Metal-to-ligand Charge Transfer Transitions Research Articles

Extended Dipolar Nonlinear Optical Chromophores Based on trans-Bis[1,2-phenylenebis(dimethylarsine)]chlororuthenium(II) Centers

Four new complex salts trans[RuIICl(pdma)2LA][PF6]n [pdma = 1,2-phenylenebis(dimethylarsine); LA = 1,4-bis[E-2-(4-pyridyl)ethenyl]benzene (bpvb), n = 1, 1; LA = N-methyl-1,4-bis(E-2-(4-pyridyl)ethenyl)benzene (Mebpvb+), n = 2, 2; LA = N-phenyl-1,4-bis(E-2-(4-pyridyl)ethenyl)benzene (Phbpvb+), n = 2, 3; LA = N-(2-pyrimidyl)-1,4-bis(E-2-(4-pyridyl)ethenyl)benzene (Pymbpvb+), n = 2, 4] have been prepared. The electronic absorption spectra of 1-4 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, with lambda(max) values in the range 432-474 nm in acetonitrile. Intense intraligand charge-transfer (ILCT) bands due to LA are also observed, with lambda(max) values in the range 350-416 nm. Cyclic voltammetric studies in acetonitrile reveal reversible RuIII/II waves with E(1/2) values of ca. 1.05 V vs Ag/AgCl, together with LA-based reduction processes that are irreversible with the exception of 1. Salts 1-4 have been investigated by using Stark (electroabsorption) spectroscopy in butyronitrile glasses at 77 K. These studies have afforded dipole moment changes, Deltamu12, for the MLCT and ILCT transitions which have been used to calculate molecular static first hyperpolarizabilities, Beta0, according to the two-state equation Beta0 = 3Deltamu12(mu12)2/(Emax)2 (mu12 = transition dipole moment, Emax = MLCT/ILCT energy). In contrast with related RuII ammine complexes, replacement of a central E-ethylene bond with a 1,4-phenylene unit does not appear to be an especially effective strategy for combating the NLO transparency-efficiency tradeoff in these pdma complexes. Single-crystal X-ray studies with the complex salts 2 and 3 and also with the pro-ligand salt [Phbpvb+] PF6.0.5HPF6 show that these materials all adopt centrosymmetric packing structures.

Synthesis, structure, solution chemistry and the electronic effect of para substituents on the vanadium center in a family of mixed-ligand [V VO(ONO)(ON)] complexes

Four tridentate dibasic ONO donor hydrazone ligands derived from the condensation of benzoylhydrazine with either 2-hydroxyacetophenone or its para substituted derivatives (H 2L 1–4, general abbreviation H 2L) have been used as primary ligands and 8-hydroxyquinoline (Hhq, a bidentate monobasic ON donor species) has been used as auxiliary ligand. The reaction of [V IVO(acac) 2] with H 2L in methanol followed by the addition of Hhq in equimolar ratio under aerobic condition afforded the mixed-ligand oxovanadium(V) complexes of the type [V VO(L)(hq)] ( 1– 4) in excellent yield. The X-ray structure of the compound [V VO(L 4)(hq)] ( 4) indicates that the H 2L 4 ligand is bonded with vanadium meridionally in a tridentate dinegative fashion through its deprotonated phenolic-O, deprotonated enolic-O and imine-N atoms. The V–O bond length order is: oxo < phenolato < enolato. 1H NMR spectra of 4 in CDCl 3 solution indicates that it’s solid-state structure is retained in solution. Complexes are diamagnetic and exhibit only ligand to metal charge transfer (LMCT) transition band near 530 nm in CH 2Cl 2 solution in addition to intra-ligand π → π ∗ transition band near 335 nm and they display quasi-reversible one electron reduction peak near − 0.10 V versus SCE in CH 2Cl 2 solution. λ max (for LMCT transition) and the reduction peak potential ( E p c ) values of the complexes are found to be linearly related with the Hammett ( σ) constants of the substituents in the aryloxy ring of the hydrazone ligands. λ max and E p c values show large dependence d λ max/d σ = 32.54 nm and d E p c / d σ = 0.19 V, respectively, on the Hammett constant.

