Branching mechanism of photoswitching in Fe(II) polypyridyl complexes explained by full singlet-triplet-quintet dynamics

It has long been known that irradiation with visible light converts Fe(II) polypyridines from their low-spin (singlet) to high-spin (quintet) state, yet mechanistic interpreation of the photorelaxation remains contraversal. Herein, we simulate the full singlet-triplet-quintet dynamics of the [Fe(terpy)2]2+ (terpy = 2,2’:6’,2’’-terpyridine) complex in full dimension, in order to clarify the complex photodynamics. Importantly, we report a branching mechanism with two sequential components: 3MLCT → 3MC(3T2g) → 5MC and 3MLCT → 3MC(3T2g) → 3MC(3T1g) → 5MC (MLCT = metal-to-ligand charge transfer, MC = metal-centered). While the direct 3MLCT → 5MC mechanism is considered as a relevant alternative, we show that it could only be operative and thus lead to competing pathways in the absence of 3MC states. The quintet state is populated on the sub-picosecond timescale involving non-exponential dynamics and coherent Fe-N breathing oscillations. The results are in agreement with the available time-resolved experimental data on Fe(II) polypiridines, and fully describe the photorelaxation dynamics.