Ferrous to Ferric Transition in Fe‐Phthalocyanine Driven by NO2 Exposure

Abstract Due to its unique magnetic properties offered by the open‐shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d‐states of FePc and the sp‐band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature.


Introduction
In the last decades, ag reat effort, both from the fundamental research and technological applicationp oint of view,h as been made to exploit organic-metal interfaces in the engineering of functional molecular-based spintronics devices. [1][2][3][4] Among others, planar and aromatic metalorganic molecules, such as metal-phthalocyanines (MPcs),m etal-porphyrins (MPs) and their derivatives, have been widely employed in molecular spintronics because they may manifest ab road variety of spinrelatedp henomena, ranging from magnetic anisotropy [5,6] to the Kondo effect. [7] However,o ne of the main challenges is to preserve and enhance the magnetic properties of these complexes upon adsorption on am etal electrode whileb eing able to precisely manipulatet heir spin states by external stimuli.I n this context, the FePc molecules show ap articularp otential due to their unique magnetic properties as molecular magnets. [8,9] In general, upon adsorption on am etal surface, the iron center of at etrapyrrolicc ompound can be subject to charge transfer,w hose extent depends on both the nature of the chelated metal and the substrate reactivity.I np articular,o nc oinage metals,t he molecule-substrate interaction is weaker when the FePc is deposited on gold; it increaseso ns ilver,r eaching its maximum on copper. No changes in the electronic structure of the FePc occur upon interaction with Au, [10,11] while in the case of Ag and Cu, the open Fe 3d shell hybridizes with the surface, leadingt oac harget ransfer from the substrate to the molecular layer. [10][11][12] The coupling between the FePc andt he substrate can alter or even completely quencht he magnetic momento ft he iron ion, because of the interaction occurring between the 3d shell and the substrate atoms. [13][14][15] In fact, the magnetic moment of the iron ion is retained upon the deposition of FePc on Au, whereas it almost vanishes when the molecule is deposited on Ag [15,16] and it is completely quenched on Cu. [12] The introduction of ab uffer layer at the organic-metal interface can be exploitedt ot une the molecular-surface interaction, [17,18] even for restoring the gas phase triplet spin state of the iron ion together with its net magnetic moment. The latter has been achievedo nt he oxygen-reconstructed coppers urface, where the covalent nature of the CuÀOi nteraction yields as trong localizationo ft he surfacee lectrons inhibiting the charge transfer from the metal to the organic overlayer. [17,18] Metal-phthalocyanines and metal-porphyrins exhibit two possible sites for axially binding ligands to the central metal ion [19] that can be used for spin manipulation, i.e.,b yc hanging the ligand field of the chelated ion. [20] When these molecules are adsorbed on as urface, one of the two available binding sites is coordinated to the underlying substrate atom. The vacant site can be used both to influence the molecular-surface interaction via the so-called surface trans-effect, [21][22][23][24][25] and to directly manipulate the spin and oxidation state of the ion within the tetrapyrrolicm acrocycle.F or instance, the local spin on the iron atom in FePc interfaced with the Au(111)s ubstrate can be switched in ac ontrolled way by ammonia and hydrogen adsorption. [26,27] In some cases, the anchoring of atomic speciest ot he metal ion in an organic array can switch-on or enhance the magnetic momenti nt he metalorganic layer. [16,21,28] In the present work, we use ad ifferent strategy,w hichl eads to the finely controlled modulation of the magneticp roperties of the chelated ion the FePc array.B ye xploiting the FePc/ Cu(100) interface, we demonstrate the possibility to restore, via nitrogen dioxide (NO 2 )d issociation at the interface, the magnetic moment of the metal ion of aF e-phthalocyanine molecule, whichi sq uenched when the molecule is adsorbed on the copper( 100) surface. Moreover,i ncreasing the exposure of the FePc/Cu(100) interfacet oN O 2 gas leads to the stabilization of at hird spin configuration of the Fe ion (see Scheme 1).
