Highlights

 
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New kids in lactide polymerization: highly active and robust iron guanidine complexes as superior catalysts

The development of novel and sustainable catalysts for the ring-opening polymerization of lactide is one of the research fields of the Herres-Pawlis group. In order to obtain an alternative to the currently applied tin(II)octanoate, the applicability of the developed catalysts in industrial conditions is of special interest.

In contrast to the so far mainly investigated zinc(II)-complexes, this publication deals with iron(II)-complexes. Three novel and promising iron(II)-complexes where synthesized from iron(II)chloride and a N,N-donor-hybrid guanidine and two N,O-donor-hybrid guanidine ligands, respectively. The X-ray structures of these complexes were determined. Moreover, the complexes where applied in the bulk polymerization of technical-grade rac-lactide. The polymerizations were monitored with in-situ Raman spectroscopy.

The observed activity of the three complexes proved that the catalysts are robust and tolerate the impurities present in technical-grade lactide. However, the two investigated iron(II)-N,O-donor complexes showed a significantly higher activity than the iron(II)-N,N-complex. The two complexes exhibit an even higher activity than the fastest known robust zinc(II)-catalysts.

In addition, the rate constant of propagation k p was determined for the two N,O-donor complexes: One of the complexes shows an activity similar to tin(II)-octanoate, the other complex surpasses the activity of the tin catalyst by one order of magnitude. With molar masses above 90.000 g mol-1, the produced polymers are suited for industrial processing.

In conclusion, the presented novel iron(II)-hybrid guanidine catalysts provide promising results and represent a decisive advance towards the tin-free lactide polymerization.

The article New kids in lactide polymerization: highly active and robust iron guanidine complexes as superior catalysts is available on the webpage of the publisher.

 
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Next Generation of Guanidine Quinoline Copper Complexes for Highly Controlled ATRP: Influence of Backbone Substitution on Redox Chemistry and Solubility

One aim of the Herres-Pawlis group is the development of new homogeneous catalysts based on guanidinoquinoline copper complexes, which are suitable for the atom transfer radical polymerisation (ATRP). The first generation of ATRP catalysts (with an unsubstituted ligand) exhibited limited solubility in the chosen monomer (styrene) and thus limited polymerisation control (see Chem. Eur J. 2016). Since guanidinoquinolines represent a promising class of ligands, they should be improved by targeted substitution. Consequently, a strategy for the synthesis of substituted guanidinoquinolines was developed. The strategy allows the synthesis of various guanidinoquinolines in high yields. The second generation of ligands possesses an alkylated quinoline backbone. n-Butylation of the backbone leads to a dramatic increase of the catalysts solubility. The higher solubility of the catalysts is furthermore affiliated with an increased polymerisation control. Consequently, polymer samples, produced with second-generation catalysts, exhibit polydispersities (PDs) of 1.1, which are associated with an extremely narrow molar mass distribution. Additionally, the influence of the substituent on the electronic properties of the ligands were comprehensively studied with various methods. As expected, alkylated TMG ligands reveal a higher activity in ATRP as the unsubstituted ligands. The knowledge of a new strategy for the synthesis of substituted guanidinoquinoline ligands allows future development of tailor-made complexes for homogeneous catalysis. The whole article is available on the webpage of the publisher.

 
  Reaction Cycle Copyright: J. Moegling

Terminal Copper Nitrenes and their Application in Catalytic C–H Aminations

The discovery of terminal copper nitrenes led us to find a long-missing link in the family of copper nitrenes. They have been predicted by theory and are pursued since decades. Their remarkable reactivity in terms of direct C–H-aminations of unsaturated C–H-compounds is direcetly linked to the existence of terminal copper nitrenes. The term terminal simply describes the occurrence of an end-on bound nitrene fragment to the copper centre. To date, only a few copper nitrenes are reported, but neither a terminal species or monometallic species was found. Our low-temperature stabilised nitrenes are based on bis(pyrazolyl)methane ligands and have been directly observed by Cryo-ESI mass spectrometry and UV/Vis spectroscopy. Their reactivity in stoichiometric reactions was followed by UV/Vis spectroscopy. In the end, a catalytic protocol was discovered, leading to the C–H-Amination of cyclohexane and toluene, compounds with very strong C–H bonds. This research will stimulate further efforts for the atom-efficient synthesis of organic compounds and helps to better understand the optimisation of crucial components in C–H insertions. The article Designed to react: Terminal Copper Nitrenes and their Application in Catalytic C-H Aminations is available on the webpage of the publisher.

