Spectroscopic investigation of photocatalytic reactions: elucidating the mechanism and reactivity of photocatalysts

Abstract

Photocatalysis is an invaluable tool in reaction method development due to the ease of using visible light to enhance redox activity, allowing for the activation of challenging bonds under mild conditions. Unlike other catalysts, photocatalysts can be probed through optical spectroscopy with techniques that survey ground state (UV-Vis absorption spectroscopy) and excited state properties (transient absorption and time-correlated single photon counting). The methodological and innovative way of utilizing these steady-state and time-resolved spectroscopic techniques by photophysicists has allowed for the identification of many intermediate and transient species formed uniquely in photoinduced reactions. Spectroscopy has provided valuable insights into the underlying reactivities present in complex organic matrices, allowing the mechanistic understanding of photocatalytic systems. However, these mechanisms often differ from those proposed, revealing the complexity of light-driven reactions and the need for specialised analyses through spectroscopic methods. In this thesis, we strive to understand the underlying reactivity in Rhodamine 6G and heteroleptic Ir(III) photoredox catalytic systems through spectroscopically led mechanistic investigation, allowing us to gain clarity of the mechanisms in these reactions. Here, we have identified the interaction of substrate molecules with photocatalysts as a crucial reaction that determines the efficiency of the reaction. Irradiation of photocatalysts has often been accompanied by its degradation. However, we have found that Ir(III) catalysts undergo in situ photosubstitution process rapidly, completely converting all the catalysts into spectroscopically unique catalysts. These in situ photosubstituted catalysts are redox-active and act as the true active species in the reaction. Using this knowledge, we successfully synthesized a class of catalysts that remain non-modifiable in the reaction in an effort to study the innate reactivity of photocatalysts in reaction. Through this thesis, we hope to relay the importance of spectroscopically led mechanistic investigation as a powerful technique for identifying novel reactivities and ultimately developing synthetically useful protocols. These investigations must play a decisive role throughout the entirety of the project, rather than being relegated to the final phase, to ensure an elegant development of the method.