Unraveling the photoredox chemistry of a molecular ruby

In contrast to well-studied 4d⁶ and 5d⁶ transition metal complexes such as the modern-day drosophila of photochemistry, Ru(ii)-tris(bipyridine), which often feature a typical triplet metal-to-ligand charge transfer emission in the nanosecond timescale, the photophysics of Cr(iii) complexes are drastically different. The 3d³ configuration of the chromium(iii) allows for an unusual spin-flip emission from the low-lying metal-centered (MC; ²T₁ and ²E) states, exhibiting lifetimes up to the milliseconds to seconds timescale. In this fully computational contribution, the photophysical properties as well as the application of such long-lived excited states in the context of photoredox chemical transformations are investigated for the recently introduced [Cr(dqp)₂]³⁺ [Cr(iii)-(2,6-bis(8'-quinolinyl)pyridine)₂]³⁺, otherwise known as a type of molecular ruby. Our in-depth theoretical characterization of the complicated electronic structure of this 3d³ system relies on state-of-the-art multiconfigurational methods, i.e. the restricted active space self-consistent field (RASSCF) method followed by second-order perturbation theory (RASPT2). This way, the light-driven processes associated with the initial absorption from the quartet ground state, intersystem crossing to the doublet manifold as well as the spin-flip emission were elucidated. Furthermore, the applicability of the long-lived excited state in [Cr(dqp)₂]³⁺ in photoredox chemistry, i.e. reductive quenching by N,N-dimethylaniline, was investigated by ab initio molecular dynamics (AIMD). Finally, the thermodynamics and kinetics of these underlying intermolecular electron transfer processes were analyzed in the context of semiclassical Marcus theory.