Speaker: Olga S. Bokareva, Leibniz Institute for Catalysis; University of Rostock

Transition-metal complexes exhibit rich photochemical and photophysical properties due to their ability to access long-lived electronically excited states. Replicating such behavior in first-row (3d) transition metals presents significant challenges that warrant further investigation. Recent progress in ligand-field photocatalysis, particularly with cobalt(III) cyclometalated complexes, has revealed novel excited-state reactivity. However, the photophysics and photochemistry of cobalt(III) complexes remain underexplored.

In this work, I present excited-state dynamical simulations using mixed quantum-classical non-adiabatic surface-hopping and quantum multi-layer MCTDH methods to unravel the excited-states dynamics of cobalt(III) photosensitizers. For polypyridyl cobalt(III) complexes, ground-state recovery occurs in the Marcus inverted region,[1] a notable deviation from typical first-row transition-metal behavior, such as that of isoelectronic iron(II) systems. This regime supports simultaneous enhancement of redox potential and excited-state lifetime, enabling light-to-chemical energy conversion, including activation of oxidatively resistant substrates in photoredox catalysis. Initial triplet population timescales are captured through simulated population dynamics, while long-time recovery is recovered using transition state theory.

As a second example, I discuss how alkyl substituent functionalization affects the photodynamics of newly reported cobalt(III) complexes with imidazole-based NHC ligands.[2] The simulations reveal that ultrafast relaxation is driven by spin-orbit coupling, vibrational coherence, and structural dynamics, all influenced by the ligand environment. Overall, our findings highlight the necessity of integrating mixed quantum-classical and quantum dynamical approaches to accurately describe excited-state processes in transition-metal complexes.

[1] A. Ghosh, J. T. Yarranton, J. K. McCusker. Nat. Chem. 2024, 16, 1665.
[2] A. Krishna, L. Fritsch, J. Steube, M. A. Arguello Cordero, R. Schoch, A. Neuba, S. Lochbrunner and M. Bauer, Inorg. Chem. 2025, 64, 1401.