Ternary Iron(III) complex showing photocleavage of DNA in the photodynamic therapy window

A new ternary iron(III) complex [FeL(dpq)] containing dipyridoquinoxaline (dpq) and 2,2-bis(3,5-di-tert-butyl-2-hydroxybenzyl)aminoacetic acid (H3L) is prepared and structurally characterized by X-ray crystallography. The high-spin complex with a FeN3O3 core shows a quasi-reversible iron(III)/iron(II) redox couple at -0.62 V (vs SCE) in DMF/0.1 M TBAP and a broad visible band at 470 nm in DMF/Tris buffer. Laser photoexcitation of this phenolate (L)-to-iron(III) charge-transfer band at visible wavelengths including red light of >= 630 nm leads to cleavage of supercoiled pUC19 DNA to its nicked circular form via a photoredox pathway forming hydroxyl radicals.

Electrochemical reduction of Cu(II)-EDTA complex at dropping mercury electrode. An impedance approach

The study of electrochemical reduction of Cu(II)-EDTA system by phase sensitive a.c. impedance method at dropping mercury electrode reveals several interesting features. The complex plane polarograms exhibit loop like shape in contrast to the classical zinc ion reduction where crest like shape is found. Again, the relative placement of peaks of in-phase and quadrature components, and the relative placement of portions before and after the peaks of complex plane polarograms are different from that of zinc ion reduction. The complex plane plots suggest that electrochemical reduction of Cu-EDTA is charge transfer controlled.

Electron donor-acceptor complex of ICl with diethyl ether - He I photoelectron spectroscopy and ab initio molecular orbital study

The He I photoelectron spectrum of the diethyl ether-ICl complex has been obtained. The oxygen orbitals are shifted to higher binding energies and that of ICl to lower binding energies owing to complex formation. Ab initio molecular orbital (MO) calculations of the complex molecule showed that the bonding is between the sigma-type lone pair of oxygen and the I atom and that the complex has C-2v symmetry. The binding energy of the complex is computed to be 8.06 kcal mol(-1) at the MP2/3-21G* level. The orbital energies obtained from the photoelectron spectra of the complex are compared and assigned with orbital energies obtained by MO calculations. Natural bond orbital analysis (NBO) shows that charge transfer is from the sigma-type oxygen lone pair to the iodine atom and the magnitude of charge transfer is 0.0744 e.

Quantum and non-Markovian effects in the electron transfer reaction dynamics in the Marcus inverted region

Electron transfer reactions in large molecules may often be coupled to both the polar solvent modes and the intramolecular vibrational modes of the molecule. This can give rise to a complex dynamics which may in some systems, like betaine, be controlled more by vibrational rather than by solvent effects. Additionally, a significant contribution from an ultrafast relaxation component in the solvation dynamics may enhance the complexity. To explain the wide range of behavior that has been observed experimentally, Barbara et al. recently proposed that a model of an electron transfer reaction should minimally consist of a low-frequency classical solvent mode (X), a low-frequency vibrational mode (Q), and a high-frequency quantum mode (q) (J. Phys. Chem. 1991, 96, 3728). In the present work, a theoretical study of this model is described. This study generalizes earlier work by including the biphasic solvent response and the dynamics of the low-frequency vibrational mode in the presence of a delocalized, extended reaction zone. A novel Green's function technique has been developed which allowed us to study the non-Markovian dynamics on a multidimensional surface. The contributions from the high-frequency vibrational mode and the ultrafast component in the non-Markovian solvent dynamics are found to be primarily responsible for the dramatic increase in charge transfer rate over the prediction of the classical theories that neglect both these factors. These, along with a large coupling between the reactant and the product states, may combine to render the electron transfer rate both very large and constant over a wide range of solvent relaxation rates. A study on the free energy gap dependence of the electron transfer rate reveals that the rates are sensitive to changes in the quantum frequency particularly when the free energy gap is very large.

