Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution

A Heterodox Approach for Designing Iron Photosensitizers: Pentacyanoferrate(II) Complexes with Monodentate Bipyridinium/Pyrazinium-Based Acceptor Ligands

The main obstacle in replacing well-established precious ruthenium photosensitizers with earth-abundant iron analogs is the short excited state lifetimes of metal-to-ligand charge transfer (MLCT) states due to relatively weak octahedral field splitting and relaxation via metal-centered (MC) states. In this study, we address the issue of short lifetime by using pentacyanoferrate(II) complexes and combat facile photodissociation by utilizing positively charged pyrazinium or bipyridinium ligands. We utilize femtosecond transient absorption spectroscopy alongside quantum chemical calculations to probe the excited states of three 4,4'-bipyridinium- or pyrazinium-based pentacyanoferrate(II) complexes. The 4,4'-bipyridinium-based complexes exhibit 3MLCT lifetimes of about 20 ps, while the pyrazinium-based complex exhibits a lifetime of 61 ps in an aqueous solution, setting a benchmark for cyanoferrate complexes. These results mark the foundation for a new group of easy-to-prepare iron photosensitizers that can be used for harvesting visible light.

Investigation of Vibrational Cooling in a Photoexcited Dichloro-Ruthenium Charge Transfer Complex Using Transient Electronic Absorption Spectroscopy

Vibrational cooling of molecules in excited electronic states is ubiquitous in photochemical reactions in solution but challenging to infer in time-resolved electronic absorption experiments. We report the ultrafast photophysics of cis-dichlorobis(2,2'-bipyridine)ruthenium(II), Ru(bpy)2Cl2, a precursor molecule commonly utilized in synthetic modifications of a vast array of ruthenium complexes. Femtosecond time-resolved electronic absorption spectroscopy is used to track an ultrafast spectral narrowing of the excited-state absorptions at 475 nm (21,050 cm-1) and 505 nm (19,800 cm-1) due to the reduced ligand in the photoexcited molecular complex. These sharp features, which overlap with a broader ground-state bleach spanning 450 nm (22,220 cm-1) to 600 nm (16,670 cm-1), evolve rapidly with time constants of 16 ± 5, 15 ± 3, and 18 ± 2 ps, respectively, for ligand-centered (π → π*, 266 nm) and charge-transfer (t2 → π*, 400 and 550 nm) excitations and constitute a direct signature of picosecond vibrational cooling.

A mononuclear iron(II) complex constructed using a complementary ligand pair exhibits intrinsic luminescence-spin-crossover coupling

Molecular materials that exhibit synergistic coupling between luminescence and spin-crossover (SCO) behaviors hold significant promise for applications in molecular sensors and memory devices. However, the rational design and underlying coupling mechanisms remain substantial challenges in this field. In this study, we utilized a luminescent complementary ligand pair as an intramolecular luminophore to construct a new Fe-based SCO complex, namely [FeL1L2](BF4)2·H2O (1-Fe, L1 is a 2,2':6',2''-terpyridine (TPY) derivative ligand and L2 is 2,6-di-1H-pyrazol-1-yl-4-pyridinecarboxylic acid), and two isomorphic analogs (2-Co, [CoL1L2](BF4)2·H2O and 3-Zn, [ZnL1L2](BF4)2·H2O). Magnetic studies reveal that 1-Fe exhibits thermally induced SCO within the temperature range of 150-350 K. Variable-temperature fluorescence emission spectral analysis of the three complexes confirmed the occurrence of SCO-luminescence coupling in 1-Fe. Furthermore, variable-temperature UV-vis absorption spectra and time-dependent density functional theory (TD-DFT) calculations elucidate the intramolecular luminescence emission behavior, highlighting the critical role of charge transfer processes between the L1 ligand and FeII ions with different spin states. Our research presents a novel construction strategy for synthesizing synergistic SCO-luminescent materials and contributes to the understanding of the mechanisms underlying SCO-luminescence coupling.

Leveraging Intramolecular π-Stacking to Access an Exceptionally Long-Lived 3MC Excited State in an Fe(II) Carbene Complex

The ability to manipulate excited-state decay cascades using molecular structure is essential to the application of abundant-metal photosensitizers and chromophores. Ligand design has yielded some spectacular results elongating charge-transfer excited state lifetimes of Fe(II) coordination complexes, but triplet metal-centered (3MC) excited states─recently demonstrated to be critical to the photoactivity of isoelectronic Co(III) polypyridyls─have to date remained elusive, with temporally isolable examples limited to the picosecond regime. With this report, we show how strong-field donors and intramolecular π-stacking can conspire to stabilize a long-lived 3MC excited state for a remarkable 4.1 ± 0.3 ns in fluid solution at ambient temperature. Analysis of variable-temperature time-resolved absorption data using theoretical models ranging from Arrhenius to semiclassical Marcus theory, combined with computational modeling and X-ray crystallography, reveal a Jahn-Teller stabilized excited state with a high activation barrier for ground-state recovery. The net result is a chromophore with a 3MC excited-state lifetime that is orders of magnitude longer than anything yet observed for an Fe(II) complex.

Femtosecond Spin-State Switching Dynamics of Fe(II) Complexes Condensed in Thin Films

The tailoring of spin-crossover films has made significant progress over the past decade, mostly motivated by the prospect in technological applications. In contrast to spin-crossover complexes in solution, the investigation of the ultrafast switching in spin-crossover films has remained scarce. Combining the progress in molecule synthesis and film growth with the opportunities at X-ray free-electron lasers, we study the photoinduced spin-state switching dynamics of a molecular film at room temperature. The subpicosecond switching from the S = 0 low-spin ground state to the S = 2 high-spin state is monitored by analyzing the transient evolution of the Fe L3 X-ray absorption edge fine structure, i.e. element-specifically at the switching center of the Fe(II) complex. Our measurements show the involvement of an intermediate state in the switching. At large excitation fluences, the fraction of high-spin molecules saturates at ≈50%, which is likely due to molecule-molecule interaction within the film.