 
 

Transferring the entatic state principle into copper photochemistry

So-called pre-distorted states accelerate photochemical reactions too

What enables electrons to be transferred swiftly, for example during photosynthesis? An interdisciplinary team of researchers has worked out the details of how important bioinorganic electron transfer systems operate. Using a combination of very different, time-resolved measurement methods at DESY's X-ray source PETRA III and other facilities, the scientists were able to show that so-called pre-distorted states can speed up photochemical reactions or make them possible in the first place. The group headed by Sonja Herres-Pawlis from the RWTH Aachen University (RWTH) Michael Rübhausen from the University of Hamburg and Wolfgang Zinth from Munich’s Ludwig Maximilian University, is presenting its findings in the journal Nature Chemistry.

 
 

Entatic state model complexes optimize the energies of starting and final configuration to enable fast reaction rates (illustrated by the hilly ground). The authors demonstrate that the entatic state principle can be used to tune the photochemistry of copper complexes.

 
 

The scientists had studied the pre-distorted, “entatic” state using a model system. An entatic state is the term used by chemists to refer to the configuration of a molecule in which the normal arrangement of the atoms is modified by external binding partners such that the energy threshold for the desired reaction is lowered, resulting in a higher speed of reaction. One example of this is the metalloprotein plastocyanin, which has a copper atom at its centre and is responsible for important steps in the transfer of electrons during photosynthesis. Depending on its oxidation state, the copper atom either prefers a planar configuration, in which all the surrounding atoms are arranged in the same plane (planar geometry), or a tetrahedral arrangement of the neighbouring ligands. However the binding partner in the protein forces the copper atom to adopt a sort of intermediate arrangement. This highly distorted tetrahedron allows a very rapid shift between the two oxidation states of the copper atom.

“Pre-distorted states like this play an important role in many biochemical processes,” explains Rübhausen, who works at the Centre for Free-Electron Laser Science (CFEL) in Hamburg, a cooperation between DESY, the University of Hamburg and the Max Planck Society. “The principle of the entatic state helps the electron transfer reactions that occur everywhere in nature and also in human beings, for example when we breathe or a plant photosynthesises,” adds Herres-Pawlis.

Biologically relevant, pre-distorted states always involve a metal atom. The scientists examined a model system consisting of a copper complex with specially tailored molecules bound to it, so-called ligands. Using a wide range of observation methods as well as theoretical calculations, the scientists showed that the ligands used did indeed put the copper complex into a pre-distorted (entatic) state and were then able to observe the details of the reaction that occurred when light was absorbed.

The combination of time-dependent UV, infra-red, X-ray and visual fluorescence spectroscopy produces a detailed picture of the dynamics of the structural changes on a timescale of pico- to nanoseconds (trillionths to billionths of a second). “We are now able for the first time to understand how pre-distorted states favour charge transfer,” explains Rübhausen. “Also, our studies demonstrate that pre-distorted states are important for photochemical reactions, in other words for certain biochemical processes which are triggered by light,” explains Herres-Pawlis.

The study shows in detail how the process proceeds: from the initial state (copper in an oxidation state of +1) an electron is transferred from the copper to one of the ligands, by optical excitation. Within femtoseconds (trillionths of a second) the excited state created decays into another, still excited state, known as the S1 state. In this configuration, the geometry is slightly relaxed.

Shortly afterwards, the electron undergoes a change in spin. The spin of an electron is comparable to the direction in which a top rotates. Although one of the electrons has so far remained on the ligand, this electron and its corresponding partner on the copper were spin-coupled. The spin of the electron on the ligand now reverses, and this very rapid transition to the so-called triplet state, within just about two picoseconds, removes the spin coupling. This T1 state exists for 120 picoseconds and drops back into the original state again after once again reversing its spin. All the time constants are distinctly shorter compared with other copper complexes. “A complete understanding of all the processes taking place has only become possible through the unique combination of different methods of study,” emphasises Zinth.

