States Of Transition Metal Complexes Research Articles

Controlling of Photophysical Behavior of Rhenium(I) Complexes with 2,6-Di(thiazol-2-yl)pyridine-Based Ligands by Pendant π-Conjugated Aryl Groups.

The structure–property correlations and control of electronic excited states in transition metal complexes (TMCs) are of high significance for TMC-based functional material development. Within these studies, a series of Re(I) carbonyl complexes with aryl-substituted 2,6-di(thiazol-2-yl)pyridines (Arn-dtpy) was synthesized, and their ground- and excited-state properties were investigated. A number of condensed aromatic rings, which function as the linking mode of the aryl substituent, play a fundamental role in controlling photophysics of the resulting [ReCl(CO)3(Arn-dtpy-κ2N)]. Photoexcitation of [ReCl(CO)3(Arn-dtpy-κ2N)] with 1-naphthyl-, 2-naphthyl-, 9-phenanthrenyl leads to the population of 3MLCT. The lowest triplet state of Re(I) chromophores bearing 9-anthryl, 2-anthryl, 1-pyrenyl groups is ligand localized. The rhenium(I) complex with appended 1-pyrenyl group features long-lived room temperature emission attributed to the equilibrium between 3MLCT and 3IL/3ILCT. The excited-state dynamics in complexes [ReCl(CO)3(9-anthryl-dtpy-κ2N)] and [ReCl(CO)3(2-anthryl-dtpy-κ2N)] is strongly dependent on the electronic coupling between anthracene and {ReCl(CO)3(dtpy-κ2N)}. Less steric hindrance between the chromophores in [ReCl(CO)3(2-anthryl-dtpy-κ2N)] is responsible for the faster formation of 3IL/3ILCT and larger contribution of 3ILCTanthracene→dtpy in relation to the isomeric complex [ReCl(CO)3(9-anthryl-dtpy-κ2N)]. In agreement with stronger electronic communication between the aryl and Re(I) coordination centre, [ReCl(CO)3(2-anthryl-dtpy-κ2N)] displays room-temperature emission contributed to by 3MLCT and 3ILanthracene/3ILCTanthracene→dtpy phosphorescence. The latter presents rarely observed phenomena in luminescent metal complexes.

Capturing Atom-Specific Electronic Structural Dynamics of Transition-Metal Complexes with Ultrafast Soft X-Ray Spectroscopy.

The atomic specificity of X-ray spectroscopies provides a distinct perspective on molecular electronic structure. For 3d metal coordination and organometallic complexes, the combination of metal- and ligand-specific X-ray spectroscopies directly interrogates metal-ligand covalency-the hybridization of metal and ligand electronic states. Resonant inelastic X-ray scattering (RIXS), the X-ray analog of resonance Raman scattering, provides access to all classes of valence excited states in transition-metal complexes, making it a particularly powerful means of characterizing the valence electronic structure of 3d metal complexes. Recent advances in X-ray free-electron laser sources have enabled RIXS to be extended to the ultrafast time domain. We review RIXS studies of two archetypical photochemical processes: charge-transfer excitation in ferricyanide and ligand photodissociation in iron pentacarbonyl. These studies demonstratefemtosecond-resolution RIXS can directly characterize the time-evolving electronic structure, including the evolution of the metal-ligand covalency.

Chasing unphysical TD-DFT excited states in transition metal complexes with a simple diagnostic tool.

Transition Metal Complexes (TMCs) are known for the rich variety of their excited states showing different nature and degrees of locality. Describing the energies of these excited states with the same degree of accuracy is still problematic when using time-dependent density functional theory in conjunction with the most current density functional approximations. In particular, the presence of unphysically low lying excited states possessing a relevant Charge Transfer (CT) character may significantly affect the spectra computed at such a level of theory and, more relevantly, the interpretation of their photophysical behavior. In this work, we propose an improved version of the MAC index, recently proposed by the authors and collaborators, as a simple and computationally inexpensive diagnostic tool that can be used for the detection and correction of the unphysically predicted low lying excited states. The analysis, performed on five prototype TMCs, shows that spurious and ghost states can appear in a wide spectral range and that it is difficult to detect them only on the basis of their CT extent. Indeed, both delocalization of the excited state and CT extent are criteria that must be combined, as in the MAC index, to detect unphysical states.

