Thermal and near-infrared light induced spin crossover in a mononuclear iron( II ) complex with a tetrathiafulvalene-fused dipyridophenazine ligand †

A mononuclear Fe( II ) complex involving a tetrathiafulvalene-based ligand exhibits thermal spin-crossover (around 143 K) with pronounced hysteresis behaviour (48 K). The chromophoric and π -extended ligand allows Near-Infrared (NIR) sensitization for the light-induced excited spin-state trapping (LIESST) with T (LIESST) = 90 K.

Molecular magnetism is a very active field where both chemist and physicist communities are looking for systems with highdensity memory capabilities.In this context, Single Molecule Magnets (SMM) 1 and Spin Crossover (SCO) materials 2 are intensively studied because of their potential applications. 3CO behaviour has been known since the 1930s. 4In this respect, the Fe(II) ion is the most popular magnetic centre owing to its large variation of the spin value (S = 2 → S = 0), the largest possible for 3d metal centres along the high-spin (HS) to low-spin (LS) transition.A crucial parameter for device applications is the presence of magnetic bistability that is driven mainly by the cooperativity induced by intermolecular interactions in the crystal lattice.Thus, the switching of spin states can be induced by changing the size of the coordinated ligands, counter ions and/or the molecules of crystallization. 5he magnetic bistability can also be reached under kinetic conditions by light irradiation at low temperatures (lightinduced excited spin state trapping, LIESST). 6All these aspects have already been studied on several derivatives of [Fe(H 2 Bpz 2 ) 2 (X)] complexes (H 2 Bpz 2 = dihydrobis(1-pyrazolyl)borate, X = 2,2′-bipyridine, 1,10-phenanthroline, dipyrido-[3,2a:2′,3′-c]phenazine (dppz)). 5,7Moreover, the SCO behaviour of such derivatives has recently been observed upon deposition on a gold surface and in thin films. 8Herein, we propose to extend the π-system of the dppz ligand through fusion with a tetrathiafulvalene (TTF) derivative in order (i) to combine redox activity with SCO behaviour, 9 (ii) to increase the cooperativity via enhancement of π-stacking interactions, thus the hysteretic behaviour around the transition temperature, 10 and (ii) to induce strong intra-ligand charge transfer (ILCT) transitions reaching the NIR spectral region for LIESST investigation.Sensitization of the LIESST effect through ILCT excitations could present some advantages such as (i) the ILCT bands allow lower-energy excitations than the MLCT and (ii) they can be easily modulated in energy through chemical changes of the push-pull ligands and (iii) the absorption intensity is higher than forbidden MLCT transitions.
The cyclic voltammogram of 1 shows three single-electron redox waves at −1.11 V, 0.71 V and 1.06 V corresponding to the formation of the dppz radical anion, the radical cation and the dication of the TTF fragment, respectively (Fig. S1 †).These oxidation potentials are comparable to a range of functionalized TTF donors. 15Upon coordination to Fe(II), the dppz-centred reduction wave is anodically shifted from −1.23 V (ref.12) for the free L to −1.11 V for 1 while the TTF-centred oxidation waves remain unchanged. 16These electrochemical properties look promising for controlling magnetic properties by changing oxidation states.
The UV-visible absorption of 1 was studied in CH 2 Cl 2 solution (Fig. 3) and in the solid state (Fig. S2 †).TD-DFT calculations were performed to rationalize the absorption spectrum (Fig. S3, S4 and Table S2 †), however considering the Fe(II) ion in its LS (S = 0) state (see the ESI † for computational details).The lowest energy bands were attributed to the ILCT transitions from the TTF to the dppz moieties which in the solid state gives rise to an absorption tail that stretches out into the NIR (800 nm, 12 500 cm −1 ) region.These electronic excitations as well as those at higher energies were in good agreement with the calculated ones (Table S2

