Down to the Photoinduced Hole: A DFT insight into Ruthenium Polypyridines Excited State

GERMAN E. PIESLINGER

San Sebastián

Congreso; AEBIN Photochemistry School; 2020

Asociación Española de Bioinorgánica

DFT calculations are a powerful tool to analyse the configuration of photoinduced mixed-valence (PIMV) systems. In bimetallic cyanide-bridged ruthenium polypyridines with a (dπ)6(dπ)6 configuration, MLCT light absorption produces transient PIMV species with a triplet multiplicity, where the properties of the photogenerated (dπ)6(dπ)5 mixed-valence core are modulated by the presence of the polypyridine radical anion. It is well known that the intersystem crossing process that leads from the singlet to the triplet MLCT manifold of ruthenium polypyridines is downhill and takes around 100 fs. Therefore, the photoinduced IVCT (PIIVCT) bands observed using picosecond transient absorption spectroscopy techniques correspond to triplet excited states. DFT optimizations of the lowest-lying triplet state for a family of cyanide-bridged ruthenium polypyridines yielded, in fact, PIMV systems. PIIVCT transitions calculated by TD-DFT methods, nicely agree with those experimentally observed using ultrafast transient absorption spectroscopy. Spin density isosurface plots clearly show the mixed-valence character of these photoinduced systems that, in contrast to the ground-state analogues, also include spin density over the polypyridinic ligand corresponding to the radical anion. This allows to unambiguously identify these states as MLCT instead of MC states that only show spin density over metal centers. Moreover, these diruthenium systems show substantial delocalization of the charge. Currently, we are studying other bimetallic ruthenium polypyridines combining ultrafast transient absorption spectroscopy and DFT calculations in order to exploit their unique properties.