CLB18: A new structural database with unusual carbon–carbon long bonds

We introduce a new database called Carbon Long Bond 18 (CLB18), composed of 18 structures with one long C–C bond. We use this new database to evaluate the performance of several low-cost methods commonly used for geometry optimization of medium and large molecules. We found that the long bonds in CLB18 are electronically different from those found in barrier heights databases. We also report the unexpected correlation between the results of CLB18 and those of the energetics of spin states in transition-metal complexes. Given this unique property, CLB18 can be a useful tool for assessing existing electronic structure calculation methods and developing new ones.

Transition Metal Spin-State Energetics by MC-PDFT with High Local Exchange.

The energetics of the spin states of transition metal complexes have been explored with a variety of electronic structure methods, but the calculations require a compromise between accuracy and affordability. In this work, the spin splittings of several iron complexes are studied with multiconfiguration pair-density functional theory (MC-PDFT). The results are compared to previously published results obtained by complete active space second-order perturbation theory (CASPT2) and CASPT2 with coupled-cluster semicore correlation (CASPT2/CC). In contrast to CASPT2's systematic overstabilization of high-spin states with respect to the CASPT2/CC reference, MC-PDFT with the tPBE on-top functional understabilizes high-spin states. This systematic understabilization is largely corrected by revising the exchange and correlation contributions to the on-top functional using the high local-exchange approximation (tPBE-HLE). Moreover, tPBE-HLE correctly predicts the spin of the ground state in most cases, while CASPT2 incorrectly predicts high-spin ground states in all cases. This is encouraging for practical work because tPBE and tPBE-HLE are faster than CASPT2 by a factor of 50 even in a moderately sized example.

Steric and electronic control of an ultrafast isomerization.

Synthetic control of the influence of steric and electronic factors on the ultrafast (picosecond) isomerization of penta-coordinate ruthenium dithietene complexes (Ru((CF3)2C2S2)(CO)(L)2, where L = a monodentate phosphine ligand) is reported. Seven new ruthenium dithietene complexes were prepared and characterized by single crystal X-ray diffraction. The complexes are all square pyramidal and differ only in the axial vs. equatorial coordination of the carbonyl ligand. Fourier Transform Infrared (FTIR) spectroscopy was used to study the ν(CO) bandshapes of the complexes in solution, and these reveal rapid exchange between two or three isomers of each complex. Isomerization is proposed to follow a Berry psuedorotation-like mechanism where a metastable, trigonal bipyramidal (TBP) intermediate is observed spectroscopically. Electronic tuning of the phosphine ligands L = PPh3, P((p-Me)Ph)3, ((p-Cl)Ph)3, at constant cone angle is found to have little effect on the kinetics or thermodynamic stabilities of the axial, equatorial and TBP isomers of the differently substituted complexes. Steric tuning of the phosphine ligands over a range of phosphine cone angles (135 < θ < 165°) has a profound impact on the isomerization process, and in the limit of greatest steric bulk, the axial isomer is not observable. Temperature dependence of the FTIR spectra was used to obtain the relative thermodynamic stabilities of the different isomers of each of the seven ruthenium dithietene complexes. This study details how ligand steric effects can be used to direct the solution state dynamics on the picosecond time scale of discrete isomers energetically separated by <2.2 kcal mol-1. This work provides the most detailed description to date of ultrafast isomerization in the ground states of transition metal complexes.

Deactivation of Excited States in Transition-Metal Complexes: Insight from Computational Chemistry.

Investigation of the excited-state decay dynamics of transition-metal systems is a crucial step for the development of photoswitchable molecular based materials with applications in growing fields as energy conversion, data storage, or molecular devices. The photophysics of these systems is an entangled problem arising from the interplay of electronic and geometrical rearrangements that take place on a short time scale. Several factors play a role in the process: various electronic states of different spin and chemical character are involved, the system undergoes important structural variations and several nonradiative processes can occur. Computational chemistry is a useful tool to get insight into the microscopic description of the photophysics of these materials, since it provides unique information about the character of the electronic spin states involved, the energetics and time evolution of the system. In this review article, we present an overview of the state of the art methodologies available to address the several aspects that have to be incorporated to properly describe the deactivation of excited states in transition-metal complexes. The most recent developments in theoretical methods are discussed and illustrated with examples.

Zero-Magnetic-Field Splitting in the Excited Triplet States of Octahedral Hexanuclear Molybdenum(II) Clusters: [{Mo6X8}Y6]2- (X, Y = Cl, Br, I).