By employing photoemission tomography,a bsorption, and photoemission spectroscopies, we show that as trong charge transfer takes place upon FePc adsorption on Cu(100), leading to as pin transition and the stabilizationo ft he Fe ioni nt he low singlet spin state, totally quenching the magnetic moment of the organic layer.T he gas-phase triplet spin state of the Fe ion can be restored by exposure of the organic layer to intermediate doses of NO 2 .T his process proceeds via NO 2 dissociation at the interfacea nd interaction of as ingle oxygen atom with the iron ion followed by the decoupling of the FePc moleculesf rom the copper substrate. The exposure to higher doses of NO 2 further changes the oxidation state of the central ion, and the ferrous to ferric transition is accompanied by a stronge nhancement of the magnetic moment, as the iron oxidation increases the hole density.T his corresponds to af inal adsorption configuration where two oxygen atoms are interacting with the iron ion in cis position, formingt he FePc(h 2 -O 2 ) complex. Both the intermediate and final oxygen adsorption configurations are stable at room temperature.
The presentr esultsd emonstrate explicitlyt hat the magnetic momento ft he iron ion can be manipulated by trimming the dose of the (Scheme 1) NO 2 gas, which ultimately allows stabilizing three different spin configurations of the iron through the modification of its local oxidation state.

Results and Discussion
Quenching of the magneticmomentatt he FePc/Cu(100) interface Thankst oi ts relatively high reactivity,c opperp romotes as ignificanta mount of charget ot etrapyrrolic compounds,a s proved, for example, in the case of nickel-containing porphyrins, where the lowest unoccupied molecular orbitals (LUMOs) are filled up to the LUMO + 3 [29] and, at the same time, the molecule-substrate interaction stabilizes the Ni ion in the uncommon + 1o xidation state. [30] To study the energy level alignment of the FePc frontier orbitals upon interaction with copper, followed by ap ossible charget ransfer caused by the molecule-metal interaction, we performedp hotoemission tomography( PT) measurements combining momentum resolved photoemission experiments and theoretical calculations (see Photoemission To mography simulations in Experimental Section). [31] Withint he PT approach, the squarem odulus of the Fourier transform( FT) of the real space molecular orbitals can be directly relatedt ot he measured momentum distribution of the Scheme1.Schematicrepresentation of the systemsl eading to the stabilization of threedifferent spin configurations of the Fe center.Left:att he FePc/ Cu(100)i nterface the Fe II is stabilizedi nthe singlet spin state due to the charget ransfer from the copper substrate;middle:a tl ow dose, the atomic oxygen generated from the NO 2 dissociation at the interface partially decouples the molecule from the substrate stabilizing the Fe II in the triplet spin state;right:a fter high NO 2 dose the iron undergoes the ferrous to ferric transition due to the coordination of two oxygen atomsi ncis position.Both at low and high NO 2 doses the surface is involvedi nthe oxidation process. photoemitted electrons (momentum map) at defined binding energy (BE). [29,[31][32][33] As ar esult, this procedure allows to unequivocally assign an experimentalv alence band feature to a specific molecular frontier orbital. Figure 1a shows the momentumi ntegrated photoelectron spectrum of the FePc/ Cu(100) interface measured at 30 eV using p-polarized synchrotron radiation. While the valence band spectrum of the bare coppers ubstrate shows ar ather featureless plateau associated with sp-bands, [17] two prominentf eatures are presenti nt he FePc/Cu(100) spectrum,p eakeda tB Es 1.6 eV and 0.6 eV.T o identify their origin, the momentum maps at corresponding BE were measureda nd the results are presented in the bottom row of Figure 1b.T he experimental pattern, in addition to the features originating from the molecular states, also contains sharp sp-band contributions from the coppers urface(visible at j k j % 1.2 À1 ). The flat adsorption geometry of FePc,a sd etermined from the NK -edges pectrum (Figure 1c), and the coexistence of two rotational domains due to the two-fold symmetry of the Cu(100) substrate have been taken into accounti n the simulated maps (see Figure 1b,t op row). Furthermore, the intensity gradient along k y ,d eriving from the experimental geometry (258 incidence angle with respectt ot he surface), is well-reproduced in the theoretical maps by including the j A·k j polarization factor. [31] The FePc molecules result to be 29 AE 58 mirrored with respect to the [100] high symmetry direction of the substrate, in reasonable agreementw ith the data reported in ref. [34].B ased on the excellent match between the experimentala nd theoretical data, the features at 1.6 eV and 0.6 eV BE, observed in the photoelectrons pectrum of FePc/Cu(100) can be assigned to the emissions from the highest occupied molecular orbital (HOMO) of a 1u symmetry and the two degenerate LUMO/LUMO + 1o fe g symmetry of the FePc, respectively.Therefore, the occupation of the former LUMOs is associated with astrongm olecular-substrate interaction at the FePc/Cu(100)i nterface and these orbitals are populated due to charged onation from the metal substratet ot he molecular system.FePc has previouslys hown asimilar behavior when depositedo nA g(100), [35] but not on Au(100), suggesting am uch weaker interaction of FePc with less reactive substrates and, as ac onsequence, no induced changes in the energy level alignment of the molecular electronic state upon adsorption. [10] Complementary information on the charge transfer mechanism can be obtained by performingn ear-edge X-ray absorption fine structure (NEXAFS) spectroscopy. [36] While PT is probing the occupied molecular states,N EXAFS is an efficient tool to study unoccupied and partially occupied molecularo rbitals. The combination of these two techniques allows us to determine whether the molecular states are fully or partially filled by the charget ransferp henomenono ccurring at the organicmetal interface. Changes in the electronic structure can be determined using NEXAFS by comparing the absorption spectra measured in the monolayer and multilayer regime. Thel atter is used as ar eference for the gas-phase like molecule and it is only weakly influenced by intermolecular and molecular-substrate interactions. While, in the monolayer regime, the intensity and energy positionofthe absorption resonances is strongly influenced by the latter.