†).
Magnetic measurements clearly indicate that 1•(CH 2 Cl 2 ) 2 remains in the HS state (S = 2; χ M T = 3.5 cm 3 K mol −1 ) down to low temperatures (Fig. 4).Thermogravimetric analysis reveals that the sample loses the solvent of crystallization when heated up to 120 °C (Fig. S5 †).After the initial loss of the wetting solvent of a fresh sample, the observed mass loss (14%) conforms to a full desolvation process (theo, 15%).After desolvation, the magnetic behaviour of 1 is radically different since it shows a HS to LS transition centred at 143 K with a wide thermal hysteresis (48 K) which can be cycled (Fig. S6 †).The transition occurs at T C↓ = 119 K in the cooling mode and T C↑ = 167 K in the warming mode.Both curves have been recorded at a rate of 0.3 K min −1 .The transition occurs in the same temperature range as in the parent H 2 Bpz 2 ligand based compounds, 5b,7 but with larger thermal hysteresis.It is expected that the loss of solvent from the crystal structure increases the overlap between TTF moieties (Fig. 2), leading to the extension of the interaction between the iron(II) centres.Unfortunately, the structural modifications upon the desolvation process are so severe that crystals crack and do not diffract anymore.
Starting from the desolvated form (1), irradiation at 780 nm (12 820 cm −1 ) in the tail of the ILCT band (Fig. S2 †), at 10 K, induces within minutes a complete spin change from LS to HS (Fig. 4).After the light has been switched off, the population of the metastable 5 T 2g (O h ) state persists up to T LIESST = Fig. 2 Crystal packing of 1 (HS) in 1•(CH 2 Cl 2 ) 2 highlighting the "headto-tail" dimers.90 K at a sweep rate of 0.3 K min −1 .This value is much higher than the one (44 K) reported for [Fe(H 2 Bpz 2 ) 2 phen].7b Irradiation at 530 nm (18 870 cm −1 ) and 660 nm (15 150 cm −1 ) produces the same quantitative LIESST effect while the irradiation at 404 nm (24 750 cm −1 ), outside the HOMO → LUMO band (Fig. S2 †), is less efficient.Excitation in the lowenergy singlet ILCT band (780 nm) might induce fast relaxation to the triplet T 1 level of Fe II and progressive population of the quintet T 2 state of the HS Fe II ion.Even if we are aware that the sensitization process is controversial, 17 a hypothetical proposed mechanism for the LIESST effect is given in Fig. S7.† The spectacular difference in magnetic behaviour of 1•(CH 2 Cl 2 ) 2 and 1 calls for further investigation.Electronic structure calculations have been conducted on the periodic crystal structure using the density functional theory (DFT) framework as implemented in the VASP code. 18The Perdew-Buke-Ernzerhof form generalized gradient approximation was used for the exchange and correlation functional. 19Wave functions were expanded using a plane-wave basis set with a cutoff energy of 400 eV.Core electrons were treated within the projector augmented wave method. 20The simulation cell contains two molecules of 1 and, in the case of the solvated structure, three molecules of CH 2 Cl 2 and two of H 2 O.The latter structure was chosen to account for (i) the presence of two CH 2 Cl 2 molecules and (ii) the disorder created at the two half-occupied sites in the crystal structure of 1•(CH 2 Cl 2 ) 2 , that are mimicked by two H 2 O molecules.Given the size of the cell and the nature of the system, Γ-point calculations turn out to give satisfactory accuracy.Geometries were optimized until the forces were smaller than 0.01 eV Å −1 .Forces were corrected for the missing van der Waals interactions using the Grimme scheme. 21It is known that usual functionals fail to take into account the strongly correlated character of iron d-electrons.Therefore, a Hubbard-like term is added to the Kohn-Sham Hamiltonian, 22 controlled by an effective term U eff . 23Several propositions have been formulated regarding the appropriate value for U eff in the case of SCO compounds ranging from 1.55 to 2.65 eV. 24As we did not intend to reach a quantitative agreement, but rather a qualitative description of the physical processes, we used a value U eff = 2.00 eV.We checked that a value of 3.00 eV was not affecting dramatically the geometry of the systems (variations are less than 0.5%).For both solvated and non-solvated compounds, the HS (S = 2) and LS (S = 0) solutions have been considered.In the case of the HS state with solvents, the resulting optimized cell is in good agreement with the experimental structure (Table S3 †).In particular, the optimized HS molecule shows deviations of less than 3% in bond lengths (Table S4 †).
In order to achieve SCO, a system has to possess specific features.In particular, the enthalpy should favour the LS state since the entropic contributions (electronic and vibrational) will benefit to the HS state.This is characterized by the adiabatic gap ΔE adia = E HS − E LS , i.e. the energy difference between the HS and LS states each considered in their own geometry.As a necessary, but not sufficient condition, ΔE adia should be positive.In the case of solvated 1 (Fig. 5a), ΔE adia = −0.40eV, which leads to an HS system whatever the temperature, in agreement with the experimental observation.The situation is different with 1. Indeed, in the absence of solvents, the LS state is stabilized and ΔE adia reaches 0.41 eV and SCO behaviour becomes possible.
When considering the optimized geometries of the HS and LS solutions with and without solvents (Fig. S8 and Table S4 †), it becomes clear that the variation of the adiabatic gap can be traced back to a competition between Fe-coordination and π-π interactions.In particular, when considering the HS solution with and without solvents (Fig. 5b, c and Table S4 †), one can notice that the H 2 Bpz 2 ligand opts either for a Fe-coordination which leads to an expected Fe-N bond of 2.15 Å, or without solvent, an interaction with the π-system of L which leads to a much longer bond (2.42 Å).The distortion of the coordination sphere leads to a destabilization of the HS solution and makes SCO possible.
As a final remark, let us note that in the absence of solvent, the Fe-Fe distance goes from 13.48 Å to 13.42 Å when going from HS to LS.It is known that a limited contraction in the Fe-Fe distance along the HS to LS transition favours the electrostatic contribution to cooperativity hence to the opening of a hysteresis loop. 25In addition to the existing π-network, this strongly supports the potential of TTF-based compounds as candidates for SCO systems with large hysteresis.To summarize, compound 1 was obtained from the association of the TTF-fused dipyrido-[3,2-a:2′,3′-c]phenazine ligand (L) and the metallo-precursor Fe(H 2 Bpz 2 ) 2 .This neutral heteroleptic complex presents both thermal and light-induced spin transitions.The π-extended chromophoric ligand L exhibits an intense and energetically low-lying ILCT absorption band, which enables sensitization of LIESST in the NIR spectral region.The analogous NIR antenna effect of this ligand has recently been described for the NIR emission of Yb(III) doped silica nanoparticles. 26The large π-system of L increases as well the interaction between the magnetic centres which leads to one of the largest thermal hysteresis for a mononuclear spin transition complex of Fe(II).The use of a TTF-based ligand provides a promising possibility to have high temperature LIESST effects and large thermal hysteresis loops as well as to use the redox activity of the TTF ligand to switch the magnetic properties.

