Density Functional Theory Prediction of the Electrocatalytic Mechanism of Proton Reduction by a Dicobalt Tetrakis(Schiff Base) Macrocycle.

A dicobalt tetrakis(Schiff base) macrocycle has recently been reported to electrochemically catalyze the reduction of H to H in an acetonitrile solution. Density functional theory (DFT) calculations using the ωB97X-D functional are shown to produce structural and thermodynamic results in good agreement with the experimental data. A mechanistic model based on thermodynamics is developed that incorporates electrochemical and magnetic details of the complex, accounting for electron-spin reorganization of the metal center after redox steps.

Electrochemical Rectification of Redox Mediators Using Porphyrin-Based Molecular Multilayered Films on ITO Electrodes.

Electrochemical charge transfer through multilayer thin films of zinc and nickel 5,10,15,20-tetra(4-ethynylphenyl) porphyrin constructed via copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) "click" chemistry was examined. Current rectification toward various outer-sphere redox probes is revealed with increasing numbers of layers, as these films possess insulating properties over the neutral potential range of the porphyrin, then become conductive upon reaching its oxidation potential. Interfacial electron transfer rates of mediator-dye interactions toward [Co(bpy)3](2+), [Co(dmb)3](2+), [Co(NO2-phen)3](2+), [Fe(bpy)3](2+), and ferrocene (Fc), all outer-sphere redox species, were measured by hydrodynamic methods.

Temperature-dependent and bistable current-voltage measurements in zinc porphyrin molecular junctions.

We report bistability in current-voltage curves from di(PEP)PorZn in an electromigrated molecular junction. Bistability was observed at ±0.3 V at 300 K but did not occur at 4 K.

Synthesis, Characterization, and Fluorescence Properties of Mixed Molecular Multilayer Films of BODIPY and Zn(II) Tetraphenylporphyrins.

A new azido functionalized 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) has been synthesized to achieve spectral complementarity to a Zn(II) tetraphenylethynyl porphyrin (ZnTPEP). Mixed multilayer films were assembled on glass and quartz up to 10 bilayers thick in a layer-by-layer (LbL) fabrication process using copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) to couple these two dyes together with a tris-azido linker. By varying the amount of BODIPY in the CuAAC reaction solutions for the azido linker layers, we achieve tunable doping of BODIPY within the porphyrin films.

Electrocatalytic proton reduction by a dicobalt tetrakis-Schiff base macrocycle in nonaqueous electrolyte.

A series of dicobalt complexes, Co2L(2+) and Co2LAc(+), where L is a N6O2 coordinating bis(phenolate) tetrakis-Schiff base ligand, have been synthesized and characterized via electrochemical and spectroscopic techniques. [Co2LAc](ClO4) crystallizes in the monoclinic space group P21/n, and the structure reveals a highly distorted octahedral geometry for the Co(II) ions, which are bridged by an acetate with a Co-Co distance of 3.2 Å.

Structural, electrochemical, and spectroscopic investigation of acetate bridged dinuclear tetrakis-Schiff base macrocycles of Mn and Zn.

The synthesis of Mn2LAc+, Zn2LAc+, and H4L2+ is described, where L is a tetrakis-Schiff base macrocycle formed using 4-tert-butyl-2,6-diformylphenol and 2,2′-diamino-N-methyldiethylamine resulting in an N6O2 coordination environment. In Mn2LAc+ and Zn2LAc+, the two metal centers are bridged by an acetate ligand. [Mn2LAc](ClO4)·(DMF)0.

Photocurrent enhancement by multilayered porphyrin sensitizers in a photoelectrochemical cell.

Multilayer Zn(II) tetraphenylporphyrin chromophores, assembled using copper-catalyzed azide-alkyne cycloaddition (CuAAC), provide a new sensitization scheme that could be useful in dye-sensitized solar cells (DSSCs). We report on the photoelectrochemical responses of multilayer films of Zn(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin (1) assembled on planar ITO substrates operating as a p-type DSSC using three different redox mediators. The traditional I(-)/I3(-) redox couple results in the greatest short circuit current densities (JSC) but very low open circuit potentials (VOC).

Efficient Förster resonance energy transfer in 1,2,3-triazole linked BODIPY-Zn(II) meso-tetraphenylporphyrin donor-acceptor arrays.

Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC) reactivity was successfully employed to synthesize three donor-acceptor energy transfer (EnT) arrays that contain one (Dyad), three (Tetrad) and four (Pentad) 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) donors connected to a Zn-tetraphenylporphyrin acceptor via 1,2,3-triazole linkages. The photophysical properties of the three arrays, along with individual donor and acceptor chromophores, were investigated by UV-vis absorption and emission spectroscopy, fluorescence lifetimes, and density functional theory (DFT) electronic structure modeling. Comparison of the UV-vis absorption spectra and frontier molecular orbitals from DFT calculations of the three arrays with ZnTPP, ZnTTrzlP, and Trzl-BODIPY shows that the electronic structure of the chromophores is essentially unperturbed by the 1,2,3-triazole linkage.

Structural analysis of porphyrin multilayer films on ITO assembled using copper(I)-catalyzed azide-alkyne cycloaddition by ATR IR.

We report the use of grazing-angle attenuated total reflectance (GATR) IR and polarized UV-vis to determine the molecular structure of porphyrin based molecular multilayer films grown in a layer-by-layer (LbL) fashion using copper-catalyzed azide-alkyne cycloaddition (CuAAC). The molecular orientation and bonding motif present in multilayer films of this type could impact their photophysical and electrochemical properties as well as potential applications. Multilayer films of M(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin (1 M = Zn, 2 M = Cu) and azido based linkers 3-5 were used to fabricate the films on ITO substrates.

Thickness, surface morphology, and optical properties of porphyrin multilayer thin films assembled on Si(100) using copper(I)-catalyzed azide-alkyne cycloaddition.

We report the structure, optical properties and surface morphology of Si(100) supported molecular multilayers resulting from a layer-by-layer (LbL) fabrication method utilizing copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), also known as "click" chemistry. Molecular based multilayer films comprised of 5,10,15,20-tetra(4-ethynylphenyl)porphyrinzinc(II) (1) and either 1,3,5-tris(azidomethyl)benzene (2) or 4,4'-diazido-2,2'-stilbenedisulfonic acid disodium salt (3) as a linker layer, displayed linear growth properties up to 19 bilayers. With a high degree of linearity, specular X-ray reflectivity (XRR) measurements yield an average thickness of 1.

A versatile molecular layer-by-layer thin film fabrication technique utilizing copper(I)-catalyzed azide-alkyne cycloaddition.

We have developed a rapid and versatile layer-by-layer (LbL) thin film fabrication method using copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) or "click" chemistry in the construction of molecular multilayer assemblies. Multilayers containing synthetic porphyrins, perylene diimides, and mixtures of the two have been constructed in order to highlight the versatility of this method. Characterization of thin films using UV-vis absorption, water contact angle, and electrochemical techniques indicate that multilayer growth is consistent over tens of layers.

A new class of mixed-valence systems with orbitally degenerate organic redox centers. Examples based on hexa-rhenium molecular prisms.

The ligand-centered mixed-valence (LCMV) properties of two supramolecular complexes are reported: triangular prisms of the form ([Re(CO)3]2X)3-mu,mu',mu' '-[tPyTz]2, where X is 2,2'-bisbenzimidazolate (1) or a pair of benzylthiols (2), and tPyTz is tri-(4-pyridyl)-1,3,5-triazine. Cyclic voltammetry demonstrates that the redox-accessible bridging ligands, tPyTz, are reduced in sequential, one-electron reactions. The singly reduced prisms, which represent an unusual type of mixed-valence compound in which the tPyTz ligands themselves are the redox centers, show intense, broad intervalance transfer (IT) bands in the NIR, consistent with highly coupled MV species.

Active-site models of bacterial nitric oxide reductase featuring tris-histidyl and glutamic acid mimics: influence of a carboxylate ligand on Fe(B) binding and the heme Fe/Fe(B) redox potential.

Active-site models of bacterial nitric oxide reductase (NOR) featuring a heme Fe and a trisimidazole- and glutaric acid-bound non-heme Fe (Fe(B)) have been synthesized. These models closely replicate the proposed active site of native NORs. Examination of these models shows that the glutamic acid mimic is required for both Fe(B) retention in the distal binding site and proper modulation of the redox potentials of both the heme and non-heme Fe's.

Underlying spin-orbit coupling structure of intervalence charge transfer bands in dinuclear polypyridyl complexes of ruthenium and osmium.

The mixed-valence systems meso- and rac-[{M(bpy)2}2(mu-BL)]5+ {M = Ru, Os; BL = a series of polypyridyl bridging ligands such as 2,3-bis(2-pyridyl)benzoquinoxaline (dpb)} are characterized by multiple intervalence charge transfer (IVCT) and interconfigurational (IC) bands in the mid-infrared and near-infrared (NIR) regions. Differences in the relative energies of the IC transitions for the fully oxidized (+6) states of the osmium systems demonstrate that stereochemical effects lead to fundamental changes in the energy levels of the metal-based dpi orbitals, which are split by spin-orbit coupling and ligand-field asymmetry. An increase in the separation between the IC bands as BL is varied reflects the increase in the degree of electronic coupling through the series of ruthenium and osmium complexes.

