Surface-enhanced Raman scattering of 4,4′-dimercaptoazobenzene trapped in Au nanogaps

Abstract

The surface-enhanced Raman scattering (SERS) of 4,4′-dimercaptoazobenzene (4,4′-DMAB), an alpha, omega-dithiol possessing also an azo moiety, has seen a surge of interest recently, since 4,4′-DMAB might be able to form from 4-aminobenzenethiol (4-ABT) via a surface-induced photoreaction. An understanding of the intrinsic SERS characteristics of 4,4′-DMAB is thus very important to evaluate the possibility of such a photoreaction. We found in this work that 4,4′-DMAB should adsorb on a flame-annealed Au substrate via one of its two thiol groups such that Au nanoparticles could adsorb further on the pendent thiol group, forming a SERS hot site. The most distinctive feature in the SERS of 4,4′-DMAB was the appearance of a_g bands, which were quite similar to the b₂-type bands occurring in the SERS of 4-ABT. In an electrochemical environment, the a_g bands of 4,4′-DMAB at 1431, 1387, and 1138 cm⁻¹ became weakened at lower potentials, completely disappearing at −1.0 V, but the bands were restored upon increasing the electrode potential, implying that neither electro- nor photo-chemical reaction to break the azo group took place, in agreement with data from a cyclic voltammogram. The appearance and disappearance of these a_g bands are thus concluded to be associated with the charge transfer phenomenon: 4,4′-DMAB must then be one of a unique group of compounds exhibiting chemical enhancement when subjected to a SERS environment.