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Photoinduced Spin Crossover in Iron(II) Systems

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Chemistry in Action: Making Molecular Movies with Ultrafast Electron Diffraction and Data Science

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Abstract

Some transition-metal complexes can change spin states under certain perturbation of their environment. This process is called spin crossover (SCO) and this chapter is focused on its role in the light-induced dynamics of two iron(II) compounds: [FeII(bpy)3](PF6)2 and [FeII(PM-AzA)2(NCS)2], abbreviated herein to BPY and AZA respectively.

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Notes

  1. 1.

    Linus Carl Pauling (1901–1994) was awarded the 1954 Nobel Prize in Chemistry for his research on the nature of the chemical bond [399].

  2. 2.

    Transition-metal complexes are chemical compounds that are formed by various molecules attaching to a central transition-metal atom via coordination bonds; the coordinating molecules are referred to as ‘ligands’ [58].

  3. 3.

    In the case of iron(III) in [FeIIIL6]3+, the five electrons in the 3d orbitals are either all unpaired or paired twice with one left; the former is the ‘high-spin’ (HS) state, with total spin quantum number \(S = \frac {5}{2}\); the latter is the ‘low-spin’ (LS) state, with \(S = \frac {1}{2}\).

  4. 4.

    These were a homologuous series of tris(N,N’-dialkyldithiocarbamate)iron(III) derivatives, (R2NCS2)FeIII where R = CnH2n+1.

  5. 5.

    Named after Pierre Curie (1859–1906) and Pierre-Ernest Weiss (1865–1940), this law describes the magnetic susceptibility χ of a material as a function of its temperature T: χ = χ 0 + C(T − T C)−γ, where χ 0, C, T W, γ are the Pauli susceptibility, Curie constant, Weiss temperature, and critical exponent [13, 398].

  6. 6.

    British chemist Leslie E. Orgel (1927–2007) is most known for his early application of LFT to transition metal chemistry and later proposal of the ‘RNA World’ hypothesis for the origin of life on Earth [271, 408].

  7. 7.

    German physicist Rudolf Ludwig Mössbauer (1929–2011) carried out the first experimental observation of recoilless nuclear resonance absorption of gamma rays in 1955–1957 [380]; this discovery earned him a share of the 1961 Nobel Prize in Physics [399].

  8. 8.

    The first excited state of the 57Fe nucleus has a non-zero electric quadrupole moment which gives rise to a doublet in its Mössbauer absorption spectrum; this splitting, ΔE Q, is narrowed in the LS electronic configuration due to depopulation of metal–ligand orbitals and thus reduced anisotropy of the electric-field gradient [196].

  9. 9.

    Note that, for two spin states to be spectrally resolved, it is necessary that: (1) the relaxation time for the \(\text{LS} \rightleftharpoons \text{HS}\) fluctuation is longer than the half-life of the nuclear isomer used (t 1∕2 = 98.3 ns for 57Fe∗) and (2) their Lamb-Mössbauer factors f HS, f LS are equal [197].

  10. 10.

    See footnote 36.

  11. 11.

    Friedrich Hund (1896–1997) formulated three simple empirical rules to determine the ground electronic state of an atom: maximize S and L while minimizing J, where S, L, J are the total spin, orbital, and net angular momentum numbers [234].

  12. 12.

    Japanese chemist Tsuchida Ryutaro (1903–1962) measured the absorption spectra of several transition-metal complexes and concluded that ligand substitution can modulate the energy of electronic transitions [462, 463]; the approximate sequence at which this occurs in increasing order is known as the spectrochemical series: I−, Br−, SCN−, Cl−, NO\(_3^-\), N\(_3^-\), F−, OH−, OH2, NCS−, NH3, py, en, bpy, phen, NO\(_2^-\), PPh3, CN−, CO, NO+ [161].

  13. 13.

