Exploring C-C Coupling Conditions of Metalloporphyrins on Au(Ⅲ) Using Pulsed Power Application
Authors: Takao Nene , Yamashita Daisuke , Wang Douyan , Namihira Takao , Yoshimoto Soichiro 
Published: December 2024

Covalent Organic Frameworks (COFs) are promising materials for applications in electrocatalysts, sensors, and gas storage owing to their advantages such as large surface area, high crystallinity, and high thermal and chemical stability. COFs can be synthesized via the Ullmann condensation reaction, which is a C-C coupling method. However, the synthesis typically requires long reaction times and high-temperature and high-pressure conditions, which limits the practical application of COFs.
In this study, we utilized the pulsed power method to reduce the reaction time and simplify the synthesis conditions while generating various energy states as a reaction field. Our investigation concentrates on intermolecular C-C coupling conditions by applying high-current pulse rapid heating temperature control to Co/Fe porphyrin thin films deposited on gold substrates. We evaluated the conditions for preparing porphyrin films and the impact of high-current pulsed power on films formed on the deposited gold using atomic force microscopy (AFM) and cyclic voltammetry (CV). Deposited gold was prepared by heating a gold wire on a clean mica substrate at room temperature in vacuum and then annealing at 600 °C for 6 h in an electric furnace. The thin films were prepared by spin-coated with 5.0 μM benzene solutions containing either tetraiodophenylporphyrin cobalt(Ⅱ) or iron(Ⅲ) chloride (CoTIPP or FeTIPPCl). Subsequently, the thin-film substrate was dried under vacuum. Atomic force microscopy (AFM) observations were performed under ambient conditions. The electrochemical properties were characterized by CV in 0.1 M H2SO4 before and after the application of high current pulses (35–300 V). A relatively smooth surface was observed on the unmodified deposited gold before the application of the pulsed voltage, indicating that atomically flat terraces were exposed under the experimental conditions. When a pulsed voltage was applied to the Au substrate, island structures with heights of approximately 3.5 nm were generated along the Au atomic step lines. After confirming the presence of the Au surface, CoTIPP molecules were deposited on the Au surface. The Au surface was entirely covered with CoTIPP molecules. When a pulsed voltage (50 V) was applied to the CoTIPP adlayer on the Au substrate, the surface morphology changed drastically. The AFM image reveals the formation of several large domains and aggregated islands with a width of approximately 50 nm and a height of 1.2 ~ 1.7 nm at domain boundaries. In addition, several high protrusions with a height of approximately 3.3 nm were found in the domain boundaries. This morphological change in the AFM images suggests that pulse application and Ullmann condensation reaction were induced on the Au substrate, and the C–C coupling occurred by the desorption of halogen groups at the ends of each porphyrin molecule. The CV profiles of the substrates are shown in Fig. 1. In the CV profile, spikes were observed after the pulse application in the electric double-layer region. This indicates the specific adsorption of hydrogen sulfate ions (HSO4–) on the Au(111) electrode surface, suggesting that the porphyrin molecules that were sparsely deposited on the terrace became linked domains by the pulse application because vacancies (Au bare sites) can be formed in the COFs of CoTIPP molecules. Further high-resolution analysis using scanning tunneling microscopy (STM) is currently underway.

Figure 1