Ultrafast transient absorption spectroelectrochemistry: femtosecond to nanosecond excited-state relaxation dynamics of the individual components of an anthraquinone redox couple.

Sofia Goia,Michael D Horbury,Matthew A P Turner,Jack M Woolley,Ben R Robinson,Joshua J Tully,Adam Burriss,Alexandra J Borrill,Michael Staniforth,Samuel J Cobb,Julie V Macpherson,Vasilios G Stavros,Nicholas D M Hine

doi:10.1039/d1sc04993c

Sofia Goia, Michael D Horbury + Show 11 more

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https://doi.org/10.1039/d1sc04993c

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Abstract

Many photoactivated processes involve a change in oxidation state during the reaction pathway and formation of highly reactive photoactivated species. Isolating these reactive species and studying their early-stage femtosecond to nanosecond (fs–ns) photodynamics can be challenging. Here we introduce a combined ultrafast transient absorption-spectroelectrochemistry (TA-SEC) approach using freestanding boron doped diamond (BDD) mesh electrodes, which also extends the time domain of conventional spectrochemical measurements. The BDD electrodes offer a wide solvent window, low background currents, and a tuneable mesh size which minimises light scattering from the electrode itself. Importantly, reactive intermediates are generated electrochemically, via oxidation/reduction of the starting stable species, enabling their dynamic interrogation using ultrafast TA-SEC, through which the early stages of the photoinduced relaxation mechanisms are elucidated. As a model system, we investigate the ultrafast spectroscopy of both anthraquinone-2-sulfonate (AQS) and its less stable counterpart, anthrahydroquinone-2-sulfonate (AH2QS). This is achieved by generating AH2QS in situ from AQS via electrochemical means, whilst simultaneously probing the associated early-stage photoinduced dynamical processes. Using this approach we unravel the relaxation mechanisms occurring in the first 2.5 ns, following absorption of ultraviolet radiation; for AQS as an extension to previous studies, and for the first time for AH2QS. AQS relaxation occurs via formation of triplet states, with some of these states interacting with the buffered solution to form a transient species within approximately 600 ps. In contrast, all AH2QS undergoes excited-state single proton transfer with the buffered solution, resulting in formation of ground state AHQS− within approximately 150 ps.

Highlights

Photoinduced intra- or intermolecular reactions are ubiquitous in many chemical and biological processes,[1,2] with the latter typically comprising proton transfer or electron transfer[3] between the solvent or a second chemical species
Previous transient absorption-spectroelectrochemistry (TA-SEC) studies have employed a variety of electrodes and set-ups
A compact spectroelectrochemical methodology based on boron doped diamond (BDD) mesh electrodes, that can be used in a wide range of spectroscopic techniques, from primary photodynamics all the way through to steady-state measurements, has been successfully developed to assess the time-resolved photochemical and photophysical properties of both the oxidised and reduced forms of a quinone (AQS/anthraquinone-2-sulfonate (AQS)/anthrahydroquinone2-sulfonate (AH2QS))

Summary

Introduction

Photoinduced intra- or intermolecular reactions are ubiquitous in many chemical and biological processes,[1,2] with the latter typically comprising proton transfer or electron transfer (or both, proton-coupled electron transfer)[3] between the solvent or a second chemical species. Such reactions are of vital importance in many areas including photosynthesis,[4] photoredox catalysis,[5,6] photoelectrocatalysis,[7] electrophotocatalysis,[8] and photoacidity.[9] Understanding the transient dynamics of these processes is critically important in elucidating mechanistic pathways and in determining the photostability of different species.

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