Correlating Valence and 2p3d RIXS Spectroscopies: A Ligand-Field Study of Spin-Crossover Iron(II)

The molecular spin-crossover phenomenon between high-spin (HS) and low-spin (LS) states is a promising route to next-generation information storage, sensing applications, and molecular spintronics. Spin-crossover complexes also provide a unique opportunity to study the ligand field (LF) properties of a system in both HS and LS states while maintaining the same ligand environment. Presently, we employ complementing valence and core-level spectroscopic methods to probe the electronic excited-state manifolds of the spin-crossover complex [FeII(H2B(pz)2)2phen]0. Light-induced excited spin-state trapping (LIESST) at liquid He temperatures is exploited to characterize magnetic and spectroscopic properties of the photoinduced HS state using SQUID magnetometry and magnetic circular dichroism spectroscopy. In parallel, Fe 2p3d RIXS spectroscopy is employed to examine the ΔS = 0, 1 excited LF states. These experimental studies are combined with state-of-the-art CASSCF/NEVPT2 and CASCI/NEVPT2 calculations characterizing the ground and LF excited states. Analysis of the acquired LF information further supports the notion that the spin-crossover of [FeII(H2B(pz)2)2phen]0 is asymmetric, evidenced by a decrease in eπ in the LS state. The results demonstrate the power of cross-correlating spectroscopic techniques with high and low LF information content to make accurate excited-state assignments, as well as the current capabilities of ab initio theory in interpreting these electronic properties.

The sensitive aspects of modelling polymer-ceramic composite solid-state electrolytes using molecular dynamics simulations

Solid-state composite electrolytes have arisen as one of the most promising materials classes for next-generation Li-ion battery technology. These composites mix ceramic and solid-polymer ion conductors with the aim of combining the advantages of each material. The ion-transport mechanisms within such materials, however, remain elusive. This knowledge gap can to a large part be attributed to difficulties in studying processes at the ceramic-polymer interface, which are expected to play a major role in the overall ion transport through the electrolyte. Computational efforts have the potential of providing significant insight into these processes. One of the main challenges to overcome is then to understand how a sufficiently robust model can be constructed in order to provide reliable results. To this end, a series of molecular dynamics simulations are here carried out with a variation of certain structural (surface termination and polymer length) and pair potential (van der Waals parameters and partial charges) models of the Li7La3Zr2O12 (LLZO) poly(ethylene oxide) (PEO) system, in order to test how sensitive the outcome is to each variation. The study shows that the static and dynamic properties of Li-ion are significantly affected by van der Waals parameters as well as the surface terminations, while the thickness of the interfacial region - where the structure-dynamic properties are different as compared to the bulk-like regime - is the same irrespective of the simulation setup.

Tailoring the Photophysical Properties of a Homoleptic Iron(II) Tetra N-Heterocyclic Carbene Complex by Attaching an Imidazolium Group to the (C∧N∧C) Pincer Ligand─A Comparative Study

We here report the synthesis of the homoleptic iron(II) N-heterocyclic carbene (NHC) complex [Fe(miHpbmi)2](PF6)4 (miHpbmi = 4-((3-methyl-1H-imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the π-electron-withdrawing 3-methyl-1H-imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi)2](PF6)2 (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region. However, compared to the carboxylic acid derivative [Fe(cpbmi)2](PF6)2 (cpbmi = 1,1'-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe(miHpbmi)2](PF6)4 is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi)2](PF6)4 a promising pH-insensitive analogue of [Fe(cpbmi)2](PF6)2. Finally, the excited-state dynamics of the title compound [Fe(miHpbmi)2](PF6)4 was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

Influence of ligand variation on the deactivation process of metal-to-ligand charge transfer excited states in quadruply bonded dimolybdenum complexes

To explore the dynamics of metal-to-ligand charge transfer (MLCT) excited states involving covalently bonded dimetal units, a series of quadruply bonded dimolybdenum (Mo2) complexes, namely, [Mo2]-ph, [Mo2]-naph, and [Mo2]-anth, were synthesized and characterized. Our investigations reveal a non-radiative process associated with the deactivation of the MLCT state into a low-lying dimetal-centered triplet state (3Mo2-δδ*), resulting in the populated MLCT states in these molecular systems exhibiting either extremely weak emission or being non-emissive. The influence of ligand variation on the dynamics of MLCT states was examined using femtosecond transient absorption spectroscopy, with deactivation time constants determined to be 1.9 ps for [Mo2]-ph, 6.5 ps for [Mo2]-naph, and 49 ps for [Mo2]-anth. This electron transfer behaviour follows an inverse energy-gap law, contrary to the general guideline that applies to the decay of the MLCT state back to the electronic ground state. This result offers valuable insights into understanding the photochemical and photophysical properties of covalently bonded dimetal complexes.

