Molecular Structure and Vibrational Spectra of Spin-Crossover Complexes in Solution and Colloidal Media: Resonance Raman and Time-Resolved Resonance Raman Studies

Spin crossover in dinuclear iron(II) complexes bridged by bis-bipyridine ligands: dimer effects on electronic structure, spectroscopic properties and spin-state switching

Inspired by the well-studied mononuclear spin crossover compound [Fe(H2B(pz)2)2(bipy)], the bipyridine-based bisbidentate ligands 1,2-di(2,2'-bipyridin-5-yl)ethyne (ac(bipy)2) and 1,4-di(2,2'-bipyridine-5-yl)-3,5-dimethoxybenzene (Ph(OMe)2(bipy)2) are used to bridge two [Fe(H2B(pz)2)2] units, leading to the charge-neutral dinuclear iron(II) compounds [{Fe(H2B(pz)2)2}2 μ-(ac(bipy)2)] (1) and [{Fe(H2B(pz)2)2}2 μ-(Ph(OMe)2(bipy)2)] (2), respectively. The spin-crossover properties of these molecules are investigated by temperature-dependent PPMS measurements, Mössbauer, vibrational and UV/Vis spectroscopy as well as X-ray absorption spectroscopy. While compound 1 undergoes complete SCO with T1/2 = 125 K, an incomplete spin transition is observed for 2 with an inflection point at 152 K and a remaining high-spin fraction of 40% below 65 K. The spin transitions of the dinuclear compounds are also more gradual than for the parent compound [Fe(H2B(pz)2)2(bipy)]. This is attributed to steric hindrance between the molecules, limiting intermolecular interactions such as π-π-stacking.

Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and noninvasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands, L4-L6, were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III) or Eu(III)) in acetonitrile to give C₃-symmetrical dinuclear triple-stranded helical [LnM(Lk)₃]⁵⁺ cations, which can be crystallized with noncoordinating counter-anions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)₃]⁵⁺ (strictly high-spin), [LnFe(L5)₃]⁵⁺ (spin-crossover (SCO) around room temperature), and [LnFe(L6)₃]⁵⁺ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)₃]⁵⁺ results in europium-centered luminescence modulated by variable intramolecular Eu(III) → Fe(II) energy-transfer processes. The kinetic analysis implies Eu(III) → Fe(II) quenching efficiencies close to 100% for the low-spin configuration and greater than 95% for the high-spin state. Consequently, the sensitivity of indirect luminescence detection of Fe(II) spin crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring at the molecular scale.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: III. [Fe(tpen)]Cl2 in acetonitrile

In order to achieve an in-depth understanding of the role played by the solvent in the photoinduced low-spin (LS) → high-spin (HS) transition in solvated Fe(ii) complexes, an accurate description of the solvated complexes in the two spin states is required. To this end, we are applying state-of-the-art ab initio molecular dynamics (AIMD) simulations to the study of the structural and vibrational properties of iron(ii) polypyridyl complexes. Two aqueous LS complexes were investigated in this framework, namely, [Fe(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) [Lawson Daku and Hauser, J. Phys. Chem. Lett., 2010, 1, 1830; Lawson Daku, Phys. Chem. Chem. Phys., 2018, 20, 6236] and [Fe(tpy)₂]²⁺ (tpy = 2,2′:6′,2′′-ter-pyridine) [Lawson Daku, Phys. Chem. Chem. Phys., 2019, 21, 650]. For aqueous [Fe(bpy)₃]²⁺, combining the results of forefront wide-angle X-ray scattering experiments with those of the AIMD simulations allowed the visualization of the interlaced coordination and solvation spheres of the photoinduced HS state [Khakhulin et al., Phys. Chem. Chem. Phys., 2019, 21, 9277]. In this paper, we report the extension of our AIMD studies to the spin-crossover complex [Fe(tpen)]²⁺ (tpen = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine) in acetonitrile (ACN). The determined LS and HS solution structures of the complex are in excellent agreement with the experimental results obtained by high-resolution transient X-ray absorption spectroscopy [Zhang et al., ACS Omega, 2019, 4, 6375]. The first solvation shell of [Fe(tpen)]²⁺ consists of ACN molecules located in the grooves defined by the chelating coordination motif of the tpen ligand. Upon the LS → HS change of states, the solvation number of the complex is found to increase from ≈9.2 to ≈11.9 and an inner solvation shell is formed. This inner solvation shell originates from the occupancy by about one ACN molecule of the internal cavity which results from the arrangement of the 4 pyridine rings of the tpen ligand, and which becomes accessible to the solvent molecules in the HS state only thanks to the structural changes undergone by the complex. The presence of this inner solvation shell for the solvated HS complex probably plays a key role in the spin-state dependent reactivity of [Fe(tpen)]²⁺ in liquid solutions.

AIMD study of the SCO [Fe(tpen)]²⁺ complex in acetonitrile: radial distribution functions and running coordination numbers characterizing its solvation structure in the HS state.

