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Infrared Spectroelectrochemical Studies of Redox-Active Self Assembled Monolayers: Structure and Kinetics

Date

Journal Title

Journal ISSN

Volume Title

Publisher

ORCID

0009-0009-3400-3027

Type

Thesis

Degree Level

Doctoral

Abstract

Long-range bridge-mediated electron transfer proceeding through outer-sphere pathways attracts scientific interest beyond fundamental studies due to its relevance in technological systems like molecular electronics and biosensors. Deep knowledge of the structure and dynamics of these molecular interfaces and the kinetics of the electron transfer processes are critical to improving the performance of such technological systems. Interfacial charge transfer between the electrode and redox molecules can be manipulated as an outer sphere electron transfer process by employing organic molecules as bridging moieties between the electron donor and acceptor. Electrode surfaces can be made suitable for charge transfer studies via the self assembled monolayers (SAMs) of redox-substituted alkanethiols. Alternatively, redox species can be covalently tethered to the terminals of preformed monolayers of alkane chains. Electroactive surfaces prepared via both methodologies are explored for studying heterogeneous electron transfer (ET) processes using conventional electrochemical techniques such as cyclic voltammetry and chronocoulometry. Butler-Volmer (BV) formalism and Marcus-Hush-Chidsey (MHC) theory are some of the widely accepted models to predict the kinetic parameters of electron transfer processes. In-situ surface characterization techniques such as surface-enhanced infrared absorption spectroscopy (SEIRAS) offer the potential to provide deeper insights into molecular processes occurring in organized systems during electron transfer. Time-resolved SEIRAS technique is an advanced method capable of correlating structural changes in both the redox-active moiety and the scaffold supporting the redox centre preceding/during/following the electron transfer process. This thesis reports a combination of time-resolved SEIRAS with conventional electrochemistry techniques to study the electron transfer process across different electroactive layers. The time-resolved SEIRAS technique is applied here to follow the molecular restructuring of alkane-bridging moieties during the electron transfer process in ferrocene-SAM systems. The behaviour of surrounding SAM structures to the redox moieties during the electron transfer process is also explored using deuterated alkanethiols as diluents. An amide-coupling reaction is explored to link electroactive moieties to prefabricated alkanethiol SAM terminals. Studying the reaction mechanism of the amide-coupling process offers an opportunity to improve the reaction efficiency. Therefore, the potential of electrochemical-SEIRAS has been leveraged to monitor the amide-coupling process on the monolayers under various reaction conditions. SEIRAS analysis identified that the reaction intermediates change their rate of formation under the electrode potential control, which establishes proof for potential-dependent reaction pathways for amide-coupling reactions. Another redox species studied in this body of research is 2,2,6,6- tetramethylpiperidine-1-oxyl (TEMPO ̇ ), a prominent organic free radical used as a catalyst in various industrial-scale processes. Electron transfer studies of TEMPO ̇ tethered to various lengths of alkanethiols are reported in this thesis using conventional electrochemical techniques. Time-resolved SEIRAS studies of TEMPO ̇ -alkanethiol monolayers for structural and kinetic analysis are reported here for the first time. The SEIRAS analysis provides a molecular model showing the conformational change of TEMPO ̇ moieties along with structural reorientation of the alkane chain adlayers during the electron transfer process.

Description

Keywords

Electrochemical SEIRAS, redox- active monolayers, ferrocene, TEMPO, EDC coupling, chronocoulometry

Citation

Degree

Doctor of Philosophy (Ph.D.)

Department

Chemistry

Program

Chemistry

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DOI

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