Experimental capabilities for liquid jet samples at sub-MHz rates at the FXE Instrument at European XFEL

The Femtosecond X-ray Experiments (FXE) instrument at the European X-ray Free-Electron Laser (EuXFEL) provides an optimized platform for investigations of ultrafast physical, chemical and biological processes. It operates in the energy range 4.7-20 keV accommodating flexible and versatile environments for a wide range of samples using diverse ultrafast X-ray spectroscopic, scattering and diffraction techniques. FXE is particularly suitable for experiments taking advantage of the sub-MHz repetition rates provided by the EuXFEL. In this paper a dedicated setup for studies on ultrafast biological and chemical dynamics in solution phase at sub-MHz rates at FXE is presented. Particular emphasis on the different liquid jet sample delivery options and their performance is given. Our portfolio of high-speed jets compatible with sub-MHz experiments includes cylindrical jets, gas dynamic virtual nozzles and flat jets. The capability to perform multi-color X-ray emission spectroscopy (XES) experiments is illustrated by a set of measurements using the dispersive X-ray spectrometer in von Hamos geometry. Static XES data collected using a multi-crystal scanning Johann-type spectrometer are also presented. A few examples of experimental results on ultrafast time-resolved X-ray emission spectroscopy and wide-angle X-ray scattering at sub-MHz pulse repetition rates are given.

In situ X-ray spectroscopies beyond conventional X-ray absorption spectroscopy on deciphering dynamic configuration of electrocatalysts

Realizing viable electrocatalytic processes for energy conversion/storage strongly relies on an atomic-level understanding of dynamic configurations on catalyst-electrolyte interface. X-ray absorption spectroscopy (XAS) has become an indispensable tool to in situ investigate dynamic natures of electrocatalysts but still suffers from limited energy resolution, leading to significant electronic transitions poorly resolved. Herein, we highlight advanced X-ray spectroscopies beyond conventional XAS, with emphasis on their unprecedented capabilities of deciphering key configurations of electrocatalysts. The profound complementarities of X-ray spectroscopies from various aspects are established in a probing energy-dependent "in situ spectroscopy map" for comprehensively understanding the solid-liquid interface. This perspective establishes an indispensable in situ research model for future studies and offers exciting research prospects for scientists and spectroscopists.

Ultrafast Spin Crossover Photochemical Mechanism in [FeII(2,2'-bipyridine)3]2+] Revealed by Quantum Dynamics

Photoexcitation of [FeII(2,2'-bipyridine)3]2+ induces a subpicosecond spin crossover transformation from a low-spin singlet to a high-spin quintet state. The mechanism involves metal-centered (MC) and metal-ligand charge transfer (MLCT) triplet intermediates, but their individual contributions to this efficient intersystem crossing have been object of debate. Employing quantum wavepacket dynamics, we show that MC triplets are catalyzing the transfer to the high-spin state. This photochemical pathway is made possible thanks to bipyridine stretching vibrations, facilitating the conversion between the MLCT bands to such MC triplets. We show that the lifetime of the MLCT states can be increased to tens of picoseconds by breaking the conjugation between pyridine units, which increases the energetic gap between MLCT and MC states. This opens the route for the design of new chelating ligands inducing long-lived MLCT states in iron complexes.

Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [FeII(ImP)2] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its 3MLCT (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(ii) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(ii) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher (2MLCT) state of its Fe(iii) analogue [FeIII(ImP)2]+, which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [FeIII(ImP)2]+, we confirmed however that higher (2MLCT) states of [FeIII(ImP)2]+ are short-lived (<10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [FeIII(ImP)2]+ prepared by us only shows red emission from its lower 2LMCT state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [FeIII(ImP)2]+ is instead attributed to an impurity, most probably a photoproduct of the Fe(ii) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [FeIII(ImP)2]+ itself.

Manipulating fluorescence by photo-switched spin-state conversions in an iron(ii)-based SCO-MOF

Manipulating fluorescence by photo-switched spin-state conversions is an attractive prospect for applications in smart magneto-optical materials and devices. The challenge is how to modulate the energy transfer paths of the singlet excited state by light-induced spin-state conversions. In this work, a spin crossover (SCO) Fe^II-based fluorophore was embedded into a metal-organic framework (MOF) to tune the energy transfer paths. Compound 1 {Fe(TPA-diPy)[Ag(CN)₂]₂}·2EtOH (1) has an interpenetrated Hofmann-type structure, wherein the Fe^II ion is coordinated by a bidentate fluorophore ligand (TPA-diPy) and four cyanide nitrogen atoms and acts as the fluorescent-SCO unit. Magnetic susceptibility measurements revealed that 1 underwent an incomplete and gradual spin crossover with T_1/2 = 161 K. Photomagnetic studies confirmed photo-induced spin state conversions between the low-spin (LS) and high-spin (HS) states, where the irradiation of 532 and 808 nm laser lights converted the LS and HS states to the HS and LS states, respectively. Variable-temperature fluorescence spectra study revealed an anomalous decrease in emission intensity upon the HS → LS transition, confirming the synergetic coupling between the fluorophore and SCO units. Alternating irradiation of 532 and 808 nm laser lights resulted in reversible fluorescence intensity changes, confirming spin state-controlled fluorescence in the SCO-MOF. Photo-monitored structural analyses and UV-vis spectroscopic studies demonstrated that the photo-induced spin state conversions changed energy transfer paths from the TPA fluorophore to the metal-centered charge transfer bands, ultimately leading to the switching of fluorescence intensities. This work represents a new prototype compound showing bidirectional photo-switched fluorescence by manipulating the spin states of iron(ii).

