Surface-Enhanced Raman Scattering Characteristics of 4-Aminobenzenethiol Derivatives Adsorbed on Silver

The most unusual feature in the surface-enhanced Raman scattering (SERS) of 4-aminobenzenethiol (4-ABT) is the appearance of b 2 -type bands in the region of 1100–1500 cm –1, which, in recent years, has given rise to a dispute over their origin. In order to resolve this dispute, we have examined the SERS spectral pattern of 4-ABT derivatives adsorbed on Ag substrates. At first, we found in the SERS spectra of 4-(diethylamino)benzenethiol and 4-(benzylideneamino)benzenethiol that no photoreaction had taken place at all, but the b 2 -type bands appeared distinctly as is usual in the SERS of 4-ABT. The b 2 -type bands were also observed in the SERS spectra of other 4-ABT derivatives while, to our surprise, no such b 2 -type bands were observed at all in the SERS spectra of para-substituted (but not amino) benzenethiol derivatives. The appearance of the b 2 -type bands in the SERS spectra seemed thus a common feature of 4-ABT derivatives. The difference between 4-ABT derivatives and para-substituted (but not amino) benzenethiol derivatives was supposedly associated with the LUMO–HOMO energy gap. The energy gap computed for 4-ABT derivatives (∼300 nm) was consonant with the interpretation that the appearance of the b 2 -type bands in the SERS spectrum of 4-ABT is associated with an intensity borrowing from an intense π → π* molecular transition ( 1 A 1 → 1 B 2 ) at 300 nm.