Studying the dynamic behaviour of porphyrins as prototype functional molecules by scanning tunnelling microscopy close to room temperature

Abstract

Scanning tunnelling microscopy (STM) enables us to directly observe the dynamic behaviour of organic molecules on surfaces. While imaging atoms and molecules using STM is certainly fascinating by itself, corresponding temperature-dependent measurements allow for the quantitative determination of the energetics and kinetics of the underlying molecular surface processes. Herein, we review recent advances in the STM investigation of the dynamic behaviour of adsorbed porphyrins at and close to room temperature.Three different case studies are discussed, providing insight into the dynamics of diffusion, rotation, reaction, and molecular switching at surfaces, based on isothermal STM measurements. The reviewed examples demonstrate that variable temperature STM can be a suitable tool to directly monitor the dynamic behaviour of individual adsorbed molecules, at and close to room temperature. Free base porphyrins on Cu(111) proved to be particularly suitable for these studies due to the strong bonding interaction between the iminic nitrogen atoms in the porphyrin macrocycle and the Cu substrate atoms. As a consequence, the corresponding activation energies for surface diffusion, self-metalation reaction and conformational switching are of a magnitude that allows for monitoring the processes at and around room temperature, in contrast to most previous studies, which were performed at cryogenic temperatures. The kinetic analysis of the surface diffusion and self-metalation was performed using an Arrhenius approach, yielding the corresponding activation energies and preexponential factors. In contrast, the conformational switching process was analysed in the framework of transition state theory, based on the Eyring equation. This approach provides a more detailed insight into interpretable thermodynamic potentials, i.e., the enthalpic and entropic contributions to the activation barrier. The analysis shows that at room temperature the adsorption and switching behaviour of the investigated free base porphyrin on Cu(111) is dominated by entropic effects. Since the entropic energy contribution vanishes at low temperatures, the importance of experiments conducted at temperatures close to room temperature is emphasized.

Dynamics and thermal stability of surface-confined metal–organic chains

“…In previous studies, the surface diffusion of molecules and supramolecules has been explored experimentally [11,[37][38][39][40][41][42][43][44][45][46] and theoretically [46][47][48][49]. In our study we distinguish three thermal regimes.…”

Self-assembly and coverage dependent thermally induced conformational changes of Ni(ii)-meso-tetrakis (4-tert-butylphenyl) benzoporphyrin on Cu(111)

“…In this respect, scanning tunnelling microscopy (STM) is a powerful tool to investigate such systems in real space. 7,[9][10][11] One important group of such organic molecules are porphyrins, which have numerous technological applications due to their versatile functionalities and distinct chemical and physical properties. 12,13 During the past few years, extensive attention has been paid to porphyrins self-assembled on metal surfaces in order to gain more information about their electronic properties, inherent conformational flexibility and reactivity.…”

Orthogonal Insertion of Lanthanide and Transition‐Metal Atoms in Metal–Organic Networks on Surfaces

“…[31] Thus,our procedure based on at wo-step consecutive exposure of 2H-TPCN arrays to Gd and Co provides an example of the fabrication of d-f bimetallic networks with distinct metal centers embedded in specific environments,p rovided that metalation of the porphyrin macrocycle is accomplished at the appropriate temperature (300 K). In addition, this procedure has the further advantage of using as ingle molecular module (2H-TPCN) to create such bimetallic arrays,without the necessity of synthesizing ex situ metalated compounds.…”