Nanosecond Metal-to-Ligand Charge-Transfer State in an Fe(II) Chromophore: Lifetime Enhancement via Nested Potentials

Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong σ-donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal Fe^II complex, Fe(HMTI)(CN)₂ (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene), with a 1.25 ns metal-to-ligand charge-transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties have been examined in a variety of solvents. The HMTI ligand is highly π-acidic due to low-lying π*(C═N), which enhances Δ_Fe via stabilizing t_2g orbitals. The inflexible geometry of the macrocycle results in short Fe-N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid-base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an Fe^II macrocyclic species.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

Many coordination complexes and organometallic compounds with the 4d⁶ and 5d⁶ valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d⁶ complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr⁰, Mn^I, Fe^II, and Co^III compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Electronic structure of cobalt valence tautomeric molecules in different environments

Future molecular microelectronics require the electronic conductivity of the device to be tunable without impairing the voltage control of the molecular electronic properties. This work reports the influence of an interface between a semiconducting polyaniline polymer or a polar poly-D-lysine molecular film and one of two valence tautomeric complexes, i.e., [Co^III(SQ)(Cat)(4-CN-py)₂] ↔ [Co^II(SQ)₂(4-CN-py)₂] and [Co^III(SQ)(Cat)(3-tpp)₂] ↔ [Co^II(SQ)₂(3-tpp)₂]. The electronic transitions and orbitals are identified using X-ray photoemission, X-ray absorption, inverse photoemission, and optical absorption spectroscopy measurements that are guided by density functional theory. Except for slightly modified binding energies and shifted orbital levels, the choice of the underlying substrate layer has little effect on the electronic structure. A prominent unoccupied ligand-to-metal charge transfer state exists in [Co^III(SQ)(Cat)(3-tpp)₂] ↔ [Co^II(SQ)₂(3-tpp)₂] that is virtually insensitive to the interface between the polymer and tautomeric complexes in the Co^II high-spin state.

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.

Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence

Recently reported manganese(I) complexes with chelating arylisocyanide ligands exhibit luminescent metal-to-ligand charge-transfer (MLCT) excited states, similar to ruthenium(II) polypyridine complexes with the same d6 valence electron configuration used for many different applications in photophysics and photochemistry. However, chelating arylisocyanide ligands require substantial synthetic effort, and therefore it seemed attractive to explore the possibility of using more readily accessible monodentate arylisocyanides instead. Here, we synthesized the new Mn(I) complex [Mn(CNdippPhOMe2)6]PF6 with the known ligand CNdippPhOMe2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide. This complex was investigated by NMR spectroscopy, single-crystal structure analysis, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) measurements, IR spectroscopy supported by density functional theory (DFT) calculations, cyclic voltammetry, and time-resolved as well as steady-state UV-vis absorption spectroscopy. The key finding is that the new Mn(I) complex is nonluminescent and instead undergoes arylisocyanide ligand loss during continuous visible laser irradiation into ligand-centered and charge-transfer absorption bands, presumably owed to the population of dissociative d-d excited states. Thus, it seems that chelating bi- or tridentate binding motifs are essential for obtaining emissive MLCT excited states in manganese(I) arylisocyanides. Our work contributes to understanding the basic properties of photoactive first-row transition metal complexes and could help advance the search for alternatives to precious metal-based luminophores, photocatalysts, and sensors.

Resolving Femtosecond Solvent Reorganization Dynamics in an Iron Complex by Nonadiabatic Dynamics Simulations

The ultrafast dynamical response of solute–solvent interactions plays a key role in transition metal complexes, where charge transfer states are ubiquitous. Nonetheless, there exist very few excited-state simulations of transition metal complexes in solution. Here, we carry out a nonadiabatic dynamics study of the iron complex [Fe(CN)₄(bpy)]^2– (bpy = 2,2′-bipyridine) in explicit aqueous solution. Implicit solvation models were found inadequate for reproducing the strong solvatochromism in the absorption spectra. Instead, direct solute–solvent interactions, in the form of hydrogen bonds, are responsible for the large observed solvatochromic shift and the general dynamical behavior of the complex in water. The simulations reveal an overall intersystem crossing time scale of 0.21 ± 0.01 ps and a strong reliance of this process on nuclear motion. A charge transfer character analysis shows a branched decay mechanism from the initially excited singlet metal-to-ligand charge transfer (¹MLCT) states to triplet states of ³MLCT and metal-centered (³MC) character. We also find that solvent reorganization after excitation is ultrafast, on the order of 50 fs around the cyanides and slower around the bpy ligand. In contrast, the nuclear vibrational dynamics, in the form of Fe–ligand bond changes, takes place on slightly longer time scales. We demonstrate that the surprisingly fast solvent reorganizing should be observable in time-resolved X-ray solution scattering experiments, as simulated signals show strong contributions from the solute–solvent scattering cross term. Altogether, the simulations paint a comprehensive picture of the coupled and concurrent electronic, nuclear, and solvent dynamics and interactions in the first hundreds of femtoseconds after excitation.

Dynamics of Spin Crossover Molecular Complexes

We review the current understanding of the time scale and mechanisms associated with the change in spin state in transition metal-based spin crossover (SCO) molecular complexes. Most time resolved experiments, performed by optical techniques, rely on the intrinsic light-induced switching properties of this class of materials. The optically driven spin state transition can be mediated by a rich interplay of complexities including intermediate states in the spin state transition process, as well as intermolecular interactions, temperature, and strain. We emphasize here that the size reduction down to the nanoscale is essential for designing SCO systems that switch quickly as well as possibly retaining the memory of the light-driven state. We argue that SCO nano-sized systems are the key to device applications where the “write” speed is an important criterion.

Selective Photocatalyst for styrene epoxidation with atmospheric O2 using covalent organic frameworks

Fe@POG-OH was synthetized and used to photo-catalyze styrene epoxidation with high selectivity and high conversion at room temperature. O2˙− plays crucial roles in the effective and selective oxidation of styrene to styrene oxide.

