APPLICATIONS OF TRANSIENT ABSORPTION SPECTROSCOPY IN MECHANISTIC INVESTIGATIONS OF HOMOGENEOUS PHOTOCATALYSIS

Abstract

Homogeneous photocatalysis has emerged as a striking methodology revolution to the field of organic synthesis that redefines the traditionally challenging chemical transformations in many ways. The improvements in the substrate scope, the ease of reaction condition, and the reaction performance were exceptional when a light-absorbing photocatalyst was introduced with visible light irradiation, the origin of which was proposed to be the photogenerated highly reactive molecular excited states of the photocatalyst, granting access to unconventional reaction pathways at mild conditions. However, these molecular excited states are elusive to be captured by traditional steady-state post-analysis methods such as nuclear magnetic resonance and mass spectroscopy because of their transient and thermodynamic nonequilibrium nature, making mechanistic investigations challenging. Recognizing the power of time-resolved spectroscopy to probe extremely short-lived species, experimental efforts to address the aforementioned challenge were described by applying ultrafast transient absorption spectroscopy to the mechanistic studies of three different photocatalytic systems in this dissertation. In chapter 2, the mechanistic insights gained from time-resolved spectroscopy studies were reported for a model system, an iridium/nickel photocatalytic C–O cross-coupling reaction. With the discovery of a novel energy-transfer-mediated pathways, the scale of the research was then escalated from model system study to a full mechanistic analysis on a similar iridium/nickel system for a photocatalytic C–N cross-coupling reaction in chapter 3. It is in this project that the perplexity of homogeneous photocatalysis was exposed that a non-trivial switch from oxygen to nitrogen atoms on the coupling partner could alter the final mechanistic picture to a great extent. In chapter 4 time-resolved spectroscopy was exploited to investigate the detailed photoactivation mechanisms for a photocatalytic hydrogenation reaction catalyzed by piano-stool iridium hydride complexes, with a hope to observe the real-time dynamics of one of the simplest chemical reactions, hydrogen atom transfer on the setup. Instead, the unconventional role of iridium hydride as a photosensitizer for this type of reaction was discovered by a combination of spectroscopic measurements, isotopic labeling, structure–reactivity relationships, and computational studies. With these examples, the defining role of time-resolved spectroscopy was consolidated.