The temperature ( T) dependences of the emissions from the tetra- n-butylammonium salts of [{Mo6X8}Y6]2- (X, Y = Cl, Br, and I) in optically transparent polyethylene glycol dimethacrylate matrixes were studied in the T range of 3-300 K. [{Mo6Cl8}Y6]2-, [{Mo6Br8}Y6]2-, and [{Mo6I8}I6]2- showed the T-dependent emission characteristics similar to those of other hexanuclear Mo(II), Re(III), and W(II) clusters reported previously, while [{Mo6I8}Br6]2- and [{Mo6I8}Cl6]2- exhibited the emission properties different from those of other [{Mo6X8}Y6]2- clusters. The photophysical behavior of these clusters was explained by the excited triplet state spin-sublevel ( Φn, n = 1-4) model irrespective of the nature of X and Y. The zero-magnetic-field splitting energies between the lowest energy (Φ1) and the higher energy spin sublevels (Φ4 or Φ3) caused by the first- or second-order spin-orbit coupling, Δ E14 or Δ E13, were evaluated to be 620-870 or 50-99 cm-1, respectively. We found the linear correlation between the Δ E14 or Δ E13 value and the fourth power of the atomic number ( Z) of the inner halide X: Δ E14 or Δ E13 vs { Z(X)}4 (correlation coefficient: cc = ∼ 0.999). Furthermore, we also found the correlation between Δ E14 or Δ E13 and the 95Mo NMR chemical shift of the cluster. These findings gave very important insight into the spin-orbit coupling and zero-magnetic-field splitting in the excited triplet states of transition metal complexes.

Accessing the Long-Lived Triplet Excited States in Transition-Metal Complexes: Molecular Design Rationales and Applications

Transition-metal complex triplet photosensitizers are versatile compounds that have been widely used in photocatalysis, photovoltaics, photodynamic therapy (PDT) and triplet-triplet annihilation (TTA) upconversion. The principal photophysical processes in these applications are the intermolecular energy transfer or electron transfer. One of the major challenges facing these triplet photosensitizers is the short triplet-state lifetime, which is detrimental to the above-mentioned photophysical processes. In order to address this challenge, transition-metal complexes showing long-lived triplet excited states are highly desired. This review article summarizes the development of this fascinating area, including the molecular design rationales, the principal photophysical properties, and the applications of these complexes in PDT and TTA upconversion.

Full-electron ligand-to-ligand charge transfer in a compact Re(I) complex.

Ligand-to-ligand charge transfer (LLCT) states in transition metal complexes are often characterized by fractional electron transfer due to coupling of the LLCT state with many other states via the metal. We designed and characterized a compact Re(I) complex that displays essentially full-electron charge transfer in the LLCT excited state. The complex, [Re(DCEB)(CO)3(L)](+) (DCEB = 4,4'-dicarboxyethyl-2,2'-bipyridine), referred to as ReEBA, features two redox active ligands with strong electron accepting (DCEB) and electron donating (L is 3-dimethylaminobenzonitrile (3DMABN)) properties. The lowest energy excited state formed with a ca. 10 ps time constant and was characterized as the full-electron 3DMABN → DCEB LLCT state using time-resolved infrared spectroscopy (TRIR), transient absorption spectroscopy, and DFT computations. Analysis of a range of vibrational modes helped to assign the charge transfer characteristics of the complex. The LLCT state lifetime in ReEBA shows a strong dependence on the solvent polarity and features solvent dependent frequency shifts for several vibrational reporters. The formation of a full-electron LLCT state (∼92%) was enabled by tuning the redox properties of the electron accepting ligand (DCEB) and simultaneously decoupling the redox active group of the electron donating ligand (3DMABN) from the metal center. This strategy is generally applicable for designing compact transition metal complexes that have full-electron LLCT states.

Evaluating the Extent of Intramolecular Charge Transfer in the Excited States of Rhenium(I) Donor–Acceptor Complexes with Time-Resolved Vibrational Spectroscopy