Ultrafast Photodynamics of an Azopyridine-Functionalized Iron(II) Complex: Implications for the Concept of Ligand-Driven Light-Induced Spin Change

We report on the ultrafast photodynamics of an iron(II) complex with a photoisomerizable pentadentate azo-tetrapyridylamino ligand after irradiation with ultraviolet light. The results of femtosecond transient electronic absorption spectroscopy performed on the low-spin (LS) form of the title complex show that initial excitation of the ππ* state of the azopyridine unit in the ligand at λ_pump = 312 nm is followed by an ultrafast intersystem crossing (ISC) that leads to the formation of a metal-centered (MC) ⁵T state, in competition with the intended photoswitching of the azopyridine unit. Additional measurements carried out upon excitation of the singlet metal-to-ligand charge-transfer (¹MLCT) transition at λ_pump = 455 nm suggest that this energy transfer occurs via an MLCT state. The resulting high-spin (HS) ⁵T state of the complex is metastable and recovers to the LS ground state with a time constant of ∼3 ns. The implications of these observations on the ligand-driven light-induced spin change concept are discussed.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: II. aqueous [Fe(tpy)2]Cl2

LS and HS Fe–O radial distribution functions and running coordination numbers for aqueous [Fe(tpy)₂]Cl₂: in both spin states, the first hydration shell of [Fe(tpy)₂]²⁺ consists in a chain of ∼15 hydrogen-bonded water molecules wrapped around the ligands.

Spin-state dependence of the structural and vibrational properties of solvated iron(ii) polypyridyl complexes from AIMD simulations: aqueous [Fe(bpy)3]Cl2, a case study

LS and HS IR spectra of aqueous [Fe(bpy)₃]²⁺ and corresponding HS–LS difference IR spectrum as obtained from state-of-the-art ab initio molecular dynamics simulations applied to the determination of the structural and vibrational properties of the solvated complex.

Bis-meridional Fe2+ spincrossover complexes of phenyl and pyridyl substituted 2-(pyridin-2-yl)-1,10-phenanthrolines

Fe²⁺ spincrossover complexes [Fe(L)₂]²⁺ (L = substituted (pyridin-2-yl)-1,10-phenanthroline) were prepared and SCO with changing coordination numbers was identified by ¹H NMR spectroscopy and in silico modeling.

Selective signalling of glyphosate in water using europium luminescence

Selective sensing of glyphosate in the micromolar regime in water.

Simulating picosecond iron K-edge X-ray absorption spectra by ab initio methods to study photoinduced changes in the electronic structure of Fe(II) spin crossover complexes

Recent time-resolved X-ray absorption experiments probing the low-spin to high-spin photoconversion in Fe(II) complexes have monitored the complex interplay between electronic and structural degrees of freedom on an ultrafast time scale. In this study, we use transition potential (TP) and time-dependent (TD) DFT to simulate the picosecond time-resolved iron K-edge X-ray absorption spectrum of the spin crossover (SCO) complex, [Fe(tren(py)(3))](2+). This is achieved by simulating the X-ray absorption spectrum of [Fe(tren(py)(3))](2+) in its low-spin (LS), (1)A(1), ground state and its high-spin (HS), (5)T(2), excited state. These results are compared with the X-ray absorption spectrum of the high-spin analogue (HSA), [Fe(tren(6-Me-py)(3))](2+), which has a (5)T(2) ground state. We show that the TP-DFT methodology can simulate a 40 eV range of the iron K-edge XANES spectrum reproducing all of the major features observed in the static and transient spectra of the LS, HS, and HSA complexes. The pre-edge region of the K-edge spectrum, simulated by TD-DFT, is shown to be highly sensitive to metal-ligand bonding. Changes in the intensity of the pre-edge region are shown to be sensitive to both symmetry and π-backbonding by analysis of relative electric dipole and quadrupole contributions to the transition moments. We generate a spectroscopic map of the iron 3d orbitals from our TD-DFT results and determine ligand field splitting energies of 1.55 and 1.35 eV for the HS and HSA complexes, respectively. We investigate the use of different functionals finding that hybrid functionals (such as PBE0) produce the best results. Finally, we provide a detailed comparison of our results with theoretical methods that have been previously used to interpret Fe K-edge spectroscopy of equilibrium and time-resolved SCO complexes.

Photoenhanced oxidative DNA cleavage with non-heme iron(II) complexes

The DNA cleavage activity of iron(II) complexes of a series of monotopic pentadentate N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine (N4Py)-derived ligands (1-5) was investigated under laser irradiation at 473, 400.8, and 355 nm in the absence of a reducing agent and compared to that under ambient lighting. A significant increase in activity was observed under laser irradiation, which is dependent on the structural characteristics of the complexes and the wavelength and power of irradiation. Under photoirradiation at 355 nm, direct double-stand DNA cleavage activity was observed with Fe(II)-1 and Fe(II)-3-5, and a 56-fold increase in the single-strand cleavage activity was observed with Fe(II)-2. Mechanistic investigations revealed that O(2)(•-), (1)O(2), and OH(•) contribute to the photoenhanced DNA cleavage activity, and that their relative contribution is dependent on the wavelength. It is proposed that the origin of the increase in activity is the photoenhanced formation of an Fe(III)OOH intermediate as the active species or precursor.