Adsorbed structures of 4,4′-bipyridine on Cu(111) in acid studied by STM and IR

The adsorption of 4,4′-bipyridine (BiPy) on Cu(111) has been investigated in 0.1 M HClO₄ by cyclic voltammetry, electrochemical scanning tunneling microscopy (STM), and surface-enhanced infrared adsorption spectroscopy (SEIRAS). Cyclic voltammetry showed the double layer region extending from -0.2 to 0.26 V and a pair of redox waves superposing on hydrogen evolution wave at more negative potentials. Diprotonated BiPy, BiPyH2 ²⁺, is adsorbed flat on the Cu(111) (1 × 1) surface and forms a well-ordered monolayer with a (3 × 4) symmetry in the double-layer potential region. At more negative potential, BiPyH₂²⁺ is reduced to its monocation radical, BiPyH₂²⁺, and forms another well-ordered structure in which the radicals are stacked in molecular rows with a face-to-face self-dimer as the building unit. The SEIRA spectra of both BiPyH₂²⁺ and BiPy Ha₂^.+ are dominated by gerade modes which should be IR-inactive for the centrosymmetric species. The breakdown of the selection rule of IR absorption is ascribed to the vibronic coupling associated with charge transfer between BiPyH ₂⁺ and the surface and between the radicals.