Ligand-metal Charge Transfer Band Research Articles

Ultrafast Spin Crossover Photochemical Mechanism in [FeII(2,2'-bipyridine)3]2+] Revealed by Quantum Dynamics.

Photoexcitation of [FeII(2,2'-bipyridine)3]2+ induces a subpicosecond spin crossover transformation from a low-spin singlet to a high-spin quintet state. The mechanism involves metal-centered (MC) and metal-ligand charge transfer (MLCT) triplet intermediates, but their individual contributions to this efficient intersystem crossing have been object of debate. Employing quantum wavepacket dynamics, we show that MC triplets are catalyzing the transfer to the high-spin state. This photochemical pathway is made possible thanks to bipyridine stretching vibrations, facilitating the conversion between the MLCT bands to such MC triplets. We show that the lifetime of the MLCT states can be increased to tens of picoseconds by breaking the conjugation between pyridine units, which increases the energetic gap between MLCT and MC states. This opens the route for the design of new chelating ligands inducing long-lived MLCT states in iron complexes.

Influence of nitro ruthenium isomerization on photochemically induced nitric oxide release: Vasorelaxant activities

We have synthesized cis-[Ru(bpy)2(NO2-κN)Ln-](n-1) and cis-[Ru(bpy)2(NO2-κO)L n-](n-1) (bpy = 2,2′-bipyridine; k = indication of the coordinated center to Ruthenium; L = pyridine type ligand) by reacting cis-[Ru(bpy)2(H2O)Ln-](n-2) with sodium nitrite or conducting basic cis-[Ru(bpy)2NO(Ln-)](n-3) hydrolysis. Photolysis at the metal-ligand charge transfer band (MLCT) of the isomers yielded nitric oxide (NO) as determined by NO measurement. The NO photorelease rates obtained upon 447 nm laser irradiation of the ruthenium complexes showed that cis-[Ru(bpy)2(NO2-κO)Ln-](n-1) released NO three times faster than cis-[Ru(bpy)2(NO2-κN)Ln-](n-1). We investigated endothelium-dependent vasodilation induced by cis-[Ru(bpy)2(4-pic)(NO2-κN)]+ and cis-[Ru(bpy)2(4-pic)(NO2-κO)]+ (4-pic = 4-picoline) in isolated 3 mm aortic rings precontracted with L-phenylephrine. Maximum vasodilation was achieved under 447 nm laser irradiation of 0.5 μMol.L−1 ruthenium complexes for 100 s.

Molecular mechanics of glove‐like re(I) metallacycles: Toward light‐activated molecular catchers

AbstractTwo novel glove‐shaped rhenium(I)‐based metallacycle complexes, composed of two bidentate ligands α,α′‐bis(benzimidazol‐1‐yl)‐meta(orortho)‐xylene(bbim) and a tetradentate ligand, 2,5‐pyrazinedicarboxylate(pzdc) with two Re(I) metal centers each are developed and reported. The opening and closure motion of the metaxylene‐linked metallacycle complex were studied via variable temperature proton nuclear magnetic resonance spectroscopy experiment, as well as density functional theory calculation. Our results indicate that the motion of the “capping” phenyl moiety in bbim is accompanied by the swing of the two benzimidazol groups. Both complexes show two different emission bands when excited at two different wavelengths, a violation of the Kasha's rule. Theoretical calculation indicate that the lower‐energy transitions are metal–ligand charge transfer bands, arising from electrons localized to one of the two Re(I) centers to the pzdc ligand, while the higher‐energy ones are assigned to a ligand–ligand charge‐transfer excitation from bbim to pzdc. Results from excited‐state optimized structures confirm that the higher‐energy excitation leads to a closure of the molecule with meta‐bbim, a desirable “molecular catcher” behavior. Our findings provide an important insight for a strategy to design a light‐activated molecular catcher for many interesting applications.

