Niéli Daffé
email: nieli.daffe@psi.ch

Marie-Anne Arrio

Juan-Ramón Jiménez

Michal Studniarek

Amina Benchohra

Rodrigue Lescouëzec

Jan Dreiser
email: jan.dreiser@psi.ch

Michał Studniarek

Direct Observation of Charge Transfer and Magnetism in Fe4 Co4 Cyanide-Bridged Molecular Cubes

Keywords: Photomagnetism, Prussian blue analogues, Charge transfer, Cyanometalates, XMCD, XAS

The photocontrol of the magnetic and optical properties of switchable compounds is of great interest in view of possible implementations as sensors, optical switches or memories in organic electronics devices. [START_REF] Sato | Control of Magnetic Properties through External Stimuli[END_REF][2][START_REF] Létard | Towards Spin Crossover Applications[END_REF][START_REF] Ricci | Sensor and Biosensor Preparation, Optimisation and Applications of Prussian Blue Modified Electrodes[END_REF][START_REF] Karyakin | Prussian Blue Based Nanoelectrode Arrays for H2O2 Detection[END_REF][6][START_REF] Kahn | Spin-Transition Polymers: From Molecular Materials Toward Memory Devices[END_REF] FeCo Prussian Blue Analogues 8 (PBAs) are excellent candidate materials exhibiting a photoinduced metastable state as reported in the inorganic K 0.2 Co 1.4 [Fe(CN) 6 ]•6.9H 2 O PBA. [START_REF] Sato | Photoinduced Magnetization of a Cobalt-Iron Cyanide[END_REF] At the origin of the substantial changes of the magnetic properties in these three-dimensional networks of metallic ions connected by cyanide ligands, there is a metal-to-metal electron transfer within the Fe-CN-Co pairs associated with a spin transition of the Co ions, known as the electron transfer coupled to spin transition (ETCST) phenomenon. [START_REF] Tokoro | Novel Magnetic Functionalities of Prussian Blue Analogs[END_REF][START_REF] Bleuzen | Photoinduced Ferrimagnetic Systems in Prussian Blue Analogues CIxCo4[Fe(CN)6]y (CI = Alkali Cation). 1. Conditions to Observe the Phenomenon[END_REF] The ground state is formed from pairs of Co and Fe ions both in the configuration. After the ETCST, in the excited state the formerly diamagnetic (Co III , , S = 0) (Fe II , , S = 0) pairs are converted to the paramagnetic (Co II , , S = 3/2) (Fe III , , S = 1/2) ones. A schematic representation of the ETCST process in FeCo 3D-PBA is depicted Figure 1a.

Lately, cyanide-bridged FeCo systems have attracted a renewed attention when the lightinduced magnetic properties of PBAs were successfully transferred to discrete molecular FeCo models. First demonstrations include the report of thermally-induced ETCST in a Fe 2 Co 3 pentanuclear complex 12 and the photomagnetic effect in a zero-dimensional molecular cube of Fe 4 Co 4 was reported for the first time a decade ago. [START_REF] Li | Magnetic and Optical Bistability Driven by Thermally and Photoinduced Intramolecular Electron Transfer in a Molecular Cobalt-Iron Prussian Blue Analogue[END_REF] Since then, the many advantages of transferring the photomagnetic properties to zero-dimensional molecular systems were corroborated. [START_REF] Koumousi | Metal-to-Metal Electron Transfer in Co/Fe Prussian Blue Molecular Analogues: The Ultimate Miniaturization[END_REF][15][START_REF] Li | An S = 6 Cyanide-Bridged Octanuclear Fe III 4 Ni II 4 Complex That Exhibits Slow Relaxation of the Magnetization[END_REF][START_REF] Yuanzhu | Reversible Thermally and Photoinduced Electron Transfer in a Cyano-Bridged {Fe2Co2} Square Complex[END_REF][START_REF] Mondal | On/Off Photoswitching in a Cyanide-Bridged {Fe 2 Co 2 } Magnetic Molecular Square[END_REF][START_REF] Shiga | Cyanide-Bridged Decanuclear Cobalt-Iron Cage[END_REF][START_REF] Nihei | Cyanide-Bridged Iron(II,III) Cube with Multistepped Redox Behavior[END_REF] These include less structural complexity than in 3D-PBAs facilitating rationalization and optimization of their magnetic properties, the availability of single crystals and good solubility. Similar to 3D-PBA, the light-induced properties of the molecular systems are also attributed to the mechanism of ETCST.