Pentacyanoiron(II) as an Electron Donor Group for Nonlinear Optics: Medium-Responsive Properties and Comparisons with Related Pentaammineruthenium(II) Complexes

In this article, we describe a series of complex salts in which electron-rich {Fe(II)(CN)(5)}(3)(-) centers are coordinated to pyridyl ligands with electron-accepting N-methyl/aryl-pyridinium substituents. These compounds have been characterized by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Molecular quadratic nonlinear optical (NLO) responses have been determined by using hyper-Rayleigh scattering (HRS) at 1064 nm, and also via Stark (electroabsorption) spectroscopic studies on the intense, visible d --> pi* metal-to-ligand charge-transfer (MLCT) bands. The relatively large static first hyperpolarizabilities, beta(0), increase markedly on moving from aqueous to methanol solutions, accompanied by large red-shifts in the MLCT transitions. Acidification of aqueous solutions allows reversible switching of the linear and NLO properties, as shown via both HRS and Stark experiments. Time-dependent density functional theory and finite field calculations using a polarizable continuum model yield relatively good agreement with the experimental results and confirm the large decrease in beta(0) on protonation. The Stark-derived beta(0) values are generally larger for related {Ru(II)(NH(3))(5)}(2+) complexes than for their {Fe(II)(CN)(5)}(3)(-) analogues, consistent with the HRS data in water. However, the HRS data in methanol show that the stronger solvatochromism of the Fe(II) complexes causes their NLO responses to surpass those of their Ru(II) counterparts upon changing the solvent medium.

Electrogenerated chemiluminescence of (bis-bipyridyl)ruthenium(II) acetylacetonate complexes

The synthesis, electrochemistry, spectroscopy and electrogenerated chemiluminescence (ECL) of five bis-bipyridine ruthenium(II) complexes containing acetylacetonate complexes are reported. More specifically, (bpy) 2Ru(BA) 2(PF 6) (bpy = 2,2′-bipyridine; BA = benzoylacetonate), (bpy) 2Ru(TTFA)(PF 6) (TTFA = thenoyltrifluoroacetonate), (bpy) 2Ru(TFPB)(PF 6) (TFPB = 4,4,4-trifluoro-1-phenyl-1,3-butanedionate), (bpy) 2Ru(TFPD)(PF 6) (TFPD = 1,1,1-trifluoro-2-4-pentanedionate), and (bpy) 2Ru(DBM)(PF 6) (DBM = dibenzoylmethide) display UV–Vis, photoluminescence, electrochemical and ECL properties characteristic of ruthenium bipyridyl complexes. All complexes display absorptions in the UV and visible regions of the spectra, with visible absorptions ranging from 350 to 700 nm, typical of metal-to-ligand charge transfer (MLCT) transitions. Photoluminescence emission maxima are also characteristic of MLCT transitions with wavelength maxima from 575 to 600 nm depending on the nature of the acetylacetonate ligand. ECL efficiencies for the complexes ( ϕ ecl ∼ 0.013–0.051) are much lower than a Ru ( bpy ) 3 2 + standard ( ϕ ecl = 1) with electron-withdrawing substituents resulting in lower efficiencies.

Ultrafast Dynamics and Excited State Deactivation of [Ru(bpy)2Sq]+ and Its Derivatives