Optical bandgap of Cd, Zn, and Ag nitroprussides. A combined experimental and computational study

The optical bandgap (Eg) of transition metal nitroprussides remains practically undocumented, probably because these materials are usually considered wide bandgap solids, characterized by a strong metal-ligand charge-transfer band below 500 nm. In this contribution, we are reporting experimental and DFT computed data for both, 3D and 2D phases of Zn and Cd nitroprussides to get physical insight into the metal and structure effect on the optical bandgap (Eg) of these materials. 3D Ag nitroprusside, an analog solid for which the Eg value has been reported, was included for comparison. Eg values were obtained from both, UV–vis–NIR spectra and DFT calculations. The solids under study show a direct bandgap with diverse Urbach tails, which was ascribed to the presence of structural defects. Metal to ligand charge transfer within nitroprusside ion dominates the electronic structure in the valence-gap-conduction region for the 3D systems. Both, the outer metal, and the incorporation of 1-Methyl-2-pyrrolidone as an organic pillar molecule between adjacent layers for the 2D analogs, influence the electronic structure and consequently, the Eg values obtained. In these materials, the polarizing power of outer metal has a relevant role in the observed Eg value.

Syntheses of Square Planar Gallium Complexes and a Proton NMR Correlation Probing Metalloaromaticity.

Syntheses of square planar (SP) coordination complexes of gallium(III) are reported herein. Using the pyridine diimine ligand (PDI), we prepared both (PDI2-)GaH (4) and (PDI2-)GaCl (5), which were spectroscopically and structurally characterized. Reduction of PDI using Na metal afforded "Na2PDI", which reacts with in situ-prepared "GaHCl2" or GaCl3 to afford the SP 4 and 5. The planar geometry of these and previously reported SP Al(III) complexes is attributed to energetic stabilization derived from a ring-current effect, or metalloaromaticity. Typically, aromaticity in metal-containing ring systems can be difficult to characterize or confirm experimentally. An experimental approach employing proton NMR spectroscopy and described here provided an estimate of a downfield chemical shift promoted by a small ring-current associated with metalloaromaticity. Near infrared spectroscopic analyses display ligand-metal charge transfer bands which support the assignment of aromaticity. The SP complexes (PDI2-)AlH (1), (PDI2-)AlCl (2), (PDI2-)AlI (3), 4, and 5 are all discussed in this report, using aromaticity as a model for their electronic structure and reactivity properties.

Tunable Chiroptical Properties from the Plasmonic Band to Metal-Ligand Charge Transfer Band of Cysteine-Capped Molybdenum Oxide Nanoparticles.

Understanding the interactions between a semiconducting nanocrystal surface and chiral anchoring molecules could resolve the mechanism of chirality induction in nanoscale and facilitate the rational design of chiral semiconducting materials for chiroptics. Now, chiral molybdenum oxide nanoparticles are presented in which chirality is transferred via a bio-to-nano approach. With facile control of the amount of chiral cysteine molecules under redox treatment, circular dichroism (CD) signals are generated in the plasmon region and metal-ligand charge-transfer band. The obtained enhanced CD signals with tunable lineshapes illustrate the possibility of using chiral molybdenum oxide nanoparticles as potentials for chiral semiconductor nanosensors, optoelectronics, and photocatalysts.

Tunable Chiroptical Properties from the Plasmonic Band to Metal–Ligand Charge Transfer Band of Cysteine‐Capped Molybdenum Oxide Nanoparticles

AbstractUnderstanding the interactions between a semiconducting nanocrystal surface and chiral anchoring molecules could resolve the mechanism of chirality induction in nanoscale and facilitate the rational design of chiral semiconducting materials for chiroptics. Now, chiral molybdenum oxide nanoparticles are presented in which chirality is transferred via a bio‐to‐nano approach. With facile control of the amount of chiral cysteine molecules under redox treatment, circular dichroism (CD) signals are generated in the plasmon region and metal–ligand charge‐transfer band. The obtained enhanced CD signals with tunable lineshapes illustrate the possibility of using chiral molybdenum oxide nanoparticles as potentials for chiral semiconductor nanosensors, optoelectronics, and photocatalysts.