A perspective of biological supramolecular electron transfer

Electron transfer is an essential activity in biological systems. The migrating electron originates from water-oxygen in photosynthesis and reverts to dioxygen in respiration. In this cycle two metal porphyrin complexes possessing circular conjugated system and macrocyclic pi-clouds, chlorophyll and hems, play a decisive role in mobilising electrons for travel over biological structures as extraneous electrons. Transport of electrons within proteins (as in cytochromes) and within DNA (during oxidative damage and repair) is known to occur. Initial evaluations did not favour formation of semiconducting pathways of delocalized electrons of the peptide bonds in proteins and of the bases in nucleic acids. Direct measurement of conductivity of bulk material and quantum chemical calculations of their polymeric structures also did not support electron transfer in both proteins and nucleic acids. New experimental approaches have revived interest in the process of charge transfer through DNA duplex. The fluorescence on photoexcitation of Ru-complex was found to be quenched by Rh-complex, when both were tethered to DNA and intercalated in the base stack. Similar experiments showed that damage to G-bases and repair of T-T dimers in DNA can occur by possible long range electron transfer through the base stack. The novelty of this phenomenon prompted the apt name, chemistry at a distance. Based on experiments with ruthenium modified proteins, intramolecular electron transfer in proteins is now proposed to use pathways that include C-C sigma-bonds and surprisingly hydrogen bonds which remained out of favour for a long time. In support of this, some experimental evidence is now available showing that hydrogen bond-bridges facilitate transfer of electrons between metal-porphyrin complexes. By molecular orbital calculations over 20 years ago. we found that "delocalization of an extraneous electron is pronounced when it enters low-lying virtual orbitals of the electronic structures of peptide units linked by hydrogen bonds". This review focuses on supramolecular electron transfer pathways that can emerge on interlinking by hydrogen bonds and metal coordination of some unnoticed structures with pi-clouds in proteins and nucleic acids, potentially useful in catalysis and energy missions.

On the ultrafast charge migration and subsequent charge directed reactivity in Cl center dot center dot center dot N halogen-bonded clusters following vertical ionization

In this article, we have presented ultrafast charge transfer dynamics through halogen bonds following vertical ionization of representative halogen bonded clusters. Subsequent hole directed reactivity of the radical cations of halogen bonded clusters is also discussed. Furthermore, we have examined effect of the halogen bond strength on the electron-electron correlation-and relaxation-driven charge migration in halogen bonded complexes. For this study, we have selected A-Cl (A represents F, OH, CN, NH2, CF3, and COOH substituents) molecules paired with NH3 (referred as ACl:NH3 complex): these complexes exhibit halogen bonds. To the best of our knowledge, this is the first report on purely electron correlation-and relaxation-driven ultrafast (attosecond) charge migration dynamics through halogen bonds. Both density functional theory and complete active space self-consistent field theory with 6-31+G(d, p) basis set are employed for this work. Upon vertical ionization of NCCl center dot center dot center dot NH3 complex, the hole is predicted to migrate from the NH3-end to the ClCN-end of the NCCl center dot center dot center dot NH3 complex in approximately 0.5 fs on the D-0 cationic surface. This hole migration leads to structural rearrangement of the halogen bonded complex, yielding hydrogen bonding interaction stronger than the halogen bonding interaction on the same cationic surface. Other halogen bonded complexes, such as H2NCl:NH3, F3CCl:NH3, and HOOCCl:NH3, exhibit similar charge migration following vertical ionization. On the contrary, FCl:NH3 and HOCl:NH3 complexes do not exhibit any charge migration following vertical ionization to the D-0 cation state, pointing to interesting halogen bond strength-dependent charge migration. (C) 2015 AIP Publishing LLC.

Direct simulation of quantum dynamics in complex systems

Accurate simulation of quantum dynamics in complex systems poses a fundamental theoretical challenge with immediate application to problems in biological catalysis, charge transfer, and solar energy conversion. The varied length- and timescales that characterize these kinds of processes necessitate development of novel simulation methodology that can both accurately evolve the coupled quantum and classical degrees of freedom and also be easily applicable to large, complex systems. In the following dissertation, the problems of quantum dynamics in complex systems are explored through direct simulation using path-integral methods as well as application of state-of-the-art analytical rate theories.

