Ligand Field States Research Articles

Spin States and Other Ligand-Field States of Aqua Complexes Revisited with Multireference ab Initio Calculations Including Solvation Effects.

High-level multireference (CASPT2, NEVPT2) calculations are reported for transition metal aqua complexes with electronic configurations from (3d)1 to (3d)8. We focus on the experimentally evidenced excitation energies to their various ligand-field states, including different spin states. By employing models accounting for both explicit and implicit solvation, we find that solvation effect may contribute up to 0.5 eV to the excitation energies depending on the charge of ion and character of the electronic transition. We further demonstrate that with an adequate choice of the active space and the energetics extrapolated to the complete basis set limit, the presently computed excitation energies are in a good agreement with the experimental data. This allows us to conclusively resolve significant discrepancies reported in earlier theory works [e.g., J. Phys. Chem. C 2014 , 118 , 29196 - 29208 ]. For the benchmark set of 19 spin-forbidden and 24 spin-allowed transitions (for which experimental data are unambiguous), we find the mean absolute error of 0.15 or 0.13 eV and the maximum error of 0.56 or 0.42 eV for CASPT2 or NEVPT2 calculations, respectively. For the particularly challenging sextet-quartet gap for [Fe(H2O)6]3+, we support our interpretation by additional calculations with multireference configuration interaction (MRCI) and coupled cluster theory up to the CCSDT(Q) level. By underlining a rather subtle interplay between the solvation and correlation effects, the findings of this Article are relevant not only for modeling and interpretation of optical spectra of transition metal complexes but also in further benchmarking of theoretical methods for the challenging problem of spin-state energetics.

Photolytic Properties of Antivitamins B12.

Antivitamins B12 represent an important class of vitamin B12 analogues that have gained recent interest in several research areas. In particular, 4-ethylphenylcobalamin (EtPhCbl) and phenylethynylcobalamin (PhEtyCbl) exemplify two such antivitamins B12 which have been characterized structurally and chemically. From a spectroscopic point of view, EtPhCbl is photolabile with a very low quantum yield of photoproducts, while PhEtyCbl is incredibly photostable. Herein, DFT and TD-DFT computations are provided to explore the photolytic properties of these compounds to shed light on the electronic properties that are indicative of these differences. Potential energy surfaces (PESs) were constructed to investigate the mechanisms of photodissociation leading to radical pair (RP) formation and the mechanisms of deactivation to the ground state. The S1 PESs for each antimetabolite contain two energy minima, one being the metal-to-ligand charge transfer (MLCT) and another the ligand-field (LF) state. There are two possible pathways for photodissociation that can be identified for EtPhCbl but only one (path B) is energetically feasible and involves the lengthening of the Co-NIm bond through the MLCT region followed by the lengthening of the Co-C bond through the LF region. For PhEtyCbl, there is not an energetically favorable path for photolysis; rather, internal conversion (IC) is the significantly preferred photophysical event.

Ru(II) Polypyridyl Complexes Derived from Tetradentate Ancillary Ligands for Effective Photocaging.