The NEXAFS spectra measured with p-a nd s-polarized light across the NK -edge, for both mono-a nd multilayer FePc depositedo nC u(100), are reported in Figure 1c.T he intense spectralf eatures observed in the photon energy range of 397-404 eV in the multilayer spectrum measured using p-polarization are assigned to the transition of N1se lectron to the p*symmetry unoccupied molecular orbitals, while the resonances above the 404 eV are attributed to the 1s!s*t ransitions. [37] The linear dichroism observed in the NK -edge spectra for the FePc monolayer on Cu surfaces, i.e.,t he maximum intensity of the p*t ransitions in p-polarization and the almost vanishing intensity of these resonances in the spectra measured in s-polarization,indicates that the FePc molecules are highly oriented on the copper surface, with the molecular plane lying parallel to the substrate. [38] Moreover,t he decrease in intensity of the low energy p*-symmetry resonances around 399 eV,i nt he FePc/Cu(100) monolayer spectrum (compared to the multilayer) supports the filling of the low energy LUMOs via the charge donation from the substrate to the adsorbed layer.T his agrees with the PT measurements discussed previously (see Figure 1a). Having studied the charge transfer taking place at the interface, in the following, we discusshow the charge transfer influences the electronic and magnetic properties of the iron ion in the adsorbed FePc. To address this point, we performed XPS measurements of the Fe 2p 3/2 core level,together with NEXAFS and XMCD experiments at the Fe L-edge.
In Figure 2a,w ec omparet he Fe 2p 3/2 core level signals of FePc multilayer and monolayer adsorbed on the Cu(100) substrate. Them onolayer spectrum, whichc onsists of am ainline peakeda t7 07.05eVa nd ah igh-intensity satellite at higher BE (with the maximum at 708.8 eV), clearly resembles that of the FePc monolayer on Cu(110) substrate. [12] The measurement of the Fe 2p 3/2 core level of the multilayerp hase (about 8ML) displays as ignificant shift of the main peak to higherb inding energies ( % 1.5 eV) with respect to the monolayer case, as well as an otable change in the satellite features (see Figure 2a). In the multilayer spectrum,acontribution from the first layer of FePc in direct contact with copper is still visible at 707.05 eV, suggesting that FePc deposition proceeds in aS transki-Krastanov regime of growth.
Notably,t he energy shift between mono-and multilayer of the Fe 2p 3/2 mainline is much larger than the one observed for the peaks in the C1sa nd N1ss pectra,n amely 1.5 eV versus 0.1 eV (see Figure S1 in the Supporting Information).B oth initial and final state effects may contribute to this large energy shift at the Fe 2p 3/2 .F rom XPS measurements alone, we cannot disentangle these two contributions to the chemical shifts and the change of the spectrals hapeo ft he satellite features in the monolayer and multilayer spectra.
To get direct access to the oxidationa nd spin state states of the Fe ion, we acquired absorption spectra forb oth multi-and monolayer FePc coveragesa tt he Fe L 3 -edge (Figure 2b). Two groups of Fe 2p 3/2 excitationsw ith different polarization dependence are clearly visible from Figure 2b:i np-polarization, the features at 707.4 and7 08.9 eV (features A p and B p )a re dominanti nt he spectra, whereas in s-polarizationt he strongest peak C s is observed at higherp hoton energy (709.8 eV).