Notes and references
were dissolved in 5 mL of degassed H 2 O under argon and then 14.8 mg of solid KH 2 Bpz 2 (0.08 mmol) 27 were added in one portion under stirring.The light yellow solid formed was dissolved addition of 10 mL of CH 2 Cl 2 .The aqueous phase was removed and the organic phase was dried over MgSO 4 .A solution of 5 mL of CH 2 Cl 2 containing 12.1 mg of L (0.02 mmol) 12 was added to the organic phase.After 15 min of stirring, n-hexane was layered for a few days leading to the formation of dark blue single crystals of (1)•(CH 2 Cl 2 ) 2 which are suitable for X-ray diffraction studies.Yield (dried in air for two hours, based on L), 1

Fig. 1
Fig. 1 ORTEP view of the structure of 1•(CH 2 Cl 2 ) 2 .Atomic displacement ellipsoids are drawn at the 50% probability level.Hydrogen atoms are omitted for clarity.

Fig. 4
Fig. 4 Thermal dependencies of χ M T of the solvated (empty circles) and desolvated (full circles) forms of 1. Wine colour represents χ M T in the low temperature range after light irradiation at 780 nm (12 820 cm −1 , 5 mW) for 5 min.

Fig. 5
Fig. 5 (a) Adiabatic gap ΔE adia = E HS − E LS for 1 with and without solvent molecules (CH 2 Cl 2 and H 2 O), calculated at the PBE + D3 + U level (U eff = 2.0 eV).(b), (c) Optimized geometry of the HS state with and without solvent, respectively.Fe, Cl, S, O, N, C, B and H atoms are depicted in orange, light green, yellow, red, blue, brown, dark green, and pink, respectively.Without solvent molecules, the HS geometry shows a competition between Fe-coordination and the π-π interaction between H 2 Bpz 2 and the ligand L.