Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.

We show that the magnetic state of individual manganese phthalocyanine (MnPc) molecules on a Bi(110) surface is modified when the Mn2þ center coordinates to CO molecules adsorbed on top. Using scanning tunneling spectroscopy we identified this change in magnetic properties from the broadening of a Kondo-related zero-bias anomaly when the CO-MnPc complex is formed. The original magnetic state can be recovered by selective desorption of individual CO molecules. First principles calculations show that the CO molecule reduces the spin of the adsorbed MnPc from S ¼ 1 to S ¼ 1=2 and strongly modifies the respective screening channels, driving a transition from an underscreened Kondo state to a state of mixed valence. DOI: 10.1103/PhysRevLett.109.147202 PACS numbers: 75.20.Hr, 68.37.Ef, 82.37.Gk, 68.43.Bc Control over the magnetic moment of a molecule and its interaction with a substrate is a key issue in the emerging field of molecular spintronics [1]. In metal-phthalocyanines and metal-porphyrins the metal center is usually coordinatively unsaturated and presents a local reactive site, which opens a unique possibility of controlling the magnetic moment in situ by external chemical stimuli [2][3][4][5][6][7][8]. The axial coordination of small molecules like CO, NO or O 2 to those complexes substantially alters their electronic properties, which has led to a successful implementation of phthalocyanines and porphyrins in gas sensors [9,10]. Studies specifically addressing the magnetic state are, however, scarce. Only recently it has been shown that the attachment of a NO molecule to a cobalt-tetraphenylporphyrin (CoTPP) quenches its spin due to the oxidation process [6]. The general picture is however more complicated, as the chemical bond to the reactant molecule causes the redistribution of charge in the d orbitals of the metal center and modifies the ligand field of the metal ion. This has critical consequences for the magnetic ground state of the complex [7,8]. On metal surfaces, the formation of a new ligand bond may additionally alter the hybridization of molecular and substrate states, thus affecting the electronic and magnetic coupling of the metal ion to the substrate [3][4][5]. Understanding the response of these effects to the change in chemical coordination is crucial to gain full control over the functionality of the magnetic system.Here, we show that the magnetic moment of a manganese phthalocyanine (MnPc) molecule [see Fig. 1(a)] on a Bi(110) surface is reduced when coordinated to a CO molecule. Using a combination of a low temperature scanning tunneling microscopy (STM) and density functional theory (DFT) we find, first, that the spin of the MnPc molecule is reduced from S ¼ 3=2 to S ¼ 1 upon adsorption and, second, that CO further reduces the spin of the MnPc from S ¼ 1 to S ¼ 1=2. The change in the magnetic ground state upon CO attachment is detected by the broadening of a characteristic zero-bias anomaly (ZBA). We interpret this broadening as a transition from a Kondo regime, for the bare MnPc on...

The electron acceptor molecule TCNQ is found in either of two distinct integer charge states when embedded into a monolayer of a charge transfer-complex on a gold surface. Scanning tunneling spectroscopy measurements identify these states through the presence/absence of a zero-bias Kondo resonance. Increasing the (tip-induced) electric field allows us to reversibly induce the oxidation/reduction of TCNQ species from their anionic or neutral ground state, respectively. We show that the different ground states arise from slight variations in the underlying surface potential, pictured here as the gate of a three-terminal device.A goal of molecular electronics is to use single molecules as transistors [1][2][3][4]. To achieve this paradigm a high degree of charge localization in the device is required [5]. Charge localization favors that a molecular device can only change its electron occupation in integer numbers, leading to discrete changes of its transconductance [6,7]. However, a molecule contacted by metal electrodes generally shows non-integer charge states, in which the electron density is redistributed throughout the bond with the leads [5,8]. To achieve a large degree of charge localization, the interaction of the molecule with the metal leads has to be sufficiently weak. This is best achieved by introducing an "effective tunneling barrier" to electronically decouple molecular states from the metal. Several STM studies on single molecules adsorbed on atomically clean and well-structured metal surfaces have shown that this is achievable using thin insulating layers (ionic or oxide thin films) separating the molecule from the surface [9-12, 18, 19]. A comparable level of decoupling has been observed when combining two different molecules in a monolayer directly adsorbed on a metal surface [13,14]. In this case, the proximity of the metal surface screens more efficiently charges localized at the molecule, leading to a reduction of Coulomb charging energy U and, hence, of the energy cost to change the electron occupation of the molecule [15]. Here, we find a similar degree of charge localization on the electron acceptor molecule tetracyanoquinodimethane (TCNQ) mixed with the electron donor tetramethyl-tetrathiafulvalene (TMTTF) in a stoichiometrically ordered monolayer. The methylation of the TTF backbone reduces the interaction of the molecular ad-layer with the metal substrate in comparison to the parent compound TTF-TCNQ [14,16], while maintaining ground states with integer charge occupation.A second requirement for molecules acting as electronic building blocks is the tunability of their charge state which would allow us to control the molecular transconductance [17]. To change the charge state one needs an external handle (i.e. a potential) that shifts the alignment of molecular levels around the leads' chemical potential. A successful approach consists in using the local electric field induced by the proximity of a metal STM tip to induce such shift [20][21][22][23][24]. In this case, the critical field f...

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On surfaces with strong spin-orbit coupling, backscattering is forbidden since it requires flipping of the spin of the electron. It has been proposed that the forbidden scattering channels in such systems can be activated if time reversal symmetry is locally broken, for example, by a magnetic scattering center. Scanning tunneling spectroscopic maps of quasiparticle interference patterns around a single magnetic MnPc molecule on a Bi(110) surface reveal only spin-conserving scattering events. Simulations based on the Green's functions approach confirm that the charge-density interference patterns are unaffected by the magnetic state of the impurity. A fingerprint of backscattering processes appears, however, in the magnetization patterns, suggesting that only spin-polarized measurements can access this information.