Woormileela Sinha

Experimental and Theoretical Investigations of the Existence of Cu(II), Cu(III), and Cu(IV) in Copper Corrolato Complexes.

The most common oxidation states of copper in stable complexes are +I and +II. Cu(III) complexes are often considered as intermediates in biological and homogeneous catalysis. More recently, Cu(IV) species have been postulated as possible intermediates in oxidation catalysis. Despite the importance of these higher oxidation states of copper, spectroscopic data for these oxidation states remain scarce, with such information on Cu(IV) complexes being non-existent. We herein present the synthesis and characterization of three copper corrolato complexes. A combination of electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, XANES measurements, and DFT calculations points to existence of three distinct redox states in these molecules for which the oxidation states +II, +III, and +IV can be invoked for the copper centers. The present results thus represent the first spectroscopic and theoretical investigation of a Cu(IV) species, and describe a redox series where Cu(II), Cu(III), and Cu(IV) are discussed within the same molecular platform.

Silver corrole complexes: unusual oxidation states and near-IR-absorbing dyes.

Macrocycles such as porphyrins and corroles have important functions in chemistry and biology, including light absorption for photosynthesis. Generation of near-IR (NIR)-absorbing dyes based on metal complexes of these macrocycles for mimicking natural photosynthesis still remains a challenging task. Herein, the syntheses of four new Ag(III) corrolato complexes with differently substituted corrolato ligands are presented. A combination of structural, electrochemical, UV/Vis/NIR-EPR spectroelectrochemical, and DFT studies was used to decipher the geometric and electronic properties of these complexes in their various redox states. This combined approach established the neutral compounds as stable Ag(III) complexes, and the one-electron reduced species of all the compounds as unusual, stable Ag(II) complexes. The one-electron oxidized forms of two of the complexes display absorptions in the NIR region, and thus they are rare examples of mononuclear complexes of corroles that absorb in the NIR region. The appearance of this NIR band, which has mixed intraligand charge transfer/intraligand character, is strongly dependent on the substituents of the corrole rings. Hence, the present work revolves round the design principles for the generation of corrole-based NIR-absorbing dyes and shows the potential of corroles for stabilizing unusual metal oxidation states. These findings thus further contribute to the generation of functional metal complexes based on such macrocyclic ligands.

A comparative study of optical nonlinearities of trans-A2B-corroles in solution and in aggregated state

A series of novel A3-corrole and trans-A2B-corroles have been synthesized with the aim of developing organic materials with improved nonlinear optical (NLO) properties. All three newly synthesized corroles have been characterized by various spectroscopic techniques including single crystal X-ray structural analysis of the representative one. The crystal structure analysis of 10-(4-hydroxyphenyl)-5,15-bis(2-bromo-5-fluorophenyl) corrole shows several O–H⋯N interactions. The self aggregates of all three corroles were prepared on a silicon wafer as well as on a quartz substrate by using a drop-casting method in a dichloromethane and methanol (1 : 2) solvent mixture. In all three free base corroles, well defined and nicely organized three-dimensional objects with diameter of ca. 320 nm (nanospheres), 450 nm (nanobulbs), and 120 nm (nanodiscs) were obtained. The NLO properties (nonlinear refractive index, n2 and two-photon absorption coefficient, β) of all of the corrole derivatives in toluene solution and in aggregated form were measured by the Z-scan technique. The nonlinear refractive indices, n2 of the free base corroles (in toluene solution) were found out to be −16.8 × 10−18 m2 W−1, −7.8 × 10−18 m2 W−1 and −25.9 × 10−18 m2 W−1 respectively and for the corresponding aggregates (nanoparticles of the free base corroles), it was found out to be −1.1 × 10−15 m2 W−1, −1.9 × 10−15 m2 W−1, and 71.8 × 10−15 m2 W−1 respectively. Similarly, the two-photon absorption coefficient, β of all the synthesized free base corroles (in toluene solution) were found out to be 5.7 × 10−15 m W−1, 1.9 × 10−15 m W−1 and 17.2 × 10−15 m W−1 respectively and for the corresponding aggregates (nanoparticles), the values were 4.0 × 10−13 m W−1, 2.0 × 10−13 m W−1, and 444.0 × 10−13 m W−1 respectively. These NLO properties of the free base corrole derivatives (in solution and in aggregates) have been explored with a specific aim to identify the possibility of their applications in ultrafast switching devices for use in high-speed fiber-optic communications and photonic integrated circuits.

Synthesis, electron transport, and charge storage properties of fullerene–zinc porphyrin hybrid nanodiscs

A donor (D)–acceptor (A) based system has been developed with the help of a newly synthesized Zn–porphyrin complex and C60 fullerene. A series of spectroscopic methods, absorption spectroscopy (UV-Vis-NIR), powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements, confirm the existence of a charge transfer based complex between Zn–porphyrin and C60 fullerene. By using this D–A based system, circular disc-like nano objects have been generated. With the help of these nano objects, a diode has been fabricated. Current–voltage (I–V) and the capacitance–voltage (C–V) analyses of the diode were performed. The critical role of the Ag–C60–Zn–porphyrin and C60–Zn–porphyrin–SiOx interfaces are clearly understood from the I–V characteristics of the present device. Existence of a hysteresis loop in the C–V plot indicates that the device is indeed suitable for charge storage applications.

Chromium Complexes with Oxido and Corrolato Ligands: Metal-Based Redox Processes versus Ligand Non-Innocence

Metal- versus ligand-centered redox processes and the effects of substituents on the ligands on the spectroscopic properties of the metal complexes are at the heart of research on metal complexes with non-innocent ligands. This work presents three examples of chromium complexes that contain both oxido and corrolato ligands, with the substituents on the corrolato ligands being different in the three cases. Combined X-ray crystallographic, electrochemical, UV/Vis/NIR/EPR spectroelectrochemical, and EXAFS/XANES measurements, together with DFT calculations, have been used to probe the complexes in three different redox forms. This combined approach makes it possible to address questions related to chromium- versus corrolato-centered redox processes, and the accessibility (or not) of CrIV , CrV , and CrVI in these complexes, as well as their spin states. To the best of our knowledge, these are the first EXAFS/XANES investigations on Cr-corrolato complexes in different redox forms, and hence these data should set benchmarks for future investigations on such complexes by this method.