Development of an experimental apparatus to observe ultrafast phenomena by tender X-ray absorption spectroscopy at PAL-XFEL

Understanding the ultrafast dynamics of molecules is of fundamental importance. Time-resolved X-ray absorption spectroscopy (TR-XAS) is a powerful spectroscopic technique for unveiling the time-dependent structural and electronic information of molecules that has been widely applied in various fields. Herein, the design and technical achievement of a newly developed experimental apparatus for TR-XAS measurements in the tender X-ray range with X-ray free-electron lasers (XFELs) at the Pohang Accelerator Laboratory XFEL (PAL-XFEL) are described. Femtosecond TR-XAS measurements were conducted at the Ru L3-edge of well known photosensitizer tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)3]2+) in water. The results indicate ultrafast photoinduced electron transfer from the Ru center to the ligand, which demonstrates that the newly designed setup is applicable for monitoring ultrafast reactions in the femtosecond domain.

Ligand-Field Effects in a Ruthenium(II) Polypyridyl Complex Probed by Femtosecond X-ray Absorption Spectroscopy

We employ femtosecond X-ray absorption spectroscopy of [Ru(m-bpy)₃]²⁺ (m-bpy = 6-methyl-2,2'-bipyridine) to elucidate the time evolution of the spin and charge density upon metal-to-ligand charge-transfer (MLCT) excitation. The core-level transitions at the Ru L₃-edge reveal a very short MLCT lifetime of 0.9 ps and relaxation to the lowest triplet metal-centered state (³MC) which exhibits a lifetime of about 300 ps. Time-dependent density functional theory relates ligand methylation to a lower ligand field strength that stabilizes the ³MC state. A quarter of the ³MLCT population appears to be trapped which may be attributed to intramolecular vibrational relaxation or further electron transfer to the solvent. Our results demonstrate that small changes in the ligand field allow control of the photophysical properties. Moreover, this study underscores the high information content of femtosecond L-edge spectroscopy as a probe of valence charge density and spin-state in 4d transition metals.

Multi-stimuli Photo and Redox-active Nanostructured Mesoporous Silica Films on Transparent Electrodes

Silica matrices hosting transition metal guest complexes may offer remarkable platforms for the development of advanced functional devices. We report here the elaboration of ordered and vertically oriented mesoporous silica thin films containing covalently attached tris(bipyridine)iron derivatives using a combination of electrochemically assisted self-assembly (EASA) method and Huisgen cycloaddition reaction. Such a versatile approach is primarily used to bind nitrogen-based chelating ligands such as (4-[(2-propyn-1-yloxy)]4'-methyl-2,2'-bypiridine, bpy') inside the nanochannels. Further derivatization of the bpy'-functionalized silica thin films is then achieved via a subsequent in-situ complexation step to generate [Fe(bpy)₂ (bpy')]²⁺ inside the mesopore channels. After giving spectroscopic evidences for the presence of such complexes in the functionalized film, electrochemistry is used to transform the confined diamagnetic (S=0) F e L S b p y 2 b p y ' 2 + species to paramagnetic (S=1/2) oxidized F e L S b p y 2 b p y ' 3 + species in a reversible way, while blue light irradiation (λ=470 nm) enables populating the short-lived paramagnetic (S=2) F e H S b p y 2 b p y ' 2 + excited state. [Fe(bpy)₂ (bpy')]²⁺ -functionalized ordered films are therefore both electro- and photo-active through the manipulation of the oxidation state and spin state of the confined complexes, paving the way for their integration in optoelectronic devices.

Resonance Raman Excitation Profiles of Fe(II)-Terpyridine Complexes: Electronic Effects of Ligand Modifications

Metal 2,2':6',2″-terpyridine (tpy) complexes are readily used as building blocks in metallo-supramolecular polymers that stand out for their photophysical properties in solar energy assemblies. Furthermore, Resonance Raman (RR) excitation profiles are sensitive indicators of the electronic properties of chromophores. Previously, using RR spectroscopy, we studied the [Fe(tpy)₂]²⁺ complex and metallo-supramolecular polymers formed by tpy derivatives and Fe(II) ions. Here, we compare RR spectra of iron (Fe(II)) complexes with 4'-substituted tpy ligands─[Fe(4'-R-tpy)₂]²⁺, with R = H (1a), Cl (2a), 4-chlorophenyl (3a), and 2-thienyl (4a) to describe changes in their electronic structure after functionalization. By combining theoretical calculations, RR, and UV/vis spectra, we elucidated differences in the RR excitation profiles of 1a, 2a, and 4a complexes. In all Raman modes, complexes 1a and 2a showed maximal enhancement only at 532 nm excitation, whereas complex 4a exhibited maximal enhancement selectively at either 532 or 633 nm excitations. Based on our calculations, the mixed metal/ligand character of the highest occupied molecular orbital (HOMO) of 4a complex manifests itself through selective enhancement of vibration modes, mainly localized on the 2-thienyl unit at 633 nm excitation, which may explain the unique behavior of this complex. Therefore, complex 4a is a prospective candidate for further detailed photophysical explorations toward developing sensitizers for solar cells.