Efficient synthesis of trisimidazole and glutaric acid bearing porphyrins: ligands for active-site models of bacterial nitric oxide reductase.

Ligands (1) for active-site models of bacterial nitric oxide reductase (NOR) have been efficiently synthesized. These compounds (1) feature three imidazolyl moieties and one carboxylic acid residue at the FeB site, which represent the closest available synthetic model ligands of NOR active center. The stereo conformations of these ligands are established on the basis of steric effects and 1H NMR chemical shifts under the ring current effect of the porphyrin.

Selective functionalization of independently addressed microelectrodes by electrochemical activation and deactivation of a coupling catalyst.

We demonstrate selective functionalization of independently addressed microelectrodes by electrochemical activation and deactivation of a coupling catalyst. 1,2,3-Triazole formation between terminal acetylenes and organic azides is efficiently catalyzed by copper(I) complexes (a Sharpless "click" reaction), while the oxidized copper(II) complexes are inactive. By electrochemically activating or deactivating the catalyst by switching its redox state, we demonstrate control over triazole formation between surface-immobilized azides and ethynylferrocene.

C- and Z-shaped coordination compounds. synthesis, structure, and spectroelectrochemistry of cis- and trans-[Re(CO)3(L)]2-2,2'-bisbenzimidizolate with L = 4-phenylpyridine, 2,4'-bipyridine, or pyridine.

A series of C- and Z-shaped complexes of the form cis- and trans-[Re(CO)3(L)]2BiBzIm, where L = 4-phenylpyridine, 2,4'-bipyridine, or pyridine and BiBzIm = 2,2'-bisbenzimidizolate, have been synthesized by the reaction of [Re(CO)4]2BiBzIm with a slight excess of L in refluxing tetrahydrofuran. Five of the six compounds have been isolated and crystallographically and electrochemically characterized. Formation of the sixth, the cis form of the [Re(CO)3(4-phenylpyridine)]2BiBzIm, is evidently inhibited by the torsional steric demands of proximal 4-phenylpyridines.

Borderline class II/III ligand-centered mixed valency in a porphyrinic molecular rectangle.

A molecular rectangle of the form ([Re(CO)(3)](2)BiBzIm)(2)-mu,mu'-(LL)(2), where BiBzIm is 2,2'-bisbenzimidazolate and LL are cofacially aligned [5,15-bis(4-ethynylpyridyl)-10,20-bis(n-hexyl)-porphyrinato]zinc ligands, has been examined via electrochemical, spectroelectrochemical, and electronic Stark effect methods. The rectangle displays three electrochemically accessible reductions assigned as LL based. The singly reduced rectangles are part of an unusual class of mixed-valence complexes where cofacial ligands, in this case porphyrins, comprise the degenerate redox centers.

Tetra-rhenium molecular rectangles as organizational motifs for the investigation of ligand-centered mixed valency: three examples of full delocalization.

Molecular rectangles having the form ([Re(CO)3]2(X)2)(2)-mu,mu'-(LL)2, where X is either a bridging alkoxide or phenylthiolate group and LL is 4,4'-bipyridine or pyrazine, are characterized by cofacial LL pairs that are in van der Waals contact across the "long" side of the rectangle. Cyclic voltammetry shows that the redox-accessible bridging ligands, LL, are reduced in sequential, one-electron reactions. The singly reduced rectangles represent an unusual type of mixed-valence compound in which the LL ligands themselves are the redox centers.

Rhenium-based molecular rectangles as frameworks for ligand-centered mixed valency and optical electron transfer.

A series of six neutral, tetrametallic, molecular rectangles has been synthesized that have the form ([Re(CO)(3)](2)BiBzIm)(2)-mu,mu'-(LL)(2), where BiBzIm is 2,2'-bisbenzimidazolate and LL is a reducible, dipyridyl or diazine ligand. X-ray crystallographic studies of the six show that the rectangle frameworks, as defined by the metal atoms, range in size from 5.7 A x 7.

Shape-selective transport through rectangle-based molecular materials: thin-film scanning electrochemical microscopy studies.

Microporous thin films (approximately equal to 50 to 400 nm) composed of discrete, cavity-containing molecular rectangles have been prepared. The films, which contain both amorphous and microcrystalline domains, display shape-selective transport behavior. They are permeable to small molecules and to molecules that are short or narrow in at least one dimension--for example, elongated planar molecules--but are impermeable to molecules lacking a narrow dimension.