Excited states in transition-metal complexes, even in those featuring ligands with strong electron donating and accepting properties, often involve only partial charge transfer between the donor and acceptor ligands. The excited-state properties of [Re(bpy)(CO)3L](+) compounds were studied, where L is 4-dimethylaminobenzonitrile (Re4DMABN), 3-dimethylaminobenzonitrile (Re3DMABN), and benzonitrile (ReBN) using time-resolved infrared (TRIR) and electronic spectroscopy methods as well as electronic structure computations. The DMABN complexes exhibit strongly solvent-dependent luminescence; the excited state lifetime decreases from microseconds in dichloromethane to several nanoseconds in mixed MeOH:DCM (1:1) solvent. Despite the similarities in the solvent dependence of the excited state dynamics and redox properties for Re3DMABN and Re4DMABN, the nature of the lowest energy excited states formed in these two compounds is drastically different. For example, the lowest energy excited state for Re4DMABN in the mixed solvent is assigned to the (4DMABN → bpy) ligand-to-ligand charge transfer (LLCT) state featuring partial charge transfer character. An equilibrium between a 3DMABN intraligand triplet ((3)IL) and a metal-ligand-to-ligand charge transfer (MLLCT) state is found for Re3DMABN in the mixed solvent with the latter at ca. 400 cm(-1) lower energy. The origin of such a drastic difference between the states involved in Re4DMABN and Re3DMABN is attributed to a difference in the energies of polarized quinoidal resonance structures in 4DMABN and 3DMABN ligands.

Molecular symmetry determines the mechanism of a very efficient ultrafast excitation-to-heat conversion in Ni-substituted chlorophylls

In the Ni-substituted chlorophylls, an ultrafast (<60fs) deactivation channel is created, which is not present in Ni-porphyrins. This observation prompted us to investigate in detail the mechanism of excitation-to-heat conversion in Ni-substituted chlorophylls, experimentally, using time-resolved laser-induced optoacoustic spectroscopy, and theoretically, using group theory approach. The Ni-substituted chlorophylls show exceptional photostability and the optoacoustic measurements confirm the prompt and very efficient (100%) excitation-into-heat conversion in these complexes. Considering their excellent spectral properties and the loss-free excitation-into-heat conversion they are likely to become a new class of versatile photocalorimetric references. The curious features of the Ni-substituted chlorophylls originate from the symmetry of a ligand field created in the central cavity. The central NNi2+ bonds, formed via the donation of two electrons from each of the sp2 orbitals of two central nitrogens to an empty s−dx2−y2 hybrid centered on Ni2+, have a considerable covalent character. The extreme rate of excited state relaxation is then not due to a ladder of the metal centered d-states, often invoked in metalloporphyrins, but seems to result from a peculiar topology of the potential energy surface (a saddle-shaped crossing) due to the covalent character of the NNi2+ bonds. This is confirmed by a strong 0→0 character of electronic transitions in these complexes indicating a similarity of their equilibrium geometries in the ground (S0) and the excited states (both QX and QY). The excitation energy is very efficiently converted into molecular vibrations and dissipated as heat, involving the central Ni2+. These Ni-substituted pigments pose a fine exemplification of symmetry control over properties of excited states of transition metal complexes.

ChemInform Abstract: Excited States of Transition Metal Complexes Studied by Time-Resolved Infrared Spectroscopy

Abstract This article describes the application of time-resolved infrared spectroscopy (TRIR) to probe structure and reactivity of excited states of transition metal complexes.

Excited-state molecular structures captured by X-ray transient absorption spectroscopy: a decade and beyond

Transient molecular structures along chemical reaction pathways are important for predicting molecular reactivity, understanding reaction mechanisms, as well as controlling reaction pathways. During the past decade, X-ray transient absorption spectroscopy (XTA, or LITR-XAS, laser-initiated X-ray absorption spectroscopy), analogous to the commonly used optical transient absorption spectroscopy, has been developed. XTA uses a laser pulse to trigger a fundamental chemical process, and an X-ray pulse(s) to probe transient structures as a function of the time delay between the pump and probe pulses. Using X-ray pulses with high photon flux from synchrotron sources, transient electronic and molecular structures of metal complexes have been studied in disordered media from homogeneous solutions to heterogeneous solution-solid interfaces. Several examples from the studies at the Advanced Photon Source in Argonne National Laboratory are summarized, including excited-state metalloporphyrins, metal-to-ligand charge transfer (MLCT) states of transition metal complexes, and charge transfer states of metal complexes at the interface with semiconductor nanoparticles. Recent developments of the method are briefly described followed by a future prospective of XTA. It is envisioned that concurrent developments in X-ray free-electron lasers and synchrotron X-ray facilities as well as other table-top laser-driven femtosecond X-ray sources will make many breakthroughs and realise dreams of visualizing molecular movies and snapshots, which ultimately enable chemical reaction pathways to be controlled.

