Simulation of Ultrafast Fe(II) Photoswitching Dynamics Triggered by Excitation within the Quintet Manifold
Year: 2026
Type: preprint
Abstract: The quintet-to-singlet back-switching process in [Fe(tpy)2]2+(tpy=2,2’:6’,2"-terpyridine), [Fe(dftpy)2]2+ (dftpy=6,6"-difluoro-2,2’:6’,2"-terpyridine) and [Fe(dctpy)2]2+(dctpy=6,6"-dichloro-2,2’:6’,2"-terpyridine), together with the photorelaxation dynamics of the [Fe(dftpy)2]2+ spin-crossover and the [Fe(dctpy)2]2+ high-spin complexes were investigated by full-dimensional trajectory surface hopping (TSH) dynamics. Following photoexcitation, the initial population of the 5Eg (5MC) state decays quickly via a branching mechanism of direct (5Eg → 5T2g) and 3MC-crossed (5Eg → 3MC→5T2g) processes, with the latter dominating the dynamics. The quintet-to-singlet photoconversion, the key step of the back-switching process, remains inefficient (<10%). This low efficiency arises because the singlet–triplet–quintet crossing region is readily accessible only during the initial sub-picosecond dynamics (<500 fs). Shifting the crossing region to improve its accessibility could substantially enhance the quintet-to-singlet conversion efficiency, suggesting that more extensive chemical modification could significantly optimize the back-switching process. Upon excitation to the quintet MLCT manifold, the population decays within <200 fs to the dark 5Eg (5MC) state (followed by the relaxation to the 5T2g (5MC) in less than 1 ps); only 1-2% of the initial population remains in the ligand-centered states in all complexes. In contrast, previous transient absorption spectroscopy experiments identified a long-lived 5/7MLCT state for [Fe(dftpy)2]2+ and [Fe(dctpy)2]2+, potentially because the analysis was restricted to picosecond timescales and probe wavelengths were used at which 5MC states are dark.
Source: ChemRxiv
Institution: HUN-REN Wigner Research Centre for Physics
Language: English
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7212025383