Femtosecond transient absorption spectroscopy has been employed to understand the excited state dynamics of [Ru(bpy)(2)Sq](+) (I; bpy is 2,2'-bipyridyl, and Sq is the deprotonated species of the semiquinone form of 1,2-dihydroxy benzene) and its derivatives, a widely studied near-infrared (NIR) active electrochromic dye. Apart from the well-defined dpi(Ru) --> pi(bpy)-based metal-to-ligand charge transfer (MLCT) transition bands at approximately 480 nm, this class of molecules generally shows another dpi(Ru) --> pi(Sq)(SOMO)-based intense MLCT band at around 900 nm, which is known to be redox active and bleaches reversibly upon a change in the oxidation state of the coordinated dioxolene moiety. To have better insight into the photoinduced electron transfer dynamics associated with this MLCT transition, detailed investigations have been carried out on exciting this MLCT band at 800 nm. Immediately after photoexcitation, bleach at 900 nm has been observed, whose recovery is found to follow a triexponential function with major contribution from the ultrafast component. This ultrafast component of approximately 220 fs has been ascribed to the S(1) to S(0) internal conversion process. In addition to the bleach, we have detected two transient species absorbing at 730 and 1000 nm with a formation time approximately 220 fs for both species. The excited state lifetimes for these two transient species have been measured to be 1.5 and 11 ps and have been attributed to excited singlet ((1)MLCT) and triplet ((3)MLCT) states, respectively. Transient measurements carried out on the different but analogous derivatives (II and III) have also shown similar recovery dynamics except that the rate for the internal conversion process has increased with the decrease in the S(1) to S(0) energy gap. The observed results are consistent with the energy gap law for nonradiative decay from S(1) to S(0).

Overexpression and Mechanistic Characterization of Blastula Protease 10, a Metalloprotease Involved in Sea Urchin Embryogenesis and Development

Blastula protease 10 (BP10) is a metalloenzyme involved in sea urchin embryogenesis, which has been assigned to the astacin family of zinc-dependent endopeptidases. It shows greatest homology with the mammalian tolloid-like genes and contains conserved structural motifs consistent with astacin, tolloid, and bone morphogenetic protein 1. Astacin, a crustacean digestive enzyme, has been proposed to carry out hydrolysis via a metal-centered mechanism that involves a metal-coordinated "tyrosine switch." It has not been determined if the more structurally complex members of this family involved in eukaryotic development share this mechanism. The recombinant BP10 has been overexpressed in Escherichia coli, its metalloenzyme nature has been confirmed, and its catalytic properties have been characterized through kinetic studies. BP10 shows significant hydrolysis toward gelatin both in its native zinc-containing form and copper derivative. The copper derivative of BP10 shows a remarkable 960% rate acceleration toward the hydrolysis of the synthetic substrate N-benzoyl-arginine-p-nitroanilide when compared with the zinc form. The enzyme also shows calcium-dependent activation. These are the first thorough mechanistic studies reported on BP10 as a representative of the more structurally complex members of astacin-type enzymes in deuterostomes, which can add supporting data to corroborate the metal-centered mechanism proposed for astacin and the role of the coordinated Tyr. We have demonstrated the first mechanistic study of a tolloid-related metalloenzyme involved in sea urchin embryogenesis.

Synthesis, characterization and spectroscopic and electrochemical studies of new axially coordinated cobalt(III) salen (salen = N, N′-bis(salicylidene)-1,2-ethylenediamine) complexes. The crystal structure of [Co III(salen)(aniline) 2]ClO 4

Cobalt(III) complexes with some salen ligands (salen = N, N′-bis(salicylidene)-1,2-ethylenediamine) as equatorial and potentially tetradentate ligands and with different axial amine ligands (cyclohexylamine, aniline, 4-picoline and pyridine) were synthesized in situ and characterized by IR, 1H NMR and elemental analysis. Electronic spectra and electrochemical properties of the complexes were studied in DMF solutions. Electronic structure calculations indicate that the absorption between 420 and 440 nm, which are the lowest energy transfers in the complexes, are attributed to metal–ligand charge transfer transitions. The cyclic voltammetry of the complexes show an irreversible reduction of Co(III) to Co(II) and a reversible reduction of Co(II) to Co(I). The axial ligands affect reduction potentials of Co(III) in the order of changes of axial ligands’ p K a. The crystal structure of [Co(salen)(aniline) 2]ClO 4 was determined. In the [Co(salen)(aniline) 2]ClO 4, cobalt(III) center is six coordinated surrounded by an equatorial tetradentate salen Schiff-base ligand and two aniline axial ligands and ClO 4 - acts as a counter ion.