The Fe L 3 -edge NEXAFS spectrum of the multilayer,a se xpected, resembles previously reported spectra for thin FePc film onto gold plated sapphire. 9 The Fe L 3 -edgesN EXAFS data of the free FePc molecule was recently analyzed in great detail by Carlottoe tal. [39] Briefly,t he 3d atomic orbitals of Fe ions transform as a 1g (d z 2 ),b 1g (d x 2 -y 2 ),b 2g (d xy )a nd e g (d xz ,d yz )i naD 4h symmetry.
According to the DFT/ROCIS calculations (see Absorption spectra simulations in computational details), the electronic ground state ( 3 E g )o ff ree FePc corresponds to an intermediate state (IS) with as pin quantum number S = 1a nd aa 1 1g b 1 2g e 2 g b 0 1g spin up (›)a nd b 1 2g e 1 g a 0 1g b 0 1g spin down (fl)e lectronic configuration. We would like to pointo ut that, despite the unanimous consensus about the number of unpaired electrons (two) and spin state (S = 1), different electronic terms and occupation numbersh ave been proposed in the literature. [8,9,16,19,[40][41][42][43][44] According to our calculations, the lowest-lying A p and A s features in the Fe L 3 -edge multilayer spectra (see Figure 2b)a re both generated by DS = 0s tates associated with single Fe-based 2p!3d electronic excitations involving the a 1g and e g singly occupied MOs (SOMOs). Notably,t he intensity of A p is significantly higher than that of A s clearly indicating that (a 2u !a 1g ) ? /(e u !e g ) ? excitations give as tronger contributiont ot he spectra than the (a 2u !e g ) k /(e u !a 1g ) k ones. Regarding the Ba nd C features at 708.9 and 709.7 eV,r espectively,D FT/ROCIS results allowed us to conclude that they are associated with both single and coupled-single electronic excitations involving the 3d xy virtual molecular orbitals (VMO). [39] Comparing the multilayert ot he monolayer spectra reported in Figure 2b (bottom), we notice ar eductioni nt he intensity of the low energy resonances A p and A s . Notably,t he excitations at high photon energy have not shown such strong changes. By referring at the theoretical predictions reported in ref. [39], we can conclude that the transitions from the Fe 2p 3/2 level to the a 1g and e g molecular orbitals mainly contributet ot he lower energyf eatures in the spectra. The overall NEXAFS dichroism in the monolayer range is consistent with the expect- ed out-of-plane-oriented d z 2 (a 1g ) andd xz /d yz (e g )m olecular orbitals. These molecular orbitals have as tronger z-direction component whichi sp erpendicular to the copper surface, while the b 1g (d x 2 -y 2 ),b 2g (d xy )orbitals lie mostly in the molecular plane parallel to the surface. Thus, the electrons of d z 2 and d xz / d yz orbitalsc an better couple with the electrons in the substrate than those of b 1g and b 2g orbitals. Therefore, a 1g and e g mainly participate in the molecule-surface interaction and they are partially occupiedd ue to the charge transfer between the coppersubstrate and the FePc molecules at the interface.
The rearrangement of the electronic states at the Fe center, upon interaction with the Cu surface, is also expected to influence the magnetic properties of the chelated ion. [12,15,16,45] Thus, the magnetic configuration of the iron atom was probed using X-ray magnetic circular dichroism (XMCD) measurements.
XMCD measurements were performed at 3K while applying an externalm agnetic field of 4T ,which ensures the saturation of the magnetic moments in the FePc thin film regime. [9] The magnetic moment of the Fe centeri on is quenchedo nt he Cu(100) surface, as evidenced by the absence of XMCD intensity both in in-plane and in out-of-plane directions indicating that the total magnetic momento fF ei sn ull (AE 0.05 m B ) ( Figure 2c), supporting the stabilization of the Fe II singlet state (S = 0), in agreement with ref. [12].W es uggest that this could be due to the enhanced coupling of the Fe d-states with the sp-band of copper substrate electrons.T his is in clear contrast with the FePc multilayer [9] andF ePc adsorbed on Au(111), [15] where an XMCD signal is clearly visible. The change of the spin state in the adsorbed FePc molecule on ab are coppers urface is also associated with the changes in energy positiona nd shape of the Fe 2p 3/2 core-levelspectra discussed above.

