray single crystal structure determinations.

Studies on the effect of polymer coating on solution grown hygroscopic non-linear optical single crystal of L-lysine monohydrochloride

Nonlinear optical single crystals are getting attention because of its enormous applications in the area of fiber optic communication and optical signal processing. In this article, we are reporting the single crystal growth of l-lysine monohydrochloride by slow evaporation solution growth technique, by using double distilled water as the solvent. We found that the grown single crystal is bulk in size and fairly transparent. But after a period of time, due to its hygroscopic nature, the transparency is completely vanished and became opaque. Then we have attempted to coat the poly methyl methacrylate (PMMA) polymer on the surface of l-lysine monohydrochloride (l-LMHCL) single crystal by dip coating method. This polymer coating is giving resistance to hygroscopic nature and also acting as thin protective covering layer without affecting the other properties. Then we have systematically studied the different properties of bare, polymer coated and hygroscopic l-LMCHL single crystals. Its crystalline perfection was examined by high resolution X-ray diffractometer and found major differences in crystalline quality. Its structural and optical behavior was assessed by powder X-ray diffraction, UV-vis and luminescence analyses.

Molecular structure, symmetry, and shape as design elements in the fabrication of molecular crystals for second harmonic generation and the role of molecules-in-materials

Molecular materials have emerged as a major theme in contemporary science and technology, thanks to the synergetic confluence of the power of synthetic chemistry, the predictive and analytical capability of condensed matter physics, and the versatility of materials science and engineering. The stringent demands in terms of molecular design and supramolecular assembly to be met in the fabrication of nonlinear optical (NLO) materials for applications such as optical second harmonic generation (SHG) make it a unique arena covering fascinating explorations in chemical synthesis, molecular and materials characterization, theoretical modeling, NLO studies, and photonics technology. Our search for conceptually novel and practically simple but effective approaches to the design of molecular building blocks for crystals exhibiting efficient SHG has led us to the avenues described in this Account. The focus has been on the incorporation of structural, symmetry, and shape features in dipolar NLO-phores to realize noncentrosymmetric or polar molecular assemblies suitable to elicit appreciable SHG responses. Attachment of n-alkyl chains of optimal length and remote functional groups were shown to be effective methods to steer the assembly of achiral NLO-phore units into SHG-active noncentrosymmetric lattices. Computational modeling of molecules, molecular clusters, and molecular assemblies in crystals provides valuable insight into the observed structure-function correlations. A systematic exploration of the impact of the placement of stereogenic centers in strongly zwitterionic NLO-phores on the molecular organization led us to the effective exploitation of C2-symmetric units to form helical superstructures capable of efficient SHG. New materials developed are based on organic and metal-organic molecules as well as coordination polymers. The potential utility of molecular shape could be demonstrated through the realization of perfectly polar organization in a family of screw-shaped dipolar molecules. Combination of optical transparency, thermal stability, and feasibility of fabricating thin films with orientationally ordered crystallites are important aspects of these materials. Several examples presented in this Account highlight the significance of molecules-in-materials by illustrating that not only the individual molecules and their organization in the crystal lattice but also the intermolecular interactions exert critical impact on the nonlinear optical response of the materials. The role of cooperative interactions in some of the cases is pointed out. This Account projects a range of design strategies for molecular SHG crystals and avenues for expanding further on the present observations; the need to address and exploit the contribution of intermolecular interactions is specially noted. The different examples presented illustrate not only the fabrication of new families of materials based on interesting models proposed earlier but also the emergence of new models from the novel materials developed.

N-(n-Dec-yl)-4-nitro-aniline

N-(n-Dec-yl)-4-nitro-aniline, C(16)H(26)N(2)O(2), crystallizes with two essentially planar mol-ecules in the asymmetric unit. The decyl chains are fully extended in an anti conformation. The mol-ecules pack in planar layers, within which mol-ecules are linked into chains by two approximately linear N-H⋯O hydrogen bonds between the amine N atom and one O atom of the nitro group of an adjacent mol-ecule. These mol-ecular chains propagate via inter-leaving of the decyl chains to form the two dimensional sheets. The sheets are associated exclusively via non-bonded contacts. The structure has features in common with those of other N-alkyl-4-nitro-anilines, but also subtle differences in packing.