    John H. van Vleck (1899–1980) was awarded the 1977 Nobel Prize in Physics, along with Philip W. Anderson and Nevill F. Mott (see footnote 36), for his work on the electronic structure of magnetic materials [336].

  14. 14.

    Friedrich Hund (see footnote 11), Robert S. Mulliken (1896–1986), and others calculated the electron wavefunctions of a molecule as a linear combination of those of the constituent atoms; Mulliken was awarded the 1966 Nobel Prize in Chemistry for his contribution [401].

  15. 15.

    The anti-or non-bonding nature of an MO is denoted by an ‘n’ or asterisk to the right of its term symbol in superscript.

  16. 16.

    For a typical FeIIN6-type SCO complex, r LS ≈ 2.0 Å and r HS ≈ 2.2 Å [409, 478].

  17. 17.

    Yukito Tanabe (1927–present) and Satoru Sugano (1928–present) calculated the electronic ground- and excited-state energy of all possible electronic configurations of the 3d orbitals and plotted them in diagrams that clearly explain the absorption spectra of various transition-metal complexes [508,509,510].

  18. 18.

    Dutch physical chemist Jacobus H. van’t Hoff (1852–1911) was awarded in 1901 the first Nobel prize in chemistry for his contributions to chemical thermodynamics, one of which is this formula on the temperature dependence of the equilibrium constant of a simple mixture of chemical species [398].

  19. 19.

    Josiah Willard Gibbs (1839–1903) was an American mathematical physicist whose contributions include statistical mechanics and, in particular, the concept of ‘available energy’ at constant volume and temperature [210].

  20. 20.

    From temperature-dependent XRD measurements of several Fe(II) SCO systems, the \(\text{LS} \rightleftharpoons \text{HS}\) volume contraction associated with the spin transition is about 1.5–2.5 % [193, 198].

  21. 21.

    Laser Raman temperature jump is a technique used for studying fast chemical reactions in solution; it involves an impulsive heating of the spectating solvent by a ns near-IR pulse from a Q-switched neodymium-doped glass laser, Raman-shifted from 1.41-μm to 1.41-μm to match the wavelength of the first overtone of the O–H stretch [525].

  22. 22.

    FeII(biz)\(_3^{2+}\), FeII(ppa)\(_2^{2+}\), and FeII(pyimH)\(_3^{2+}\) in water and acetonitrile–methanol, where biz = 2,2′-bi-1,4,5,6-tetrahydropyrimidine, ppa = N 2-(2-pyridylmethyl)picolinamidine, and pyimH = 2-(2-pyridylimidazole) [359].

  23. 23.

    Gallé et al. [170] showed that photoexcitation to the 5MLCT band can also lead to rLIESST.

  24. 24.

    Electronic transitions with ΔS ≠ 0 (and ΔL = 0) are prima facie forbidden since the corresponding transition moment integrals are identically zero by symmetry [209].

  25. 25.

    Through this formula [12], Swedish physicist Svante A. Arrhenius (1859–1927) related the temperature dependence of the rate of a chemical reaction to the concepts of activation energy and Boltzmann distribution; for related work, he was awarded the 1903 Nobel Prize in Chemistry [398].

  26. 26.

    This quantity is named after physicist-couple Kun Huang (1919–2005) and Avril Rhys (1928–present), who first introduced it in their seminal work [232].

  27. 27.

    From Ref. [198], \(|\langle \psi _{\text{HS}} | \hat {H}_{\text{SO}} |\psi _{\text{HS}} \rangle | \approx 150\) cm−1, ħω ≈ 250 cm−1, Δr HL ≈ 0.2 Å, k ≈ 200 N/m, S ≈ 45, \(\Delta S_{\text{HL}}^{(0)} \approx 5\) cm−1/K.

  28. 28.

    See footnote 3.

  29. 29.