Metal complexes have many proven applications in the caging and photochemical release of biologically active compounds. Photocaging groups derived from Ru(II) traditionally have been composed of ancillary ligands that are planar and bi- or tridentate, such as 2,2'-bipyridine (bpy), 2,2':6',2″-terpyridine (tpy), and 1,10-phenanthroline (phen). Complexes bearing ancillary ligands with denticities higher than three represent a new class of Ru(II)-based photocaging groups that are grossly underdeveloped. Because high-denticity ancillary ligands provide the ability to increase the structural rigidity and control the stereochemistry, our groups initiated a research program to explore the applications of such ligands in Ru(II)-based photocaging. Ru(TPA), bearing the tetradentate ancillary ligand tris(2-pyridylmethyl)amine (TPA), has been successfully utilized to effectively cage nitriles and aromatic heterocycles. Nitriles and aromatic heterocycles caged by the Ru(TPA) group show excellent stability in aqueous solutions in the dark, and the complexes can selectively release the caged molecules upon irradiation with light. Ru(TPA) is applicable as a photochemical agent to offer precise spatiotemporal control over biological activity without undesired toxicity. In addition, Ru(II) polypyridyl complexes with desired photochemical properties can be synthesized and identified by solid-phase synthesis, and the resulting complexes show properties to similar to those of complexes obtained by solution-phase synthesis. Density functional theory (DFT) calculations reveal that orbital mixing between the π* orbitals of the ancillary ligand and the Ru-N dσ* orbital is essential for ligand photodissociation in these complexes. Furthermore, the introduction of steric bulk enhances the photoliability of the caged molecules, validating that steric effects can largely influence the quantum efficiency of photoinduced ligand exchange in Ru(II) polypyridyl complexes. Recently, two new photocaging groups, Ru(cyTPA) and Ru(1-isocyTPQA), have been designed and synthesized for caging of nitriles and aromatic heterocycles, and these complexes exhibit unique photochemical properties distinct from those derived from Ru(TPA). Notably, the unusually greater quantum efficiency for the ligand exchange in [Ru(1-isocyTPQA)(MeCN)2](PF6)2, Φ400 = 0.033(3), uncovers a trans-type effect in the triplet metal-to-ligand charge transfer (3MLCT) state that enhances photoinduced ligand exchange in a new manner. DFT calculations and ultrafast transient spectroscopy reveal that the lowest-energy triplet state in [Ru(1-isocyTPQA)(MeCN)2](PF6)2 is a highly mixed 3MLCT/3ππ* excited state rather than a triplet metal-centered ligand-field (3LF) excited state; the latter is generally accepted for ligand photodissociation. In addition, Mulliken spin density calculations indicate that a majority of the spin density in [Ru(1-isocyTPQA)(MeCN)2](PF6)2 is localized on the isoquinoline arm, which is opposite to the cis MeCN, rather than on the ruthenium center. This significantly weakens the Ru-N6 ( cis MeCN) bond, which then promotes the ligand photodissociation. This newly discovered effect gives a clearer perception of the interplay between the 3MLCT and 3LF excited states of Ru(II) polypyridyl complexes, which may be useful in the design and applications of ruthenium complexes in the areas of photoactivated drug delivery and photosensitizers.

Investigations of the Magnetic and Spectroscopic Properties of V(III) and V(IV) Complexes.

Herein, we utilize a variety of physical methods including magnetometry (SQUID), electron paramagnetic resonance (EPR), and magnetic circular dichroism (MCD), in conjunction with high-level ab initio theory to probe both the ground and ligand-field excited electronic states of a series of V(IV) ( S = 1/2) and V(III) ( S = 1) molecular complexes. The ligand fields of the central metal ions are analyzed with the aid of ab initio ligand-field theory (AILFT), which allows for a chemically meaningful interpretation of multireference electronic structure calculations at the level of the complete-active-space self-consistent field with second-order N-electron valence perturbation theory. Our calculations are in good agreement with all experimentally investigated observables (magnetic properties, EPR, and MCD), making our extracted ligand-field theory parameters realistic. The ligand fields predicted by AILFT are further analyzed with conventional angular overlap parametrization, allowing the ligand field to be decomposed into individual σ- and π-donor contributions from individual ligands. The results demonstrate in VO2+ complexes that while the axial vanadium-oxo interaction dominates both the ground- and excited-state properties of vanadyl complexes, proximal coordination can significantly modulate the vanadyl bond covalency. Similarly, the electronic properties of V(III) complexes are particularly sensitive to the available σ and π interactions with the surrounding ligands. The results of this study demonstrate the power of AILFT-based analysis and provide the groundwork for the future analysis of vanadium centers in homogeneous and heterogeneous catalysts.

Quantum Chemical Study of the Interligand Electron Transfer in Ru Polypyridyl Complexes.

Quantum chemical calculations have been performed to study the photocycle of [Ru(bpy)3]2+, a complex that is extensively used as an electron donor in photocatalytic reactions. After the initial spin-allowed excitation from the nonmagnetic ground state to a singlet state of metal-to-ligand charge transfer character, the system undergoes a rapid intersystem crossing to a triplet state of equal character. The calculations indicate a lifetime of 10 fs, in good agreement with experimental estimates. Important factors for this extremely fast intersystem crossing are the large spin-orbit coupling and the large vibrational overlap of the states involved. Both MLCT states are delocalized over the three bipyridine ligands, but the delocalized electron can easily increase its degree of localization. The hopping parameters have been calculated and found to be large for the localization on two ligands and subsequently on one. The combination of localization and geometry relaxation creates a rather long-lived trapped triplet MLCT state with a calculated lifetime of 9 μs. The addition of methyl groups on the bipyridine ligands decreases the ligand field and consequently lowers the metal-centered triplet states. This could eventually lead to opening of a fast deactivation channel of the 3MLCT states to the initial nonmagnetic states via the triplet ligand field states as occurs in the corresponding Fe(II) complex.

