Metal-to-ligand Charge Transfer Band Research Articles

Visible-Light Selective Photooxidation of Aromatic Hydrocarbons via Ligand-to-Metal Charge Transfer Transition on Nb2O5

This study shows that a vacuum heat treatment of Nb2O5 permits the ligand-to-metal charge transfer (LMCT) transition between its surface and adsorbed aromatic hydrocarbons, which corresponds to visible light (>390 nm) absorption. Selective photooxidation of aromatic hydrocarbons to form the corresponding carbonyl compounds could effectively proceed under visible light irradiation by using this LMCT transition, and the catalytic activity is much superior to that of TiO2. Specifically, the LMCT bands of toluene-adsorbed Nb2O5 correlated well with the action spectrum for toluene photooxidation, suggesting that the reaction proceeded via the LMCT photoexcitation. We successfully detected toluene radical cation and its further oxidation product of benzylperoxy radical as intermediate species on toluene-adsorbed Nb2O5 through the LMCT transition. These findings suggest a novel reaction mechanism for the selective photooxidation of aromatic hydrocarbons via LMCT transition on Nb2O5.

Cobalt Porphyrin-Thiazyl Radical Coordination Polymers: Toward Metal-Organic Electronics.

Herein we delineate an unusual one-dimensional coordination polymer (CP), 3, prepared from S = 1/2 Co(TPP), 1 (TPP = 5,10,15,20-tetraphenylporphyrin dianion), and S = 1/2 4-(4'-pyridyl)-1,2,3,5-dithiadiazolyl (py-DTDA) radical, 2. The atypically long S-S distance for CP 3 (2.12 Å) reflects fractional electron transfer from the formally Co(II) ion into the antibonding π-SOMO of the metal-bound py-DTDA bridging ligand. The bonding in solid CP 3 involves noninteger redox states in a resonance hybrid repeat unit best formulated as [Co(TPP)]0.5+ hemication (Co2.5+) bound to a dithiadiazolide hemianion (py-DTDA0.5-). DFT calculations confirm the metal to ligand charge transfer (MLCT) character of the low-lying electronic states (641, 732, and 735 nm) observed for CP 3 and show that oligomer chains of length ≥14 repeat units tend toward a band structure with a limiting band gap energy of 0.669(6) eV. In dichloromethane, the reaction between radicals 1 and 2 involves coordination of the Co(II) ion by a py-DTDA ring sulfur atom, orbitally favored spin-pairing, and the formation of the thermodynamically favored diamagnetic five-coordinate S-bound adduct, Co(TPP)(S-py-DTDA), 3a. Polymerization and crystallization of 3a affords diamagnetic CP 3. Dissolution of CP 3 in DMSO favors Co-S bond heterolysis, yielding the diamagnetic six-coordinate purple N-bound CoIII(TPP)(N-py-DTDA-)(O═SMe2) complex (λmax, 436 nm). However, monomerization of CP 3 in dry 1,2-dichloroethane affords bright green diamagnetic CoIII(TPP)(N-py-DTDA-), 3b, with multiple MLCT bands in the 800-1100 nm NIR region and a red-shifted Soret band (λmax, 443 nm). Implications for the use of CP 3 in electronic devices are discussed based on its density of states.

Bright and Long‐Lived Emission from a Starburst‐Type Arylborane‐Appended Polypyridyl Ruthenium(II) Complex

The starburst‐type arylborane‐appended polypyridyl RuII complex [Ru{4,7‐bis({dimesityl}boryldurylethynyl)‐1,10‐phenanthroline}3]2+ (1) was synthesized, and its spectroscopic and photophysical properties were evaluated. In the excited state of the complex, intramolecular charge transfer (CT) between the π orbital of the aryl group and the vacant p orbital of the boron atom [π(aryl)–p(B) CT] interacts synergistically with metal‐to‐ligand charge transfer (MLCT). The synergistic CT interactions in 1 are more effective than those in previously reported arylborane‐appended polypyridyl RuII complexes and lead to bright (molar absorption coefficient of the MLCT band = 7.2 × 104 m–1 cm–1, emission quantum yield = 0.29) and long‐lived emission (8.7 µs) of 1 in CH3CN at 298 K. The unique emission characteristics of 1 are discussed in detail together with its electrochemical properties and time‐dependent DFT (TD‐DFT) calculations.