Synthesis, structure and spectral and redox properties of new mixed ligand monomeric and dimeric Ru(ii) complexes: predominant formation of the “cis-α” diastereoisomer and unusual3MC emission by dimeric complexes

The tetradentate ligands 1,8-bis(pyrid-2-yl)-3,6-dithiaoctane (pdto) and 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (bbdo) form the complexes [Ru(pdto)(mu-Cl)](2)(ClO(4))(2) 1 and [Ru(bbdo)(mu-Cl)](2)(ClO(4))(2) 2 respectively. The new di-mu-chloro dimers 1 and 2 undergo facile symmetrical bridge cleavage reactions with the diimine ligands 2,2'-bipyridine (bpy) and dipyridylamine (dpa) to form the six-coordinate complexes [Ru(pdto)(bpy)](ClO(4))(2) 3, [Ru(bbdo)(bpy)](ClO(4))(2) 4, [Ru(pdto)(dpa)](ClO(4))(2) 5 and [Ru(bbdo)(dpa)](ClO(4))(2) 6 and with the triimine ligand 2,2':6,2''-terpyridine (terpy) to form the unusual seven-coordinate complexes [Ru(pdto)(terpy)](ClO(4))(2) 7 and [Ru(bbdo)(terpy)](ClO(4))(2) 8. In 1 the dimeric cation [Ru(pdto)(mu-Cl)](2)(2+) is made up of two approximately octahedrally coordinated Ru(II) centers bridged by two chloride ions, which constitute a common edge between the two Ru(II) octahedra. Each ruthenium is coordinated also to two pyridine nitrogen and two thioether sulfur atoms of the tetradentate ligand. The ligand pdto is folded around Ru(II) as a result of the cis-dichloro coordination, which corresponds to a "cis-alpha" configuration [DeltaDelta/LambdaLambda(rac) diastereoisomer] supporting the possibility of some attractive pi-stacking interactions between the parallel py rings at each ruthenium atom. The ruthenium atom in the complex cations 3a and 4 exhibit a distorted octahedral coordination geometry composed of two nitrogen atoms of the bpy and the two thioether sulfur and two py/bzim nitrogen atoms of the pdto/bbdo ligand, which is actually folded around Ru(II) to give a "cis-alpha" isomer. The molecule of complex 5 contains a six-coordinated ruthenium atom chelated by pdto and dpa ligands in the expected distorted octahedral fashion. The (1)H and (13)C NMR spectral data of the complexes throw light on the nature of metal-ligand bonding and the conformations of the chelate rings, which indicates that the dithioether ligands maintain their tendency to fold themselves even in solution. The bis-mu-chloro dimers 1 and 2 show a spin-allowed but Laporte-forbidden t(2g)(6)((1)A(1g))--> t(2g)(5) e(g)(1)((1)T(1g), (1)T(2g)) d-d transition. They also display an intense Ru(II) dpi--> py/bzim (pi*) metal-to-ligand charge transfer (MLCT) transition. The mononuclear complexes 3-8 exhibit dpi-->pi* MLCT transitions in the range 340-450 nm. The binuclear complexes 1 and 2 exhibit a ligand field ((3)MC) luminescence even at room temperature, whereas the mononuclear complexes 3 and 4 show a ligand based radical anion ((3)MLCT) luminescence. The binuclear complexes 1 and 2 undergo two successive oxidation processes corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru(II)Ru(III) state (K(c): 1, 3.97 x 10(6); 2, 1.10 x 10(6)). The mononuclear complexes 3-7 exhibit only one while 8 shows two quasi-reversible metal-based oxidative processes. The coordinated 'soft' thioether raises the redox potentials significantly by stabilising the 'soft' Ru(II) oxidation state. One or two ligand-based reduction processes were also observed for the mononuclear complexes.

