Ammonia Coordination Introducing a Magnetic Moment in an On‐Surface Low‐Spin Porphyrin

Abstract

The controlled manipulation of spin states in atoms/molecules is of profound interest towards the design of future spin-based devices. [1,2] A prominent example of how spin states are modified (S = 2$S = 0) can be found in natures Fe II porphyrin moiety within hemoglobin and its coordination with the O 2 ligand.[3] Recently, we have implemented this concept in a synthetic on-surface arrangement using metallo-porphyrins adsorbed on ferromagnetic surfaces. By axial coordination with an external NO ligand the induced magnetic moment in the (S = 1/2) Co II porphyrin has been switched-off. [4] These experiments depend on a characteristic property of paramagnetic metallo-porphyrins as well as phthalocyanines: their interfacial chemical interaction with the ferromagnetic surface ligand induces a magnetic moment stable up to room temperature. [4,5] Axial coordination can also be used to control the magnetic anisotropy [6] as well as the strength and sign of the exchange interaction.[4b]Controlling on-surface/interface spin systems [4, 5j,k, 6, 7] is a prerequisite for applications in organic spintronics [1] which makes this research field increasingly popular. Recently, we combined chemically directed self-assembly and coordination chemistry to obtain selectively switchable, highly ordered supramolecular 2D spin arrays.[5j] Concerning chemical control of the magnetic moment, only off-switching [4, 5j] and spintuning, [4b, 5k] that is, switching spin-on!spin-off and spin-on! spin-on' (a modified spin state) have been established. So far this set of on-surface chemical spin operations was incomplete since the spin-off!spin-on case was missing. Generally, switching the spin in organometallic complexes by external ligands to the on-state is more difficult to achieve than switching to the off-state, since chemical bonding has to overcome the spin-pairing energy. An additional complication arises from the possibility that the surface can modify the spin states before as well as after the axial ligation.[4b] This can also lead to spin-quenching on the surface. [4b, 8] Here we report on the first demonstration of an on-surface chemical spin onswitch, for Ni II porphyrins (S = 0) adsorbed on a ferromagnetic (FM) Co substrate, by the diamagnetic (S = 0) external NH 3 ligand. A schematic representation of this spin on-switch (S = 0$S = 1) is shown in Figure 1 a.To study this effect, Ni II tetraphenylporphyrin (NiTPP; see Figure 1 a) molecules were thermally sublimed in ultrahigh vacuum onto clean Co thin films on Cu(001) single crystals. [4, 5e,f,j] For a description of the methods see the Supporting Information.In scanning tunneling microscopy (STM) experiments (Figure 1 b), we consistently find the molecules adsorbed in a random fashion on Co and Ni substrates, [4, 5f] in contrast to self-assembly of NiTPP on Au and Ag substrates.[9a] Most of the NiTPP molecules on the Co surface can be recognized as rectangular shapes-the so-called saddle-shape conformation [9b] (Figure 1 c), whereas a minority of the molecules is obse...

Magnetic exchange coupling of a synthetic Co(ii)-complex to a ferromagnetic Ni substrate

“…25 This adsorption behaviour corresponds well to results obtained for Mn, Fe, Co and Ni porphyrins adsorbed on Co(001) and Ni(001) thin films. [3][4][5]14 In the case of the stacking of two complexes onto each other, as observed for a small fraction, the overlapping parts are imaged as brighter protrusions. The magnetic and electronic properties of the ad-complexes are studied by XAS and XMCD spectroscopies.…”

Emergence of On-Surface Magnetochemistry

“…It has been shown recently by XMCD experiments that the NH 3 coordination can even introduce a magnetic moment in a diamagnetic low-spin porphyrin, namely, Ni(II)TPP, on a Co surface. 27 From DFT+U calculations, it was found for the NiP/Co system that the NiP molecule was in an S ≈ 0 state, with the 3d z 2 orbital doubly occupied and the d x 2 −y 2 orbital empty. Upon axial ligation with NH 3 , the orbital occupation is computed to change to singly occupied d z 2 and d x 2 −y 2 orbitals, rendering a triplet spin S ≈ 1 (magnetic moment of ∼1.91 μ B ), which in addition was FM coupled to the Co Figure 2.…”

“…7,9−11 In addition, the DFT+U model was used to explain on-surface axial ligation, which was used to control the molecular spin states in various molecule−substrate spin interfaces. 17,27 On-Surface Magnetochemistry of Planar Molecules on FM Substrates. Control of the molecular spin state and the exchange interaction with the FM substrate by means of either physical or chemical stimuli is a prerequisite for practical applications.…”

Restoring the Co Magnetic Moments at Interfacial Co-Porphyrin Arrays by Site-Selective Uptake of Iron

“…From a different perspective, a site-specific confinement of adsorbates on (metalÀ)organic layers provides suitable model systems to explore donorÀ acceptor systems on a single atom level 28 and to tailor the magnetic properties of surface-anchored supramolecular architectures by atomic doping. 29À31 Similarly, the use of gaseous ligands as nitric oxide, 32À34 ammonia 13,35 or oxygen 36 binding reversibly to metalÀorganic networks or adsorbed tetrapyrrole systems opened pathways to control and switch spin states of metal-centers, following the so-called onsurface magnetochemistry approach. 13,33À36 Here, we employ a highly regular tetragonal metalÀ organic template with a periodicity of 1.4 nm, namely a Co-tetraphenylporphyrin (Co-TPP) layer on Ag(111), guiding the positioning of individual Fe atoms and clusters upon exposure to an atomic beam of Fe in an ultrahigh vacuum environment.…”