Charge Recognition in Ru(bpy)₃²⁺ on a Metal Surface via Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy

Ruthenium-based dye molecules hold great promise in the photochemistry and electrochemistry research fields, ranging from photocatalysis and luminescence to thermal imaging and photodynamic therapy. However, despite this exciting prospect, thermal sublimation and surface characterization of these molecules under ultrahigh vacuum (UHV) are rare. Here, we achieved successful thermal sublimation of tris(2,2′-bipyridine)ruthenium(II) [Ru(bpy)32+] onto the Cu(100) surface and investigated the surface decoration using scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS). The experimental findings are complemented by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Integrating experimental and theoretical analyses facilitates the chemical characterization of Ru(bpy)32+ molecules at the single-molecule level, encompassing their charged states and surface aggregation. Our analysis indicates no quenching of charge or significant charge transfer from the Cu surface to the molecule's core. These results here provide important chemical insight into ruthenium-based dye molecules essential for photochemistry and electrochemistry-related applications.