Enhancemento fthe magneticmomentbye xternal chemical stimuli
To restore the magnetic momentofthe ion, which is quenched due to the molecule-surface interaction, two different ap-proaches were previously proposed:e lectron doping [16,46] or functionalization with an externall igand,e .g.,asmall gaseous molecule. [21][22][23][24][25] Inspired by the latter,w ee xposed the FePc/ Cu(100) interface to gaseous NO 2 to restore the magnetic momento ft he iron ion. The interface was exposed to two different NO 2 doses, 30 and 100 L( referredt oa sl ow and high dose, respectively,t hroughout the text), and the changes at the Fe ion were followed by measuring the Fe 2p 3/2 core-level at increasing NO 2 exposure (see Figure 3a). After the low dose, the Fe 2p 3/2 spectrum resembles the one of the multilayer, showingt he characteristics Fe II structures (see for comparison Figure2a), i.e.,asharp peak at lower and broad satellite features at higherB E. However,a fter the high dose, we witness a quenching of the sharp line (see Figure 3a,t op).
At this point, two different scenarios regarding the interaction of NO 2 with the Fe ion have to be considered. 1) The NO 2 molecules bind to the Fe center of surface-anchored phthalocyanines in trans position, decreasing the strength of the molecule-surface interaction via the trans-effect. [19] 2) The NO 2 molecules dissociate at the FePc/Cu(100)i nterface.I nt he latter scenario, the corresponding products can bind to the molecule in the trans position or intercalate between the substrate and the molecular overlayer.F urther considerations on the dissociation at the interface will be discussed in the following.
The absence of an ew component (expected to rise at higher binding energies) in the N1 ss pectra measured after both low andh igh dose of NO 2 (Figure 3b), as well as the conservation of the area below the peak related to the Pc nitrogen atoms, excludes the first scenario, in which intact NO 2 or other nitrogen-containing products bind to the coordinated iron atom. Therefore, the changes in the iron core-levels pectra are likely caused by the oxygen atoms createda tt he interfaceb y an on-surfacer eaction involving the dissociation of NO 2 molecules (the presence of oxygen at the FePc/Cu(100)i nterface is confirmed by the O1ss pectrar eported in Figure 3c). To elucidate whethero xygen atoms are anchored on top of the molecular layer (in trans position) or placed between the molecular layer and the copper substrate, the FePc moleculesh ave been sublimated on an oxygen pre-exposed coppers urface (OÀCu(100)), which shows a( p 2 p 2)R458 reconstruction. [48] The Fe 2p 3/2 ,O1s, N1s, C1sa nd Cu 3p spectra of FePc/OÀ Cu(100) interface are shown in Figure S3,and they very well resemble the corresponding spectra of FePc/Cu(100)a fter the exposure to the low NO 2 dose (see Figure 3a), supporting a similar chemical environmento fi ron in the two systems. This suggestst hat, upon the low NO 2 dose, oxygen atoms are formed after ad issociation reactiona tt he FePc/Cu(100) interface and are chemisorbed on the copper surface. This is well evident in the O1ss pectra,w here both core level spectra of OÀCu(100) and FePc/Cu(100)a fter low dose are characterized by asimilar BE (530.0 eV) and line shape, afingerprint of chemisorbed oxygen atoms on the copper surface. Instead, after dosing 100 Lo fN O 2 the O1 ss pectrum shows ac lear chemical shift of the main feature to higherB E( 530.3 eV) as well as the shouldera tl ower BE (529.1 eV);a ssociated with the oxidation of the copper substrate underneath the molecular layer (Cu 2 O and CuO, respectively). [51] Besides,t he linear dichroismo bserved in the OK -edge NEXAFS spectra also confirms the presenceo fa tomic oxygen chemisorbed on the copper surface, without further oxidation of the copper substrate. However, the clear changesi ns pectral shape and energy positioni nt he OK -edge NEXAFS spectrum of FePc/OÀCu(100) comparedt ot he bare OÀCu(100) substrate spectrum (see Figure S4) suggest that FePc molecules are not fully decoupled electronically and physically from the chemisorbed oxygen, neither for FePc/OÀCu(100) nor the FePc/ Cu(100) interface exposed to the low NO 2 dose.
