Orbital-Resolved Imaging of the Adsorbed State of Pyridine on GaP(110) Identifies Sites Susceptible to Nucleophilic Attack

Artificial photosynthesis by photoelectrocatalytic CO₂ reduction is dependent, as is natural photosynthesis, on interfacial electron transfer to couple light excitation energy to reaction centers. For heterogeneous systems, in the context of frontier orbital theory artificial reaction centers are defined through the interactions of filled and empty orbitals within a few electronvolts of the Fermi energy of the adsorbate complex. Here we report a scanning tunneling microscopy (STM) and density functional theory investigation of the orbital-resolved adsorption state defining the dative bonding interaction between a III-V semiconductor surface [GaP(110)] and a N-containing heteroaromatic (pyridine). This system was selected for its relevance to photoelectrocatalysis utilizing heteroaromatic cocatalysts, which has been reported to yield highly selective CO₂ reduction to fuels. By examining the distribution of unoccupied molecular orbitals, we show that STM images can be used to positively identify the sites on pyridine susceptible to nucleophilic attack, consistent with frontier orbital theory. This indicates that STM can be used to explore the local reaction centers of adsorbed ambidentate electrophiles and nucleophiles relevant to artificial photosynthesis, and more broadly to generate critical mechanistic information for various heterogeneous acid-base reactions.