The detailed analysis of the reaction principle not only improves our understanding of natural processes. It can also help to customise new bioinorganic complexes that imitate nature but whose range of reactions extend beyond those of natural molecules. These complexes could also accelerate or make possible chemical reactions associated with electron transfers in other areas, too.

Scientists from the University of Hamburg, RWTH Aachen University, the Ludwig Maximilian University in Munich, DESY, the University of Paderborn, the European research facility ELI Beamlines, the Institute of Physics of the Czech Academy of Sciences, the University of Uppsala, the Chalmers University of Technology in Göteborg, European XFEL and the Danish Technical University were all involved in the research. The study received grants from Deutsche Forschungsgemeinschaft as part of the dislocated research group FOR1405 (Dynamics of Electron Transfer Processes within Transition Metal Sites in Biological and Bioinorganic Systems) and the SFB749 (Dynamics and Intermediates of Molecular Transformations) and the cluster of excellence CIPSM.

The article Transferring the entatic state principle into copper photochemistry is available on the webpage of the publisher.

 
  Copyright: WILEY-VCH

Highly Active N,O Zinc Guanidine Catalysts for the Ring-Opening Polymerization of Lactide

The Herres-Pawlis group develops new catalysts for the polymerisation of lactide to polylactide. New guanidine ligands with N,O-donors are synthesised and the desired zinc complexes are characterised and tested as catalyst for the solvent-free ring-opening polymerisation of racemic lactide at 150°C. All complexes showed high activity and the fastest complex affords colorless polylactide within 90 minutes with high molar masses (M w=69 100, polydispersity=1.4). Different monomer-to-initiator ratios were studied to identify the rate constant kP . The polymerisation of the most active catalyst was monitored by in situ Raman spectroscopy. To obtain insights into the mechanism, a end group analysis was performed. All four complexes show robustness against impurities in the lactide and an excellent catalytic activity. These zinc guanidine complexes with N,O-donors represent the fastest robust lactide catalysts to date, opening new avenues to a sustainable ring-opening polymerisation catalyst family for industrial use. Here you can go directly to this publication.

 
  Copyright: WILEY-VCH

On the Way to a Trisanionic {Cu3O2} Core for Oxidase Catalysis: Evidence of an Asymmetric Trinuclear Precursor Stabilized by Perfluoropinacolate Ligands

In a cooperation with the working group Doerrer from Boston, USA, we study the oxygenation of copper(I) complexes with perfluoropinacolate. At low temperatures, the oxygenation results in a trisanionic bis(µ3-oxo) trinuclear copper (II,II,III) species. This species were characterised with UV/Vis spectroscopy, cryospray-ionization mass spectrometry, and X-band EPR spectroscopy. The trimeric {Cu3O2} species was furthermore studied with stopped-flow spectroscopy and the kinetics of this system were hereby elucidated. Density function theory calculations provide isights in the electronic structures. During the reaction, an asymmetric {Cu3O2} species is detectable which structure is similar to multicopper oxidases. The final species is a symmetric trinuclear species which shows catalytic activity: It oxidises para-hydroquinone to benzoquinone. Here you can go directly to this publication.

 
  Copyright: WILEY-VCH

The Federation de Recherche FERMaT "Fluids, Energy, Reactors, Materials and Transfer" from Toulouse, France, invited the members of the priority program 1740 "The Influence of Local Transport Processes on Chemical Reactions in Bubble Flows" for the symposium Non-Invasive Measuring Tools and Numerical Methods for the Investigation of Non-Reactive and Reactive Gas-Liquid Flows. On this symposium researchers from Germany and France presented the latest results of their research and discussed about yield and selectivity of chemical reactions with mass transport. The most important results of this symposium were published within an special issue of Chemical Engineering & Technology. Here you can go directly to this special issue.