Polyelectrolyte templating strategy for the fabrication of multilayer hemicyanine Langmuir-Blodgett films showing enhanced and stable second harmonic generation

Polyelectrolyte templating effectively suppresses the aggregation of cationic hemicyanine-based amphiphiles in monolayer Langmuir-Blodgett (LB) films leading to enhanced and stable optical second harmonic generation (SHG). The current study explores the impact of different polyelectrolytes (salts of poly(4-styrenesulfonic acid), deoxyribonucleic acid, and carboxymethylcellulose) on the mode of formation of multilayer LB films of the hemicyanine amphiphile and their SHG response. Pressure-area isotherms and Brewster angle microscopy reveal the impact of the polyelectrolyte complexation on the Langmuir films. Transfer ratios observed during film deposition, supported by electronic absorption spectra and atomic force microscope images of the multilayer LB films, suggest that the polyanions influence the deposition sequence, leading to significant variations in the SHG. Carboxymethylcellulose is identified as an optimal template that induces favorable z-type deposition, leading to the formation of stable multilayer films. These films exhibit the expected quadratic increase of SHG with the extent of deposition; significantly the film response is very stable under extended laser irradiation. It is proposed that structural adjustments of the sandwiched polymer layer lead to the observed deposition sequence and film stability. Polyelectrolyte templating is demonstrated to be a simple and effective strategy for the fabrication of multilayer LB films to elicit efficient quadratic nonlinear optical response.

SHG Active Crystals of a Remote Functionalized Achiral NLO-phore Assembled through Zinc(II) Complexation

An achiral nonlinear optical chromophore with a "remote functionality" that can act as a ligand is developed on the basis of 4-nitroaniline derivatized with pyridine. The molecules are assembled through complexation with simple achiral zinc(II) salts and the H-bond network mediated by the counterions, to generate noncentrosymmetric materials exhibiting optical second harmonic generation (SHG). The crystal structures of the new complexes are determined; the counterion strongly influences the ligand orientations and lattice structure. SHG of the microcrystalline materials is investigated. Correlation between the structure and SHG is rationalized using semiempirical quantum chemical estimation of the hyperpolarizabilities of molecules and molecular clusters. The metal complexation plays a significant role in molecular assembly but affects the SHG very little, enabling simplified analysis of the bulk property in terms of molecular responses. Organization of remote functionalized molecules by metal ion complexation thus offers a convenient approach to the rational design of quadratic NLO materials.

Synthesis, structural, thermal and optical studies of 1-ethyl-2,6-dimethyl-4-hydroxy pyridinium halides

1-Ethyl-2,6-dimethyl-4-hydroxy pyridinium chloride dihydrate and bromide dihydrate salts have been synthesized and their single crystals were grown by the slow evaporation of aqueous solution at 30 degrees C. The grown crystals were characterized by elemental analysis, FT-NMR and FT-IR techniques to confirm the formation of the expected compound. Optical transmittance window in aqueous solution was found to be 275-1100 nm by UV-vis-NIR technique. Thermogravimetric and differential thermal analyses reveal thermal stability and the presence of two water molecules in the crystal lattices. The crystal structure of chloride salt was also determined by X-ray diffraction method.

Chiral 1,2- and 1,3-Diol-Functionalized Chromophores as Lego Building Blocks for Coupled Structures

Chiral nitroanilines containing 1,2- or 1,3-diol functionalities have been synthesized by nucleophilic aromatic substitution of fluoronitroanilines with 1-aminopropane-2,3-diols and 2-aminopropane-1,3-diol in the melt. X-ray structure analyses confirm retention of the configuration of the chiral center. The novel chromophores are suitable to link reversibly to various substituted arylboronic acids which allows the construction of new solvatochromic sensor molecules suitable to response to solvent and anion coordination by fluoride. The solvatochromism of the new compounds has been studied using the Kamlet-Taft LSE relationship.

Influence of π-Stacking on the Resonant Enhancement of the Second-Order Nonlinear Optical Response of Dipolar Chromophores

The wavelength-dependent second-harmonic generation (SHG) efficiency of two simple dipolar chromophores, 4-NO2C6H4N(H)Bun (1) and 4-NO2C6H4SN(H)But (2), was compared in solution and in the solid state. Hyper-Rayleigh scattering measurements at 532 nm provided comparable molecular first hyperpolarizabilities. Both compounds crystallize in non-centrosymmetric space groups, but a more efficient arrangement of dipole moments results in a significantly larger deff value for 2. Kurtz-Perry experiments from 450 to 700 nm revealed an important difference in the resonant component of the nonlinear optical responses of these compounds; the SHG efficiency of crystalline 1 depends more strongly on the incident wavelength than that of 2. This would be in contradiction with the TD-DFT excitation energies calculated for these molecules, but the observation can be explained by the resonant contribution from low-energy interchromophore excitations enabled by pi-stacking in the crystal of 1.