    The absorption spectrum of the 5T2g state (or any of the other excited states) is not known a priori for Fe(II) complexes like [FeII(bpy)3]2+ since they do not exhibit thermal SCO. To assign the ESA in their TA data, the authors [49, 376, 574] used the change in absorbance ΔA(λ) due to electrochemical oxidation ([FeIIL3]2+ → [FeIIIL3]3+) and reduction ([FeIIL3]2+ → [FeIIL2(L−)]1+) as a proxy for that expected of the 1MLCT state, which is purportedly the result of \((\mathrm {t}_{2\mathrm {g}})^6 (\mathrm {e}_{\mathrm {g}^*})(\mathrm {L} \uppi ^*) \rightarrow (\mathrm {t}_{2\mathrm {g}})^5 (\mathrm {e}_{\mathrm {g}^*})(\mathrm {L} \uppi ^*)^1\).

  30. 30.

    Canadian chemist Rudolph A. Marcus (1923–present) developed in 1956 a theory of electron transfer amongst molecules which predicts that the rate constant k of such reactions do not increased monotonically with the total Gibbs free energy ΔG ∘ but has a Gaussian dependence [344, 348].

  31. 31.

    The evolution into the 3MLCT state happens to be much more easily observable in [RuII(bpy)3]2+ because of its strong luminescence and very long lifetime (τ = 630 ns at 298 K in water), properties made possible by the fact that Δoct(Ru) ≈ 2 Δoct(Fe) for a given ligand (see Table H.1 in Appendix H) and the ligand-field excited states are thus too high in energy to effectively quench the MLCT states [214, 517].

  32. 32.

    No vibrational feature characteristic of the 1MLCT state is observed at early times in Ref. [489] since it is very short-lived (\(\lesssim \)20 fs) and is associated with large lifetime spectral broadening (500 cm−1) [310].

  33. 33.

    The first observation of an X-ray absorption edge was made in 1913 by French experimental physicist Maurice de Broglie (1875–1960), the elder brother of Louis de Broglie (see footnote 35). For a detailed historical account of XAS, see Refs. [338, 546].

  34. 34.

    XANES is also called NEXAFS or near-edge X-ray absorption fine structure [546].

  35. 35.

    See Ref. [450] for a more thorough overview on the theory of EXAFS and XANES.

  36. 36.

    Of interest are the works in Refs. [88, 89, 175, 369, 464, 516, 527], where the ms–ps photoinduced dynamics of several transition metal complexes were probed by time-resolved XAS.

  37. 37.

    At least within the IRF of these measurements, which is τ IRF ≈ 110–250 fs FWHM.

  38. 38.

    German-American physicist Otto Laporte (1902–1971) recognized that electronic transitions in molecules with an inversion centre only occur if Δℓ = ±1 [115, 209, 315].

  39. 39.

    There are two competing notations for X-ray spectral lines: Siegbahn and IUPAC. For example, emission from transitions from the M3, M2 levels to the K level is denoted by Kβ1,3 or K-M3,2 [264]. Note that ‘Siegbahn’ refers to Swedish physicist Manne Siegbahn (1886–1978), who won the 1924 Nobel Prize in Physics for his work in X-ray spectroscopy [14].

  40. 40.

    Iron coordination complexes of different ground-state spin moments stand in for possible excited-state spin configurations of BPY: [FeIII(bpy)3]3+ (doublet, S = 1∕2), [FeIIPc] (triplet, S = 1), [FeII(phen)2(NCS)2] (quintet, S = 2); Pc2− = phthalocyanine (C32H16N8).

  41. 41.

    Wavelength λ, beamwidth w on the sample, pulse duration τ, and energy E (see Appendix K).

  42. 42.

    Four species could be spectrally resolved: solvated electrons \(\mathrm {e}^-_{\text{aq}}\), the 3MLCT excited state [RuIII(bpy)2(bpy−)]2+ the reduced ion [RuII(bpy)2(bpy⋅−)]+, and the oxidized ion [RuIII(bpy)3]3+.

