Spectroscopically Elucidating the Local Proton-Coupled Electron Transfer Loop from Amino to Nitro Groups via the Au...

Proton-coupled electron transfer (PCET) has been significant in understanding the reactions in solution. In a solid-gas interface, it remains a challenge to identify electron transfer or proton transfer intermediates. Here, in a Au/N₂ interface, we regulated and characterized the PCET from p-aminothiophenol (PATP) to p-nitrothiophenol (PNTP) in the plasmon-mediated conversion to p,p'-dimercaptoazobenzene by variable-temperature surface-enhanced Raman spectroscopy. The Raman bands of PATP and PNTP characteristically blue shifted and red shifted as the laser wavelength- and power density-regulated PCET from PATP to PNTP, respectively. These characteristic Raman band shifts were well reproduced by the density functional theoretical simulations of positively charged PATP and negatively charged PNTP, which explicitly evidenced the electron transfer intermediates of PATP or PNTP on the Au surface. PCET did not occur in the temperature cycle between 100 and 370 K without laser illumination. These results demonstrated a characteristic local PCET loop composed of electron transfer between PATP/PNTP and Au followed by intermolecular proton transfer between PATP and PNTP and the significance of conducting electron transfer on Au.