Accurate Computed Enthalpies of Spin Crossover in Iron and Cobalt Complexes

Despite their importance in many chemical processes, the relative energies of spin states of transition metal complexes have so far been haunted by large computational errors. By the use of six functionals, B3LYP, BP86, TPSS, TPSSh, M06, and M06L, this work studies nine complexes (seven with iron and two with cobalt) for which experimental enthalpies of spin crossover are available. It is shown that such enthalpies can be used as quantitative benchmarks of a functional's ability to balance electron correlation in both the involved states. TPSSh achieves an unprecedented mean absolute error of approximately 11 kJ/mol in spin transition energies, with the local functional M06L a distant second (25 kJ/mol). Other tested functionals give mean absolute errors of 40 kJ/mol or more. This work confirms earlier suggestions that 10% exact exchange is near-optimal for describing the electron correlation effects of first-row transition metal systems. Furthermore, it is shown that given an experimental structure of an iron complex, TPSSh can predict the electronic state corresponding to that experimental structure. We recommend this functional as current state-of-the-art for studying spin crossover and relative energies of close-lying electronic configurations in first-row transition metal systems.

LOBA: a localized orbital bonding analysis to calculate oxidation states, with application to a model water oxidation catalyst

We propose a method for calculation of oxidation states in transition metal complexes, utilizing a bonding analysis based on localized molecular orbitals in conjunction with traditional population analyses. The localized orbital bonding analysis (LOBA) is seen to accurately produce both the oxidation state and chemically intuitive views of bonding in the complexes studied. This is in contrast to simple population analyses where the oxidation states are not reproduced for even simple systems and more complex analyses which break down on problematic systems. We report the application to a manganese complex with potential activity as oxygen-evolving catalyst, determining the location of the oxidations.

Metal–ligand bonding in metallocenes: Differentiation between spin state, electrostatic and covalent bonding

We have analyzed metal–ligand bonding in metallocenes using density functional theory (DFT) at the OPBE/TZP level. This level of theory was recently shown to be the only DFT method able to correctly predict the spin ground state of iron complexes, and similar accuracy for spin ground states is found here. We considered metallocenes along the first-row transition metals (Sc–Zn) extended with alkaline-earth metals (Mg, Ca) and several second-row transition metals (Ru, Pd, Ag, Cd). Using an energy decomposition analysis, we have studied trends in metal–ligand bonding in these complexes. The OPBE/TZP enthalpy of heterolytic association for ferrocene (−658 kcal/mol) as obtained from the decomposition analysis is in excellent agreement with benchmark CCSD(T) and CASPT2 results. Covalent bonding is shown to vary largely for the different metallocenes and is found in the range from −155 to −635 kcal/mol. Much smaller variation is observed for Pauli repulsion (55–345 kcal/mol) or electrostatic interactions, which are however strong (−480 to −620 kcal/mol). The covalent bonding, and thus the metal–ligand bonding, is larger for low spin states than for higher spin states, due to better suitability of acceptor d-orbitals of the metal in the low spin state. Therefore, spin ground states of transition metal complexes can be seen as the result of a delicate interplay between metal–ligand bonding and Hund’s rule of maximum multiplicity.

Theoretical Study of Electron Density Delocalization in the Excited States of Transition Metal Complexes

This paper presents a theoretical study of electron density delocalization effects over an electron-accepting ligand in metal-to-ligand charge-transfer (MLCT) complexes in the excited states, where the ligand is 4,4'-X2-2,2'-bpy (X = H, NH2, CH3, Ph, Cl, CO2Et, NO2, bpy = 2,2'-bipyridine) or terpy (2,2':6',2''-terpyridine). Optimal geometry calculations are performed for neutral ligand molecules and their radical anions modeling the state of the ligands during MLCT excitations. Spin density distribution over atoms in the radical anions is used as a measure of the degree of delocalization. The role of spin density distribution in excitation-induced changes of geometrical parameters of the ligands is considered.

Ab initio multiconfiguration reference perturbation theory calculations on the energetics of low-energy spin states of iron(III) porphyrins.

Although a major goal of inorganic spectroscopy is to determine the energetics of the low-lying spin states of transition metal complexes, surprisingly little has been accomplished in this respect by means of accurate ab initio calculations. Against this context, we present ab initio multiconfiguration reference perturbation theory (CASPT2) calculations with large basis sets on the low-lying spin states of Fe(III)(P)Cl and [Fe(P)Cl](+) (P(2-)=porphinato). The CASPT2 results on the energetics of various low-lying spin states studied differ significantly, sometimes even dramatically, from those obtained from density functional theory calculations.

Theoretical Analysis of the Triplet Excited State of the [Pt2(H2P2O5)4] 4‐ Ion and Comparison with Time‐Resolved X‐Ray and Spectroscopic Results

AbstractFor Abstract see ChemInform Abstract in Full Text.