Molecular and electronic ground and excited structures of heteroleptic ruthenium polypyridyl dyes for nanocrystalline TiO2 solar cells

Heteroleptic ruthenium complexes of the type cis-[Ru(H2dcbpy)(L)(NCS)2], where H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine and L = 1,10-phenanthroline (phen) (1) or dipyrido[3,2-a∶2′,3′-c]-phenazine (dppz) (2), were synthesized and their photochemical properties were investigated. The complexes showed a broad and intense metal-to-ligand charge transfer (MLCT) transition band in the visible region. The complexes were anchored to nanocrystalline TiO2 film electrodes, and the photovoltaic properties of the resulting dye-sensitized solar cells were characterized and compared with the properties of cells prepared with cis-(NBu4)2[Ru(Hdcbpy)2(NCS)2] (N719). The efficiency of the 2-sensitized solar cell was 20% lower than that of the 1-sensitized solar cell, but neither was as efficient as the N719-sensitized solar cell. The electronic structures of the complexes were investigated by means of a time-dependent density functional theory method in an effort to better understand their effectiveness in TiO2-based photoelectrochemical cells. The calculation results indicated that the character of the MLCT transitions in the long wavelength region differed between 1 and2, although their energy levels are nearly the same in the protonated forms. It was suggested that the performance of the 2-sensitized solar cell could be improved by the introduction of electron-donating groups on the dppz ligand of 2.

Eilatin Complexes of Ruthenium and Osmium. Synthesis, Electrochemical Behavior, and Near-IR Luminescence

The synthesis and characterization of new Ru(II) and Os(II) complexes of the ligand eilatin (1) are described. The new complexes [Ru(bpy)(eil)(2)](2+) (2), [Ru(eil)(3)](2+) (3), and [Os(eil)(3)](2+) (4) (bpy = 2,2'-bipyridine; eil = eilatin) were synthesized and characterized by NMR, fast atom bombardment mass spectrometry, and elemental analysis. In the series of complexes [Ru(bpy)(x)(eil)(y)()](2+) (x + y = 3), the effect of sequential substitution of eil for bpy on the electrochemical and photophysical properties was examined. The absorption spectra of the complexes exhibit several bpy- and eil-associated pi-pi and metal-to-ligand charge-transfer (MLCT) transitions in the visible region (400-600 nm), whose energy and relative intensity depend on the number of ligands bound to the metal center (x and y). On going from [Ru(bpy)(2)(eil)](2+) (5) to 2 to 3, the d(pi)(Ru) --> pi(eil) MLCT transition undergoes a red shift from 583 to 591 to 599 nm, respectively. Electrochemical measurements performed in dimethyl sulfoxide reveal several ligand-based reduction processes, where each eil ligand can accept up to two electrons at potentials that are significantly anodically shifted (by ca. 1 V) with respect to the bpy ligands. The complexes exhibit near-IR emission (900-1100 nm) of typical (3)MLCT character, both at room temperature and at 77 K. Along the series 5, 2, and 3, upon substitution of eil for bpy, the emission maxima undergo a blue shift and the quantum yields and lifetimes increase. The radiative and nonradiative processes that contribute to deactivation of the excited level are discussed in detail.

Structural Changes upon Photoexcitation into the Metal-to-Ligand Charge-Transfer State of [Cu(pqx)(PPh3)2]+ Probed by Resonance Raman Spectroscopy and Density Functional Theory

The structural changes that occur when [Cu(pqx)(PPh(3))(2)](+) (pqx is 2-(2'-pyridyl)quinoxaline) undergoes excitation through a metal-to-ligand charge-transfer (MLCT) transition are investigated using resonance Raman excitation profiles coupled with density functional theory (DFT). The DFT calculations predict bond lengths to within 3 pm and absolute deviations of 7 cm(-1) for the vibrational frequencies of [Cu(pqx)(PPh(3))(2)](+). TD-DFT calculations of oscillator strengths (f = 0.089) and band positions (419 nm) showed close agreement with experiment (f = 0.07, 431 nm). Resonance Raman spectra show the 527 cm(-1) (nu(29)) and 1476 cm(-1) (nu(75)) modes undergo the largest dimensionless displacement (Delta = 1.5 and 1.1, respectively) following photoexcitation into the MLCT Franck-Condon region. The solvent couples strongly to the MLCT transition and resonance Raman intensity analysis (RRIA) gives a solvent reorganization energy of 3400 cm(-1) for dichloromethane and 2800 cm(-1) for chloroform solutions. A large inner-sphere reorganization of 3430 cm(-1) in dichloromethane solution (3520 cm(-1) in chloroform solution) was found for [Cu(pqx)(PPh(3))(2)](+), indicating that the molecule as a whole undergoes significant distortion following MLCT excitation.