Influence of platinum metals on spectral and electrochemical characteristics of metallated azole luminophors

Spectral (1H NMR and IR), optical, and voltammetry parameters of Pd(II), Pt(II), Rh(III), and Ir(III) complexes with ethylenediamine and metallated azole luminophors [2-phenylbenzoxazole, 2-phenylbenzothiazole, 2,5-diphenyloxazole, 2-(1,1'-biphenyl)-4-yl]-5-phenyloxazole, and 2-(naphthalene-l-yl)benzothiazole] have been studied. The variation in spectroscopic parameters of the complexes is related to the increase in the efficiency of donor-acceptor interaction of Pt(II) and Ir(III) with the ligands as compared to Pd(II) and Rh(III). The correlation between the energy of the metal-ligand charge transfer bands and the difference between the single-electron oxidation and reduction potentials of the complexes is observed. Quenching of the fluorescence of metallated azoles and phosphorescence of the complexes in the visible region from the metal-modified intraligand excited state are related with the increase in the efficiency of the singlettriplet conversion due to the spin-orbital interaction in platinum metals.

Influence of Bi3+ ions on the excitation wavelength of the YVO4:Eu3+ matrix

This work used the non-hydrolytic sol–gel process to prepare co-activated Eu3+/Bi3+ in YVO4 phosphors. We employed X-ray diffraction, which confirmed a tetragonal structure, and photoluminescence to characterize the structure and investigate the photoluminescence properties of the YVO4:Eu3+,Bi3+ matrixes. The large band between 250 and 380 nm in the excitation spectrum of YVO4:Eu3+,Bi3+ corresponded to a ligand-metal charge transfer band (VO43− → Eu3+). Addition of Bi3+ ions increased the charge transfer band and intensified the emission of Eu3+ ions. The x and y color coordinates of the phosphors ranged from 0.60 to 0.68 and from 0.32 to 0.39, respectively. The emission spectrum displayed an intense red emission centered at 618.5 nm, due to the 5D0 → 7F2 transition of Eu3+. The optimal Bi3+ concentration in the phosphor was 1% in mol, which provided more intense emission and longer lifetime. The results indicated that YVO4:Eu3+,Bi3+ is a potential red phosphor with application as biological marker.

Cd2+ and Pb2+ complexation by glutathione and the phytochelatins

Phytochelatins or PCn, (γGlu-Cys)n-Gly, and their glutathione (GSH) precursor are thiol-rich peptides that play an important role in heavy metal detoxification in plants and microorganisms. Complex formation between Cd2+ and Pb2+ and GSH or PCn (n = 2, 4 and 6) are investigated by microcalorimetry, absorption spectrophotometry and T-jump kinetics. Complex formation with Pb2+ or Cd2+ is exothermic, and induces ligand metal charge transfer bands in UV absorption spectral range, which implies the formation of a coordination bond between the metal and the thiol groups of the phytochelatins. Absorption spectra and microcalorimetry experiments allow the determination of the affinity constants and the stoichiometry of the complexes. We show that the three PCn interact with Pb2+ to form the 1:1 and 2:1 M:L complexes, with similar affinity constants (log K11Pb∼4.6, log K21Pb∼11.4). These affinities are independent of the number of thiols and are, moreover, lower than those determined for complex formation with Cd2+. On the other hand, with Cd2+, PC2-Cd, PC2-Cd2, (PC2)3-Cd2, PC4-Cd, PC4-Cd2, PC6-Cd, (PC6)2-Cd3 and PC6-Cd3 complexes are detected. Furthermore, for PC4-Cd, the 1:1 complex is the most stable: affinity constant (log K11Cd∼7.5). Kinetic studies indicate that complex formation between Cd2+ and GSH occurs in the ms range; direct rate constant kobs = (6.8 ± 0.3) 106 M−1 s−1 and reverse rate constant k−obs = 340 ± 210 s−1. Thus, when encapsulated in a silica matrix, PCn can be good candidates for heavy metal detection.