The characterizations of the metal-to-metal electron transfer processes are typically performed indirectly using magnetometry or X-ray structural analysis. In this respect, X-ray absorption spectroscopy (XAS) is a very powerful technique able to clarify the electronic structure of transition metals, including valence and spin states and crystal-field splitting by a direct observation of the 2p  3d dipole allowed transitions at the L 2,3 edges. The technique has been indeed successfully applied to examine the light-induced excited spin state trapping (LIESST) in spin-crossover complexes and PBA. [START_REF] Cartier Dit Moulin | Photoinduced Ferrimagnetic Systems in Prussian Blue Analogues CIxCo4[Fe(CN)6]y (CI = Alkali Cation). 2. X-Ray Absorption Spectroscopy of the Metastable State[END_REF][START_REF] Bleuzen | Photo-Induced Electron Transfer and Magnetic Switching in CoFe Cyanides: Study of the Metastable State[END_REF][START_REF] Lee | X-Ray Absorption Spectroscopic Studies on Light-Induced Excited Spin State Trapping of an Fe(II) Complex[END_REF][START_REF] Jesús Luque | Magnetic Ordering in an (Fe 0.2 Cr 0[END_REF] The direct observation of the ETCST properties to the photomagnetic molecular complexes was reported by Sekine et al. who have investigated the thermal and X-ray induced conversion in Fe 2 Co 2 cyanide bridge molecular squares using K-edge XAS. [START_REF] Sekine | X-Ray-Induced Phase Transitions by Selective Excitation of Heterometal Ions in a Cyanide-Bridged Fe-Co Molecular Square[END_REF] Very recently, Fatima et al. have evidenced the occurrence of the thermal and light-induced electron transfer in a FeCo dinuclear complex using L-edge XAS and XMCD electron transfer. [START_REF] Jafri | Atomic Scale Evidence of the Switching Mechanism in a Photomagnetic CoFe Dinuclear Prussian Blue Analogue[END_REF] Another important unknown is the nature of the Fe-Co magnetic coupling in the photoinduced paramagnetic state of the molecular systems. While in 3D-PBA an antiferromagnetic coupling between the and the was reported, [START_REF] Champion | Photoinduced Ferrimagnetic Systems in Prussian Blue Analogues C I x Co 4 [Fe(CN) 6 ] y (C I = Alkali Cation). 4. Characterization of the Ferrimagnetism of the Photoinduced Metastable State in Rb 1[END_REF] calculations on paramagnetic 1D FeCo chains [START_REF] Lescouëzec | Cyanide-Bridged Iron(III)-Cobalt(II) Double Zigzag Ferromagnetic Chains: Two New Molecular Magnetic Nanowires[END_REF] and on 0D FeCo square complexes [START_REF] Pardo | Crystal Structures, and Magnetic Properties of a New Family of Heterometallic Cyanide-Bridged FeIII2MII2 (M = Mn, Ni, and Co) Square Complexes[END_REF] were recorded at the X-Treme beamline of the Swiss Light Source. [START_REF] Piamonteze | X-Treme Beamline at SLS: X-Ray Magnetic Circular and Linear Dichroism at High Field and Low Temperature[END_REF] The sample was positioned so that the X-ray beam was incident at an angle of 30° from the sample surface as depicted in A more precise analysis and understanding of the X-ray spectra of the ground and excited states is obtained by comparison with theoretical spectra of multiplet features of , , and

(LS: low spin and HS: high spin). Spectra were calculated using LFM theory as implemented in the CRISPY software package. [START_REF] Stavitski | The CTM4XAS Program for EELS and XAS Spectral Shape Analysis of Transition Metal L Edges[END_REF] In the molecular Cs-Fe 4 Co 4 cubes, all metal ions experience a slightly distorted octahedral coordination sphere. with a pseudo C 3 axis along the boron-metal direction. In order to approximate this distorted octahedral environment, the calculations are performed using a C 3v point symmetry for the metallic ions and the strength of the crystal field potential is therefore defined using three independent parameters Dq, Dσ and Dτ.

