Adsorption-Controlled Reversible Azo Coupling of 4′-Aminobiphenyl-4-Carbonitrile on Plasmonic Metal Nanoparticles

초록

Plasmon-induced azo coupling under surface-enhanced Raman scattering (SERS) conditions is often interpreted as evidence of irreversible photochemical transformation at metal-molecule interfaces. However, increasing experimental evidence suggests that the manifestation of azo-related vibrational features can be strongly influenced by the adsorption geometry and local surface conditions. Here, we investigate the plasmon-induced surface chemistries of 4 '-aminobiphenyl-4-carbonitrile on Ag and Au nanoparticles by combining time-resolved SERS, systematic pH modulation, and density functional theory calculations. On Ag surfaces, characteristic azo bands are observed under plasmon excitation, but they can be reversibly switched on and off via pH-controlled changes in the adsorption state. Therefore, azo coupling proceeds via an adsorption-controlled surface equilibrium rather than irreversible bond formation. Conversely, no azo coupling is observed on the Au nanoparticles under comparable acidic, neutral, and basic conditions, which is attributed to stronger metal-molecule binding that restricts rearrangement and access to reactive surface states. These findings clarify that azo-related spectral features observed in SERS spectra do not necessarily signify permanent chemical conversion but may instead reflect reversible, adsorption-mediated surface states with accessibilities that depend on molecular protonation, adsorption geometry, and metal identity.

키워드