Reaction of BSA, CT-DNA, Adenine and Guanine with [Pt(RaaiR′)Cl2] and Characterization of the Products [Pt(RaaiR′)(NB)(H2O)](PF6)2] [RaaiR′=1-alkyl-2-(arylazo)imidazole

The reaction of [Pt(RaaiR′)(solvent)2]2+ with nucleobases (NB), adenine and guanine was studied and the products [Pt(RaaiR′)(NB)(H2O)](PF6)2RaaiR′ = 1-alkyl-2-(arylazo)imidazole, R′ = Me (1), Et (2); R = H (a),OMe (b), NO2(c)] characterized by i.r, u.v.–vis. and 1H-n.m.r spectroscopy. The solution spectra exhibit metal-to-ligand charge-transfer transitions (MLCT) and intra-ligand charge-transfer transitions. The position and symmetry of the bands depend on the nucleobase and arylazoimidazole. The coordination of the ligand is supported by 1H-n.m.r. spectral data. The redox property has been examined by cyclic voltammetric technique and shows the involvement of the azo group of the chelated ligand, RaaiR′ in the reduction process along with the irreversible reduction of the coordinated nucleobase. Binding with Bovine Serum Albumin (BSA) and Calf Thymus DNA was performed spectrophotometrically in the physiological buffer medium and spectral profile was recorded in 24 h.

Theoretical studies of ground and excited electronic states of complexes M(CO) 4(phen) (M = Cr, Mo, W; phen = 1,10-phenanthroline)

Electronic absorption and emission data of M(CO) 4(phen) complexes, where M = Cr, Mo or W; phen=1,10-phenanthroline are reported using density functional theory (DFT). Electronic absorption spectra of [M(CO) 4(phen)] show a strong band in the visible region of the spectrum, whose metal to ligand charge transfer (MLCT) M → phen and M → CO ax character is indicated by high molar absorptivity. This assignment is investigated by theoretical calculations, which differ from the early interplay of M → L metal to ligand charge transfer MLCT and ligand-filed (LF) electronic transitions, occurring between d-orbitals. The phosphorescence of each complex was identified as a lowest, triplet metal to ligand charge transition (MLCT) around 740 nm.

Molecular sensing behavior of the first Mn(I)-compound of di-2-pyridylketone- p-nitrophenylhydrazone (dpknph), fac-[Mn(CO) 3(dpknph)Br

The reaction between [Mn(CO) 5Br] and di-2-pyridylketone- p-nitrophenylhydrazone (dpknph) in diethyl ether under ultrasonic conditions gave fac-[Mn(CO) 3(dpknph)Br] in good yield. Optical and thermodynamic measurements on fac-[Mn(CO) 3(dpknph)Br] in non-aqueous polar solvents revealed reversible interconversion between two intense charge transfer absorption bands due to π–π* (dpk), followed by dpk → nitro intraligand charge transfer transition (ILCT), mixed with metal ligand charge transfer transition (MLCT) due to d π ( Mn ) → π ( dpk ) ∗ . In non-polar solvents, a single absorption band appeared. Extinction coefficients of 46 200 ± 2000 and 28 400 ± 2000 M −1 cm −1 were calculated in DMSO for the low- and high-energy electronic states of fac-[Mn(CO) 3(dpknph)Br] using excess NaBF 4. Changes in enthalpy (Δ H ø) of +14.0 and −12.1 kJ mol −1, entropy (Δ S ø) of +28.65 and −64.30 J mol −1 K −1, and free energy (Δ G ø) of +5.48 and +7.08 kJ mol −1 at 298 K were calculated for the interconversion between the high and low energy electronic states of fac-[Mn(CO) 3(dpknph)Br]. These results allow for the use of these systems ( fac-[Mn(CO) 3(dpknph)Br] and surrounding solvent or solute molecules) as optical sensors for a variety of physical and chemical stimuli that include metal ions. Group 12 metal ions in concentrations as low as 1.00 × 10 −9 M can be detected and determined using fac-[Mn(CO) 3(dpknph)Br] in dmso in the presence and absence of NaBH 4.