Pyrene-Decoration of a Chromium(0) Tris(diisocyanide) Enhances Excited State Delocalization: A Strategy to Improve the Photoluminescence of 3d6 Metal Complexes

There is a long-standing interest in iron(II) complexes that emit from metal-to-ligand charge transfer (MLCT) excited states, analogous to ruthenium(II) polypyridines. The 3d⁶ electrons of iron(II) are exposed to a relatively weak ligand field, rendering nonradiative relaxation of MLCT states via metal-centered excited states undesirably efficient. For isoelectronic chromium(0), chelating diisocyanide ligands recently provided access to very weak MLCT emission in solution at room temperature. Here, we present a concept that boosts the luminescence quantum yield of a chromium(0) isocyanide complex by nearly 2 orders of magnitude, accompanied by a significant increase of the MLCT lifetime. Pyrene units in the diisocyanide ligand backbone lead to an enlarged π-conjugation system and to a strongly delocalized MLCT state, from which nonradiative relaxation is less dominant despite a sizable redshift of the emission. While the pyrene moiety is electronically coupled to the core of the chromium(0) complex in the excited state, UV-vis absorption and 2D NMR spectroscopy show that this is not the case in the ground state. Luminescence lifetimes and quantum yields for our pyrenyl-decorated chromium(0) complex exhibit an unusual bell-shaped dependence on solvent polarity, indicative of two counteracting effects governing the MLCT deactivation. These two effects are identified as predominant deactivation either through an energetically nearby lying metal-centered state in the most apolar solvents, or alternatively via direct nonradiative relaxation to the ground state following the energy gap law in more polar solvents. This is the first example of a 3d⁶ MLCT emitter to benefit from an increased π-conjugation network.

Following Metal-to-Ligand Charge-Transfer Dynamics with Ligand and Spin Specificity Using Femtosecond Resonant Inelastic X-ray Scattering at the Nitrogen K-Edge

We demonstrate for the case of photoexcited [Ru(2,2'-bipyridine)₃]²⁺ how femtosecond resonant inelastic X-ray scattering (RIXS) at the ligand K-edge allows one to uniquely probe changes in the valence electronic structure following a metal-to-ligand charge-transfer (MLCT) excitation. Metal-ligand hybridization is probed by nitrogen-1s resonances providing information on both the electron-accepting ligand in the MLCT state and the hole density of the metal center. By comparing to spectrum calculations based on density functional theory, we are able to distinguish the electronic structure of the electron-accepting ligand and the other ligands and determine a temporal upper limit of (250 ± 40) fs for electron localization following the charge-transfer excitation. The spin of the localized electron is deduced from the selection rules of the RIXS process establishing new experimental capabilities for probing transient charge and spin densities.

Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis

Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with α-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compounds suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C₆F₅)₃ groups attached to the CN ^- ligands stabilize the metal-based t_2g-like orbitals by ∼0.8 eV, leading to high ³MLCT energies (up to 2.50 eV) that are more typical for cyclometalated iridium(III) complexes. Through variation of their α-diimine ligands, nonradiative excited-state relaxation pathways involving higher-lying metal-centered states can be controlled, and their luminescence quantum yields and MLCT lifetimes can be optimized. These combined properties make the respective isocyanoborato complexes amenable to photochemical reactions for which common ruthenium(II)-based sensitizers are unsuited, due to a lack of sufficient triplet energy or excited-state redox power. Specifically, this includes photoisomerization reactions, sensitization of nickel-catalyzed cross-couplings, pinacol couplings, and oxidative decarboxylative C-C couplings. Our work is relevant in the greater context of tailoring photoactive coordination compounds to current challenges in synthetic photochemistry and solar energy conversion.

Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering

Light-driven chemical transformations provide a compelling approach to understanding chemical reactivity with the potential to use this understanding to advance solar energy and catalysis applications. Capturing the non-equilibrium trajectories of electronic excited states with precision, particularly for transition metal complexes, would provide a foundation for advancing both of these objectives. Of particular importance for 3d metal compounds is characterizing the population dynamics of charge-transfer (CT) and metal-centered (MC) electronic excited states and understanding how the inner coordination sphere structural dynamics mediate the interaction between these states. Recent advances in ultrafast X-ray laser science has enabled the electronic excited state dynamics in 3d metal complexes to be followed with unprecedented detail. This review will focus on simultaneous X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) studies of iron coordination and organometallic complexes. These simultaneous XES-XSS studies have provided detailed insight into the mechanism of light-induced spin crossover in iron coordination compounds, the interaction of CT and MC excited states in iron carbene photosensitizers, and the mechanism of Fe-S bond dissociation in cytochrome c.

Evidence for a lowest energy 3MLCT excited state in [Fe(tpy)(CN)3]. Chem Commun (Camb) 2021;57:4658-4661. [PMID: 33977987 DOI: 10.1039/d1cc01090e]

Transient absorption data of [FeII(tpy)(CN)3]- reveals spectroscopic signatures indicative of 3MLCT with a ∼10 ps kinetic component. These data are supported by DFT and TD-DFT calculations, which show that excited state ordering is responsive to the number of cyanide ligands on the complex.

Interplays of electron and nuclear motions along CO dissociation trajectory in myoglobin revealed by ultrafast X-rays and quantum dynamics calculations

Ultrafast structural dynamics with different spatial and temporal scales were investigated during photodissociation of carbon monoxide (CO) from iron(II)-heme in bovine myoglobin during the first 3 ps following laser excitation. We used simultaneous X-ray transient absorption (XTA) spectroscopy and X-ray transient solution scattering (XSS) at an X-ray free electron laser source with a time resolution of 80 fs. Kinetic traces at different characteristic X-ray energies were collected to give a global picture of the multistep pathway in the photodissociation of CO from heme. In order to extract the reaction coordinates along different directions of the CO departure, XTA data were collected with parallel and perpendicular relative polarizations of the laser pump and X-ray probe pulse to isolate the contributions of electronic spin state transition, bond breaking, and heme macrocycle nuclear relaxation. The time evolution of the iron K-edge X-ray absorption near edge structure (XANES) features along the two major photochemical reaction coordinates, i.e., the iron(II)-CO bond elongation and the heme macrocycle doming relaxation were modeled by time-dependent density functional theory calculations. Combined results from the experiments and computations reveal insight into interplays between the nuclear and electronic structural dynamics along the CO photodissociation trajectory. Time-resolved small-angle X-ray scattering data during the same process are also simultaneously collected, which show that the local CO dissociation causes a protein quake propagating on different spatial and temporal scales. These studies are important for understanding gas transport and protein deligation processes and shed light on the interplay of active site conformational changes and large-scale protein reorganization.

