Literary speaking, non-covalent interactions of single molecules accompany our life every minute. Indeed, every breath is chemically expressed in binding of oxygen to iron-porphyrin based molecule present in our blood. This so called heme molecule is an essential part of many hemoproteins including hemoglobin. The interaction of heme with oxygen causes a change of the coordination state of molecule and would result in a spin transition.. Moreover, the controllable ad reversible spin transition may bring revolution in nanoelectronics representing one of the promising ways towards new functional molecular devices, so called molecular switches. From this perspective, the fundamental understanding and possibility to control and characterize such spin phenomena in single molecules are of the great importance.
So far, the spin states of single molecules have been successfully modified by applying external stimuli such as forming a strong chemical bonding, changes in temperature, light, and magnetic or electric fields. What is missing on the list is a weak non-covalent bonding, which plays fundamental role in many biological systems. In general, it is commonly accepted that weak non-covalent interactions generally do not alter significantly the electronic and magnetic structure of molecules.
To challenge this paradigm, we decided to investigate interaction of iron(II) phthalocyanine (FePc) molecule with nitrogen-doped graphene. FePc is a planar molecule showing basic structural features similar to those of heme molecule and thus, it may adopt different spin states depending on the changes in its local environment. Inspired by this behavior in biological systems, we investigated the possibility to vary electronic properties of FePc molecule by its positioning onto graphene. For that purpose, we intentionally implemented single substitutional nitrogen defects into the graphene structure using home-built procedure, see ACS Nano 8, 7318 (2014).
Our scanning probe microscopy (SPM) measurements performed in ultra-high vacuum conditions at cryogenic temperatures reveal distinct behavior of FePc molecule if located on pristine graphene or in the vicinity of nitrogen defects. In both cases, the interaction between FePc molecules and two-dimensional surface is mostly dominated by the non-covalent interactions. As evidence of this, we were able to repeatedly manipulate with single FePc molecule by SPM tip from/to single nitrogen dopant thus enabling the simple reversible molecule positioning. More importantly, we observe a strong variation of the symmetry of frontiers molecular orbitals of FePc molecule when located on/off the nitrogen dopant.
We attributed this variation to modification of magnetic state of the molecule, meaning that molecular orbitals reshuffled their energy levels. As consequence, redistribution of electron with spin up and down also changes. Namely, the molecule on bare graphene adopts magnetic configuration with two unpaired electrons with spin up, while in the vicinity of a nitrogen dopant the distribution of electrons with spin up and down is equal (non-magnetic state).
This results in transition from magnetic to nonmagnetic state of the molecule located on the bare graphene and in vicinity of nitrogen dopant, respectively. The spin transition is accompanied by an electron density redistribution within the molecule. Interestingly, we demonstrated that such change of the electron density accompanying the spin transition can be detected in high-resolution atomic force microscopy (AFM) images [acquired with functionalized carbon monoxide tip.
The possibility to reach spin sensitivity and look into molecular electron densities in the high-resolution AFM imaging is another step in the resolution of scanning probe techniques extending their unique capabilities beyond the observations of chemical structure of molecules , see e.g. J. Phys. Cond. Matt. 29, 343002 (2017).
The control of the spin state and electronic properties of molecules via weak non-covalent interactions opens the doors towards reversible changes of related optical, magnetic and biochemical features exploiting two-dimensional materials as smart tuners.
Thus, one can imagine the applicability of 2D/molecule hybrid systems as advanced optical sensors, molecular magnets or molecular switches for a new generation of electronics. Even more, graphene-based substrates may, through non-covalent interactions, control the biological activity of molecules towards new drugs. If it took our breath away, let notice, it would be due to non-covalent interactions and spin transitions in iron-based porphyrin-based molecules.
written by R. Zbořil, P. Hobza, and P. Jelínek
This work has been published in Nature Communication, see https://www.nature.com/articles/s41467-018-05163-y
B. de la Torre et al., Non-covalent control of spin-state in metal-organic complex by positioning on N-doped graphene Nat. Commun. 9 (2018) 2831(1) - 2831(9).