Plasmon Induced Decarboxylation of Aromatic Carboxylic Acids on Bipolar Electrodeposited Au- and Ag-Film Monitored by SERS
Authors: Yeasmin Humaira 
Published: September 2023

Here, we describe the decarboxylation of 4-mercaptobenzoic acid and its isomers using SERS as an in-situ probe of chemical reactions near metal nanostructures to understand the reaction path as well as confirm new species of reacting systems. To avoid the shortcomings of conventional techniques of nanostructure synthesis, such as hydrothermal processes1, chemical reduction , electrochemical synthesis2 which involve the use of surfactants, reducing agents or toxic organic3
,
4 solvents, that limit their applications in terms of the economic and environmental considerations, we have designed and developed a bipolar electrodeposited gold and silver thin film5. We have used this as the platform for the decarboxylation of mercaptobenzoic acid based on the concept of surface plasmon resonance. The effect of laser power, laser wavelength, and the exposure time was monitored as well as the effect of solution pH was observed. Reaction occurred particularly in alkaline pH due to the formation of deprotonated mercaptobenzoic acid and was further confirmed by the carboxylic acid group substitution by its methyl ester, where the ester form retained, and no decarboxylation was found. The behavior of other isomers (2-MBA and 3-MBA) along with 4-MBA was observed. 2-MBA gives the fastest decarboxylation compared to 3-MBA and 4-MBA even without alkaline condition, where the latter two isomers require an alkaline condition to form the product benzenethiol (BT). This indicates that carboxylate anion (– COO-) is closer to the nanostructured surface in the case of 2-MBA, which we assume is the possible reason for the higher rate of decarboxylation in this isomer. We also performed experiments with the ester form of 2- and 3-MBA and found similar behavior as 4-MBA-Me on the nanostructured Au surface which indicates that carboxylate anion is the crucial group for decarboxylation reaction. In support of the formation of resonance-stabilized carboxyl radical in mercaptobenzoic acid, we carried out experiments with linear chain thiocarboxylic acid (3-mercaptopropionic acid, 6-mercaptohexanoic acid). No decarboxylation was observed in an alkaline medium indicating that benzene ring is necessary to form a stable reaction intermediate.

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