Using X-ray free-electron lasers for spectroscopy of molecular catalysts and metalloenzymes

The metal centres in metalloenzymes and molecular catalysts are responsible for the rearrangement of atoms and electrons during complex chemical reactions, and they enable selective pathways of charge and spin transfer, bond breaking/making and the formation of new molecules. Mapping the electronic structural changes at the metal sites during the reactions gives a unique mechanistic insight that has been difficult to obtain to date. The development of X-ray free-electron lasers (XFELs) enables powerful new probes of electronic structure dynamics to advance our understanding of metalloenzymes. The ultrashort, intense and tunable XFEL pulses enable X-ray spectroscopic studies of metalloenzymes, molecular catalysts and chemical reactions, under functional conditions and in real time. In this Technical Review, we describe the current state of the art of X-ray spectroscopy studies at XFELs and highlight some new techniques currently under development. With more XFEL facilities starting operation and more in the planning or construction phase, new capabilities are expected, including high repetition rate, better XFEL pulse control and advanced instrumentation. For the first time, it will be possible to make real-time molecular movies of metalloenzymes and catalysts in solution, while chemical reactions are taking place.

Theoretical modeling of the singlet-triplet spin transition in different Ni(II)-diketo-pyrphyrin-based metal-ligand octahedral complexes

The structural stability, charge transfer effects and strength of the spin-orbit couplings in different Ni(ii)-ligand complexes have been studied at the DFT (B3LYP and CAM-B3LYP) and coupled cluster (DLPNO-CCSD(T)) levels of theory. Accordingly, two different, porphyrin- and diketo-pyrphyrin-based four-coordination macrocycles as planar ligands as well as pyridine (or pyrrole) and mesylate anion molecular groups as vertical ligands were considered in order to build metal-organic complexes with octahedral coordination configurations. For each molecular system, the identification of equilibrium geometries and the intersystem crossing (the minimum energy crossing) points between the potential energy surfaces of the singlet and triplet spin states is followed by computing the spin-orbit couplings between the two spin states. Structures, based on the diketo-pyrphyrin macrocycle as the planar ligand, show stronger six-coordination metal-organic complexes due to the extra electrostatic interaction between the positively charged central metal cation and the negatively charged vertical ligands. The results also show that the magnitude of the spin-orbit coupling is influenced by the atomic positions of deprotonations of the ligands, and implicitly the direction of the charge transfer between the ligand and the central metal ion.

Ultrafast excited state relaxation dynamics in a heteroleptic Ir(iii) complex, fac-Ir(ppy)2(ppz), revealed by femtosecond X-ray transient absorption spectroscopy

The experimental and calculation results demonstrate that the ³ML_ppzCT state generated by the spin-forbidden transition rapidly relaxes to ³ML_ppyCT through internal conversion process with a time constant of ∼450 fs.

Ultrafast photoinduced dynamics in Prussian blue analogues

A review on ultrafast photoinduced processes in molecule-based magnets with an emphasis on Prussian blue analogues.

Spin multiplicity effects in doublet versus singlet emission: the photophysical consequences of a single electron

Ambient-stable fluorescent radicals have recently emerged as promising luminescent materials; however, tailoring their properties has been difficult due to the limited photophysical understanding of open-shell organic systems. Here we report the experimental and computational analysis of a redox pair of π-conjugated fluorescent molecules that differ by one electron. A π-dication (DC) and π-radical cation (RC) demonstrate different absorption spectra, but similar red emission (λ_emiss,max = ∼630 nm), excitation maxima (λ_exc,max = ∼530 nm), fluorescence lifetimes (1–10 ns), and even excited-state (non-emissive) lifetimes when measured by transient absorption spectroscopy. Despite their experimental similarities, time-dependent density functional theory (TDDFT) studies reveal that DC and RC emission mechanisms are distinct and rely on different electronic transitions. Excited-state reorganization occurs by hole relaxation in singlet DC, while doublet RC undergoes a Jahn-Teller distortion by bending its π-backbone in order to facilitate spin-pairing between singly occupied molecular orbitals. This relationship between the excited-state dynamics of RC and its π-backbone geometry illustrates a potential strategy for developing π-conjugated radicals with new emission properties. Additionally, by comparing TDDFT and CIS (configuration interaction singles) excitations, we show that unrestricted TDDFT accurately reproduces experimental absorption spectra and provides an opportunity to examine the relaxed excited-state properties of large open-shell molecules like RC.

Experimental and computational studies reveal mechanistic differences in the photophysics of an open- versus closed-shell π-conjugated redox pair.

