Photoselective MLCT to d-d pathways for light-induced excited spin state trapping

Quantum dynamics simulations of the thermal and light-induced high-spin to low-spin relaxation in Fe(bpy)3 and Fe(mtz)6

TBC

Ballistic ΔS = 2 intersystem crossing in a cobalt cubane following ligand-field excitation probed by extreme ultraviolet spectroscopy

Femtosecond M_2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy is used to probe the excited-state dynamics of the cobalt cubane [Co^III₄O₄](OAc)₄(py)₄ (OAc = acetate, py = pyridine), a model for water oxidation catalysts. After ligand-field excitation, intersystem crossing (ISC) to a metal-centered quintet occurs in 38 fs. 30% of the hot quintet undergoes ballistic back-ISC directly to the singlet ground state, with the remainder relaxing to a long-lived triplet.

Exploring the light-induced dynamics in solvated metallogrid complexes with femtosecond pulses across the electromagnetic spectrum

Oligonuclear complexes of d⁴-d⁷ transition metal ion centers that undergo spin-switching have long been developed for their practical role in molecular electronics. Recently, they also have appeared as promising photochemical reactants demonstrating improved stability. However, the lack of knowledge about their photophysical properties in the solution phase compared to mononuclear complexes is currently hampering their inclusion into advanced light-driven reactions. In the present study, the ultrafast photoinduced dynamics in a solvated [2 × 2] iron(II) metallogrid complex are characterized by combining measurements with transient optical-infrared absorption and x-ray emission spectroscopy on the femtosecond time scale. The analysis is supported by density functional theory calculations. The photocycle can be described in terms of intra-site transitions, where the Fe^II centers in the low-spin state are independently photoexcited. The Franck-Condon state decays via the formation of a vibrationally hot high-spin (HS) state that displays coherent behavior within a few picoseconds and thermalizes within tens of picoseconds to yield a metastable HS state living for several hundreds of nanoseconds. Systematic comparison with the closely related mononuclear complex [Fe(terpy)₂]²⁺ reveals that nuclearity has a profound impact on the photoinduced dynamics. More generally, this work provides guidelines for expanding the integration of oligonuclear complexes into new photoconversion schemes that may be triggered by ultrafast spin-switching.

Bias induced spin state transition mediated by electron excitations

Recent experiments reported that spin state transitions were realized by applying bias voltages. However, these bias-induced transitions are not fully understood, especially the mechanism. It is well established in experiments that the metal-to-ligand charge transfer (MLCT) excitation activated by light radiation can lead to the transition from low spin (LS) to high spin (HS), and the transition from HS to LS can be achieved by light due to the metal-centered (MC) excitation. Moreover, electronic excitations are accessible by inelastic cotunneling in molecular junctions under bias voltages. Based on these two facts, we propose that the MLCT excitation is responsible for the bias-induced transition from LS to HS, and the bias-induced transition from HS to LS is attributed to the MC excitation. The rationality of our proposed mechanism is demonstrated by comparing first-principles results and experimental observations. Threshold voltages of MLCT and MC excitations predicted in theory are consistent with bias voltages used to reach the transition from LS to HS and that from HS to LS in the experiment [Miyamachi et al., Nat. Commun. 3, 938 (2012)]. Activation of MLCT or MC excitation depends on the bias polarity, which can explain the bias-polarity dependence of the transition in the experiment. Our study is important for further design of molecular spintronic devices working on the bias-controlled transition.

Revealing Hot and Long-Lived Metastable Spin States in the Photoinduced Switching of Solvated Metallogrid Complexes with Femtosecond Optical and X-ray Spectroscopies

An atomistic understanding of the photoinduced spin-state switching (PSS) within polynuclear systems of d⁴-d⁷ transition metal ion complexes is required for their rational integration into light-driven reactions of chemical and biological interest. However, in contrast to mononuclear systems, the multidimensional dynamics of the PSS in solvated molecular arrays have not yet been elucidated due to the expected complications associated with the connectivity between the metal centers and the strong interactions with the surroundings. In this work, the PSS in a solvated triiron(II) metallogrid complex is characterized using transient optical absorption and X-ray emission spectroscopies on the femtosecond time scale. The complementary measurements reveal the photoinduced creation of energy-rich (hot) and long-lived quintet states, whose dynamics differ critically from their mononuclear congeners. This finding opens major prospects for developing novel schemes in solution-phase spin chemistry that are driven by the dynamic PSS process in compact oligometallic arrays.

Controlling the Lifetime of the Triplet MLCT State in Fe(II) Polypyridyl Complexes through Ligand Modification

A computational study is presented in which two strategies of ligand modifications have been explored to invert the relative energy of the metal-to-ligand charge transfer (MLCT) and metal-centered (MC) state in Fe(II)-polypyridyl complexes. Replacing the bipyridines by stronger σ donors increases the ligand-field strength and pushes the MC state to higher energy, while the use of ligands with a larger π conjugation leads to lower MLCT energies.