Theoretical spectroscopy of transition metal complexes poses special challenges for quantum chemistry. Relativistic effects lead to spectral shifts of electronic transitions – the yellow color of gold originates from blue light absorption as opposed to absorption of UV light by silver – and to multiplicity mixing which in turn promotes intersystem crossing and phosphorescence. These effects are particularly pronounced in late third-row elements but cannot be neglected in the modeling of lighter transition metal compounds either. Differential electron correlation effects, on the other hand, are particularly large in metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer states (LMCT) states of first-row transition metal complexes due to their compact d shells. Additional complications are the multiconfigurational character of the excited states, where ligand-centered (LC) and metal-centered (MC) transitions are mixed with MLCT, LMCT and possibly ligand-to-ligand charge transfer (LLCT) transitions, and the open-shell structure of many transition metal complexes in the electronic ground state.
The DFT/MRCI method is well suited to capture the multireference character of the wave function in transition metal complexes. Due to technical limitations – requiring molecular orbitals of a closed-shell anchor state as one-electron basis functions – the method is not applicable to highly symmetric complexes featuring many open shells in the electronic ground state. (Marian-2018) Rates of spin-forbidden transitions can be efficiently modeled using the Spin-Orbit Coupling Kit and the VIBES program developed in our laboratory.
The methods have been employed extensively for investigating intersystem crossing and phosphorescence rate constants of iridium and platinum complexes (Kleinschmidt et al.) on the one hand and coinage metal complexes (Foeller-2017) on the other hand. In the framework of the DFG Priority Programme 2102 we explore the ability of ZnII complexes to form efficient phosphorescent or TADF emitters. Internal collaboration partners are Prof. Christian Ganter and his group.
Early-stage researchers (Bachelor and Master students as well as PhD candidates) who want to join our team on exploring the photophysical properties of transition metal complexes by means of quantum chemistry should contact Prof. Christel Marian or Dr. Martin Kleinschmidt.