Metal porphyrins as electrocatalysts for commercially important reactions

Transition Metal Chemistry -     

Long-lived, charge-shift states in heterometallic, porphyrin-based dendrimers formed via click chemistry

A series of multiporphyrin clusters has been synthesized and characterized in which there exists a logical gradient for either energy or electron transfer between the porphyrins. A central free-base porphyrin (FbP), for example, is equipped with peripheral zinc(II) porphyrins (ZnP) which act as ancillary light harvesters and transfer excitation energy to the FbP under visible light illumination. Additional energy-transfer steps occur at the triplet level, and the series is expanded by including magnesium(II) porphyrins and/or tin(IV) porphyrins as chromophores. Light-induced electron transfer is made possible by incorporating a gold(III) porphyrin (AuP(+)) into the array. Although interesting by themselves, these clusters serve as control compounds by which to understand the photophysical processes occurring within a three-stage dendrimer comprising an AuP(+) core, a second layer formed from four FbP units, and an outer layer containing 12 ZnP residues. Here, illumination into a peripheral ZnP leads to highly efficient electronic energy transfer to FbP, followed by charge transfer to the central AuP(+). Charge recombination within the resultant charge-shift state is intercepted by secondary hole transfer to the ZnP, which occurs with a quantum yield of around 20%. The final charge-shift state survives for some microseconds in fluid solution at room temperature.     

Panchromatic trichromophoric sensitizer for dye-sensitized solar cells using antenna effect

The first trichromophoric sensitizer, consisting of covalently linked boradiazaindacene (BODIPY), zinc porphyrin (ZnP), and squaraine (SQ) units, has been synthesized by Heck alkynylation to obtain a panchromatic dye, for dye sensitized solar cells (DSSCs). Efficient intramolecular energy transfers (ET) were observed between all chromophoric subunits and enhance the overall conversion efficiency by 25%. The antenna effect is demonstrated by the photoaction spectrum which features all of a chromophore's absorption bands.     

Nonlinear generation of sheared flows and zonal magnetic fields by electron whistlers in plasmas

The nonlinear generation of shear field and flow in whistler waves is considered. It is shown that a coherent parametric process leads to modulational instability of four waves whistler interaction. Growth rates for the flow/field are compared with published simulation results.     

Aquachlororuthenium(III) catalysis in the oxidation of substituted 4-oxo-4-arylbutanoic acids by bromate in acid medium: a kinetic and mechanistic study and validity of linear free-energy relationships

Ru(III) acts an efficient catalyst in the oxidation of substituted 4-oxo-4-arylbutanoic acids (4-oxo acids) by bromate in sulfuric acid medium, giving the corresponding benzoic acids in quantitative yields. The reaction shows first-order dependence in both [bromate] and [H₂SO₄], and a non-linear dependence on both [oxo acid] and [catalyst]. Changing solvent from H₂O to D₂O increases the rate. The rate is not affected by ionic strength but decreases with increase in dielectric constant of the medium. Electron-releasing substituents in the phenyl ring of the substrate greatly accelerate the rate, whereas the retardation by electron-withdrawing substituents, though perceptible, is small. The linear free-energy relationship is characterized by smooth curves in Hammett plots of log k versus σ; however, linear plots are obtained with excellent correlation coefficients at all the studied temperatures, when Brown’s σ⁺ values are used. The reaction constant is negative and decreases with increase in temperature. From the intersection of the lines in the Hammett and Arrhenius plots, the isokinetic relationship is evaluated. A mechanism involving a cyclic oxidant–substrate–catalyst ternary complex is proposed, in which both C–C bond-breaking and C–O bond formation are involved, and the oxidation state of Ru(III) remains unchanged. A rate law explaining all the kinetic results has been derived and verified. The reaction is an example of neighboring group participation in intramolecular catalysis and is potentially useful for the synthesis of substituted benzoic acids.     

