Debabrata Sengupta

Redox Non‐Innocence of Coordinated 2‐(Arylazo) Pyridines in Iridium Complexes: Characterization of Redox Series and an Insight into Voltage‐Induced Current Characteristics

Two examples of a rare class of di-radical azo-anion complexes of 2-(arylazo) pyridine with Ir(III) carrier are introduced. Their electronic structures have been elucidated using a host of physical methods that include X-ray crystallography, cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory. Room temperature magnetic moments of these are consistent with two nearly non-interacting azo-anion radicals. These displayed rich electrochemical properties consisting of six numbers of reversible and successive one electron CV-waves. Redox processes occur entirely at the coordinated ligands without affecting metal redox state. Apart from reporting their chemical characterization, I-V characteristics of these complexes in film state are investigated using sandwich-type devices comprising of a thin film of 100-125 nm thickness placed between two gold-plated ITO electrodes. These showed memory switching properties covering a useful voltage range with a reasonable ON/OFF ratio and also are suitable for RAM/ROM applications. I-V characteristics of two similar complexes of Rh and Cr with identical ligand environment and electronic structure are also referred for developing an insight into the memory switching ability of Ir- and Rh- complexes on the basis of comparative analysis of responses of the respective systems. In a nutshell, thorough analysis of voltage driven redox dynamics and corresponding solid and solution state current responses of all the systems are attempted and there from an unexplored class of switching devices are systematically introduced.

Robust resistive memory devices using solution-processable metal-coordinated azo aromatics

Non-volatile memories will play a decisive role in the next generation of digital technology. Flash memories are currently the key player in the field, yet they fail to meet the commercial demands of scalability and endurance. Resistive memory devices, and in particular memories based on low-cost, solution-processable and chemically tunable organic materials, are promising alternatives explored by the industry. However, to date, they have been lacking the performance and mechanistic understanding required for commercial translation. Here we report a resistive memory device based on a spin-coated active layer of a transition-metal complex, which shows high reproducibility (∼350 devices), fast switching (≤30 ns), excellent endurance (∼1012 cycles), stability (>106 s) and scalability (down to ∼60 nm2). In situ Raman and ultraviolet-visible spectroscopy alongside spectroelectrochemistry and quantum chemical calculations demonstrate that the redox state of the ligands determines the switching states of the device whereas the counterions control the hysteresis. This insight may accelerate the technological deployment of organic resistive memories.

Ligand-Centered Redox in Nickel(II) Complexes of 2-(Arylazo)pyridine and Isolation of 2-Pyridyl-Substituted Triaryl Hydrazines via Catalytic N-Arylation of Azo-Function

A series of nickel complexes of 2-(arylazo)pyridine have been synthesized, and the precise structure and stoichiometry of the complexes are controlled by the use of different metal precursors. Molecular and electronic structures of the isolated complexes are scrutinized thoroughly by various spectroscopic techniques, single crystal X-ray crystallography, and density functional theory (DFT). Two different classes of Ni(II) complexes are identified where the ligands bind as neutral or anion radicals in the respective metal complexes. These are shown to be chemically interconvertible, and their characterization confirmed that the redox series is entirely ligand-centered without affecting the bivalent oxidation state of the metal ion. An efficient method of Ni(II) catalyzed N-arylation of 2-(arylazo)pyridine substrates has been elaborated. The chemical reactions have led to isolation of strongly fluorescent 2-pyridyl-substituted hydrazine derivatives, which have been characterized thoroughly. Three-dimensional X-ray structure of a hydrazine molecule, 2-(2-(naphthalen-1-yl)-2-phenylhydrazinyl)pyridine, is reported. Isolated hydrazines satisfy all the prerequisites of an ideal dye with moderate absorptive property, large Stokes shift, high quantum yields, and high photostability.

Regioselective ortho Amination of Coordinated 2-(Arylazo)pyridine. Isolation of Monoradical Palladium Complexes of a New Series of Azo-Aromatic Pincer Ligands.