Description of excited states in [Re(Imidazole)(CO)3 (Phen)](+) including solvent and spin-orbit coupling effects: Density functional theory versus multiconfigurational wavefunction approach.

The low-lying electronic excited states of [Re(imidazole)(CO)3 (phen)](+) (phen = 1,10-phenanthroline) ranging between 420 nm and 330 nm have been calculated by means of relativistic spin-orbit time-dependent density functional theory (TD-DFT) and wavefunction approaches (state-average-CASSCF/CASPT2). A direct comparison between the theoretical absorption spectra obtained with different methods including SOC and solvent corrections for water points to the difficulties at describing on the same footing the bands generated by metal-to-ligand charge transfer (MLCT), intraligand (IL) transition, and ligand-to-Ligand- charge transfer (LLCT). While TD-DFT and three-roots-state-average CASSCF (10,10) reproduce rather well the lowest broad MLCT band observed in the experimental spectrum between 420 nm and 330 nm, more flexible wavefunctions enlarged either by the number of roots or by the number of active orbitals and electrons destabilize the MLCT states by introducing IL and LLCT character in the lowest part of the absorption spectrum. © 2016 Wiley Periodicals, Inc.

Remarkably Intense Emission from Ruthenium(II) Complexes with Multiple Borane Centers

The electrochemical, spectroscopic, and phophysical properties of a series of Ru(II) complexes having a triarylborane-appended 2,2'-bipyridine (bpy) ligand(s) (RuBbpys: [Ru(Bbpy)n(bpy)(3-n)](2+) (B1n) and [Ru(B2bpy)n(bpy)(3-n)](2+) (B2n), B = (dimesityl)boryldurylethynyl group(s) at the 4- or 4,4'-position(s) in bpy, n = 1-3) are described. In the excited states of the complexes, the intramolecular charge transfer transitions between the π-orbital of the aryl group and the vacant p-orbital on the boron atom (π(aryl)-p(B) CT) synergistically interact with the metal-to-ligand charge transfer (MLCT) transitions. The molar absorption coefficient of the MLCT band (ε(MLCT)) of the complex increased with increasing n, and B23 showed extremely intense absorption with ε(MLCT) = 5.6 × 10(4) M(-1) cm(-1) at 488 nm. Furthermore, B23 showed the highest emission quantum yield (0.43) among those of the polypyridine Ru(II) complexes hitherto reported. As one of the interesting results, we report that the radiative rate constant of B2n shows the correlation with ε(MLCT). The effects of the synergistic MLCT/π(aryl)-p(B) CT interactions on the spectroscopic and photophysical characteristics of RuBbpys are discussed in detail.

Thermochromism of Cu(I) Tetrakisguanidine Complexes: Reversible Activation of Metal-to-Ligand Charge-Transfer Bands.

Tetranuclear, intensely blue-coloured Cu(I) complexes were synthesised in which two Cu2 X3 (-) units (X=Br or I) are bridged by a dicationic GFA (guanidino-functionalised aromatic) ligand. The UV/Vis spectra show a large metal-to-ligand charge-transfer (MLCT) band around 638 nm. The tetranuclear "low-temperature" complexes are in a temperature-dependent equilibrium with dinuclear Cu(I) "high-temperature" complexes, which result from the reversible elimination of two CuX groups. A massive thermochromism effect results from the extinction of the strong MLCT band upon CuX elimination with increasing temperature. For all complexes, quantum chemical calculations predict a small and method-dependent energy difference between the possible electronic structures, namely Cu(I) and dicationic GFA ligand (closed-shell singlet) versus Cu(II) and neutral GFA ligand (triplet or broken-symmetry state). The closed-shell singlet state is disfavoured by hybrid-DFT functionals, which mix in exact Hartree-Fock exchange, and is favoured by larger basis sets and consideration of a polar medium.