Triplet Ground-State-Bridged Photochemical Process: Understanding the Photoinduced Chiral Inversion at the Metal Center of [Ru(phen)2(l-ser)]+ and Its Bipy Analogues.

One of the main concerns in the photochemistry and photophysics of ruthenium complexes is the de-excitation of the triplet metal centered ligand-field state 3MC. To understand the mechanism by which the 3MC states in some reversible photochemical reactions could avoid the fate of fast decay and ligand dissociations, the photoinduced chiral inversion at the metal center of the complexes [Ru(diimine)2(l-ser)]+ (diimine = 1,10-phenanthroline or 2,2'-bipyridine, l-ser = l-serine) has been analyzed at the first principle level of theory. The calculated equilibrium constants and ECD curves for the photoinduced equilibrium mixtures are in agreement with the observed ones. The results showed that the reversible photochemical process Δ(δS) ⇌ Λ(δS) on the potential surface of the lowest triplet excited state proceeds in three steps: 3CTΔ ↔ 3MCΔ, 3MCΔ ↔ 3MCΛ, 3MCΛ ↔ 3CTΛ, where the first and the third steps involve mainly the elongation and compression of the octahedral core of the reactant Δ(δS) and product Λ(δS), respectively. The chiral inversion Δ ↔ Λ takes place in the second step through a much distorted square-pyramid-like transition state, and actually proceeds on the triplet ground state 3MC due to the crossover of the triplet T1 and singlet S0 states. Inspecting the transient structures at the crossing points, we found that they become less distorted and their lowest or imaginary-frequency displacement vectors in triplet state still dominate the reaction path, which makes the reaction reversible without ligand release. Thus, the triplet ground-state-bridged photoinduced mechanism offers a new angle of view to understand the related reversible photochemical reactions.

Excited-State Decay Pathways of Tris(bidentate) Cyclometalated Ruthenium(II) Compounds.

The synthesis, electrochemistry, and photophysical characterization are reported for 11 tris(bidentate) cyclometalated ruthenium(II) compounds, [Ru(N^N)2(C^N)]+. The electrochemical and photophysical properties were varied by the addition of substituents on the 2,2'-bipyridine, N^N, and 2-phenylpyridine, C^N, ligands with different electron-donating and -withdrawing groups. The systematic tuning of these properties offered a tremendous opportunity to investigate the origin of the rapid excited-state decay for these cyclometalated compounds and to probe the accessibility of the dissociative, ligand-field (LF) states from the metal-to-ligand charge-transfer (MLCT) excited state. The photoluminescence quantum yield for [Ru(N^N)2(C^N)]+ increased from 0.0001 to 0.002 as more electron-withdrawing substituents were added to C^N. An analogous substituent dependence was observed for the excited-state lifetimes, τobs, which ranged from 3 to 40 ns in neat acetonitrile, significantly shorter than those for their [Ru(N^N)3]2+ analogues. The excited-state decay for [Ru(N^N)2(C^N)]+ was accelerated because of an increased vibronic overlap between the ground- and excited-state wavefunctions rather than an increased electronic coupling as revealed by a comparison of the Franck-Condon factors. The radiative (kr) and non-radiative (knr) rate constants of excited-state decay were determined to be on the order of 104 and 107-108 s-1, respectively. For sets of [Ru(N^N)2(C^N)]+ compounds functionalized with the same N^N ligand, knr scaled with excited-state energy in accordance with the energy gap law. Furthermore, an Arrhenius analysis of τobs for all of the compounds between 273 and 343 K was consistent with activated crossing into a single, fourth 3MLCT state under the conditions studied with preexponential factors on the order of 108-109 s-1 and activation energies between 300 and 1000 cm-1. This result provides compelling evidence that LF states are not significantly populated near room temperature unlike many ruthenium(II) polypyridyl compounds. On the basis of the underlying photophysics presented here for [Ru(N^N)2(C^N)]+, molecules of this type represent a robust class of compounds with built-in design features that should greatly enhance the molecular photostability necessary for photochemical and photoelectrochemical applications.