A Versatile Solid Photosensitizer: Periodic Mesoporous Organosilicas with Ruthenium Tris(bipyridine) Complexes Embedded in the Pore Walls

Here, the synthesis of periodic mesoporous organosilicas (PMOs) containing large amounts of ruthenium(II) (Ru) tris(bipyridine) complexes within the pore walls as a solid photosensitizer is reported. The PMOs containing Ru complexes (Ru‐PMOs) are synthesized from highly purified Ru complex precursors with three attached alkylsilatrane groups in the presence of a nonionic surfactant (triblock copolymer) using polyacrylic acid as a promoter. The Ru‐PMOs show strong absorption in the visible light region up to 600 nm due to a metal–ligand charge transfer band, and redox behaviour at ≈0.8 V versus Ag/AgNO3 due to one‐electron oxidation, that are very similar to those of homogeneous Ru(dmb)3(PF6)2 (dmb = 4,4′‐dimethyl‐2,2′‐bipyridine) solution, despite the fact that the Ru complexes are embedded in the pore walls at high density. Ru‐PMOs loaded with platinum metal or iridium oxide provide efficient photocatalysis for hydrogen evolution or water oxidation, respectively, under irradiation with visible light up to 600 nm in the presence of sacrificial agents. These results indicate that Ru‐PMO is a versatile solid photosensitizer for the construction of various heterogeneous photocatalytic systems simply by placing catalytic materials in the stable mesochannels.

Radical Monocationic Guanidino-Functionalized Aromatic Compounds (GFAs) as Bridging Ligands in Dinuclear Metal Acetate Complexes: Synthesis, Electronic Structure, and Magnetic Coupling.

In this work, the oxidation of several new dinuclear metal (M) acetate complexes of the redox-active guanidino-functionalized aromatic compound (GFA) 1,2,4,5-tetrakis(tetramethylguanidino)benzene (1) was studied. The complexes [1{M(OAc)2}2] (M = Ni or Pd) were oxidized to the radical monocationic complexes [1{M(OAc)2}2](+ •). From CV (cyclic voltammetry) measurements, the Gibbs free enthalpy for disproportionation of [1{M(OAc)2}2](+ •) into [1{M(OAc)2}2] and [1{M(OAc)2}2](2+) could be estimated to be roughly +20 kJ mol(-1) in CH2Cl2 solution. A characteristic feature of the [1{M(OAc)2}2](+ •) complexes is the presence of intense metal-ligand charge-transfer bands in the electronic absorption spectra. The complex [1{Ni(OAc)2}2](+ •) combines three paramagnetic centers with four metal-centered unpaired electrons and a ligand centered π-radical and exhibits a sextet electronic ground state. Spin distribution of the Ni complexes was evaluated by paramagnetic (1)H and (13)C NMR and was correlated with calculations. The strong ferromagnetic metal-ligand magnetic coupling was studied in the solid state by magnetometric (SQUID) measurements and by quantum chemical (DFT) calculations. The temperature dependence of the paramagnetic NMR shift was used for the evaluation of the magnetic coupling between the Ni centers and the π-radical in solution.

Nanospherical silica as luminescent markers obtained by sol-gel.