Influence of the Halogen Ligand on the Near-UV−Visible Spectrum of [Ru(X)(Me)(CO)2(α-diimine)] (X = Cl, I; α-Diimine = Me-DAB, iPr-DAB; DAB = 1,4-Diaza-1,3-butadiene): An ab Initio and TD-DFT Analysis

The near-UV-vis electronic spectroscopy of [Ru(X)(Me)(CO)(2)(iPr-DAB)] (X = Cl or I; iPr-DAB = N,N'-di-isopropyl-1,4-diaza-1,3-butadiene) is investigated through CASSCF/CASPT2 and TD-DFT calculations on the model complexes [Ru(X)(Me)(CO)(2)(Me-DAB)] (X = Cl or I). Convergence of the calculated transition energies for the low-lying metal-to-ligand charge-transfer (MLCT), X-to-ligand charge-transfer (XLCT, X halide ligand), or sigma-bond-to-ligand charge-transfer (SBLCT) to experimental values is analyzed for both methods. On the basis of these accurate calculations, it is shown that whereas the lowest singlet state can be assigned to a nearly pure XLCT state in [Ru(I)(Me)(CO)(2)(Me-DAB)], its character is mainly MLCT in [Ru(Cl)(Me)(CO)(2)(Me-DAB)]. These results are in agreement with time-resolved emission/IR and resonance Raman experimental data. The experimental UV-vis bands are well reproduced by the CASSCF/CASPT2 calculations. The TD-DFT transition energies to the long-range charge transfer states are dramatically affected by the nature of the functional, with lowering leading to meaningless values in the case of nonhybrid functionals. Both methods reproduce well the red shift of the absorption bands on going from the chloride to the iodide complex as well as the shift of the strongly absorbing higher MLCT transition from the visible to the UV domain of energy.

Electronic and structural effects on the nonlinear optical behavior in push-pull TTF/tricarbonyl chromiun arene complexes.

A novel D-pi-A system in which tetrathiafulvalene (TTF) and pi-extended TTFs as strong electron donors are covalently connected to a tricarbonyl (eta(6)-arene)chromium complex as the acceptor moiety through a systematically increased conjugated bridge of vinylene units (12a-c, 16a-c) have been synthesized by Wittig-Horner olefination reaction. The electronic spectra as well as the electrochemical data reveal a different behavior of TTF derivatives (12a-c) and of exTTF derivatives (16a-c). Cyclic voltammetry shows the influence of the tricarbonylchromium arene on the oxidation potentials in compounds 12a-c, and no remarkable effect is observed for exTTFs (16a-c). The nonlinear optical properties of 12a-c and 16a-c have been calculated by using the ab initio CPHF/6-31G//B3P86/6-31G model, and the time-dependent density functional theory (TD-DFT) method has been used for the calculation of the electronic transitions. The calculations reveal that an intraligand charge-transfer transition (ILCT) and the metal to ligand charge-transfer transition (MLCT) are responsible for the nonlinear response. In addition, the large angles formed by the ground-state dipole moment and the vectorial hyperpolarizability are responsible for the mubeta values determined experimentally by the EFISH technique.

A TDDFT study of the ruthenium(II) polyazaaromatic complex [Ru(dppz)(phen) 2] 2+ in solution

DFT/TDDFT calculations were performed to investigate the structural, electronic and optical properties of the [Ru(dppz)(phen) 2] 2+ complex in solution. TDDFT calculations in water show two groups of metal-to-ligand charge transfer (MLCT) transitions at ≈450 and 415 nm whose superposition gives account of the broad absorption band experimentally characterized at 440 nm. Also, a group of almost coincident MLCT transitions partially mixed with dppz intraligand π–π ∗ transitions centered at ≈380 nm is found to give rise to the narrow absorption band experimentally found at 380 nm. Our results provide insight into the hypochromic shifts experimentally characterized upon intercalation of the title complex into DNA.