 
  Copyright: WILEY-VCH

Record Broken: A Copper Peroxide Complex with Enhanced Stability and Faster Hydroxylation Catalysis

For decades, functional tyrosinase models have been sought for. Our model complexes use N-donor ligands which mimic the histidine ligands in the actice site of the enzyme. Bis(pyrazolyl)methanes with pyridinyl or imidazolyl are excellent ligands for tyrosinase model complexes and herein we substited the pyridinyl donor. The new ligand HC(3-tBuPz)2(4-CO2MePy) stabilises a room temperature stable μ-η22-peroxide dicopper(II) species upon oxygenation and show high catalytic activity. This species hydroxylates 8-hydroxyquinoline in high yields (TONs of up to 20) and much faster than all other model systems (max. conversion within 7.5 min). The stoichiometric reaction behaviour with para-substituted sodium phenolates constrains saturation kinetics and a electrophilic aromatic substitution mechanism. Density functional theory calculations show the influence of the substitution of the pyridinyl donor: the carboxymethyl group adjusts the basicity and nucleophilicity without additional steric demand. This substitution opens up new pathways in reactivity tuning. Here you can go directly to this publication.

 
  Copyright: WILEY-VCH

Implications of Guanidine Substitution on Copper Complexes as Entatic-State Models

In this publication, we study copper complexes with the guanidine-quinoline ligand DMEGqu. The bis(chelate) copper(I) and copper(II) complexes show a geometry between tetrahedral and square-planar coordination. These structures model the entatic state and have been investigated in solid state by single-crystal X-ray diffraction and in the solid state and in solution by X-ray absorption spectroscopy. The DMEG unit of DMEGqu reveals a smaller steric encumbrance than the tetramethylguanidine (TMG) counterpart. So in DMEGqu complexes, we observe slightly larger structural changes upon oxidation than those for TMGqu complexes. DFT analyses show a good aggreement to the experiment (structur and optical features) with the functional B3LYP or TPSSh with the basis set def2-TZVP and empirical dispersion correction with Becke-Johnson damping. An orbital analysis explains the electronic structure of the complexes and their charge-transfer behaviour. Here you can go directly to this publication.

  Copyright: This journal is © The Royal Society of Chemistry 2016

Decay kinetics of sensitive bioinorganic species in a SuperFocus mixer at ambient conditions

In this key publication of the Schwerpunktprogramm 1740, we determine the intrinsic kinetics (formation and decay) of the oxygenation reaction of a copper(I) complex with a guanidine ligand with a stopped-flow UV/Vis setup and with SuperFocus mixer. By means of this setup, we were able for the first time to detect the formation and decay of a thermally very sensitive bis(μ-oxo)dicopper species at ambient temperature. Comparing these data to results from a stopped-flow setup we could confirm the performance of the SuperFocus mixer setup. Here you can go directly to this publication.

Image reproduced by permission of Sonja Herres-Pawlis and The Royal Society of Chemistry from Reaction Chemistry & Engineering 2016, 1, 485-493, DOI: 10.1039/C6RE00119J, link to the Cover.

 
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Donor-driven conformational flexibility in a real-life catalytic dicopper(II) peroxo complex

In this study we reported on a comprehensive theoretical analysis of the conformers of a real-life P species [Cu2O2{HC(3-tBuPz)2(Py)}2]2+ which displays catalytic reactivity. By second-order perturbation theory we elucidated the donor competition between pyrazolyl and pyridinyl moieties and found that pyrazolyl units are the stronger donors in bis(pyrazolyl)pyridinylmethane copper complexes. Geometry optimisations and TD-DFT calculations proved to be robust in the prediction of the experimental data: the XAS distances and both charge-transfer bands are well reproduced. Four stable conformers were found which are energetically rather close. We calculated for each conformer the energies of closed-shell and open-shell P state as well as the isomeric O state. Here, the basis set 6-31g(d) fails in the O-P energetics and predict in some cases randomly the O state with too large energetic favourisation. For a correct description of the energetics, empirical dispersion correction with Becke-Johnson damping, a PCM solvent model and a 3z basis set are required. The CDA analyses gave insights into the electronic structure of the real-life P species. The classic P MOs can be relocated but they are mixing with each other or with ligand orbitals. Hence, the CDA is a great method to parse the crucial interactions. In summary, we find that the N donor interactions to the core are extremely stabilising and that in the pzpz conformer, the equatorial interactions are more stabilising than the axial ones. We relate the extraordinary catalytic activity of the [Cu2O2{HC(3-tBuPz)2(Py)}2]2+ system to the subtle interplay of the different donor moieties which allow a flexible re-orientation of the dioxygen as well as the substrate approach. Both features are crucial requirements for efficient hydroxylation catalysis. Here you can go directly to this paper.

