Characterising excited states in transition metal complexes by looking at pictures of orbitals can be a tedious task. Even more, it is hard to eliminate personal in the process and produce quantitative results. In a study led by Pedro Sánchez-Murcia from the University of Vienna, we have taken a closer look at this problem in the case of various substituted complexes deriving from the archetype Ru(bpy)3 with the aim of quantifying how different substituents influence the localisation of the excited electron. The result is presented in the article “Orbital-free photophysical descriptors to predict directional excitations in metal-based photosensitizers,” which just appeared in Chemical Science.
Splitting N2 is a challenging undertaking due to the strong triple bond holding the two nitrogen atoms together. Traditionally, this task is achieved via the Haber-Bosch process but recently interest has shifted to the possibilities of nitrogen splitting via homogeneous catalysis. Computations on the dinuclear transition metal complexes involved can be performed thanks to modern computational hardware and appropriate computer codes. However, the analysis of the excited states involved can become a significant challenge due to the large number of states and difficulties in assigning their character unambiguously. To tackle this problem, we have recently devised a strategy for a detailed analysis of the electronic wavefunctions of the states contributing to the reactive process. The associated paper, led by S. Rupp and V. Krewald from TU Darmstadt, just appeared in the European Journal of Inorganic Chemistry: Multi‐tier electronic structure analysis of Sita’s Mo and W complexes capable of thermal or photochemical N2 splitting.
In the tutorial, we explain the process of creating conditional electron densities for visualising electron correlation (ChemPhotoChem2019, 3, 702). The figure below shows a comparison between the ionic and covalent singlet and triplet B3u states of naphthalene.
The tutorial also explains the creation of bar graphs for a compact representation of excited-state character (see Coord. Chem. Rev., 2018, 361, 74 and ChemRxiv.11395314). In the picture below, the excited states of an iridium complex are decomposed into metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), and ligand centred (LC) contributions. The lowest six states are all dominated by MLCT character but the presented analysis clearly shows that the first three have enhanced LC character compared to the latter three.
Second, a more extensive paper exploring how far we can use information from excited-state wavefunction analysis tools to understand excitation energies beyond the molecular orbital picture. The energy of a correlated electron-hole pair is derived using diagrammatic techniques and this information is further used for a graphical depiction in terms of different charge distributions and their electrostatic potentials. Doing so turned out not as easy as hoped for but was very exciting. Find more here: Toward an Understanding of Electronic Excitation Energies Beyond the Molecular Orbital Picture by P. Kimber and F. Plasser.
Tomorrow Felix will give a talk at the New Horizons in Materials Modelling 2020 taking place in York. Title: A toolbox for analysing structure-property relationships in functional molecules interacting with light.
Tomorrow, Felix will give a talk at the Zernike Institute for Advanced Materials, Groningen. The talk is entitled: Understanding electronic excitation energies within and beyond the molecular orbital picture. It discusses how we can understand excited-state energies beyond simply looking at orbitals and their energies.
On Thursday, 20/06, Felix will give a talk at the CECAM workshop on Theoretical and Computational Inorganic Photochemistry in Toulouse. This talk will discuss how excited states in transition metal complexes can be assigned completely automatically without ever looking at an orbital. It is shown how this method can be used for a high-throughput analysis of excited states as well as for benchmarking excited-state computations. Finally, a quick outlook will be given on how correlation effects can be visualised using a newly developed tool for computing conditional electron/hole densities.