Effects of Electrochemical Tailoring of Monolayers on a Catalytic Redox Entity: An ON–OFF Phenomenon Regulated by the Surrounding Medium

Abstract

Here we report an ON–OFF effect on electrocatalytic amplification when the molecules are connected to each other on an electrode surface. When the molecular connection is achieved, the redox potential of the catalytic redox entity is significantly upshifted with concurrent experience of a more electron-withdrawing atmosphere, and the electron transfer to and from the catalytic center is accelerated, rendering the molecules with strong electroreducing character; however, the overall outcome is dictated by the surface charge. Outer sphere redox probes and X-ray photoelectron spectroscopy evidently revealed that a positive surface charge is preserved in acidic and neutral media after molecular connection which is in parallel with the corresponding electrocatalytic amplifications toward oxygen reduction reaction (ORR), suggesting the electrodics after molecular connection is extremely pH dependent. Surface charge present after molecular connection is significantly neutralized by the abundant hydroxyl groups in alkaline media, making the central metal ion’s atmosphere less electron deficient and downshifting its redox potential (compared to the case in acidic and neutral media), thereby reducing the disparity in electroactivity before and after molecular connection. This study underlines that after molecular connection surface charge is the key as it can turn on and turn off the redox entity responsible for electrochemical amplifications. The solvent dependence and redox potential upshift outlined here for monolayer electrodes are nullified during bulk polymerization, indicating the circumstances leading to molecular connection are different from bulk polymerization. Since the surface charge can be modulated by connecting the molecules and tuning the surrounding media, the proposed strategy brings forward a way to tune the interfacial activity and is expected to have implications in electrocatalysis, selective sensing, ion screening, and so on.

Unprecedented Isomerism–Activity Relation in Molecular Electrocatalysis

“…As noted earlier, unsubstituted CoPc was synthesized and characterized as per the literature (please refer to the Experimental section for more details and Figure S7, Supporting Information). 19 In order to prove that this isomerism-dependent electrochemical activation is the pristine behavior of molecular electrocatalysts, these molecules were adsorbed as a monolayer on an inert glassy carbon electrode (see the Experimental section for more details). The adsorption step was monitored by the in situ quartz crystal microbalance (QCM) technique (Figure S8, Supporting Information), which indicated surface coverage values in the order of 10 −11 mol/cm 2 (inset of Figure S8, Supporting Information).…”

“…It is well-known that electron-withdrawing substituents at the N 4 macrocycle of metal Pcs can activate the catalytic central metal ion toward electrochemical reactions. − However, it is not yet understood how the regioisomerism of the same functionality influences the electrocatalytic pathways. In order to shed light on this in the context of electron-withdrawing substituents, we have synthesized regioisomers of tetra-nitro cobalt Pc (TNCoPc) as per the literature by shifting the position of the −NO 2 group from the alpha (α-TNCoPc) to beta (β-TNCoPc) position, Figure a.…”

Metallophthalocyanine-nanofibre-based electrodes for electrochemical sensing of biomolecules

“…Apart from being thermally and chemically stable, they also possess rich redox chemistry, which enables them to be useful candidates for electrochemical applications including electrocatalysis, supercapacitance, fuel cells, etc. [2,[6][7][8][9]. MPcs containing transition metals are mostly planar and exhibit intense absorption ranging from ultraviolet to near-infrared region and are thus useful in sensitization of large-bandgap semiconductors in solar photovoltaics [5].…”

“…Substitution of MPcs with active functional moieties also enhances certain characteristics of MPcs, like better solubility, conductivity, electron-hole donor properties, etc. [3,[6][7][8][9]. Applications involving native unsubstituted MPcs are rare.…”

Geometrical Isomerism Directed Electrochemical Sensing

“…Recently, we have shown how isomerism of substituents influences various electrochemical oxidation reactions . To elaborate this further, particularly with respect to electrochemical sensing of substrates, we have synthesized unsubstituted cobalt phthalocyanine (CoPc) and geometrical isomers of tetranitro cobalt phthalocyanine (TNCoPc) as per the literature , by substituting the −NO 2 group at α (α-TNCoPc) and β (β-TNCoPc) positions (please refer to the Supporting Information, Experimental Section), as shown in Figure a. These molecules were characterized with the help of various physico chemical techniques.…”