Photoinduced Surface Catalytic Coupling Reactions of Aminothiophenol Derivatives Investigated by SERS and DFT

p-Aminothiophenol (PATP) is an important probe molecule in surface-enhanced Raman spectroscopy. The unique and strong SERS signals of PATP distinguished from its normal Raman spectrum were considered as a signal of an existing charge transfer mechanism. Recent theoretical and experimental studies demonstrate that PATP undergoes surface catalytic coupling reaction to produce an aromatic azo species p,p'-dimercaptoazobenzene (DMAB), which should be responsible for the abnormal signals in the observed SERS spectra of PATP. In this work, three aminothiophenol derivatives with different substitute position and conjugation degree between the amino group (-NH2) and mercapto group (-SH) were chosen to study the effects of substituent including adsorption orientation effect and conjugation effect on the reactivity of photoinduced surface catalytic coupling reactions. A combined SERS and DFT study indicated that no surface reactions occurred for compound C1 and compound C2, while compound C3 was converted to the corresponding azo species during their SERS measurements. The differences in reactivity of the selected probe molecules were investigated on the basis of proposed photoexcitation and photoreaction mechanisms.