Isomerism-Activity Relation in Molecular Electrocatalysis and Molecular Charge Storage

Abstract

Electrocatalysts play major roles in energy conversion/storage, electrochemical sensing, environmental remediation, green chemistry, etc.; which in turn are responsible for the exhaustive attempts in understanding their structure-activity relations. Among the class of various electrocatalysts, the N4 macrocycle based molecular electrocatalysts such as metal phthalocyanines and porphyrins stand out as potential non-precious electrocatalysts for an assortment of electrochemical applications due to their tunable optoelectronic properties and chemical and thermal stabilities. While targeting the surface-to-volume ratio, exposing reactive crystal planes, interfacial modifications etc., are time-tested considerations for activating conventional electrocatalysts; the catalytic properties of molecular electrocatalysts are primarily tuned by the nature of the secondary sphere (N4 macrocycle) functionality which in turn is known to modulate the electronic arrangement at the catalytic centre. However, far too little attention has been paid to the fact that this electronic arrangement is not only a function of their chemical identity across the spectrochemical series but also can be a strong function of their isomerism. In the context of charge storage, since the energy density of supercapacitors scales-up linearly with surface-confined charge density, the pursuit towards high energy and high-power supercapacitors are conventionally accomplished in solid systems by maximizing their surface-to-volume ratio. In molecular systems, a handful of reports suggest that the nature of the secondary sphere functionality can tune their supercapacitive charge storage, however, a clear knowledge gap exists in comprehending what controls their structure- charge storage relation. The primary aim of this thesis is to draw a distinction between a substituent’s chemical identity and isomerism towards electrocatalysis and molecular charge storage when regioisomerism of electron-withdrawing/electron-donating substituents is conferred at the ‘α’ and ‘β’ positions of the N4 macrocycle in metal-phthalocyanines (MPcs). The existing accounts fail to make any distinction between the chemical identity of groups and their isomerism in molecular electrocatalysis and charge storage, etc., and to the best of our knowledge no research has been found that surveyed and understood isomerism-catalytic activity relation and isomerism-charge storage relation in molecular systems. Therefore, the major focus of this thesis is on studying how the geometric and electronic factors of substituents as a result of their isomerism at the secondary sphere influence molecular charge storage and molecular electrocatalysis of challenging electrochemical transformations. This study brings to light how the isomerism of secondary sphere functionality can be tapped to design efficient interfaces for energy conversion and electrochemical sensing and molecular platforms for charge storage devices.