Synthesis, photoluminescence, and theoretical studies of novel Cu(I) complex

A novel diimine Cu(I)complex [Cu(ABPQ)(DPEphos)]BF4 [ABPQ and DPEphos are acenaphtho[1,2-b]bipyrido[2,3-h:3,2-f]quinoxaline and bis(2-(diphenylphosphanyl)phenyl) ether, respectively] is synthesized, and its photophysical properties are experimentally and theoretically characterized. The emission bands centered at ca. 400/470 and 550 nm of [Cu(ABPQ)(DPEphos)]BF4 are attributed to the ligand-centered pi -> pi* transition and the metal-to-ligand charge transfer d pi(Cu) -> pi*(N-N) transition, respectively. The luminescence quantum yield of [Cu(ABPQ)(DPEphos)]BF4 in CHCl3 is found to be about five times higher than that of [Cu(Phen)(DPEphos)]BF4.

Synthesis, photophysical properties, and theoretical studies on pyrrole-containing bromo Re(I) complex

Two bromo rhenium(I) carbonyl complexes with the formula of [Re(CO)(3)(L)Br], where L = 1,10-phenanthroline (Phen-Re) and 5-(1H-pyrrol-1-yl)-1,10-phenanthroline (Pyph-Re), were successfully synthesized with the aim to analyze the effect of the pyrrole (Py) moiety on the photophysical properties of Pyph-Re. It was found that the triplet metal-to-ligand charge-transfer d pi (Re) --> pi*(N-N) emission of Phen-Re and Pyph-Re centered at ca. 527 nm with the luminescence quantum yield (LQY) of 0.015 and ca. 578 nm with the LQY of 0.011, respectively. At the same time, the geometrical structures of the ground state and the absorption spectral properties of Phen-Re and Pyph-Re were also calculated with the 6-31G* basis set employed on C, H, N, O, and Br atoms, and LANL2DZ adopted on Re atom.

Effective energy transfer and luminescence of LB films based on europium-substituted heteropolytungstate

The europium-substituted heteropolytungstate K13Eu(SiW11O39)(2) was successfully assembled into two lipids by LB technique for the first time. X-ray diffraction has shown a well defined lamellar for the LB films. The LB films have been characterized by fluorescence spectra and the characteristic luminescence behaviors were discussed. The ligand-metal charge transfer band could be observed in the spectra of the LB films, which could not be found in that of heteropolytungstate solid. The results of fluorescence spectra indicate the energy could be effectively transferred from ligands to the Eu3+ ions in the LB films and the luminescence efficiency was increased greatly. The influences of various lipids on the luminescence of polyoxometalates were investigated. The various interactions between monolayer and polyanions have different effect on the luminescence properties of europium-substituted heteropolytungstate.

ELECTRONIC-STRUCTURE AND CHEMICAL BONDING OF EUROPIUM NITRATE COMPLEX WITH AZACROWN (2, 2)

The electronic structure and bond character of europium nitrate complex with azacrown (2, 2)(1, 7, 10, 16-tetraoxa-4, 13-diazacyclooctadecane), [Eu(NO_3)_2(2, 2)] NO_3, have been studied by means of XPS and INDO method. The data of electronic binding energies and charge distribution of atoms in the complex showed that chemical shift of less electronegative nitrogen N1s binding energy was larger than that of more electronegative oxygen O1s binding energy in coordinating atoms, and that charge transfer from N...

Raman spectroelectrochemical studies and crystal structure of a binuclear copper(I) complex with a bridging diimine ligand

Raman spectroelectrochemical and X-ray crystallographic studies have been made for the binuclear copper(I) complex, [(Ph(3)P)(2)Cu(dpq)Cu(PPh(3))(2)][BF4](2), where dpq is the bridging ligand 2,3-di(2-pyridyl)quinoxaline. The X-ray data show that the pyridine rings are twisted out of plane with respect to the quinoxaline ring which is itself non-planar. The UV/VIS spectra of the metal-to-ligand charge-transfer excited state and those of the electrochemically reduced complex are similar. The resonance-Raman spectrum of the latter species exhibits little change in the frequency of the pyridinylquinoxaline inter-ring C-C bond stretching mode, compared to the ground electronic state. This suggests minimum change in the inter-ring C-C bond order in the electrochemically or charge-transfer generated radical anion. Semiempirical molecular-orbital calculations on both the neutral dpq and radical anion show two near-degenerate lowest unoccupied orbitals in the neutral species. One is strongly bonding across the inter-ring C-C bond while the other is almost nun-bonding. The Raman data suggest that it is this latter orbital which is populated in the transient and electrochemical experiments.