Details of the parameters used are given in the Supporting Information (SI). The simulations of the Fe L-edge spectra were performed in order to match the ones of the mononuclear reference complexes K 2 In order to extract quantitative information about the relative amount of Co II /Co III and Fe II /Fe III , linear combinations presented in Figure 2 of the modeled spectra are used to visually fit the measurements obtained for Cs-Fe 4 Co 4 . We find 100% of and a mix of 16±1% of and 84±1 % of in the ground state.The amount of Co II in the ground state is lower than the 25% that were previously reported. [START_REF] Jiménez | A New {Fe4Co4} Soluble Switchable Nanomagnet Encapsulating Cs+: Enhancing the Stability and Redox Flexibility and Tuning the Photomagnetic Effect[END_REF] Due to the probing depth of TEY of a few nanometers, this result can be interpreted in terms of surface oxidation of part of the cubes. The excited state is composed of 52±1 % + 48±1 % on the one hand and 64±1 % of + 36±1 % of on the other hand (see SI Figures S4 andS5). From the compositions of the ground and excited states we conclude that 48±1 % of are transformed into while 48±1 % of the initial ions in the ground state are oxidized toward during the three hours of laser irradiation. These findings are perfectly consistent, and they demonstrate the concomitance of the Fe oxidation with the Co reduction. This is further supported by the observation of the XAS changes during the irradiation process. Figure S5 in the SI shows the comparison of the variation of Co II and Fe III in time during laser irradiation, revealing similar time constants for Co and Fe extracted from monoexponential fits. Altogether, these results show that only a charge transfer between the Fe and the Co ions associated with a Co spin transition can be at the origin of the simultaneous transformation. Figure S5 indicates that the conversion is not complete after 3 hours of irradiation. This may be due to the 650 nm wavelength deviating from 800 nm for which a full conversion was reported in these compounds, [START_REF] Mondal | On/Off Photoswitching in a Cyanide-Bridged {Fe 2 Co 2 } Magnetic Molecular Square[END_REF] and/or the presence of Co II in the ground state preventing the electron-transfer of 16% of the -pairs. 66 K (see Figure S5). No XMCD signal is detected at 200 K at the Fe L 2,3 edges. This is consistent with the above conclusion that the ground state contains solely LS Fe II . In contrast, at the Co L 2,3 edges, a clear dichroic signal of the order of 0.7% with respect to the main edge jump is detected, confirming that the ground state contains a minor contribution of in line with the XAS analysis of the ground state. After laser irradiation at 2 K, the magnetic state of Cs- Indeed, a significant XMCD signal is detected from together with a strong XMCD contribution of , leading to 21% and 50% of signal normalized to the main edge jump, respectively. The intensity of the XMCD is well reproduced by the calculations performed in C 3v point symmetry for Fe 3+ and Co 2+ (note that we did not obtain any XMCD signal for the calculations described above while performed for Fe 2+ and Co 3+ in good agreement with their expecting low spin state). From the calculations of the XMCD signals, we extract for the Fe (respectively for the Co) the orbital angular kinetic momentum <Lz> = XX (resp. XX), the spin kinetic momentum <Sz> = XX (resp. XX) and the magnetic dipole <Tz> = XX (resp. XX). From the comparison of the experimental XMCD signal with the calculated ones with the magnetic field set along the C 3v axis one finds that the observed Co and Fe magnetic moments are both oriented parallel to the external magnetic field at 6.8 T. This clearly demonstrates the absence of a significant antiferromagnetic coupling between the metal ions in the paramagnetic excited state. In order to investigate further the magnetic coupling between the ions, we have obtained the field dependent magnetization for both Fe and Co from XMCD, as shown in Figure 3c in the paramagnetic excited state. Figure 3c

ASSOCIATED CONTENT

Supporting Information: Crystal field multiplet calculations; best-fit parameters of the LFM calculations; reference spectra of Fe and best fits; reference spectra of Co and best fits; best-fit calculated spectra of the ground state; best-fit calculated spectra of the paramagnetic excited state; time dependence of Fe III and Co II spectral features during laser irradiation.

Figure 1 .

 1 Figure 1. (a) Schematic representation of the ETCST process in FeCo 3D-PBA. (b) Scheme of the XAS and XMCD experimental geometry (c) Ball-and-stick representation of Cs-Fe 4 Co 4 . Hydrogen atoms, solvent molecules, and Cs bonds are omitted for clarity. Color code: iron: orange, cobalt: blue, cesium: purple, nitrogen: light blue, carbon: grey, boron: pink. The scale bar has a length of XXX nm.

Figure 1c .

 1c Figure 1c. XAS measurements were performed in total electron yield (TEY) mode with the

Figure 2 .

 2 Figure 2. Experimental XAS of the ground and excited states measured at the Fe L 2,3 edges (700 eV -740 eV) and Co L 2,3 edges (770 eV-805 eV) of Cs-Fe 4 Co 4 and calculated best-fit curves. All fits exhibit a high coefficient of determination 0.81 ≤ R² ≤ 0.95.

