Similarity and Dissimilarity in Surface-Enhanced Raman Scattering of 4-Aminobenzenethiol, 4,4′-Dimercaptoazobenzene, and 4,4′-Dimercaptohydrazobenzene on Ag

There has been a surge of interest in the surface-enhanced Raman scattering (SERS) of 4-aminobenzenethiol (4-ABT) and 4,4′-dimercaptoazobenzene (4,4′-DMAB), since 4,4′-DMAB might be capable of being produced from 4-ABT via a surface-induced photoreaction. A problem was initially encountered in the interpretation of the SERS spectrum of 4-ABT due to difficulty in correlating several peaks therein with the normal Raman peaks, as the SERS spectral pattern of 4-ABT looked similar to that of 4,4′-DMAB. To clarify the issue, we have carefully examined the similarities and dissimilarities in the SERS of 4-ABT and 4,4′-DMAB, along with the SERS spectrum of their analogue molecule 4,4′-dimercaptohydrazobenzene (4,4′-DMHAB). Under ambient conditions, the SERS spectra of 4-ABT, 4,4′-DMAB, and 4,4′-DMHAB on Ag looked in fact comparable to one another, but the excitation wavelength dependence of the peak intensities dictated that the similarity in the SERS spectral patterns could not guarantee the photoconversion of the three molecules. On the other hand, the spectral dissimilarity was evidenced not only from the SERS spectra taken after treating the probing substrates with a borohydride solution but also from the potential-dependent SERS spectra. An electrochemical reaction did not take place for 4-ABT and 4,4′-DMHAB on Ag either by placing the substrates in contact with a borohydride solution or by lowering the electrode potential down to −1.2 V versus a saturated Ag/AgCl electrode. Conversely, however, 4,4′-DMAB on Ag was converted to 4-ABT not only by contact with a 100 mM borohydride but also by lowering the potential below −1.0 V. The reverse reaction from 4-ABT on Ag to 4,4′-DMAB appeared to be insignificant electrochemically as well as photochemically. All of these results clearly support our previous proposition that the b 2 -type bands appearing in the SERS of 4-ABT must be attributable to the chemical enhancement of 4-ABT itself.