Metal-to-ligand charge transfer in polarized metal L-edge X-ray absorption of Ni and Cu complexes

Metal L-edge X-ray absorption spectra for Ni and Cu complexes are discussed by investigating their linear polarization dependence. The origin of the characteristic bands is revealed to be one-electron transitions to ligand-centered molecular orbitals carrying metal-to-ligand charge transfer (MLCT). These MLCT transitions are related to the electronic structure of the ground state, particularly, the back-donation. Polarized L-edge X-ray absorption spectroscopy is applied to reveal electronic structures of metal complexes with doped hole and nickel–nickel bonding.

Electronic and vibrational properties of a MOF-5 metal-organic framework: ZnO quantum dot behaviour.

UV-Vis DRS and photoluminescence (PL) spectroscopy, combined with excitation selective Raman spectroscopy, allow us to understand the main optical and vibrational properties of a metal-organic MOF-5 framework. A O(2-)Zn(2+)[rightward arrow] O(-)Zn(+) ligand to metal charge transfer transition (LMCT) at 350 nm, testifies that the Zn(4)O(13) cluster behaves as a ZnO quantum dot (QD). The organic part acts as a photon antenna able to efficiently transfer the energy to the inorganic ZnO-like QD part, where an intense emission at 525 nm occurs.

Syntheses, spectroscopic and molecular quadratic nonlinear optical properties of dipolar ruthenium(II) complexes of the ligand 1,2-phenylenebis(dimethylarsine).

Six new complex salts trans-[Ru(II)Cl(pdma)2L][PF6]n [pdma = 1,2-phenylenebis(dimethylarsine); L = (E,E,E)-1,6-bis(4-pyridyl)hexa-1,3,5-triene (bph), n= 1, 5; L =N-methyl-4-[(E)-2-(4-pyridyl)ethenyl]pyridinium (Mebpe+), n= 2, 7; L =N-methyl-4-[(E,E)-4-(4-pyridyl)buta-1,3-dienyl]pyridinium (Mebpb+), n= 2, 8; L =N-methyl-4-[(E,E,E)-6-(4-pyridyl)hexa-1,3,5-trienyl]pyridinium (Mebph+), n= 2, 9; L = bis(4-pyridyl)acetylene (bpa), n= 1, 10; L =N-methyl-4-[2-(4-pyridyl)ethynyl]pyridinium (Mebpa+), n= 2, 11] have been prepared. The electronic absorption spectra of 5 and 7-11 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, with lambdamax values in the range 434-492 nm in acetonitrile. Cyclic voltammetric studies reveal reversible Ru(III/II) waves with E(1/2) values in the range 1.06-1.15 V vs. Ag-AgCl, together with irreversible L-based reduction processes. Along with a number of previously reported related compounds (B. J. Coe et al., J. Chem. Soc., Dalton Trans., 1996, 3917; 1997, 591; 2000, 797), salts 5 and 7-11 have been investigated by using Stark (electroabsorption) spectroscopy in butyronitrile glasses at 77 K. These studies have afforded dipole moment changes Deltamu12 for the MLCT transitions which have been used to calculate molecular static first hyperpolarizabilities beta0 according to the two-state equation beta0= 3Deltamu12(mu12)2/(Emax)2 (mu12 = transition dipole moment, Emax = MLCT energy). MLCT absorption and electrochemical data show that a trans-[Ru(II)Cl(pdma)2]+ centre is considerably less electron-rich than a [Ru(II)(NH3)5]2+ unit. Although the beta0 responses of the pdma complexes are only a little smaller than those of their [Ru(II)(NH3)5]2+ analogues, this result is partly attributable to unexpected changes in the relative mu12 values on freezing. Thus, substantial increases in mu12 for the arsine compounds act to partially offset the beta0-decreasing influence of their higher Emax values when compared with the analogous pentaammine species. Single crystal X-ray structures have been obtained for the salts 1(.)2.5MeCN, 4(.)2MeCN, 7 and 11, but only 1(.)2.5MeCN adopts a non-centrosymmetric space group (Fdd2) such as may show bulk NLO effects.