In an unusual reaction of [Pd(L(1))Cl2] (L(1) = 2-(arylazo)pyridine) with amines, a new series of palladium complexes [Pd(L(2•-))Cl] (L(2) = 2-((2-amino)arylazo)pyridine) (1a-1h) were isolated. The complexes were formed via N-H and N-C bond cleavage reactions of 1°/2° and 3° amines, respectively, followed by regioselective aromatic ortho-C-N bond formation reaction and are associated with ortho-C-H/ortho-C-Cl bond activation. A large variety of amines including both aromatic and aliphatic were found to be effective in producing air-stable complexes. Identity of the resultant complexes was confirmed by their X-ray structure determination. Efforts were also made to understand the mechanism of the reaction. A series of experiments were performed, which point toward initial ligand reduction followed by intraligand electron transfer. Examination of the structural parameters of these complexes (1) indicates that the in situ generated ligand coordinated to the Pd(II) center serves as the backbone of these air-stable monoradical complexes. Molecular and electronic structures of the isolated complexes were further scrutinized by various spectroscopic techniques including cyclic voltammetry, variable temperature magnetic susceptibility measurements, electron paramagnetic resonance, and UV-vis spectroscopy. Finally the electronic structure was confirmed by density functional theory calculations. The isolated monoradical complexes adopt an unusual π-stacked array, which leads to a relatively strong antiferromagnetic interaction (J = -40 cm(-1) for the representative complex 1c).

Pure white light emission from a rare earth-free intrinsic metal–organic framework and its application in a WLED

Metal–organic frameworks are a class of porous materials where the metal centre is attached to other anionic or neutral ligands for extension into 1D, 2D or 3D framework. Since the discovery of light-emitting metal–organic frameworks (MOFs), many efforts have been made to achieve white light emission.To date, most of the white light-emitting MOFs that are available in the literature are based on rare earth metal doping or encapsulation of some luminescent dye molecules or other inorganic complexes within the pores of MOF, such that the collective luminescence covers the whole visible region. However, white light emission from a non rare earth-based single MOF is still unexplored and desirable because of low power consumption, lower manufacturing costs, and environmental safety purposes. In the present study, a zinc-based MOF was synthesized, which showed white light emission with a CIE index of (0.31, 0.33) upon excitation at 260 nm, and the corresponding quantum yield was about 32.5%. The white emission arises due to three peaks, among which two peaks at 384 nm and 468 nm originate from π–π* and n–π* transitions of N3-ipa, respectively, and the new peak at 570 nm is due to the charge transfer phenomenon from the pyridine moiety to the linker. DFT calculations were carried out to estimate the energy levels of the MOF, and all the peaks in PL spectra were well explained using the DFT study. Finally, a light-emitting diode was fabricated using the MOF as the active material, which showed white electroluminescence spectra. An energy band diagram based on the DFT calculation is also presented to understand the origin of white photoluminescence and electroluminescence from this MOF-based light-emitting diode.

Ligand Redox-Controlled Tandem Synthesis of Azines from Aromatic Alcohols and Hydrazine in Air: One-Pot Synthesis of Phthalazine

A controlled tandem synthetic route to azines from various alcohols and hydrazine hydrate by the use of a Ni(II) complex of 2,6-bis(phenylazo)pyridine as a catalyst is reported. In marked contrast to the previous report, the reaction is operative using an earth-abundant metal catalyst, milder reaction conditions, and aerobic conditions, which though are desirable but unprecedented in the literature. The catalytic reaction has a vast substrate scope including a single-step synthesis of phthalazine from 1,2-benzenedimethanol and hydrazine hydrate via intramolecular coupling. Mechanistic investigation suggests that the coordinated ligand redox controls the reaction by the use of a reversible azo (N═N)/ hydrazo (NH-NH) redox couple where the metal center is used primarily as a template.

An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex

Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc(II) complex of the ligand 2,6-bis(phenylazo)pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redox-inactive metal ion Zn(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl2 (1a) along with the two hydrogenated complexes of 1a, ZnH2LCl2 (1b), and ZnH4LCl2 (1c) are reported. It has been argued that the reversible azo-hydrazo redox couple of 1a controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O2 and para-benzoquinone to H2O2 and para-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates.

Corrigendum: Robust resistive memory devices using solution-processable metal-coordinated azo aromatics

This corrects the article DOI: 10.1038/nmat5009.