Structures, Electrochemical and Spectral Properties of a Series of [MnN(CN) 3 (diimine)] – Complexes

The structures and the electrochemical and spectral properties of a series of discrete [PPh4][MnN(CN)3(diimine)] complexes {diimine = 2,2′-bipyridine [1(bpy)], 4,4′-dimethyl-2,2′-bipyridine [1(4-dmbpy)], 5,5′-dimethyl-2,2′-bipyridine [1(5-dmbpy)], 1,10-phenanthroline [1(phen)] and bipyrimidine [1(bpym)]} have been assessed. These complexes represent potential building blocks for the construction of coordination frameworks. The incorporation of diimine ligands with different π-accepting capacities allows the systematic tuning of the metal-to-ligand charge-transfer (MLCT) band as well as the MnV/MnVI redox potential. The complexes exhibit solvatochromism, and the marked sensitivity of λmax of the MLCT band to polar solvents leads the way towards their potential application in chemical sensing.

Syntheses and electronic and optical properties of complexes of the bis(2,2′-bipyrazyl)ruthenium unit

Seven new complexes of the form cis-[RuII(bpz)2(L–L)]n+ (bpz=2,2′-bipyrazyl: n=2; L–L=4,4′-bis(tert-butyl)-2,2′-bipyridyl, 4,4′-diphenyl-2,2′-bipyridyl, 4,4′-dichloro-2,2′-bipyridyl, 4,4′-diamino-2,2′-bipyridyl, 4,4′-bis(trifluoromethyl)-2,2′-bipyridyl, 4,4′-bis(methoxycarbonyl)-2,2′-bipyridyl: n=4; L–L=N″,N′″-dimethyl-4,4′:2′,2″:4″,4′″-quaterpyridinium) are prepared and isolated as their PF6− and Cl− salts. Improved methods for synthesising bpz and 4,4′-bis(trifluoromethyl)-2,2′-bipyridyl are described also. Characterisation involves various techniques including 1H NMR spectroscopy and mass spectrometry. The new compounds are studied alongside the known species where n=2 and L–L=2,2′-bipyridyl, 4,4′-dimethyl-2,2′-bipyridyl or 2,2′-bipyrimidine. Their UV–Vis spectra display intense intraligand π→π∗ absorptions, and also metal-to-ligand charge-transfer (MLCT) bands with two resolved maxima in the visible region. Red-shifts in the MLCT bands occur as the electron-donating strength of L–L increases. Cyclic voltammograms show reversible RuIII/II oxidation waves, and several ligand-based reductions that are also mostly reversible. The variations in the redox potentials correlate with changes in the MLCT energies. Time-dependent density functional theory calculations give relatively good correlations with the experimental UV–Vis spectra for selected complexes when using the M06 functional and basis sets Def2-QZVP (on Ru) and Def2-SVP (on all other atoms) in acetonitrile. The lowest energy visible absorption band is confirmed to be due to RuII→bpz MLCT, while further such transitions occur along with MLCT to L–L at higher energies.