To gain furtheri nsights into the low and high NO 2 dose trends,e speciallya bout the oxygen atoms coordination to the Fe ion, we simulated different structural arrangements for FePc on Cu(100). It is noteworthy that al eading role in determining the Fe L 3 -edge NEXAFSs pectrum [39] is played by the Fe nearest neighbors;t hus, for the low NO 2 dose, the nature of the weakly interacting oxygen with FePc determined from OKedge NEXAFS data is critical, while the charactero ft he OÀCu interaction is less relevant.T he adoptiono ft he molecular cluster approach to model ap eriodic system implies the saturation of the oxygen dangling bonds with hydrogen/pseudo-hydrogen atoms. [49] Therefore, the coordinated system resulting from low NO 2 dosing has been modelled by considering the free molecular complex I (see its optimized structure in Figure 4) characterized by the presence of as ingle oxygen atom of water molecule placed at 1.8 from the Fe II ion. [50] The Fe Ledge NEXAFS modelling has been carriedo ut for both s-a nd p-polarized excitation.T he final good agreement between theory and experiment is ac lear indication of the adopted model feasibility.
The comparison betweent heoretical results (see Figure 4, bottom row) and experimental evidencer ecorded at low (30 L) NO 2 dosing (see Figure 4, top row) encourages us to assess that NO 2 initially dissociates at the FePc/Cu(100) interface and generates single oxygen atoms, which intercalate between the FePc layer and chemisorbs on the copper surface. The intercalation of the oxygen atoms results in the partial decoupling of the molecules from the substrate and the restoring the original FePc gas-phases pin state (S = 1). As far as the detailed assignment of the L 3 -edge spectrum of the decoupled FePc is concerned, the lowest-lying feature in p (A p )a nd s (A s )p olarization (see Figure 4, bottom row) are both associatedw ith electronic states with DS = 0, generated by single Fe 2p!Fe 3d electronic excitationsi nvolving the 3d z 2 and 3d xz singly occupied MOs (SOMOs). In contrast, DS = 0, AE 1e lectronic states contribute to B s .S ingle (2p!d z 2 /p * Pc-based VMO) and coupled-single (Fe 2p!3d xz and 3d xz !3d xy /p * Pc-basedV MOs) electronic excitations generate DS = 0s tates,w hile the DS = AE 1s tates are all associated to metal-to-ligand-charge-transfer (MLCT) single electronic transitions. Only electronic states with DS = 0, À1 contributet oB p ;b otho ft hem imply single electronic transitions, the former states have aF e2 p!3d SOMOs nature;t he latter ones,a nalogously to B s ,d isplay an MLCT character.F inally,only DS = 0e lectronic states contribute to C s and D s through single and coupled-single excitations having once again an MLCT character.
To get information about the most favorable adsorption structure formed after exposing FePc/Cu(100)t oh igh NO 2 doses, we have examined different geometries considering two possible Fe oxidation states, i.e.,F e III and Fe IV .T he presence of the former has been modelled by considering two O atoms coordinated to FePc with ap seudo-peroxide coordination, the FePc(h 2 -O 2 )c omplex, (II a,s ee Figure 4) whose electronic properties have been thoroughly describedi nr ef. [39]. As far as Fe IV is concerned, two different modelsh ave been tested:t he former implied the presence of two Oa toms in a trans arrangement( II b, Figure S2) with respectt oF ePc plane, while the latter involvest he formation of an oxoiron(IV)Pc (II c) ( Figure S2) complex. Amongt he different spin states consid- ered for II b and II c,t he most stable speciesc orrespond to a triplet spin state with two unpaired electrons on the Fe=O fragment in agreement with the literature. [53] As such, it is noteworthy that linear dichroism is well evidenti nt he modeled spectra of II b and II c (see FigureS2), while it is absent in the spectrao fII a complex (see Figure 4). As aw hole, the comparisonb etweens imulated and experimental NEXAFS spectra at high (100 L) NO 2 dosages hows that II a is the most favorable complex formed at the interface and rules out the presence of ar elevant percentage of II b or II c species.
As imilars cenario hasb een observed for the FePc/Ag(110) interface exposed to oxygen. [39,47] The use of the FePc(h 2 -O 2 ) cluster shows av ery good agreement between theory and experiment thus providing support to the presence of oxygen atoms lying in between FePc and the substrate and information aboutt he relevant role of the substrate on the NO 2 dissociation. In detail,t he single peak characterizing both the s-a nd p-polarized L 3 -edge spectrum of the cis complex is mainly (80 %) due to electronic states associatedt ot ransitions having DS = 0a nd corresponding to Fe III 2p-based!3d-based single electronic excitations involving the d z 2 ,d xz and d xy SOMOs.I nterestingly,t he MLCT electronic state generated by Fe III 2p!p* MLCT excitations with DS = À1s ignificantly contributet ot he highere xcitation energy side of both peaks. Despite the overall agreement between experimental and simulated (see Figure 4, right) spectra,w eh ave to point out that in the simulated spectrum II a the evident shoulder on the lower excitation energy side of the s-a nd p-polarized spectra is not well reproduced. This feature is rather associated with the co-presence at the interfaceo fr esidual molecules in the singleoxygen structure I.