Boosting Vis/NIR Charge-Transfer Absorptions of Iron(II) Complexes by N-Alkylation and N-Deprotonation in the Ligand Backbone.

Reversing the metal-to-ligand charge transfer (3 MLCT)/metal-centered (3 MC) excited state order in iron(II) complexes is a challenging objective, yet would finally result in long-sought luminescent transition-metal complexes with an earth-abundant central ion. One approach to achieve this goal is based on low-energy charge-transfer absorptions in combination with a strong ligand field. Coordinating electron-rich and electron-poor tridentate oligopyridine ligands with large bite angles at iron(II) enables both low-energy MLCT absorption bands around 590 nm and a strong ligand field. Variations of the electron-rich ligand by introducing longer alkyl substituents destabilizes the iron(II) complex towards ligand substitution reactions while hardly affecting the optical properties. On the other hand, N-deprotonation of the ligand backbone is feasible and reversible, yielding deep-green complexes with charge-transfer bands extending into the near-IR region. Time-dependent density functional theory calculations assign these absorption bands to transitions with dipole-allowed ligand-to-ligand charge transfer character. This unique geometric and electronic situation establishes a further regulating screw to increase the energy gap between potentially emitting charge-transfer states and the non-radiative ligand field states of iron(II) dyes.

Solvent Effects on Nonradiative Relaxation Dynamics of Low-Energy Ligand-Field Excited States: A CuCl42- Complex.

Nonradiative relaxation dynamics of CuCl42- complexes photoexcited into the highest-energy ligand-field electronic state (2A1) is studied in acetonitrile, dichloromethane, and chloroform solvents, as well as in acetonitrile-water and in acetonitrile-deuterated water mixtures. Due to ultrafast internal conversion, this excited state directly converts to the electronic ground state in dichloromethane and chloroform. The nonradiative relaxation constant is similar in anhydrous acetonitrile. Addition of water to acetonitrile solutions efficiently quenches the excited ligand-field 2A1 state. The quenching is proposed to be due to the diffusion-controlled formation of an electronically excited pentacoordinated [CuCl4H2O]2- encounter complex or a short-lived exciplex of similar structure, in which the electronic excitation energy transfers into the O-H stretch of the coordinated H2O molecule. This is followed by the dissociation of the pentacoordinated species, resulting in the reformation of the ground-state CuCl42- and free H2O molecules.

Ultrafast transient IR spectroscopy and DFT calculations of ruthenium(ii) polypyridyl complexes.

Ultrafast time-resolved infrared spectroscopy of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine), [Ru(mbpy)3]2+ (mbpy = 6-methyl-2,2'-bipyridine) and [Ru(mphen)3]2+ (mphen = 2-methyl-1,10'-phenanthroline) in deuterated acetonitrile serves to elucidate the evolution of the system following pulsed excitation into the 1MLCT band at 400 nm. While for [Ru(bpy)3]2+ no intermediate state can be evidenced for the relaxation of the corresponding 3MLCT state back to the ground state, for [Ru(mbpy)3]2+ and [Ru(mphen)3]2+ an intermediate state with a lifetime of about 400 ps is observed. The species associated IR difference spectra of this state are in good agreement with the calculated difference spectra of the lowest energy 3dd state using DFT. The calculated potential energy curves for all the complexes in the triplet manifold along the metal-ligand distance show that for [Ru(bpy)3]2+ the 3dd state is at a higher energy than the 3MLCT state and that there is a substantial barrier between the two minima. For [Ru(mbpy)3]2+ and [Ru(mphen)3]2+, the 3dd state is at a lower energy than the 3MLCT state.

Quantifying covalent interactions with resonant inelastic soft X-ray scattering: Case study of Ni2+ aqua complex

We analyze the effects of covalent interactions in Ni 2p3d resonant inelastic X-ray scattering (RIXS) spectra from aqueous Ni2+ ions and find that the relative RIXS intensities of ligand-to-metal charge-transfer final states with respect to the ligand-field final states reflect the covalent mixing between Ni 3d and water orbitals. Specifically, the experimental intensity ratio at the Ni L3-edge allows to determine that the Ni 3d orbitals have on average 5.5% of water character. We propose that 2p3d RIXS at the Ni L3-edge can be utilized to quantify covalency in Ni complexes without the use of external references or simulations.

