Revealing the effects of molecular orientations on the azo-coupling reaction of nitro compounds driven by surface plasmonic resonances.

A recent report on the azo coupling of 4,4’-mercaptoazobenzene-15-crown-5-ether (4NB15C) and 4-nitrothiophenol (4NTP) indicated that the reaction barrier could be reduced greatly with surface plasmonic effects on silver dendritic nanostructures, in aqueous solution. Accordingly, an azo coupling reaction mechanism was proposed based on one or two SERS peaks. Toward a profound understanding of such azo coupling reaction mechanism, however, it is crucial to scrutinize the origin of the full SERS spectrum. Here we construct a molecular model consisting of 4NTP and 4NB15C on an Ag7 cluster that simulates a silver dendritic nanostructure, and investigate the SERS spectra of the azo coupling of these two molecules. We propose five different adsorption sites and 13 different orientations of 4NTP on the Ag7 cluster, and optimize the geometries of the five configurations. With each optimized configuration of 4NTP adsorbed on Ag7, we further consider the azo coupling product with a 4NB15C molecule and simulate the corresponding Raman spectra. Compared with the measured Raman spectra and our model analysis, we conclude that the azo coupling reaction depends decisively on a parallel molecular orientation of the adsorbed 4NTP relative to the facets of Ag¬7, which orientation further directs the subsequently reacted final product of 4NB15C-4NTP.