Formation of an unusual copper(II) complex from the degradation of a novel tricopper(II) carbohydrazone complex

An unusual copper(II) complex [Cu(L1a)2Cl2] CH3OH H2O H3O+Cl (1a) was isolated from a solution of a novel tricopper(II) complex [Cu3(HL1)Cl2]Cl3 2H2O (1) in methanol, where L1a is 3-(2-pyridyl)triazolo [1,5-a]-pyridine, and characterized with single crystal X-ray diffraction study. The tricopper(II) complex of potential ligand 1,5-bis(di-2-pyridyl ketone) carbohydrazone (H2L1) was synthesized and physicochemically characterized, while the formation of the complex 1a was followed by time-dependant monitoring of the UV–visible spectra, which reveals degradation of ligand backbone as intensity loss of bands corresponding to O?Cu(II) charge transfer

Luminescence of a new Ru(II) polypyridine complex controlled by a redox-responsive protonable anthra[1,10]phenanthrolinequinone

Redox-controlled luminescence quenching is presented for a new Ru(II)-bipyridine complex [Ru(bpy)(2)(1)](2+) where ligand 1 is an anthra[1,10] phenanthrolinequinone. The complex emits from a short-lived metal-to-ligand charge transfer, (MLCT)-M-3 state (tau = 5.5 ns in deaerated acetonitrile) with a low luminescence quantum yield (5 x 10(-4)). The emission intensity becomes significantly enhanced when the switchable anthraquinone unit is reduced to corresponding hydroquinone. On the contrary, chemical one-electron reduction of the anthraquinone moiety to semiquinone in aprotic tetrahydrofuran results in total quenching of the emission.

Spectro-electrochemical and DFT studies of a planar Cu(II)–phenolate complex active in the aerobic oxidation of primary alcohols

A square-planar compound [Cu(pyrimol)Cl] (pyrimol = 4-methyl-2-N-(2-pyridylmethylene)aminophenolate) abbreviated as CuL–Cl) is described as a biomimetic model of the enzyme galactose oxidase (GOase). This copper(II) compound is capable of stoichiometric aerobic oxidation of activated primary alcohols in acetonitrile/water to the corresponding aldehydes. It can be obtained either from Hpyrimol (HL) or its reduced/hydrogenated form Hpyramol (4-methyl-2-N-(2-pyridylmethyl)aminophenol; H2L) readily converting to pyrimol (L-) on coordination to the copper(II) ion. Crystalline CuL–Cl and its bromide derivative exhibit a perfect square-planar geometry with Cu–O(phenolate) bond lengths of 1.944(2) and 1.938(2) Å. The cyclic voltammogram of CuL–Cl exhibits an irreversible anodic wave at +0.50 and +0.57 V versus ferrocene/ferrocenium (Fc/Fc+) in dry dichloromethane and acetonitrile, respectively, corresponding to oxidation of the phenolate ligand to the corresponding phenoxyl radical. In the strongly donating acetonitrile the oxidation path involves reversible solvent coordination at the Cu(II) centre. The presence of the dominant CuII–L. chromophore in the electrochemically and chemically oxidised species is evident from a new fairly intense electronic absorption at 400–480 nm ascribed to a several electronic transitions having a mixed pi-pi(L.) intraligand and Cu–Cl -> L. charge transfer character. The EPR signal of CuL–Cl disappears on oxidation due to strong intramolecular antiferromagnetic exchange coupling between the phenoxyl radical ligand (L.) and the copper(II) centre, giving rise to a singlet ground state (S = 0). The key step in the mechanism of the primary alcohol oxidation by CuL–Cl is probably the alpha-hydrogen abstraction from the equatorially bound alcoholate by the phenoxyl moiety in the oxidised pyrimol ligand, Cu–L., through a five-membered cyclic transition state.