Spectroscopic and Computational Investigations of a Mononuclear Manganese(IV)-Oxo Complex Reveal Electronic Structure Contributions to Reactivity.

The mononuclear Mn(IV)-oxo complex [MnIV(O)(N4py)]2+, where N4py is the pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine, has been proposed to attack C-H bonds by an excited-state reactivity pattern [ Cho, K.-B.; Shaik, S.; Nam, W. J. Phys. Chem. Lett. 2012 , 3 , 2851 - 2856 (DOI: 10.1021/jz301241z )]. In this model, a 4E excited state is utilized to provide a lower-energy barrier for hydrogen-atom transfer. This proposal is intriguing, as it offers both a rationale for the relatively high hydrogen-atom-transfer reactivity of [MnIV(O)(N4py)]2+ and a guideline for creating more reactive complexes through ligand modification. Here we employ a combination of electronic absorption and variable-temperature magnetic circular dichroism (MCD) spectroscopy to experimentally evaluate this excited-state reactivity model. Using these spectroscopic methods, in conjunction with time-dependent density functional theory (TD-DFT) and complete-active space self-consistent-field calculations (CASSCF), we define the ligand-field and charge-transfer excited states of [MnIV(O)(N4py)]2+. Through a graphical analysis of the signs of the experimental C-term MCD signals, we unambiguously assign a low-energy MCD feature of [MnIV(O)(N4py)]2+ as the 4E excited state predicted to be involved in hydrogen-atom-transfer reactivity. The CASSCF calculations predict enhanced MnIII-oxyl character on the excited-state 4E surface, consistent with previous DFT calculations. Potential-energy surfaces, developed using the CASSCF methods, are used to determine how the energies and wave functions of the ground and excited states evolved as a function of Mn═O distance. The unique insights into ground- and excited-state electronic structure offered by these spectroscopic and computational studies are harmonized with a thermodynamic model of hydrogen-atom-transfer reactivity, which predicts a correlation between transition-state barriers and driving force.

Tuning the excited state properties of ruthenium(II) complexes with a 4-substituted pyridine ligand

A series of complexes of the type [Ru(tpy)(bpy)(4-R-py)]2+, where tpy=2,2′;6′,2″-terpyridine, bpy=2,2′-bipyridine, and 4-R-py=pyridine (py, 1), 4-methoxypyridine (4-CH3O-py, 2), 4-aminopyridine (4-NH2-py, 3), 4-acetylpyridine (4-Ac-py, 4) and 4-trifluoromethylpyridine (4-CF3-py, 5), were synthesized and the effect of the pyridine substitution on the ground and excited state properties was examined. Cyclic voltammetry shows that an electron-donating substituent on the pyridine ligand results in complexes that are more easily oxidized, as is the case in 2 and 3, whereas the electron-withdrawing substituents on the pyridine in 4 and 5 result in more positive oxidation potential. The electronic absorption spectra of 2 and 3 feature a detectable red shift of the metal-to-ligand charge transfer (1MLCT) as compared to 1. Additionally, the 1MLCT maxima of 4 and 5 are blue shifted relative to 1. Complexes 1–3 exhibit phosphorescence with luminescence quantum yields of 2.0×10–4, 4.3×10–4, and 3.1×10–4, respectively. The emission quantum yields of 4 and 5 are approximately an order of magnitude lower, 5×10–5 and 4×10–5. Upon irradiation in CH3CN (λirr⩾395nm) all five complexes undergo photoinduced ligand exchange with the solvent to form [Ru(tpy)(bpy)(CH3CN)]2+. This process is completed more rapidly in 4 (30min) and 5 (10min) as compared to 1 (90min), 2 (240min) and 3 (>24h). Theoretical calculations show that the electron-donating groups increase the energy difference between the ligand field state (LF) and MLCT while the electron-withdrawing groups decrease this value, giving rise to the different excited state properties. These findings demonstrate the synthetic tuning of the series of complexes, which can be useful to vary the photochemistry.