Nonlinear Optical Chromophores with Two Ferrocenyl, Octamethylferrocenyl, or 4-(Diphenylamino)phenyl Groups Attached to Rhenium(I) or Zinc(II) Centers

The compounds 4,4′-bis[(E)-2-R-vinyl]-2,2′-bipyridyl {R = ferrocenyl [(Fcv)_2bpy], octamethylferrocenyl [(Me_8Fcv)_2bpy] or 4-(diphenylamino)phenyl [(Dapv)_2bpy]} are used to prepare eight new complexes with Zn^(II)Cl_2, Zn^(II)(OAc)_2, or fac-Re^ICl(CO)_3 centers. The recently reported complex fac-Re^ICl(CO)_3[(Dapv)_2bpy] (Horvath, R. et al. Inorg. Chem. 2013, 52, 1304) is also studied. Electronic absorption spectra show intense d → π^* metal-to-ligand charge-transfer (MLCT) and π → π^* intraligand charge-transfer (ILCT) absorption bands, the relative energies of which correlate logically with the molecular structure. Cyclic voltammetry reveals a reversible oxidation wave for the Fc/Me_8Fc complexes, accompanied by quasireversible or irreversible ligand-based reductions. The Re complexes also show irreversible Re^(II/I) waves. Single-crystal X-ray structures are reported for (Me_8Fcv)_2bpy, Zn^(II)Cl_2[(Me_8Fcv)_2bpy], Zn^(II)(OAc)_2[(Fcv)_2bpy]·CHCl_3, and fac-Re^ICl(CO)_3[(Me_8Fcv)_2bpy]·0.5CHCl_3. Molecular first hyperpolarizabilities β are measured in DCM solutions via the hyper-Rayleigh scattering (HRS) technique at 1300 nm. Stark (electroabsorption) spectroscopic studies on only the MLCT bands in frozen butyronitrile allow the indirect estimation of lower limits for the overall static first hyperpolarizabilities β_0. Time-dependent density functional theory (TD-DFT) calculations on selected complexes confirm the expected assignments of their low energy absorption bands, with the best results obtained by using the M06 functional and Def2-TZVP/SVP/TZVPP mixed basis set. DFT predicts that the total static first hyperpolarizability β_(tot) increases in the Zn^(II)Cl_2 series in the order R = Fc < Me_8Fc < Dap, consistent with the HRS and Stark data. The computed β values increase substantially on moving from the gas phase to a DCM or MeCN solvent medium, and the essentially 2D nature of the chromophores leads to dominant β_(xxy) tensor components.

Ruthenium sensitizers with a hexylthiophene-modified terpyridine ligand for dye-sensitized solar cells: synthesis, photo- and electrochemical properties, and adsorption behavior to the TiO2 surface.

Two novel ruthenium sensitizers with a hexylthiophene-modified terpyridine ligand (TUS-35 and TUS-36) were synthesized to improve the molar absorptivity of the previously reported ruthenium sensitizer (TBA)[Ru{4'-(3,4-dicarboxyphenyl)-4,4″-dicarboxyterpyridine}(NCS)3], TBA = tetrabutylammonium (TUS-21). A relatively strong absorption appeared at ∼380 nm, and the molar absorption coefficient at the metal-to-ligand charge transfer (MLCT) band decreased in TUS-35 by introducing a 2-hexylthiophene unit to the 5-position of the terpyridine-derived ligand. For comparison, a relatively strong absorption was observed at ∼350 nm without decreasing the molar absorption coefficient at the MLCT band in TUS-36 by introducing a 2-hexylthiophene unit to the 4-position of the terpyridine-derived ligand. On the other hand, the energy levels of the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of these two sensitizers were found to be almost equal to those of TUS-21. The adsorption behavior of TUS-35 and TUS-36 was similar to that of (TBA)[Ru{4'-(3,4-dicarboxyphenyl)terpyridine}(NCS)3] (TUS-20), which binds to the TiO2 surface by using the 3,4-dicarboxyphenly unit, rather than that of TUS-21, which adsorbs to the TiO2 photoelectrode using one of the carboxyl groups at the terminal pyridines of the terpyridine-derived ligand. Therefore, TUS-35 and TUS-36 are considered to bind to the TiO2 surface by using the 3,4-dicarboxyphenly unit just like TUS-20. The dye-sensitized solar cells (DSCs) with TUS-35 and TUS-36 showed a relatively lower conversion efficiency (6.4% and 5.7%, respectively) compared to the DSC with TUS-21 (10.2%). Open-circuit photovoltage decay and electrochemical impedance spectroscopy measurements revealed that the promoted charge recombination and/or charge transfer of the injected electrons in the TiO2 photoelectrode is a main reason for the inferior performances of TUS-35 and TUS-36.

