Toward Simulation of Fe(II) Low-Spin - High-Spin Photoswitching by Synergistic Spin-Vibronic Dynamics

[Image: see text] A new theoretical approach is presented and applied for the simulation of Fe(II) low-spin (LS, singlet, t(2g)(6)e(g)(0)) → high-spin (HS, quintet, t(2g)(4)e(g)(2)) photoswitching dynamics of the octahedral model complex [Fe(NCH)(6)](2+). The utilized synergistic methodology heavily exploits the strengths of complementary electronic structure and spin-vibronic dynamics methods. Specifically, we perform 3D quantum dynamics (QD) and full-dimensional trajectory surface hopping (TSH, in conjunction with a linear vibronic coupling model), with the modes for QD selected by TSH. We follow a hybrid approach which is based on the application of time-dependent density functional theory (TD-DFT) excited-state potential energy surfaces (PESs) and multiconfigurational second-order perturbation theory (CASPT2) spin–orbit couplings (SOCs). Our method delivers accurate singlet–triplet–quintet intersystem crossing (ISC) dynamics, as assessed by comparison to our recent high-level ab initio simulations and related time-resolved experimental data. Furthermore, we investigate the capability of our simulations to identify the location of ISCs. Finally, we assess the approximation of constant SOCs (calculated at the Franck–Condon geometry), whose validity has central importance for the combination of TD-DFT PESs and CASPT2 SOCs. This efficient methodology will have a key role in simulating LS → HS dynamics for more complicated cases, involving higher density of states and varying electronic character, as well as the analysis of ultrafast experiments

Photoinduced Low-Spin → High-Spin Mechanism of an Octahedral Fe(II) Complex Revealed by Synergistic Spin-Vibronic Dynamics

[Image: see text] The Fe(II) low-spin (LS; (1)A(1g), t(2g)(6)e(g)(0)) → high-spin (HS; (5)T(2g), t(2g)(4)e(g)(2)) light-induced excited spin state trapping (LIESST) mechanism solely involving metal-centered states is revealed by synergistic spin-vibronic dynamics simulations. For the octahedral [Fe(NCH)(6)](2+) complex, we identify an initial ∼100 fs (1)T(1g) → (3)T(2g) intersystem crossing, controlled by vibronic coupling to antisymmetric Fe–N stretching motion. Subsequently, population branching into (3)T(1g), (5)T(2g) (HS), and (1)A(1g) (LS) is observed on a subpicosecond time scale, with the dynamics dominated by coherent Fe–N breathing wavepackets. These findings are consistent with ultrafast experiments, methodologically establish a new state of the art, and will give a strong impetus for further intriguing dynamical studies on LS → HS photoswitching

Effect of <i>tert</i>-Butyl Functionalization on the Photoexcited Decay of a Fe(II)-<i>N</i>-Heterocyclic Carbene Complex

Understanding and subsequently being able to manipulate the excited-state decay pathways of functional transition-metal complexes is of utmost importance in order to solve grand challenges in solar energy conversion and data storage. Herein, we perform quantum chemical calculations and spin-vibronic quantum dynamics simulations on the Fe-<i>N</i>-heterocyclic carbene complex, [Fe­(btbip)₂]²⁺ (btbip = 2,6-bis­(3-<i>tert</i>-butyl-imidazole-1-ylidene)­pyridine). The results demonstrate that a relatively minor structural change compared to its parent complex, [Fe­(bmip)₂]²⁺ (bmip = 2,6-bis­(3-methyl-imidazole-1-ylidene)­pyridine), completely alters the excited-state relaxation. Ultrafast deactivation of the initially excited metal-to-ligand charge transfer (^1,3MLCT) states occurs within 350 fs. In contrast to the widely adopted mechanism of Fe­(II) photophysics, these states decay into close-lying singlet metal-centered (¹MC) states. This occurs because the <i>tert</i>-butyl functionalization stabilizes the ¹MC states, enabling the ^1,3MLCT → ¹MC population transfer to occur close to the Franck–Condon geometry, making the conversion very efficient. Subsequently, a spin cascade occurs within the MC manifold, leading to the population of triplet and quintet MC states. These results will inspire highly involved ultrafast experiments performed at X-ray free electron lasers and shall pave the way for the design of novel high-efficiency transition-metal-based functional molecules

