Force involved in the in statu nascendi metalation of a single dye

K. Rothe, N. Néel, M.-L. Bocquet, J. Kröger, J. Am. Chem. Soc. (accepted for publication on 22nd March 2022)

Since its invention by Gerd Binnig and coworkers (IBM Zurich, 1986) the atomic force microscope has also turned into a powerful tool for experimental quantum chemistry at surfaces. The tip of the microscope can be controlled to an extent that enables the manipulation of matter at the atomic scale. In particular, chemical reactions between single atoms or molecules can be induced.

Working for his master thesis, Karl Rothe (Experimental Physics 1) has recently achieved to measure the attractive force that is relevant for an important chemical reaction, the metalation of a single dye. In his experiments, he has initiated a chemical reaction between a single silver atom and the dye molecule adsorbed on a crystalline silver surface. How did he proceed?

Phthalocyanine molecules are macrocycles of benzopyrrole moieties (Fig. 1a), which in the metalfree case coordinate two hydrogen atoms in the center of the macrocycle.  Prior to metalation, these atoms must be removed. According to a known protocol the first author of the studies has locally injected a tunneling current and abstracted the hydrogen atoms in a stepwise manner and imaged the products with high spatial resolution (Fig. 1b). The spatial resolution is achieved with an atomic force microscope operating in the non-contact frequency modulation mode at constant tip height. Moreover, a particularly sharp tip resulting from the termination with a single CO molecule is used (Fig. 1c). The distance between CO and the dye falls into the Pauli repulsion range and, therefore, is comparable with bonding distances. A single silver atom can now be added to the dehydrogenated phthalocyanine molecule. To this end, a microscope tip terminated with a single silver atom approaches the molecule with subpicometer control of the separation and transfers the metal atom without additional external stimulus.

Figure 1: (a) Illustration of the starting reactant (2H-Pc) and the successive products (H-P, Pc) in the single-molecule dehydrogenation reaction. (b) Atomic force microscope images of 2H-Pc, H-Pc and Pc acquired with a CO-terminated tip in the Pauli repulsion range of distances. (c) Illustration of a CO-terminated tip.

In order to measure the force involved in the in statu nascendi metalation the change in the resonance frequency of the force probe is measured simultaneously with the tip approach to the molecule. The force probe is a piezoceramic tuning fork whose free prong carries the tip (Fig. 2a).  The change of the resonance frequency of the oscillating free prong can be understood in a simple mechanical picture that considers a harmonic oscillator connected to two springs with spring constants k0 and ksp (Fig. 2b). In the course of approaching the surface, the interaction of the tip and the molecule (represented by ksp) varies, and so does the resonance frequency. The metalation of the single dye is signaled by the abrupt decrease of the resonance frequency (Fig. 2c). These data can be further used to extract the associated force and energy involved in the metalation process.  Karl Rothe obtained a lower bound to the metalation force (energy) of approximately 450 pN (275 meV).

Figure 2: (a) Sketch of the qPlus sensor. The piezoceramic tuning fork (I) is mounted to a piezoelectric actuator (II). A sinusoidal AC voltage Aextsin(𝜔0t) excites oscillations of the free prong of the tuning fork. In turn, the oscillations induce piezoelectric voltages Vac in the sensor, which can be accessed my metallic contacts (red/blue). The metallic tip is connected to the transimpedance amplifier that converts the tunneling current into a magnified voltage. The bias voltage is applied to the sample. (b) Mechanical model of the force sensor, which is represented by a harmonic oscillator of mass m connected to two springs with spring constants k0 and ksp. (c) Change in the resonance frequency Δf as a function of the tip displacement z (approach from left to right) for the clean surface (red) and the Pc molecule (blue). The bottom panel shows the associated force F (blue) and energy E (gray) involved in the metalation process.

The accompanying and supporting calculations have been performed by Prof. Marie-Laure Bocquet (Université Sorbonne, Paris) within density functional theory and hint at the relevant underlying mechanisms for the tip-induced single-molecule metalation. It is crucial for the simulations to take the entire junction – tip and dye adsorbed on a metal substrate – into account. Only the worldwide best theory groups are able to perform such model calculations. Figure 3 shows a sequence of supercells, which represent the modeled metalation. Tip and sample exhibit a small separation that is required for the atom transfer, which takes place at image 4 with an energy gain of 670 meV; that is, the transfer occurs spontaneously without the necessity of surmounting an energy barrier, in agreement with the experiments. The absence of an activation barrier can be ratinalized in terms of a concerted effect, where in the metalation process the silver atom breaks bonds to the tip and simultaneously forms bonds with the molecule.

Figure 3: Nudged-elastic-band energy profile using 8 interpolated images for the metalation process in the contact configurations connecting the initial state (0) with zero energy and the final state (9) with an energy gain of 100 meV. The encapsulation takes place at image 4 (−670 meV) and proceeds via a concerted effect.

The presented work paves the way for future studies of chemical reactions with atomic resolution at the single-molecule level. Reactive centers in catalytic processes, for instance, may be accessible and analyzed.  The paper will appear in the flagship journal of the American Chemical Society.