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

01 September 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

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 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.

Keywords

ab initio calculations
low-spin/high-spin photoswitching
spin-vibronic dynamics
ultrafast dynamics

Supplementary materials

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Supplementary information (PDF)
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Methodological details and numerical parameters of the model.
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Supplementary data files
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Numerical values of model parameters
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