Probing Spin-Vibronic Dynamics Using Femtosecond X-ray Spectroscopy

Ultrafast pump-probe spectroscopy within the X-ray regime is now possible owing to the devel- opment of X-ray Free Electrons Lasers (X-FELs) and are opening new opportunities for direct probing the correlated evolution of the nuclei, the electronic and spin degrees of freedom on the femtosecond timescale. In this contribution we use excited state wavepacket dynamics of the photoexcited decay of a new Fe(II) complex, [Fe(bmip)2]2+ (bmip=2,6-bis(3-methyl-imidazole- 1-ylidine)pyridine), to simulate the experimental observables associated with femtosecond Fe K- edge X-ray absorption near-edge structure (XANES) and X-ray emission (XES) spectra. We show how the evolution of the nuclear wavepacket is translated into the experimental observable and the sensitivity of these approaches for following excited state dynamics

Excited-State Solvation Structure of Transition Metal Complexes from Molecular Dynamics Simulations and Assessment of Partial Atomic Charge Methods

In this work, we investigate the excited-state solute and solvation structure of $\mathrm{[Ru(bpy)_3]^{2+}}$, $\mathrm{[Fe(bpy)_3]^{2+}}$, $\mathrm{[Fe(bmip)_2]^{2+}}$ and $\mathrm{[Cu(phen)_2]^{+}}$ (bpy=2,2'-pyridine; bmip=2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine; phen=1,10-phenanthroline) transition metal complexes (TMCs) in terms of solute-solvent radial distribution functions (RDFs) and evaluate the performance of some of the most popular partial atomic charge (PAC) methods for obtaining these RDFs by molecular dynamics (MD) simulations. To this end, we compare classical MD of a frozen solute in water and acetonitrile (ACN) with quantum mechanics/molecular mechanics Born-Oppenheimer molecular dynamics (QM/MM BOMD) simulations. The calculated RDFs show that the choice of a suitable PAC method is dependent on the coordination number of the metal, denticity of the ligands, and type of solvent. It is found that this selection is less sensitive for water than ACN. Furthermore, a careful choice of the PAC method should be considered for TMCs that exhibit a free direct coordination site, such as $\mathrm{[Cu(phen)_2]^{+}}$. The results of this work show that fast classical MD simulations with ChelpG/RESP or CM5 PACs can produce RDFs close to those obtained by QM/MM MD and thus, provide reliable solvation structures of TMCs to be used, e.g. in the analysis of scattering data

Characterizing the Solvated Structure of Photoexcited [Os(terpy)₂]²⁺ with X-ray Transient Absorption Spectroscopy and DFT Calculations

Characterizing the geometric and electronic structures of individual photoexcited dye molecules in solution is an important step towards understanding the interfacial properties of photo-active electrodes. The broad family of “red sensitizers” based on osmium(II) polypyridyl compounds often undergoes small photo-induced structural changes which are challenging to characterize. In this work, X-ray transient absorption spectroscopy with picosecond temporal resolution is employed to determine the geometric and electronic structures of the photoexcited triplet state of [Os(terpy)2]2+ (terpy: 2,2′:6′,2″-terpyridine) solvated in methanol. From the EXAFS analysis, the structural changes can be characterized by a slight overall expansion of the first coordination shell [OsN6]. DFT calculations supports the XTA results. They also provide additional information about the nature of the molecular orbitals that contribute to the optical spectrum (with TD-DFT) and the near-edge region of the X-ray spectra