53 research outputs found
Comparison of Electrochemically and Photochemically Induced Electron-Transfer Processes of a Series of Copper(II) Schiff Base Complexes with Thiolate Coordination
Comparison of Electrochemically and Photochemically Induced Electron-Transfer Processes of a Series of Copper(II) Schiff Base Complexes with Thiolate Coordination
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Structural variability of 4f and 5f thiocyanate complexes and dissociation of uranium(III)āthiocyanate bonds with increased ionicity
A series of complexes [Et4N][Ln(NCS)4(H2O)4] (Ln = Pr, Tb, Dy, Ho, Yb) have been structurally characterized, all showing the same structure, namely a distorted square antiprismatic coordination geometry, and the LnāO and LnāN bond lengths following the expected lanthanide contraction. When the counterion is Cs+, a different structural motif is observed and the eight-coordinate complex Cs5[Nd(NCS)8] isolated. The thorium compounds [Me4N]4[Th(NCS)7(NO3)] and [Me4N]4[Th(NCS)6(NO3)2] have been characterized, and high coordination numbers are also observed. Finally, attempts to synthesize a U(III) thiocyanate compound has been unsuccessful; from the reaction mixture, a heterocycle formed by condensation of five MeCN solvent molecules, possibly promoted by U(III), was isolated and structurally characterized. To rationalize the inability to isolate U(III) thiocyanate compounds, thin-layer cyclic voltammetry and IR spectroelectrochemistry have been utilized to explore the cathodic behavior of [Et4N]4[U(NCS)8] and [Et4N][U(NCS)5(bipy)2] along with a related uranyl compound [Et4N]3[UO2(NCS)5]. In all examples, the reduction triggers a rapid dissociation of [NCS]ā ions and decomposition. Interestingly, the oxidation chemistry of [Et4N]3[UO2(NCS)5] in the presence of bipy gives the U(IV) compound [Et4N]4[U(NCS)8], an unusual example of a ligand-based oxidation triggering a metal-based reduction. The experimental results have been augmented by a computational investigation, concluding that the U(III)āNCS bond is more ionic than the U(IV)āNCS bond
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[M(CO)4(2,2ā²-bipyridine)] (M = Cr, Mo, W) as efficient catalysts for electrochemical reduction of CO2 to CO at a gold electrode
Groupā
6 complexes of the type [M(CO)4(bpy)] (M=Cr, Mo, W) are capable of behaving as electrochemical catalysts for the reduction of CO2 at potentials less negative than those for the reduction of the radical anions [M(CO)4(bpy)].ā. Cyclic voltammetric, chronoamperometric and UV/Vis/IR spectro-electrochemical data reveal that five-coordinate [M(CO)3(bpy)]2ā are the active catalysts. The catalytic conversion is significantly more efficient in N-methyl-2-pyrrolidone (NMP) compared to tetrahydrofuran, which may reflect easier CO dissociation from 1eā-reduced [M(CO)4(bpy)].ā in the former solvent, followed by second electron transfer. The catalytic cycle may also involve [M(CO)4(H-bpy)]ā formed by protonation of [M(CO)3(bpy)]2ā, especially in NMP. The strongly enhanced catalysis using an Au working electrode is remarkable, suggesting that surface interactions may play an important role, too
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Accurate description of low-lying excited states in a series of photoreactive clusters [Os3(CO)10(Ī±-diimine)] by DFT calculations
Density functional theory (DFT) calculations were performed on the clusters [Os3(CO)10(Ī±-diimine)], for Ī±-diimine = 2,2'-bipyridine (BPY), N-isopropyl 2-iminomethylpyridine (IMP), and N,N'-diisopropyl-l,4-diaza-1,3-butadiene (DAB), together with their spectroscopic study. This important family of clusters is known to convert upon irradiation with visible light into short-lived biradicals and long-lived zwitterions from a Ļ* (SBLCT) excited state that, however, has not been described accurately thus far by quantum mechanical calculations. Based on the combined DFT, UV-vis absorption and resonance Raman data, the lowest-lying visible absorption band is assigned to a Ļ(Os1āOs3)-to-*(Ī±-diimine) CT transition for Ī±-diimine = bpy and IMP, and to a strongly delocalized Ļ(Os1āOs3)*-to-Ļ*(Os1āOs3)* Ā¬transition for conjugated non-aromatic Ī±-diimine = DAB. The DFT calculations rationalize the experimentally determined characteristics of this electronic transition in the studied series: (i) the corresponding absorption band is the dominant feature in the visible spectral region, (ii) the CT character of the electronic excitation declines from Ī±-diimine = bpy to IMP and vanishes for DAB, (iii) the excitation energies decrease in the order Ī±-diimine = DAB > BPY > IMP, (iv) the oscillator strength shrinks in the order Ī±-diimine = DAB > IMP > BPY. Reference photoreaction quantum yields measured accurately for the formation of a cluster zwitterion from [Os3(CO)10(IMP)] in strongly coordinating pyridine, demonstrate that the optical population of the lowest-energy 1Ļ* and relaxed 3Ļ* excited states in the DFT model scheme is still capable of inducing the initial homolytic Os1āOs3 Ļ-bond splitting, although less efficiently than the optical excitation into neighbor higher-lying electronic transitions due to a higher potential barrier for the reaction from a dissociative (ĻĻ*) state
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Mononuclear piano-stool iron 2-ethynylbenzo[b]thiophene complex: crystal structure and reversible oxidation studied by spectro-electrochemical and DFT methods
A mononuclear iron complex with 2-ethynylbenzo[b]thiophene C-coordinated to the
(Ī·5
-Cp*)(Ī·2
-dppe)Fe ( Cp* = pentamethylcyclopentadienyl, dppe = 1,2-diphenylphosphinoethane)