The effect of thermal motion on the electron localization in metal-to-ligand charge transfer excitations in [Fe(bpy)3]2+

Accurate electronic structure calculations of the lowest excited states have been performed on twenty snapshots of a molecular dynamics simulation of [Fe(bpy)3](2+) dissolved in water. The thermal motion distorts the structure of the complex from its average D3 symmetry, causing the localization on one bipyridine ligand of the excited electron in the metal-to-ligand charge transfer (MLCT) state. The excitation energy is about 0.25 eV lower than that for the delocalized description of the MLCT state and is in good agreement with experiments. The composition of the MLCT band is carefully analyzed and the effect of thermal motion on the mechanism of light-induced spin crossover is discussed.

Crystal structure, electronic structure, and photoluminescence properties of La₃BW(1-x)Mo(x)O₉:Eu³⁺ red phosphor.

A series of La3BW(1-x)Mo(x)O9:Eu(3+) (x = 0-0.4) polycrystalline powders were prepared by using solid-state reactions. The phase structure, UV-vis absorption spectra, and photoluminescence properties were studied as a function of the Mo/W ratio. When Mo(6+) ions are incorporated into the lattice, the characteristic sharp lines in the excitation spectra of Eu(3+) monitored at 617 nm are prominently enhanced, which join the ligand-to-metal charge transfer (LMCT) band of La3BW(1-x)Mo(x)O9:Eu(3+) into a broad band ranging from 250 to 450 nm centered at 375 nm. The intensity of the broad excitation band reaches a maximum when the content of Mo(6+) ions increases to x = 0.3. On the other hand, the LMCT band around 306 nm decreases and shifts toward the longer wavelength. These features are advantageous to near-UV or blue light GaN-based LED applications. Orbital population analysis by density functional theory calculation (DFT) reveals that the near-UV excitation of La3BW(1-x)Mo(x)O9:Eu(3+) red phosphor is due to the electronic transition from the O 2p orbital to the W 5d and Mo 4d orbitals, respectively. With the introduction of Mo(6+) into the lattice, the band gap of La3BW(1-x)Mo(x)O9 becomes narrower than that of the pure phase La3BWO9.

Design strategies to improve the sensitivity of photoactive metal carbonyl complexes (photoCORMs) to visible light and their potential as CO-donors to biological targets.