Image reproduced by permission of Sonja Herres-Pawlis and The Royal Society of Chemistry from Phys. Chem. Chem. Phys., 2016, 18, 6430-6440, DOI: 10.1039/C5CP05009J link to the cover.

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Efficient Biomimetic Hydroxylation Catalysis with a Bis(pyrazolyl)imidazolylmethane Copper Peroxide Complex

We have reported that fine-tuning of biomimetic ligands leads to enhanced catalytic activity of a catalytic tyrosinase model. We have provided syntheses and structural characterisation of a new bis(pyrazolyl)imidazolylmethane ligand and its copper(II) halide complexes. The reaction of dioxygen with its copper(I)hexafluoridoantimonate complex gives a peroxo species that has been characterised by UV/Vis and XAS spectroscopy as well as cryo-UHR-ESI mass spectrometry. The classical CT absorptions are strongly blueshifted and have been analysed by TD-DFT and NTO analysis. They show the special role of the third donor function. The new peroxo species displays stoichiometric and catalytic hydroxylation activity. The catalytic activity towards 8-hydroxyquinoline is more rapid than that observed
for the parent pyridinyl–peroxo species. Analysis of the substrate-binding kinetics quantitatively proves that the electrondeficient substrate p-carbomethoxyphenolate is hydroxylated approximately six times faster. This possibility of efficiently tuning catalytic activity by donor exchange opens up new directions for industrial applications of this mild, selective and atom-economic hydroxylation chemistry. Here you can go directly to this paper.

 
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The Cu2O2 torture track for a real-life system: [Cu2(btmgp)2O2]2+ oxo and peroxo species in density functional calculations

The structural, vibrational, electronic, and optical properties of [Cu2(btmgp)2(µ-O)2] 2+ and [Cu2(btmgp)2(\(\eta\) 2: \(\eta\) 2-O2)]2+ have been calculated within DFT using semilocal as well as hybrid density functionals in conjunction with both localized and plane wave basis sets. The geometrical data derived within the various theoretical approaches largely agree with each and are close to the experimental data available. This does also hold for the general shape of the energy landscape that describes the oxo–peroxo transition. Details, and in particular, the position of the global energy minimum of the PES, however, are strongly dependent on the XC functional and, in cases of small basis sets, depend additionally on the type of basis function. Notably, we find that hybrid-DFT calculations tend to favor the P species over O. If one additionally allows for spin relaxation – which leads to a favorable BS solution for P-this trend is enhanced. This behavior is not observed for local XC functionals, however. The inclusion of dispersion correction, conversely, leads to a small bias toward the O core. The CDA analysis for the O state reveals the strong stabilization by the guanidine donors and highlights the role of the guanidine as \(\sigma\)- and \(\pi\)-donor. The structural modification accompanying the O-P transition leads to pronounced changes of the excitation energies. In particular, the interchange of the O LUMO and P LUMO13 at the TS geometry leads to strongly dispersive excited-state PESs that might be used for optically induced isomerization. The geometry changes along the O-P transition give rise to significant changes of the optical absorption features as well as modify the vibrational fingerprints of the Cu complex. Here you can go directly to this paper.

 
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Geometrical and optical benchmarking of copper(II) guanidine-quinoline complexes: Insights from TD-DFT and many-body perturbation theory (Part II)