Synthesis and characterization of phosphorescent cyclometalated Ir and Pt heteroleptic complexes using cyclophane-based chelates

The synthesis and characterization of a series of neutral cyclometalated Pt(II) and Ir(III) heteroleptic complexes containing cyclophane substituted chelates is reported. The complexes have the general formula of (C^N)Pt(O^O) or (C^N)2Ir(O^O) where C^N is a monoanionic cyclophane ligand 2-([2.2]-paracyclophane-4-yl)pyridyl (pCpy) or 1-([2.2]-paracyclophane-4-yl)pyrazolyl (pCpz) and (O^O) is an ancillary ligand; acetylacetonato (acac) or dipivolylmethanato (dpm). The pCpyH and pCpzH ligand precursors were obtained as racemates due to the planar chirality of the cyclophane ring system. Two diastereomers of (pCpz)2Ir(acac), one with C1 symmetry (ΛRS, ΔSR) and the other with C2 symmetry (ΛRR, ΔSS) were isolated, whereas only one C1 diastereomer was obtained for (pCpy)2Ir(acac). Inherent strain in the cyclophane core and transannular interaction between the bridged phenyl rings leads to a 250–400mV cathodic shift in the oxidation potentials relative to analogous complexes with ppy or ppz ligands. The destabilization of the HOMO leads to a corresponding red shift in the absorption and emission spectra from the complexes with pCpy and pCpz ligands. The Pt and Ir complexes with pCpy ligands are strongly emissive at room temperature, whereas complexes with pCpz ligands are only weakly emissive as they still suffer from temperature-dependent nonradiative deactivation to metal-centered ligand field states.

Tuning the Energy Emission from Violet to Yellow with Bidentate Phosphine Gold(III) Complexes

The synthesis and characterization of luminescent gold(III) compounds, obtained by coordination of the metal center to different phosphines, is described. To avoid deactivation of luminescence by the presence of low-energy d–d ligand field states in the gold(III) center, the ligands bonded to the metallic center have been carefully chosen, among which we used bidentate phosphines with different numbers of phenylene or alkynyl-phenylene spacers and pentafluorophenyl groups. The reaction of [Au(C6F5)3(tht)] (tht = tetrahydrothiophene) with the corresponding diphosphines gave the complexes [{Au(C6F5)3}2(1,4-PPh2(C6H4)nPPh2)] (n = 1–3) and [{Au(C6F5)3}2(PPh2C≡C(C6H4)nC≡CPPh2)] (n = 0–2). The study of their optical behavior reveals emission color variations from violet to yellow for the compounds containing the phosphines with one, two, and three phenylene spacers, respectively, and much more fine-tuning, from deep blue to brilliant blue for those intercalating alkynyl and phenylene spacers. Four of the new co...

Spectroscopic signatures of ligand field states in {Ru(II)(imine)} complexes.

Ligand field (LF) states have been present in discussions on the photophysics and photochemistry of ruthenium-iminic chromophores for decades, although there is very little documented direct evidence of them. We studied the picosecond transient absorption (TA) spectroscopy of four {Ru(II)(imine)} complexes that respond to the formula trans-[Ru(L)4(X)2], where L is either pyridine (py) or 4-methoxypyridine (MeOpy) and X is either cyanide or thiocyanate. Dicyano compounds behave as most ruthenium polypyridines and their LF states remain silent. In contrast, in the dithiocyanate complexes we found clear spectroscopic evidence of the participation of LF states in the MLCT decay pathway. These states are of donor and acceptor character simultaneously and this is manifested in the presence of MLCT and LMCT transient absorption bands of similar energy. Spectroelectrochemical techniques supported the interpretation of the absorption features of MLCT states, and DFT methods helped to assign their spectroscopic signatures and provided strong evidence on the nature of LF states.

TD-DFT study of the light-induced spin crossover of Fe(III) complexes.