The recent surprising discovery of the beneficial effects of carbon monoxide (CO) in mammalian physiology has drawn attention toward site-specific delivery of CO to biological targets. To avoid difficulties in handling of this noxious gas in hospital settings, researchers have focused their attention on metal carbonyl complexes as CO-releasing molecules (CORMs). Because further control of such CO delivery through light-triggering can be achieved with photoactive metal carbonyl complexes (photoCORMs), we and other groups have attempted to isolate such complexes in the past few years. Typical metal carbonyl complexes release CO when exposed to UV light, a fact that often deters their use in biological systems. From the very beginning, our effort therefore was directed toward identifying design principles that could lead to photoCORMs that release CO upon illumination with low-power (5-15 mW/cm(2)) visible and near-IR light. In our work, we have utilized Mn(I), Re(I), and Ru(II) centers (all d(6) ground state configuration) to ensure overall stability of the carbonyl complexes. We also hypothesized that transfer of electron density from the electron-rich metal centers to π* MOs of the ligand frame via strong metal-to-ligand charge transfer (MLCT) transitions in the visible/near-IR region would weaken metal-CO back-bonding and promote rapid CO photorelease. This expectation has been realized in a series of carbonyl complexes derived from a variety of designed ligands and smart choice of ligand/coligand combinations. Several principles have emerged from our systematic approach to the design of principal ligands and the choice of auxiliary ligands (in addition to the number of CO) in synthesizing these photoCORMs. In each case, density functional theory (DFT) and time-dependent DFT (TDDFT) study afforded insight into the dependence of the CO photorelease from a particular photoCORM on the wavelength of light. Results of these theoretical studies indicate that extended conjugation in the principal ligand frames as well as the nature of the donor groups lower the energy of the lowest unoccupied MOs (LUMOs) while auxiliary ligands like PPh3 and Br(-) modulate the energy of the occupied orbitals depending on their strong σ- or π-donating abilities. As a consequence, the ligand/coligand combination dictates the energy of the MLCT bands of the resulting carbonyl complexes. The rate of CO photorelease can be altered further by proper disposition of the coligands in the coordination sphere to initiate trans-effect or alter the extent of π back-bonding in the metal-CO bonds. Addition of more CO ligands blue shift the MLCT bands, while intersystem crossing impedes labilization of metal-CO bonds in several Re(I) and Ru(II) carbonyl complexes. We anticipate that our design principles will provide help in the future design of photoCORMs that could eventually find use in clinical studies.

Pyranopterin dithiolene distortions relevant to electron transfer in xanthine oxidase/dehydrogenase.

The reducing substrates 4-thiolumazine and 2,4-dithiolumazine have been used to form MoIV-product complexes with xanthine oxidase (XO) and xanthine dehydrogenase. These MoIV-product complexes display an intense metal-to-ligand charge-transfer (MLCT) band in the near-infrared region of the spectrum. Optical pumping into this MLCT band yields resonance Raman spectra of the Mo site that are devoid of contributions from the highly absorbing FAD and 2Fe2S clusters in the protein. The resonance Raman spectra reveal in-plane bending modes of the bound product and low-frequency molybdenum dithiolene and pyranopterin dithiolene vibrational modes. This work provides keen insight into the role of the pyranopterin dithiolene in electron-transfer reactivity.

Synthesis, Structures, and Optical Properties of Ruthenium(II) Complexes of the Tris(1-pyrazolyl)methane Ligand

Four new complex salts [Ru(II)Cl(Tpm)(L(A))2][PF6]n [Tpm = tris(1-pyrazolyl)methane; n = 1, L(A) = pyridine (py) 1 or ethyl isonicotinate (EIN) 2; n = 3, L(A) = N-methyl-4,4'-bipyridinium (MeQ(+)) 3 or N-phenyl-4,4'-bipyridinium (PhQ(+)) 4] have been prepared and characterized. Electronic absorption spectra show intense d → π* metal-to-ligand charge-transfer (MLCT) absorption bands, while cyclic voltammetry reveals a reversible Ru(III/II) wave, accompanied by quasireversible or irreversible L(A)-based reductions for all except 1. Single crystal X-ray structures have been obtained for 1•Me2CO, 2, and 3•Me2CO. For 2-4, molecular first hyperpolarizabilities β have been measured in acetonitrile solutions via the hyper-Rayleigh scattering (HRS) technique at 800 nm. Stark (electroabsorption) spectroscopic studies on the MLCT bands in frozen butyronitrile allow the indirect estimation of static first hyperpolarizabilities β0. The various physical data obtained for 3 and 4 are compared with those reported previously for related cis-{Ru(II)(NH3)4}(2+) species [ Coe, B. J. et al. J. Am. Chem. Soc. 2005 , 127 , 4845 ]. TD-DFT calculations on the complexes in 1-4 confirm that their lowest energy absorption bands are primarily Ru(II) → L(A) MLCT in character, while Ru(II) → Tpm MLCT transitions are predicted at higher energies. DFT agrees with the Stark, but not the HRS measurements, in showing that β0 increases with the electron-accepting strength of L(A). The 2D nature of the chromophores is evidenced by dominant βxxy tensor components.

