On the electronic structure of 2,2'-bipyridine, a XMS-RASPT2 study of the photophysics of this common transition metal ligand.

2,2'-Bipyridine is one of the most widely used ligands in coordination chemistry due to its electronic versatility and ability to stabilize a wide range of transition metal complexes. Despite the above, a comprehensive ab initio study of the intrinsic electronic and photophysical properties of the free ligand is lacking. In this work, we employ the multireference second-order perturbation theory on top of restricted active space (the so called RASPT2/RASSCF approach) to close the gap. Systematic benchmarking of extended restricted active spaces demonstrates that a π-only description is insufficient to reproduce the correct energetic ordering of ππ* and nπ* electronic states in the Franck-Condon point, calling for an active space augmented by extra valence orbitals. Excited-state geometry optimizations uncover the most probable deactivation mechanism involving ultrafast 1ππ* → 1nπ* internal conversion followed by a slow decay to the ground state through a low-lying conical intersection seam involving a puckered one-ring structure, thus rationalizing the lack of fluorescence in free 2,2'-bipyridine.