In summary, our results indicate a strong influence of the exchange and correlation functional on the structural properties of copper(II) guanidine–quinoline complexes. In addition, calculations performed with a plane-wave basis show that the influence of the basis set on the structural details of the molecular complexes studied here is of similar magnitude as the one due to various XC functionals. The structural benchmarking indicates that B3LYP/def2-TZVP and TPSSh/def2-TZVP offer both reasonable structural descriptions (predictions within the error bars of the experimental structures) but that the addition of dispersion with Becke–Johnson damping offers best structural agreement for extremely low computational costs. With def2-TZVP and Becke–Johnson damping, the functionals B3LYP and TPSSh give the accuracy of crystallographically determined structures (error: 0.012 A, 0.5% of bond lengths). Concerning the optical response calculations, it is found from IPA and DSCF calculations that adiabatic linear-response TD-DFT is a viable approach to copper(II) charge-transfer systems while the Bethe–Salpeter approach, at least using a static kernel with a simplified screening, may give rise to spurious excitations. In any event, a realistic description of the molecular optical response within MBPT requires the inclusion of the resonant–nonresonant coupling terms in the exciton Hamiltonian as well as selfconsistency effects in the Greens functions. The TD-DFT optical benchmarking demonstrates that B3LYP/def2-TZVP calculations model well the experimental data (error <0.1 eV) with little influence of the description of dispersion interaction and that the Becke–Johnson damping does not change this picture. Here you can go directly to this paper.

 
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Insights into the influence of dispersion correction in the theoretical treatment of guanidine-quinoline copper(I) complexes

In summary, we could show that the correct description of copper bis(chelate) complexes requires modern dispersion correction using BJ damping. The triple-zeta basis set def2-TZVP of the Ahlrichs series is balanced and converged for the structural description. The best structural description is obtained with the TPSSh functional but B3LYP is very suited as well. Especially when application for the calculation of optical transitions (and the resulting structures) as well as Raman spectra prediction is targeted, B3LYP seems to be the best choice. Cutting of ligand substituents leads to distortions which limit the predictive ability of such calculations. We recommend the calculation of “full” chemical systems with inclusion of dispersion correction using BJ damping. In the further analysis of the regarded copper bis(chelate) complexes, we found that the theoretical description of optical and Raman spectra is not much affected by the dispersion although charge transfer excitations come into play. Hence, we can derive the result that the correct structural description with dispersion serves as crucial basis for subsequent calculation steps and very much attention has to be paid here as all errors propagate from this step. Here you can go directly to this paper.

 
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Hiking on the potential energy surface of a functional tyrosinase model – implications of singlet, broken-symmetry and triplet description

In summary, we herein present the first full PES analysis of a P/O core equilibrium for the three important states. Moreover, this complete analysis has been accomplished for a real-life catalytic tyrosinase model. We could show that the P core lies in a deep global minimum with additional broken-symmetry stabilisation. With regard to the simple optical excitation by visible light, the low-lying triplet P state appears as a viable alternative for induction of biological hydroxylation activity. Here you can go directly to this paper.

Image reproduced by permission of Sonja Herres-Pawlis and The Royal Society of Chemistry from Chem. Commun., 2014, 50, 403-405, DOI: 10.1039/C3CC46893C link to the cover.

 
  Cover pictures of the highlights Copyright: Wiley

Geometrical and optical benchmarking of copper guanidine–quinoline complexes: Insights from TD-DFT and many-body perturbation theory

In summary, we could show that the correct description of bis(chelate) copper complexes requires extensive benchmarking. The geometric benchmarking recommends the BP86/6-311G(d) methodology for best accordance of gas-phase calculations to the solid state structures from X-ray measurements whereas the optical benchmarking gives best resemblance to experimental spectra when applying the combination B3LYP/def2-TZVP. With the large real-life copper complex, we have thoroughly studied the spectral sensitivity on torsion of copper coordination, isomer formation as well as choice of functionals and basis sets. Here, we found a very strong influence of the angle between the chelate planes. The correct description of the copper coordination is crucial for the prediction of optical spectra. In order to assess the methodological influence on the calculated ground- and excited-state properties, we performed in addition to (TD-)DFT also MBPT calculations, considering a small model system. It is found that large quasiparticle energies of the order of several electronvolts are largely offset by exciton binding energies for optical excitations. For this reason, optical-response calculations on the IPA level of theory are in qualitative (and in case of the B3LYP functional near quantitative) agreement with the TD-DFT results. From the comparison between TD-DFT and MBPT, we conclude that at least the low-energy excitations are remarkably robust with respect to the approximations made in their description. While MBPT methods are for computational reasons not yet applicable to large bis(chelate) copper complexes, the present results indicate that their excitations can be meaningfully modelled within TD-DFT. Hence, provided a careful benchmarking is performed, CT excitations in copper complexes are accessible to a quantitative analysis based on DFT. Here you can go directly to this paper.