Intersystem Crossing and Triplet Dynamics in an Iron(II) N-Heterocyclic Carbene Photosensitizer

The electronic excited states of the iron(II) complex [Fe^II(tpy)(pyz-NHC)]²⁺ [tpy = 2,2′:6′,2″-terpyridine; pyz-NHC = 1,1′-bis(2,6-diisopropylphenyl)pyrazinyldiimidazolium-2,2′-diylidene] and their relaxation pathways have been theoretically investigated. To this purpose, trajectory surface-hopping simulations within a linear vibronic coupling model including a 244-dimensional potential energy surface (PES) with 20 singlet and 20 triplet coupled states have been used. The simulations show that, after excitation to the lowest-energy absorption band of predominant metal-to-ligand charge-transfer character involving the tpy ligand, almost 80% of the population undergoes intersystem crossing to the triplet manifold in about 50 fs, while the remaining 20% decays through internal conversion to the electronic ground state in about 300 fs. The population transferred to the triplet states is found to deactivate into two different regions of the PESs, one where the static dipole moment is small and shows increased metal-centered character and another with a large static dipole moment, where the electron density is transferred from the tpy to pyz-NHC ligand. Coherent oscillations of 400 fs are observed between these two sets of triplet populations, until the mixture equilibrates to a ratio of 60:40. Finally, the importance of selecting suitable normal modes is highlighted—a choice that can be far from straightforward in transition-metal complexes with hundreds of degrees of freedom.

Trajectory surface-hopping simulations with a linear vibronic coupling model reveal the competition of major intersystem crossing versus minor internal conversion dynamics in an iron(II) N-heterocyclic carbene dye. The triplet population bifurcates into two regions of the potential energy surfaces, characterized by small and large static dipole moments due to different electronic character and showing coherent oscillations of 400 fs until both triplet populations coexist in a mixture of 60:40.

Electronic Structures, Spectroscopy, and Electrochemistry of [M(diimine)(CN-BR3)4]2- (M = Fe, Ru; R = Ph, C6F5) Complexes

Complexes with the formula [M(diimine)(CN-BR₃)₄]^2-, where diimine = bipyridine (bpy), phenanthroline (phen), 3,5-trifluoromethylbipyridine (flpy), R = Ph, C₆F₅, and M = Fe^II, Ru^II, were synthesized and characterized by X-ray crystal structure analysis, UV-visible spectroscopy, IR spectroscopy, and voltammetry. Three highly soluble complexes, [Fe^II(bpy)(CN-B(C₆F₅)₃)₄]^2-, [Ru^II(bpy)(CN-B(C₆F₅)₃)₄]^2-, and [Ru^II(flpy)(CN-B(C₆F₅)₃)₄]^2-, exhibit electrochemically reversible redox reactions, with large potential differences between the bpy^0/- or flpy^0/- and M^III/II couples of 3.27, 3.52, and 3.19 V, respectively. CASSCF+NEVPT2 calculations accurately reproduce the effects of borane coordination on the electronic structures and spectra of cyanometallates.

Probing Solute-Solvent Interactions of Transition Metal Complexes Using L-Edge Absorption Spectroscopy

In order to tailor solution-phase chemical reactions involving transition metal complexes, it is critical to understand how their valence electronic charge distributions are affected by the solution environment. Here, solute-solvent interactions of a solvatochromic mixed-ligand iron complex were investigated using X-ray absorption spectroscopy at the transition metal L_2,3-edge. Due to the selectivity of the corresponding core excitations to the iron 3d orbitals, the method grants direct access to the valence electronic structure around the iron center and its response to interactions with the solvent environment. A linear increase of the total L_2,3-edge absorption cross section as a function of the solvent Lewis acidity is revealed. The effect is caused by relative changes in different metal-ligand-bonding channels, which preserve local charge densities while increasing the density of unoccupied states around the iron center. These conclusions are corroborated by a combination of molecular dynamics and spectrum simulations based on time-dependent density functional theory. The simulations reproduce the spectral trends observed in the X-ray but also optical absorption experiments. Our results underscore the importance of solute-solvent interactions when aiming for an accurate description of the valence electronic structure of solvated transition metal complexes and demonstrate how L_2,3-edge absorption spectroscopy can aid in understanding the impact of the solution environment on intramolecular covalency and the electronic charge distribution.

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.

Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers

Photoinduced intramolecular electron transfer dynamics following metal-to-ligand charge-transfer (MLCT) excitation of [Fe(CN)4(2,2'-bipyridine)]2- (1), [Fe(CN)4(2,3-bis(2-pyridyl)pyrazine)]2- (2) and [Fe(CN)4(2,2'-bipyrimidine)]2- (3) were investigated in various solvents with static and time-resolved UV-Visible absorption spectroscopy and Fe 2p3d resonant inelastic X-ray scattering (RIXS). This series of polypyridyl ligands, combined with the strong solvatochromism of the complexes, enables the 1MLCT vertical energy to be varied from 1.64 eV to 2.64 eV and the 3MLCT lifetime to range from 180 fs to 67 ps. The 3MLCT lifetimes in 1 and 2 decrease exponentially as the MLCT energy increases, consistent with electron transfer to the lowest energy triplet metal-centred (3MC) excited state, as established by the Tanabe-Sugano analysis of the Fe 2p3d RIXS data. In contrast, the 3MLCT lifetime in 3 changes non-monotonically with MLCT energy, exhibiting a maximum. This qualitatively distinct behaviour results from a competing 3MLCT → ground state (GS) electron transfer pathway that exhibits energy gap law behaviour. The 3MLCT → GS pathway involves nuclear tunnelling for the high-frequency polypyridyl breathing mode (hν = 1530 cm-1), which is most displaced for complex 3, making this pathway significantly more efficient. Our study demonstrates that the excited state relaxation mechanism of Fe polypyridyl photosensitizers can be readily tuned by ligand and solvent environment. Furthermore, our study reveals that extending charge transfer lifetimes requires control of the relative energies of the 3MLCT and the 3MC states and suppression of the intramolecular distortion of the acceptor ligand in the 3MLCT excited state.