Hybrid nanosilicas constitute a broad study field. They find application as catalysts, pigments, drug delivery systems, and biomaterials, among others, and it is possible to obtain them via the sol-gel methodology. Lanthanide ions present special properties like light emission. Their incorporation into a silica matrix can enhance their luminescent properties, which enables their application as luminescent markers. This work reports on (i) the preparation of luminescent spherical hybrid silica nanoparticles by the hydrolytic sol-gel methodology, (ii) doping of the resulting matrix with the europium(III) ion or its complex with 1,10-phenanthroline, and (iii) characterization of the final powders by scanning electron microscopy, infrared spectroscopy, X-ray diffraction, and europium(III) ion photoluminescence. The synthesized materials consisted of hybrid, amorphous, polydispersed nonspherical silicas with average size of 180nm. Photoluminescence confirmed incorporation of the europium(III) ion and its complex into the silica matrix-the ligand-metal charge transfer band emerged in the excitation spectra. The emission spectra presented the bands corresponding to the transition of the excited state (5)D0 level to (7)FJ (J=0, 1, 2, 3 and 4). The main emission occurred in the red region; the lifetime was long. These characteristics indicated that the prepared nanospherical hybrid silicas could act as luminescent markers.

Band-gap energy of heteropoly compounds containing Keggin polyanion-[PVxMo12-xO40]−(3+x) relates to counter-cations and temperature studied by UV-VIS diffuse reflectance spectroscopy

The band gap energy (absorption edge energies) of the pure H3[PMo12O40]·13H2O and H4[PVMo11O40]·13H2O, respectively, supported on SiO2 and SiC and some of its NH4+ and Cs+ salts were determined by different methods. The influence of the counter-cations and the temperature on band gap energy was studied. In this purpose, the diffuse reflectance spectra of above mentioned compounds were registered at different temperatures, and it were transposed in the curves of the Kubelka-Munk function vs. wavelength. The band gap energies were determined by processing of low field energy of the ligand-metal charge transfer band (O2− → Mo6+ and O2− → V5+) usually observed between 200 and 400 nm on these curves. In this aim, the Tauc's relation was adapted for Kubelka-Munk function use and it was plotted for n = 1/2 (direct transition) and 2 (indirect transition) vs. wave energy (photon energy). The intersection of the curves’ tangent drawn to their point of inflection with horizontal axis gives the band gap energy. The other method for calculation of band gap energy was the differential calculus on the Kubelka-Munk function vs. wave energy curve where the x value corresponding to curves’ maximum is the found value. The comparison between experimental band gap values and literature data showed their partial fit. The higher temperature produces the band gap energy diminution as a result of a stronger interaction between Keggin Units, which occurs especially by the crystallization water loss. The Keggin Units isolation one from another by voluminous counter-ion or their spreading on a support leads to a weaker interaction between them and as a consequence, the increasing of absorption edge energy. A linear correlation of the crystallites size with band gap energy was observed.

A Donor–Acceptor Tetrathiafulvalene Ligand Complexed to Iron(II): Synthesis, Electrochemistry, and Spectroscopy of [Fe(phen)2(TTF-dppz)](PF6)2

The synthesis and photophysical properties of the complex [Fe(phen)(2)(TTF-dppz)](2+) (TTF-dppz = 4',5'-bis-(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine, phen = 1,10-phenanthroline) are described. In this complex, excitation into the metal-ligand charge transfer bands results in the population of a high-spin state of iron(II), with a decay lifetime of approximately 1.5 ns, in dichloromethane, at room temperature. An intraligand charge transfer state can also be obtained and has a lifetime of 38 ps. A mechanism for the different states reached is proposed based on transient absorption spectroscopy.