Heteroleptic Ru(II) complexes containing aroyl hydrazone and 2,2′-bipyridyl: Synthesis, X-ray crystal structures, electrochemical and DFT studies

Four heteroleptic Ru(II) complexes [Ru(bpy)2(L)]X (L=bidentate N, O− donor hydrazone ligands, X=ClO4 or PF6) have been synthesized and characterized. Single crystal X-ray structures of two of the complexes are also reported. The complexes show strong absorptions in the 400–600nm region of the visible radiation, with very similar room temperature emission spectra to that of reference [Ru(bpy)3]X2 but with the lowest energy MLCT (metal to ligand charge transfer) band bathochromically shifted by about 50nm. Results obtained from both electrochemical experiments and DFT calculations show remarkable differences in the HOMO–LUMO properties of the hydrazone complexes compared to [Ru(bpy)3]X2. TDDFT calculations have been used to understand the nature of the electronic transitions.

Ruthenium(II) and Osmium(II) Complexes Bearing Bipyridine and the N-Heterocyclic Carbene-Based C^N^C Pincer Ligand: An Experimental and Density Functional Theory Study

Ruthenium(II) and osmium(II) complexes [M(C^N^C)(N^N)L](n+) (L = Cl(-), n = 1; L = CH3CN, t-BuNC, n = 2) containing a neutral tridentate N-heterocyclic carbene (NHC)-based pincer ligand, either 2,6-bis(1-butylimidazol-2-ylidene)pyridine (C(1)^N^C(1)) or 2,6-bis(3-butylbenzimidazol-2-ylidene)pyridine (C(2)^N^C(2)), and a neutral 2,2'-bipyridine-type aromatic diimine have been prepared. Investigations into the effects of varying M (Ru and Os), C^N^C, N^N, and L on the structural, electrochemical, absorption, and emission characteristics associated with [M(C^N^C)(N^N)L](n+) are presented. Interestingly, spectroscopic findings and time-dependent density functional theory (TD-DFT) calculations in this work support a dπ(Ru(II)/Os(II)) → π*(N^N) metal-to-ligand charge transfer (MLCT) assignment for the lowest-energy transition in [M(C^N^C)(N^N)L](n+) and not a dπ(Ru(II)/Os(II)) → π*(C^N^C) MLCT assignment. This is in stark contrast to [Ru(tpy)(bpy)Cl](+) and [Os(tpy)(bpy)Cl](+) (tpy = 2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine) for which the lowest-energy transitions are assigned as dπ(Ru/Os) → π*(tpy) MLCT transitions. [Ru(II)(C^N^C)(N^N)L](n+) is emissive with emission maxima of around 600-700 nm observed upon photoexcitation of their dπ(Ru(II)) → π*(N^N) MLCT bands. The electronic structures for [Ru(C^N^C)(N^N)Cl](0) have also been probed by spectroelectrochemistry, electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations, which reveal that the lowest unoccupied molecular orbitals (LUMOs) for [Ru(C^N^C)(N^N)Cl](+) are N^N-based.

Complexes with Redox-Active Ligands: Synthesis, Structure, and Electrochemical and Photophysical Behavior of the Ru(II) Complex with TTF-Annulated Phenanthroline