The NEXAFS spectra at OK -edge measured after the high dose (see Figure S4) are in good agreement with previous measurements for Cu 2 O, [51,52] thus supporting the oxidation of the coppers ubstrate after the exposure of FePc/Cu(100)i nterface to high NO 2 dose.
To summarize, the calculation shows that the I structure (low NO 2 dose) is associated with Fe II species with an intermediate spin state (S = 1), while binding of the second oxygen atom (II a configuration) induces the oxidation of the iron ion (Fe II !Fe III transition). In agreement with theoretical and experimental data, the trans adsorption configuration( II b structure) is not formed att he present experimental condition.
At this point, we can analyze the changes of the spin states and oxidation states in the I and II a structures in comparison with XMCD data measured at Fe L 3,2 -edge after stepwise increasingt he dose of NO 2 (see Figure 5).
As previously remarked, the Fe L-edge XMCD of the FePc is completely quenched on Cu(100). However,e xposing the system to increasing NO 2 doses, the iron magnetic moment gradually increases and develops as izeablei n-plane magnetic anisotropy.T he orbital and spin components of the Fe magnetic moment projected along the field direction for ag iven incidencea ngle can be obtained from the sum rule analysis. [54,55] It has to be noticed that in the effective spin magnetic momento btained from the sum rule analysis, the dipolar term T z could induce big discrepancies between the effective m S eff and m S .T ot ake account of this, based on previous multiplet calculations on the FePc system,w ec onsidered an error of 30 %o nm S . [56] For calculating the magnetic moments, we have assumed 4h oles h d for the intermediate dose and 5h oles for the higherd ose, with the holes localized on the SOMO d z 2 ,d xy and d xz ando nt he completelye mpty d yz . [39] The XMCD measurements for the lower NO 2 dose confirmt he recovery of the triplets pin state (S = 1). Indeed, the total magnetic moment m tot in the molecular plane (0.46 m B ,m S and m L values for the two doses are given in Ta ble S6 fort he in-plane direction) is comparable with the one reported for afilm of 0.5 ML FePc depositedo naferromagnetic Co(001) substrate (0.56 m B ). [14] Interestingly, the molecule-surface interaction on copper, hence the charge transfer,a ppearst ob es tronger than on cobalt, where the Fe magnetic momenti sp reserved. Comparing the intermediate and high dose XMCD spectra, as trong increase of the m tot to 2.14 m B (see Ta ble S6) furtherc onfirms the change in the oxidation state, as the transition from ferrous to ferric phthalocyanine is followed by the increasing of the number of unpaired electrons (from 2t o3 )t hat contribute to the organic layer magnetism.
As the XMCD intensity is proportionalt ot he projectiono f the magnetic moment along the X-ray incidence direction, the higher intensity of the measuredX MCD at grazing (q = 708) rather than normal (q = 08)i ncidence leads us to conclude that the system exhibits ap referentiali n-plane magnetic anisotropy at both low and high NO 2 dosing. The observedc hanges in the magnetic anisotropy induced by oxygen coordination are consistentwith previous studies. [9,28] Conclusion By combining the PT,X PS and NEXAFS experimental techniquesa long with theoretical simulations, we have shown that the controlled modulation of the spin state of am etalorganic network can be achieved by coordination of the chelated ion with small ligandst hat modify the molecule-substrate interaction. While the copper surface quenches the magnetic momento ft he metal ion, the exposure to NO 2 and, consequently,t he coordination with atomic oxygen, formed due to the NO 2 dissociation att he interface, gradually modifies the magnitude and orientation of the magnetization. For low NO 2 doses, FePc is decoupled from the copper substrate by the intercalation of atomic oxygen and the molecular network recovers the typical gas phase magnetic moment. In this regime, a single oxygen atom binds to the iron ion weakening the hybridization and the charge transfer effect at the interface, the two phenomenaw hich are responsible for the quenching of the magnetic moment at the FePc/Cu(100)i nterface. With in-creasingN O 2 doses, the central iron ion interacts with two oxygen atoms in aF ePc(h 2 -O 2 )c onfiguration, both transferred to the surface, and all the coordinated sites undergo the ferrous to ferric transition (from Fe II to Fe III ), with astrong increase of the magnetic momenta nd the in-plane anisotropy.T he FePc(h 2 -O 2 )c omplex, where two Oa toms are coordinated to the Fe ion in ap seudo-peroxide geometry,s hows to be most favorable from among of the different final structures considered.