Electron Injection Efficiency in Ru-Dye Sensitized TiO2 in the Presence of Room Temperature Ionic Liquid Solvents Probed by Femtosecond Transient Absorption Spectroscopy: Effect of Varying Anions

We investigated the electron injection efficiency of a Ru-complex dye (N719) sensitized TiO2 system using femtosecond transient absorption in the presence of two different room temperature ionic liquids (ILs) varying their anionic counterparts (tetracyanoborate [TCB]− and dicyanoamide [DCA]−), keeping the cationic counterpart (ethylmethylimidazolium [EMIm]+) invariant. We also investigated the effect of varying IL environments on the metal–ligand charge transfer band of the adsorbed N719 on TiO2 film using steady state absorption spectroscopy and the effect on cyano stretching frequency of ILs in the presence of TiO2 films using attenuated total reflectance-IR method. The downshift of conduction band (CB) edge energy of TiO2 nanoparticles in presence of [EMIm][TCB] IL environment has recently been reported (J. Phys. Chem. C2011, 115, 816–822). Based on CB edge energy modulation and transient absorption spectroscopic data, we found that the electron injection efficiency is higher in the N719/TiO2 system in the presence of [EMIm][TCB] IL environment than in the presence of [EMIm][DCA] IL environment by ∼20%.

Palladium(II) complexes with 1-allyl-3-(2-pyridyl)thiourea: Synthesis and spectroscopic characterization

New Pd(II) complexes with 1-allyl-3-(2-pyridyl)thiourea (APTU) of the formulas [Pd(C9H11N3S)Cl2] (I) and [Pd(C9H11N3S)2]Cl2 (II) were obtained and examined by UV-Vis, IR, and 1H NMR spectroscopy. The conditions for the complexation reactions were optimized. The instability constants and molar absorption coefficients of these complexes were calculated. Comparison of the characteristic bands in the UV-Vis and IR spectra of the complexes and free APTU revealed that the ligand in both complexes is coordinated to the metal atom in the thione form in the bidentate chelating mode through the S atom of the thiourea group and the pyridine N atom. In the UV-Vis spectra of the complexes, the charge transfer bands (π → π* Py) and n → π* (C=NPy), (C=S) experience hypsochromic shifts by 450–470 cm−1 caused by the coordination of APTU to the metal ion, which gives rise to ligand-metal charge-transfer bands (C=NPy → Pd, n → π* (C=S)) and (SPd). The protons in the 6-, 4-, and 3-positions of the pyridine ring and the thiourea NH proton in the chelate ring are most sensitive to the complexation.

Strong ferromagnetic metal–ligand exchange in a nickel bis(3,5-dipyridylverdazyl) complex

A new 1,5-dipyridyl verdazyl, synthesized from the corresponding dipyridyl hydrazone, coordinates nickel(ii) to form a structurally characterized, pseudooctahedral complex analogous to Ni(terpy)(2)(2+). The unusually short Ni-verdazyl distance results in strong ferromagnetic exchange (J(Ni-rad) = +300, J(rad-rad) = +160 cm(-1)) between all three paramagnetic species along with a metal-ligand charge transfer band in the electronic spectrum.

Specific features of absorption and DSC for the DEA-CuCl 4 nanoparticles incorporated into the PMMA polymer matrices

Diethylammonium tetrachlorcuprate NH 2(C 2H 5) 2CuCl 4 (DEA-CuCl 4) nanoparticles with sizes about 10 nm were synthesized and embedded into the PMMA polymer matrices. Using DSC temperature study a substantial influence of the polymer matrix on the phase transition temperatures was shown, reflecting a disturbing effect of principal 3d Cu–3p Cl metal–ligand charge transfer bands. Dependence of the absorption spectra on the nanocrystallites (NC) concentration was studied. It was established that an increase of the NC content results in spectral shift of CuCl 4 absorption bands. Explanation of this phenomenon has been suggested within a framework of first principle crystal field quantum chemical calculations.

Photo-induced chirality switching in a cobaloxime complex crystal

Switching of molecular chirality under photo-irradiation was studied in a cobaloxime complex crystal. Excitation of the d-d transition of the Co(III) ion appeared to be much more effective in inducing the chirality change than excitation of the ligand-metal charge transfer band although the latter is more effective in breaking the Co-C bond that initiates the chirality switching. The chirality change versus irradiation time showed a steplike behavior suggesting that chirality switching of molecules occurred in correlation with their nearest neighbors.