Ru(II) complexes with chelating ligands, 4',5'-ethylenedithiotetrathiafulvenyl[4,5-f][1,10]phenanthroline (L1), 1,3-dithiole-2-thiono[4,5-f][1,10]phenanthroline (L2), and 1,3-dithiole-2-ono[4,5-f][1,10]phenanthroline (L3), have been prepared and their structural, electrochemical, and photophysical properties investigated. Density functional theory (DFT) calculations indicate that the highest occupied molecular orbital of [Ru(bpy)2(L1)](PF6)2 (1) is located on the tetrathiafulvalene (TTF) subunit and appears ~0.6 eV above the three Ru-centered d orbitals. In agreement with this finding, 1 exhibits three reversible oxidations: the two at lower potentials take place on the TTF subunit, and the one at higher potential is due to the Ru(3+)/Ru(2+) redox couple. Complexes [Ru(bpy)2(L2)](PF6)2 (2) and [Ru(bpy)2(L3)](PF6)2 (3) exhibit only the Ru(3+)/Ru(2+)-related oxidation. The optical absorption spectra of all complexes reveal a characteristic metal-to-ligand charge transfer (MLCT) band centered around 450 nm. In addition, in the spectrum of 1 the MLCT band is augmented by a low-energy tail that extends beyond 500 nm and is attributed to the intraligand charge transfer (ILCT) transition of L1, according to time-dependent DFT calculations. The substantial decrease in the luminescence quantum yield of 1 compared to those of 2 and 3 is attributed to the reductive quenching of the emissive state via electron transfer from the TTF subunit to the Ru(3+) center, thus allowing nonradiative relaxation to the ground state through the lower-lying ILCT state. In the presence of O2, complex 1 undergoes a photoinduced oxidative cleavage of the central C═C bond of the TTF fragment, resulting in complete transformation to 3. This photodegradation process was studied with (13)C NMR and optical absorption spectroscopy.

Heptametallic, Octupolar Nonlinear Optical Chromophores with Six Ferrocenyl Substituents

New complexes with six ferrocenyl (Fc) groups connected to Zn(II) or Cd(II) tris(2,2'-bipyridyl) cores are described. A thorough characterisation of their BPh4(-) salts includes two single-crystal X-ray structures, highly unusual for such species with multiple, extended substituents. Intense, visible d(Fe(II))→π* metal-to-ligand charge-transfer (MLCT) bands accompany the π→π* intraligand charge-transfer absorptions in the near UV region. Each complex shows a single, fully reversible Fe(III/II) wave when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined by using hyper-Rayleigh scattering and Stark spectroscopy. The latter gives static first hyperpolarisabilities β0 reaching as high as approximately 10(-27) esu and generally increasing with π-conjugation extension. Z-scan cubic NLO measurements reveal high two-photon absorption cross-sections σ2 of up to 5400 GM in one case. DFT calculations reproduce the π-conjugation dependence of β0, and TD-DFT predicts three transitions close in energy contributing to the MLCT bands. The lowest energy transition has octupolar character, whereas the other two are degenerate and dipolar in nature.

Synthesis, Structure, and Electronic Properties of Extended π-Conjugated Group 6 Fischer Alkoxy-Bis(carbene) Complexes

The synthesis, structure and electronic properties of novel Group 6 Fischer alkoxy-bis(carbene) complexes are reported. The UV/Vis spectra of these species display two main absorptions at approximately 350 and 550 nm attributable to a ligand-field (LF) and metal-to-ligand charge-transfer (MLCT) transitions, respectively. The planarity of the system and the cooperative effect of both pentacarbonyl metal moieties greatly enhance the conjugation between the group at the end of the spacer and the metal carbene fragment provoking dramatic changes in the LF and MLCT absorptions. This is in contrast to related push-pull Fischer monocarbenes, where the position of the MLCT band remains mostly unaltered regardless the substituent attached to the donor fragment. In addition, the MLCT maxima can be tuned with subtle modifications of the electronic nature of the central aryl fragment in the novel A-π-D-π-A (A = acceptor, D = donor) systems. DFT and time-dependent (TD) DFT quantum chemical calculations at the B3LYP/def2-SVP level have also been performed to determine the minimum-energy molecular structure of this family of compounds and to analyse the nature of the vertical one-electron excitations associated to the observed UV/Vis absorptions as well as to rationalise their electrochemical behaviour. The ability of tuning up the electronic properties of the compounds studied herein may be of future use in material chemistry.