Two light-induced spin-crossover Fe(III) compounds have been studied with time-dependent density functional theory (TD-DFT) to investigate the deactivation mechanism and the role of the ligand-field states as intermediates in this process. The B3LYP* functional has previously shown its ability to accurately describe (light-induced) spin-crossover in Fe(II) complexes. Here, we establish its performance for Fe(III) systems using [Fe(qsal)2](+) (Hqsal = 2-[(8-quinolinylimino)methyl]phenol) and [Fe(pap)2](+) (Hpap = 2-(2-pyridylmethyleneamino)phenol) as test cases comparing the B3LYP* results to experimental information and to multiconfigurational wave function results. In addition to rather accurate high spin (HS) and low spin (LS) state geometries, B3LYP* also predicts ligand-to-metal charge transfer (LMCT) states with large oscillator strength in the energy range where the UV-VIS spectrum shows an intense absorption band, whereas optically allowed π-π* excitations on the ligands were calculated at higher energy. Subsequently, we have generated a two-dimensional potential energy surface of the HS and LS states varying the Fe-N and Fe-O distances. LMCT and metal centered (MC) excited states were followed along the approximate minimal energy path that connects the minima of the HS and LS on this surface. The (2)LMCT state has a minimum in the same region as the initial LS state, where we also observe a crossing with the intermediate spin (IS) state. Upon the expansion of the coordination sphere of the Fe(III) ion, the IS state crosses with the HS state and further expansion of the coordination sphere leads to the excited spin state trapping as observed in experiment. The calculation of the intersystem crossing rates reveals that the deactivation from (2)LMCT → IS → HS competes with the (2)LMCT → IS → LS pathway, in line with the low efficiency encountered in experiments.

Deep blue phosphorescent organic light-emitting diodes with very high brightness and efficiency.

The combination of both very high brightness and deep blue emission from phosphorescent organic light-emitting diodes (PHOLED) is required for both display and lighting applications, yet so far has not been reported. A source of this difficulty is the absence of electron/exciton blocking layers (EBL) that are compatible with the high triplet energy of the deep blue dopant and the high frontier orbital energies of hosts needed to transport charge. Here, we show that N-heterocyclic carbene (NHC) Ir(III) complexes can serve as both deep blue emitters and efficient hole-conducting EBLs. The NHC EBLs enable very high brightness (>7,800 cd m(-2)) operation, while achieving deep blue emission with colour coordinates of [0.16, 0.09], suitable for most demanding display applications. We find that both the facial and the meridional isomers of the dopant have high efficiencies that arise from the unusual properties of the NHC ligand-that is, the complexes possess a strong metal-ligand bond that destabilizes the non-radiative metal-centred ligand-field states. Our results represent an advance in blue-emitting PHOLED architectures and materials combinations that meet the requirements of many critical illumination applications.

Spectroscopic and Crystal Field Consequences of Fluoride Binding by [Yb⋅DTMA]3+ in Aqueous Solution.

Yb⋅DTMA forms a ternary complex with fluoride in aqueous solution by displacement of a bound solvent molecule from the lanthanide ion. [Yb⋅DTMA⋅F]2+ and [Yb⋅DTMA⋅OH2]3+ are in slow exchange on the relevant NMR timescale (<2000 s−1), and profound differences are observed in their respective NMR and EPR spectra of these species. The observed differences can be explained by drastic modification of the ligand field states due to the fluoride binding. This changes the magnetic anisotropy of the YbIII ground state from easy‐axis to easy‐plane type, and this change is easily detected in the observed magnetic anisotropy despite thermal population of more than just the ground state. The spectroscopic consequences of such drastic changes to the ligand field represent important new opportunities in developing fluoride‐responsive complexes and contrast agents.

Spectroscopic and Crystal Field Consequences of Fluoride Binding by [Yb⋅DTMA](3+) in Aqueous Solution.

Yb⋅DTMA forms a ternary complex with fluoride in aqueous solution by displacement of a bound solvent molecule from the lanthanide ion. [Yb⋅DTMA⋅F]2+ and [Yb⋅DTMA⋅OH2]3+ are in slow exchange on the relevant NMR timescale (<2000 s−1), and profound differences are observed in their respective NMR and EPR spectra of these species. The observed differences can be explained by drastic modification of the ligand field states due to the fluoride binding. This changes the magnetic anisotropy of the YbIII ground state from easy-axis to easy-plane type, and this change is easily detected in the observed magnetic anisotropy despite thermal population of more than just the ground state. The spectroscopic consequences of such drastic changes to the ligand field represent important new opportunities in developing fluoride-responsive complexes and contrast agents.