  43. 43.

    From kinetic considerations (Eq. (5.5)), a LS—HS equilibrium temperature of [FeII(bpy)3]2+ can be estimated to be ca. 700 K; however, it is purely theoretical since the molecule would have thermochemically dissociated already [198].

  44. 44.

    Sapphire is chosen here for its high thermal conductivity, which allows laser-deposited heat to dissipate in between laser shots.

  45. 45.

    Absorption of an isotropic sample after photoexcitation is given by \(\Delta A(t, \beta ) \propto N(t) [ 1 + (3 \cos ^2 \beta - 1) R(t) ]\), where N(t), R(t), and β are respectively the excited-state population, the alignment of the transition dipoles, and the relative pump–probe polarization angle. Setting \(\beta = \arccos \frac {1}{\sqrt {3}}\) eliminates the contribution of dipole-alignment relaxation [162].

  46. 46.

    See Fig. L.1 in Appendix L for plots of these principal components.

  47. 47.

    Caveat: depending on the relative polarization of the probe.

  48. 48.

    See Fig. L.2 in Appendix L for plots of the principal components.

  49. 49.

    To avoid noise, GSA” is evaluated numerically by calculating the double finite difference of not the measured GSA values directly but an analytical approximation generated by Voigt fitting [1].

  50. 50.

    Johannes Stark (1874–1919) found that an external electric field can shift the spectral lines of an atom by coupling with its electric dipole moment; for this discovery, he was awarded the 1919 Nobel Prize in Physics [398].

  51. 51.

    \(v = \frac {1}{c \epsilon }(\frac {\Delta T}{\Delta A})\), where v is the speed of sound, \(\frac {\Delta T}{\Delta A}\) is the period/absorbance linear slope, c = 2.13 M and 𝜖 = 9160 M−1 cm−1 are the molar concentration and absorptivity at 533 nm of single-crystal BPY; B = ρv 2, where B and ρ = 1.73 g/cm3 are its bulk modulus and density.

  52. 52.

    In some older works [116, 288], much longer lifetimes—(810 ± 70), (830 ± 70) ps—were described under similar conditions (aqueous BPY at room temperature).

  53. 53.

    See footnote 20.

  54. 54.

    Note that there is a gross mismatch between the volume values in Tab. 1 of Ref. [467] and those plotted in all subsequent referencing figures (e.g. Refs. [198, 216,217,218]).

  55. 55.

    In Appendix C, the unit cell volume of BPY is given as 1561.4 Å3 from Ref. [127]. The present value is calculated for an unit cell that has undergone a coordinate transformation () to match the one in Ref. [467].

  56. 56.

    cis-bis(isothiocyanato)bis[(N-2′-pyridylmethylene)- 4-(phenylazo)aniline]iron(II).

  57. 57.

    A Schiff base—named after German-Italian chemist Hugo Schiff (1834–1915)—is a molecule that is formed by reacting an amine (R–NH2) with an aldehyde (R–(CO)–H) or a ketone (R–(CO)–R′); a diimine is a compound with two imine (R,R′–C=N–R″) groups [202, 468].

  58. 58.

    Several pump wavelengths have been used to induce SCO in AZA: 400 nm[431], 647.1–676.4 nm [74, 324], 800 nm [269, 353], 830 nm [353], and 850 nm [350].

  59. 59.

    At 110 K and 300 K respectively, as determined by Guionneau et al. [193] using XRD.

  60. 60.

    There are some reports of much slower IVR time constants (ca. 8–10 ps) that could be explained by the absence of more efficient relaxation pathways in those systems [376, 489].

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Liu, L.C. (2020). Photoinduced Spin Crossover in Iron(II) Systems. In: Chemistry in Action: Making Molecular Movies with Ultrafast Electron Diffraction and Data Science. Springer Theses. Springer, Cham. https://doi.org/10.1007/978-3-030-54851-3_5

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