Convenient method for the synthesis of some novel chiral methyl 2-(2-oxo-2H-benzo[e][1,3]oxazin-3(4H)-yl)propanoate derivatives and biological evaluation of their antioxidant,cytotoxic, and molecular docking properties

Ten chiral methyl 2-(2-oxo-2H-benzo[e][1,3]oxazin-3(4H)-yl)propanoate derivatives 6a-6j have been synthesized from optically pure amino methyl phenol 5 and 4-nitrophenyl chloroformate. These derivatives 6a-6j are characterized by ¹H NMR, ¹³C NMR, FT-IR, and HRMS spectral techniques. Optical purity of these derivatives was confirmed by chiral HPLC method. Ten synthesized ester derivatives 6a-6j were screened for their in vitro antioxidant activity. Among the compounds 6b-d and 6h-j have exhibited comparable antioxidant activity with ascorbic acid as a standard. Compounds 6a and 6e-g have shown moderate antioxidant activity. Further, the in vitro cytotoxicity of these compounds were studied through MTT cell proliferation assay in addition the effect on LDH leakage and NO release. Among the derivatives, 6j showed extremely best activity and the IC₅₀ value (12.54 ± 0.71 μM) is very close to doxorubicin (7.2 ± 0.58 μM) as a standard. Compounds 6b , 6h , and 6i showed better inhibition next to compound 6j on the viability of HepG2 cells with an IC₅₀ value (μM) of 56.02 ± 1.4, 41.76 ± 0.58, and 38.17 ± 0.34, respectively. Also, molecular docking studies have been carried out with STAT-3 (PDB ID: 1BG1) and BCL-2 (PDB ID: 4AQ3) proteins against the four active compounds 6b , 6h , 6i , and 6j . The binding energies of the tested compounds were in the range of −7.76 to −8.41 kcal/mol, which is very close to doxorubicin (−8.53 kcal/mol) as a standard. These molecular docking results are in good agreement with the in vitro studies.     

Charge‐Transfer State and Large First Hyperpolarizability Constant in a Highly Electronically Coupled Zinc and Gold Porphyrin Dyad

We report the synthesis and the characterizations of a novel dyad composed of a zinc porphyrin (ZnP) linked to a gold porphyrin (AuP) through an ethynyl spacer. The UV/Vis absorption spectrum and the electrochemical properties clearly reveal that this dyad exhibits a strong electronic coupling in the ground state as evidenced by shifted redox potentials and the appearance of an intense charge‐transfer band localized at λ=739 nm in dichloromethane. A spectroelectrochemical study of the dyad along with the parent homometallic system (i.e., ZnP–ZnP and AuP–AuP) was undertaken to determine the spectra of the reduced and oxidized porphyrin units. Femtosecond transient absorption spectroscopic analysis showed that the photoexcitation of the heterometallic dyad leads to an ultrafast formation of a charge‐separated state (⁺ZnP–AuP^.) that displays a particularly long lifetime (τ=4 ns in toluene) for such a short separation distance. The molecular orbitals of the dyad were determined by DFT quantum‐chemical calculations. This theoretical study confirms that the observed intense band at λ=739 nm corresponds to an interporphyrin charge‐transfer transition from the HOMO orbital localized on the zinc porphyrin to LUMO orbitals localized on the gold porphyrin. Finally, a Hyper–Rayleigh scattering study shows that the dyad possesses a large first molecular hyperpolarizability coefficient (β=2100×10^?30 esu at λ=1064 nm), thus highlighting the valuable nonlinear optical properties of this new type of push–pull porphyrin system.     

Self-assembled and electrochemically deposited mono/multilayers for molecular electronics applications

For the development of molecular electronics, it is desirable to investigate characteristics of organic molecules with electronic device functionalities. In near future, such molecular devices could be integrated with silicon to prepare hybrid nanoelectronic devices. In this paper, we review work done in our laboratory on study of characteristics of some functional molecules. For these studies molecular mono and multilayers have been deposited on silicon surface by self-assembly and electrochemical deposition techniques. Both commercially available and specially designed and synthesized molecules have been utilized for these investigations. We demonstrate dielectric layers, memory, switching, rectifier and negative differential resistance devices based on molecular mono and multilayers.     

Repetitive operation of a helium-free mini TEA CO2 laser

A helium-free mini TEA CO₂ laser has been operated at a repetition rate of 100 Hz wherein the gas re-circulatory loop consisting of heat exchanger and catalytic converter, mandatory for conventional repetitive operation of such lasers, have been totally dispensed with. The laser utilized inexpensive molecular gases alone that were simply flown out transversely through the inter-electrode region by an open gas flow system. The active medium was energized by making use of a spiker-sustainer excitation scheme. At this repetition rate, the laser produced an average power of 3.50 W.