framework (1) was successfully prepared and characterized by 1H NMR, elemental analysis and single
crystal X-ray diffraction. The redox behavior of complex 1 was investigated by voltammetric methods and
anodic spectroelectrochemistry in the UV-vis-NIR-IR region and compared with reference complexes
including 2-ferrocenylbenzo[b]thiophene (2) and the 2-ethynylpyridine derivative of 1. The spin density
distribution along the linear molecular backbone in stable 1
+
was analyzed by DFT (BLYP35) and TDDFT
calculations of a truncated model complex. The combined experimental and theoretical results have
revealed an important role of the ethynylene linker in determining the redox properties of this family of
complexes and a sizable participation the 2-ethynylbenzo[b]thiophene framework in the largely iron-based
anodic electron transfe
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Excited-state electronic asymmetry prevents photoswitching in terthiophene compounds
The diarylethene moiety is one of the most extensively used switches in the field of molecular electronics. Here we report on spectroscopic and quantum chemical studies of two diarylethene-based compounds with a non-C3-symmetric triethynyl terthiophene core symmetrically substituted with RuCp*(dppe) or trimethylsilyl termini. The ethynyl linkers are strong IR markers that we use in time-resolved vibrational spectroscopic studies to get insight into the character and dynamics of the electronically excited states of these compounds on the picosecond to nanosecond time scale. In combination with electronic transient absorption studies and DFT calculations, our studies show that the conjugation of the non-C3-symmetric triethynyl terthiophene system in the excited state strongly affects one of the thiophene rings involved in the ring closure. As a result, cyclization of the otherwise photochromic 3,3ā³-dimethyl-2,2ā²:3ā²,2ā³-terthiophene core is inhibited. Instead, the photoexcited compounds undergo intersystem crossing to a long-lived triplet excited state from which they convert back to the ground state
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Conjugated, rod-like viologen oligomers: correlation of oligomer length with conductivity and photoconductivity
An iterative synthesis has been used to produce conjugated, monodisperse, viologen-based aromatic oligomers containing up to 12 aromatic/heterocyclic rings. The methoxy-substituted oligomers were soluble in common organic solvents and could be processed by spin coating. The conductivities of the resulting films (30 to 221 nm thick) increased by more than an order of magnitude as the oligomer length increased from unimer (1, 2.20Ć10-11 S cm-1) through dimer (2) to trimer (3, 6.87Ć10-10 S cm-1). The bandgaps of the materials were estimated from the absorption spectra of these thin films. The longest oligomer, 3, exhibited a noticeably narrower bandgap (2.3 eV) than the shorter oligomers (1 and 2 both 2.7 eV). Oligomer 3 also showed photoconductivity under irradiation across a wide range of wavelengths in the visible spectral region. In conjunction with DFT calculations of these systems our results suggest that structurally related viologen-type oligomers may find use in optoelectronic devices
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DNA cleavage and antitumour activity of platinum(II) and copper(II) compounds derived from 4-methyl-2-N-(2-pyridylmethyl)aminophenol: spectroscopic, electrochemical and biological investigation
The reaction of the redox-active ligand, Hpyramol (4-methyl-2-N-(2-pyridylmethyl)aminophenol) with K2PtCl4 yields monofunctional square-planar [Pt(pyrimol)Cl], PtL-Cl, which was structurally characterised by single-crystal X-ray diffraction and NMR spectroscopy. This compound unexpectedly cleaves supercoiled double-stranded DNA stoichiometrically and oxidatively, in a non-specific manner without any external reductant added, under physiological conditions. Spectro-electrochemical investigations of PtL-Cl were carried out in comparison with the analogue CuL-Cl as a reference compound. The results support a phenolate oxidation, generating a phenoxyl radical responsible for the ligand-based DNA cleavage property of the title compounds. Time-dependent in vitro cytotoxicity assays were performed with both PtL-Cl and CuL-Cl in various cancer cell lines. The compound CuL-Cl overcomes cisplatin-resistance in ovarian carcinoma and mouse leukaemia cell lines, with additional activity in some other cells. The platinum analogue, PtL-Cl also inhibits cell-proliferation selectively. Additionally, cellular-uptake studies performed for both compounds in ovarian carcinoma cell lines showed that significant amounts of Pt and Cu were accumulated in the A2780 and A2780R cancer cells. The conformational and structural changes induced by PtL-Cl and CuL-Cl on calf thymus DNA and phi X174 supercoiled phage DNA at ambient conditions were followed by electrophoretic mobility assay and circular dichroism spectroscopy. The compounds induce extensive DNA degradation and unwinding, along with formation of a monoadduct at the DNA minor groove. Thus, hybrid effects of metal-centre variation, multiple DNA-binding modes and ligand-based redox activity towards cancer cell-growth inhibition have been demonstrated. Finally, reactions of PtL-Cl with DNA model bases (9-Ethylguanine and 5'-GMP) followed by NMR and MS showed slow binding at Guanine-N7 and for the double stranded self complimentary oligonucleotide d(GTCGAC)(2) in the minor groove
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