Ultrafast Two-Color X-Ray Emission Spectroscopy Reveals Excited State Landscape in a Base Metal Dyad

Effective photoinduced charge transfer makes molecular bimetallic assemblies attractive for applications as active light-induced proton reduction systems. Developing competitive base metal dyads is mandatory for a more sustainable future. However, the electron transfer mechanisms from the photosensitizer to the proton reduction catalyst in base metal dyads remain so far unexplored. A Fe─Co dyad that exhibits photocatalytic H2 production activity is studied using femtosecond X-ray emission spectroscopy, complemented by ultrafast optical spectroscopy and theoretical time-dependent DFT calculations, to understand the electronic and structural dynamics after photoexcitation and during the subsequent charge transfer process from the FeII photosensitizer to the cobaloxime catalyst. This novel approach enables the simultaneous measurement of the transient X-ray emission at the iron and cobalt K-edges in a two-color experiment. With this methodology, the excited state dynamics are correlated to the electron transfer processes, and evidence of the Fe→Co electron transfer as an initial step of proton reduction activity is unraveled.

Using Iron L-Edge and Nitrogen K-Edge X-ray Absorption Spectroscopy to Improve the Understanding of the Electronic Structure of Iron Carbene Complexes

Iron-centered N-heterocyclic carbene compounds have attracted much attention in recent years due to their long-lived excited states with charge transfer (CT) character. Understanding the orbital interactions between the metal and ligand orbitals is of great importance for the rational tuning of the transition metal compound properties, e.g., for future photovoltaic and photocatalytic applications. Here, we investigate a series of iron-centered N-heterocyclic carbene complexes with +2, + 3, and +4 oxidation states of the central iron ion using iron L-edge and nitrogen K-edge X-ray absorption spectroscopy (XAS). The experimental Fe L-edge XAS data were simulated and interpreted through restricted-active space (RAS) and multiplet calculations. The experimental N K-edge XAS is simulated and compared with time-dependent density functional theory (TDDFT) calculations. Through the combination of the complementary Fe L-edge and N K-edge XAS, direct probing of the complex interplay of the metal and ligand character orbitals was possible. The σ-donating and π-accepting capabilities of different ligands are compared, evaluated, and discussed. The results show how X-ray spectroscopy, together with advanced modeling, can be a powerful tool for understanding the complex interplay of metal and ligand.

Borylation in the Second Coordination Sphere of FeII Cyanido Complexes and Its Impact on Their Electronic Structures and Excited-State Dynamics

Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and Lewis acids are known to influence their electronic structures, whereby the non-labile attachment of B(C₆F₅)₃ resulted in several particularly interesting new compounds lately. Here, we investigate the effects of borylation on the properties of two Fe^II cyanido complexes in a systematic manner by comparing five different compounds and using a range of experimental techniques. Electrochemical measurements indicate that borylation entails a stabilization of the Fe^II-based t_2g-like orbitals by up to 1.65 eV, and this finding was confirmed by Mössbauer spectroscopy. This change in the electronic structure has a profound impact on the UV–vis absorption properties of the borylated complexes compared to the non-borylated ones, shifting their metal-to-ligand charge transfer (MLCT) absorption bands over a wide range. Ultrafast UV–vis transient absorption spectroscopy provides insight into how borylation affects the excited-state dynamics. The lowest metal-centered (MC) excited states become shorter-lived in the borylated complexes compared to their cyanido analogues by a factor of ∼10, possibly due to changes in outer-sphere reorganization energies associated with their decay to the electronic ground state as a result of B(C₆F₅)₃ attachment at the cyanido N lone pair.

Borylation in the second coordination sphere of two well-known Fe^II cyanido complexes leads to isocyanoborato complexes. The effects of borylation on their electronic structure and photophysical properties are thoroughly investigated with a range of experimental techniques.

Excitation Energy-Dependent Branching Dynamics Determines Photostability of Iron(II)-Mesoionic Carbene Complexes

Photoactive metal complexes containing earth-abundant transition metals recently gained interest as photosensitizers in light-driven chemistry. In contrast to the traditionally employed ruthenium or iridium complexes, iron complexes developed to be promising candidates despite the fact that using iron complexes as photosensitizers poses an inherent challenge associated with the low-lying metal-centered states, which are responsible for ultrafast deactivation of the charge-transfer states. Nonetheless, recent developments of strongly σ-donating carbene ligands yielded highly promising systems, in which destabilized metal-centered states resulted in prolonged lifetimes of charge-transfer excited states. In this context, we introduce a series of novel homoleptic Fe-triazolylidene mesoionic carbene complexes. The excited-state properties of the complexes were investigated by time-resolved femtosecond transient absorption spectroscopy and quantum chemical calculations. Pump wavelength-dependent transient absorption reveals the presence of distinct excited-state relaxation pathways. We relate the excitation-wavelength-dependent branching of the excited-state dynamics into various reaction channels to solvent-dependent photodissociation following the population of dissociative metal centered states upon excitation at 400 nm.

Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compounds based on iron are especially interesting, as iron is the most common transition metal element in the Earth’s crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compounds with significantly improved photophysical and photochemical properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochemistry of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochemical properties of iron carbenes and related complexes for photovoltaic, photoelectrochemical and photocatalytic applications.

Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering

The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110 fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

The use of iron in photoactive metal complexes has been investigated for decades. In this respect, the charge transfer (CT) states are of particular interest, since they are usually responsible for the photofunctionality of such compounds. However, only recently breakthroughs have been made in extending CT excited state lifetimes that are notoriously short-lived in classical polypyridine iron coordination compounds. This success is in large parts owed to the use of strongly σ-donating N-heterocyclic carbene (NHC) ligands that help manipulating the photophysical and photochemical properties of iron complexes. In this review we aim to map out the basic design principles for the generation of photofunctional iron NHC complexes, summarize the progress made so far and recapitulate on the synthetic methods used. Further, we want to highlight the challenges still existing and give inspiration for future generations of photoactive iron complexes.

Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-Edge X-ray Absorption Near-Edge Structure Spectroscopy

Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a long-lived quintet state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)₃²⁺ and conclusively identify a ³T intermediate that forms in 170 fs and decays to a vibrationally hot ⁵T_2g state in 39 fs. A coherent vibrational wavepacket with a period of 249 fs and damping time of 0.63 ps is observed on the ⁵T_2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe-N symmetric stretch. The results show that the shape of the M_2,3-edge X-ray absorption near-edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high-spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.