Experimental Section
Methods and equipment:T he valence band photoemission spectra were measured at the NanoESCA beamline of Elettra, the Italian synchrotron radiation facility in Trieste, using an electrostatic photoemission electron microscope (PEEM) set-up described in detail in ref. [57].T he data were collected with ap hoton energy of 30 eV and at otal energy resolution of 100 meV,u sing p-linearly polarized light. The NEXAFS measurements of bare FePc/Cu(100) interface were performed at the ALOISA beamline, also located at Elettra synchrotron. [58] The spectra across C, Na nd OK -edge were taken in partial electron yield mode using aC hanneltron multiplier, [58] and they have been further analyzed following the procedure described in ref. [38].T he orientation of the surface with respect to the linear polarization (s and p)o ft he synchrotron beam was changed by rotating the sample around the beam axis while keeping the incident angle (68)o ft he synchrotron light fixed.
The NEXAFS and XMCD spectra at the Fe L-edge were measured at the X-Treme beamline [59] of the Swiss Light Source by recording the sample drain current in total electron yield mode. The X-ray beam was impingent at normal (q = 08)o rg razing (q = 708)i ncidence with respect to the sample surface, with the magnetic field collinear with the beam propagation direction. No spectral changes over time were observed, indicating the absence of beam damage. The temperature at the sample surface was 3K.
The clean Cu(100) surface was prepared by as tandard procedure: cycles of Ar + ion sputtering at 2.0 keV followed by annealing at 800 K. FePc molecules (Sigma-Aldrich, ! 95 %purity) were thermally sublimated at 570 Kf rom ah ome-made Knudsen cell type evaporator onto the copper substrate kept at room temperature. As the monolayer coverage is long-range ordered, the achievement of the desired coverage was monitored using reflective high-energy electron diffraction (RHEED, at ALOISA) or low energy electron diffraction (LEED). The NO 2 gas was introduced through ap recision leak valve (partial pressure of 5 10 À7 mbar) and its dosing took place in the preparation chamber while keeping the sample at room temperature. The oxygen-covered Cu(100) surface, which shows a ( p 2 p 2R)458 reconstruction confirmed by LEED pattern was prepared by exposing the Cu(100) surface to 800 Lo fO 2 while keeping the sample temperature at 500 K. [48] Computational details Absorption spectra simulations:O ptimization calculations are performed by exploiting the Amsterdam Density-Functional (ADF) software package. [60] Numerical experiments have been carried out by running spin-unrestricted, nonrelativistic DFT calculations with generalized gradient corrections self-consistently included through the Becke-Perdew formula [61,62] and by adopting at riple-z with a polarization function Slater-type basis set for all the atoms. MPc L 2,3 -edges XA spectra [39,49,[63][64][65] and Fe complexes [66][67][68] have been modelled by evaluating excitation energies and corresponding oscillator strengths (f) for transitions having the M2 p-based MOs as initial spin orbitals through the use of the module ROCIS of the ORCA program package. [69] Spectra have been simulated with the DFT/ROCIS method, [70] which includes SOC in am olecular Russell-Saunders fashion, [70] by adopting the B3LYP exchange-correlation (XC) functional [72] and by using the def2-TZVP(-f) basis set. [72,73] The combined use of DFT and configuration interaction needs as et of three semi-empirical parameters (c1 = 0.21, c2 = 0.49, and c3 = 0.29). [69] Moreover,t he resolution of identity approximation has been used with the def-TZVP/J basis set [72,73] and the zeroth-order regular approximation (ZORA) has been adopted to treat the scalar relativistic effects. [74] Photoemission tomography simulations:T heoretical photoemission simulations were based on results obtained within the framework of DFT.T he calculations of the gas phase FePc have been performed by the NWCHEM [75] DFT code, using the Becke three-parameter Lee-Yang-Parr (B3LYP) [61,76,77] hybrid. The simulated momentum maps of the gas-phase FePc molecule were obtained as the FTsoft he respective Kohn-Sham (KS) orbitals. [31]