Fe 4

 4 toward higher energies due to the shortening of the Co-ligand bonds as compared to Co II . This well-defined separation in energy allows the comparison of the respective spectral features. The results obtained on the mononuclear reference compounds for Co II and Co III are presented and discussed in the SI Figure S3. The contributions of Co II are well modeled using the C3v symmetry which takes into account the deviation from the octahedral symmetry because of the non-equivalent ligands. A weak crystal field of 10 Dq = 0.11 eV is found for the Co II ions, in agreement with the axial distortion factor (Δ = -922 cm -1 ) previously reported and compatible with the expected HS state. In contrast, the best simulation in the case of the Co III ions corresponds to the higher 10Dq potential value of 0.5 eV consistent with a LS state. The straightforward comparison of typical spectral features of both Fe and Co ions in Cs-Co 4 as visible in Figure 2, along with the LFM calculations described in the above, yields clear evidence of the occurrence of the light-induced excited state at 2 K. The spectra obtained after irradiation with the laser show an increase of the signatures of and compared to the ground state at room temperature, which mainly includes and . The signatures of and in the excited state do not disappear completely, which indicates that the conversion is not complete for both Fe and Co ions. This is expected because of the initial presence of Co II ions in the ground state which implies the presence of Fe II ions in the excited state.

Figure 3 .

 3 Figure 3. Experimental and calculated XMCD spectra measured after laser excitation at 2 K(a) at the Fe L 2,3 edges and (b) at the Co L 2,3 edges in a magnetic field of 6.8 T and at 2 K. Symbols and solid lines denote experimental data and calculations, respectively. (c) Experimental, element specific and normalized M(H) curves extracted from XMCD are plotted as symbols.Beyond the averaged properties provided by SQUID magnetometry, XMCD helps to determine

Fe 4

 4 Co 4 is considerably modified as shown by the XMCD signals reported in Figure 3a-b.

  shows a superposition of the element-specific magnetization curves M(H) which do not saturate at 6.8 T.This result contrasts with the previous studies which have demonstrated a strong antiferromagnetic coupling between Co and Fe in PBA 3D networks. Recently reported XMCD measurements of the dinuclear FeCo complex have led to similar conclusions. A ferromagnetic coupling cannot be excluded, however from the current set of data it is not possible to discriminate between a ferromagnetic Fe-Co exchange interaction and a paramagnetic behavior of the ions with their spins aligned along the direction of the magnetic field applied. Considering the Heisenberg-Dirac Hamiltonian .and by comparison with the temperature in the experiment we estimate a lower bound for the Fe-Co superexchange coupling of . This can be interpreted in that a very weak antiferromagnetic coupling, no coupling or a ferromagnetic coupling are consistent with our observations. A strong antiferromagnetic coupling can be ruled out.In summary, using L-edge XAS as an element-specific probe of the valence and spin states of Fe and Co ions in Cs-Fe 4 Co 4 discrete molecular cubes we have observed directly the concurrent electronic and magnetic changes of both types of ions upon light irradiation. These results demonstrate beyond the shadow of doubt the existence of the light-induced ETCST process and its thermal relaxation in the molecular cages. The XMCD measurements directly reveal a ferromagnetic arrangement of the Fe and Co spins in the photoexcited state at 2 K for all applied magnetic field values. Furthermore, XMCD rules out the possibility of a strong antiferromagnetic coupling. Our study paves the way toward the integration of the present Cs-Fe 4 Co 4 cages, featuring excellent photomagnetic properties and a high solubility, into organic electronics devices.

  [Fe II (Tp)(CN) 3 ] and NBu 4 [Fe III (Tp)(CN) 3 ] (see Supporting Information Figures S2 and S3 possessing well-known oxidation numbers. The best-fit calculations of the X-ray spectra Concerning the Co spectra, the peaks centered at 775.8 eV, 776.8 eV and 778.2 eV are fingerprints of Co II in a sixfold coordination environment, while the signature of Co III is characterized by peaks at 779.7 eV and 783.1 eV. The features corresponding to Co III are shifted

	are presented in Figure S4. The best simulation of K 2 [Fe II (Tp)(CN) 3 ] is obtained for Fe II ions
	with relatively strong cubic crystal field of 2.8 eV consistent with a LS state. To reproduce the
	spectral shape of	, the same parameters reported for the precursor [TpFe III CN 3 ] by Jafri and
	co-workers are used (10 Dq = 2.8 eV; Dσ = 0.07 eV, Dτ = 0.12 eV). The Fe L-edge spectrum
	measured on Cs-Fe 4 Co 4 at 300 K before photoexcitation plotted in Figure 2 is manifestly very
	similar to the simulations obtained for	. In contrast, the peak at 705.2 eV is the signature of
	, which is observed in the excited state of Cs-Fe 4 Co 4 . This peak is known to originate from
	the 2p 6 t 2g 5	2p 5 t 2g 6 transitions that are absent in the	configuration. 11

ACKNOWLEDGMENTS

The authors thank Stefan Zeugin for his technical support during the experiments. Furthermore, the authors thank Marius Retegan for his help with the CRISPY software. Funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 701647 (M.S.) and from the Swiss National Science Foundation (grant nos. 200021_165774/1 and 200020_182599/1, M.S., N.D. and J.D.) is acknowledged.