Geometrical Isomerism Directed Electrochemical Sensing

Abstract

We report the independent role of isomerism of secondary sphere substituents over their nature, a factor often overlooked in molecular electrocatalysis pertaining to electrochemical sensing, by establishing that isomerism redefines the electronic structure at the catalytic reaction center via geometrical factors. UV–vis spectroscopy and X-ray photoelectron spectroscopy suggest that a substituent’s isomerism in molecular catalysts conjoins molecular planarity and catalytic activation through competing field effects and resonance effects. As a classical example, we demonstrate the influence of isomerism of the −NO2 substituents for the electrocatalytic multi electron oxidation of As­(III), a potentially important electrochemical pathway for water remediation and arsenic detection. The isomerism dependent oxidative activation of catalytic center leads to a nonprecious molecular catalyst capable for direct As­(III) oxidation with an experimental detection limit close to WHO guidelines. This work opens up an unusual approach in analytical chemistry for developing various sensing platforms for challenging chemical and electrochemical reactions.

Isomerism‐Activity Relation in Molecular Electrocatalysis: A Perspective

“…This suggests that for the direct As (III) oxidation, isomerism of functionality does not change the overall reaction mechanism but it can influence the rate-limiting step. Since direct As (III) oxidation on these molecular catalysts follow an EC mechanism wherein Co (III) is the active intermediate, the higher activity of β-isomer over its α counterpart originates from the isomerism induced oxidative activation of catalytic metal centre in the former as explained earlier [22]. The β isomer's sensor response towards direct As (III) oxidation demonstrated good reproducibility and high cyclic stability and FTIR spectral response of the β isomer electrode showed a durable sensing architecture, Figure 4.…”

“…When α and β -NO 2 isomeric molecules were employed for the direct oxidation of As (III), the latter has been found to outperform the former in terms of activity and stability. Hydrodynamic voltammetry revealed that the overall number of electrons transferred is 2 in both the cases, however, the catalytic rate constants were significantly amplified in the case of β-NO 2 isomer compared to the α-NO 2 isomer [22]. Compared to the 'α' isomer, the 'β' isomer demonstrated high sensitivity and~170 mV reduction (at 0.15 mA/cm 2 ) in driving force towards the multi electron As (III) oxidation ( Figure 3); thus making it a viable candidate for water remediation and arsenic detection with detection limit close to the WHO guidelines [22].…”

“…Hydrodynamic voltammetry revealed that the overall number of electrons transferred is 2 in both the cases, however, the catalytic rate constants were significantly amplified in the case of β-NO 2 isomer compared to the α-NO 2 isomer [22]. Compared to the 'α' isomer, the 'β' isomer demonstrated high sensitivity and~170 mV reduction (at 0.15 mA/cm 2 ) in driving force towards the multi electron As (III) oxidation ( Figure 3); thus making it a viable candidate for water remediation and arsenic detection with detection limit close to the WHO guidelines [22]. This suggests that for the direct As (III) oxidation, isomerism of functionality does not change the overall reaction mechanism but it can influence the rate-limiting step.…”

“…We note that this investigation was specific to regioisomers of substituted metal phthalocyanines (MPcs) when regioisomerism of substituents are effected at the α and β positions on the N 4 macrocycle of the molecule (Figure 1). We have shown that isomerism of the functionality impacts the electronic arrangement and electrocatalytic activity and hence makes a distinct contribution to molecular electrocatalysis over the contribution of its chemical identity [21,22].…”

“…The existing accounts fail to attach any significance to the latter aspect i. e., the role of isomerism of functionality in molecular electrocatalysis and to the best of our knowledge, no literature has been found that paid attention to this isomerism-activity relation. Our group recently shed some light in this direction where a distinction is made between substituent's chemical identity and regioisomerism towards elec-trocatalysis [21,22]. We note that this investigation was specific to regioisomers of substituted metal phthalocyanines (MPcs) when regioisomerism of substituents are effected at the α and β positions on the N 4 macrocycle of the molecule (Figure 1).…”