Branching Mechanism of Photoswitching in an Fe(II) Polypyridyl Complex Explained by Full Singlet-Triplet-Quintet Dynamics

Rozgonyi, Tamás and Vankó, György and Pápai, Mátyás (2023) Branching Mechanism of Photoswitching in an Fe(II) Polypyridyl Complex Explained by Full Singlet-Triplet-Quintet Dynamics. COMMUNICATIONS CHEMISTRY, 6. No.-7. ISSN 2399-3669

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Abstract

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 interpretation of the photorelaxation remains controversial. 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 involving two sequential processes: a dominant (MLCT)-M-3 & RARR;(MC)-M-3(T-3(2g))& RARR;(MC)-M-3(T-3(1g))->(MC)-M-5, and a minor (MLCT)-M-3 & RARR;(MC)-M-3(T-3(2g))& RARR;(MC)-M-5 component. (MLCT = metal-to-ligand charge transfer, MC = metal-centered). While the direct (MLCT)-M-3 & RARR;(MC)-M-5 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 (MC)-M-3 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) polypyridines, and fully describe the photorelaxation dynamics.

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