Consecutive photoinduced electron transfer (ConPET) utilizes the combined energy of two photons within the visible spectrum to drive photocatalytic reductions and has recently gained increasing attention from synthetic organic chemists. However, in the decade following its conceptual proposal, the absence of suitable model reaction systems hindered detailed experimental investigation of ConPET processes due to the extreme sensitivity and ultrashort lifetimes of the reactive intermediates to water, light, and additives. As a result, the mechanistic understanding of how electrons transfer from the photoactivated high-energy intermediate PC•−* (formed upon secondary photon excitation of the sensitizer) to the substrate or reaction intermediates remains incomplete.
Figure 1. New Mechanistic Insights Gained into Continuous Photoinduced Electron Transfer (ConPET).
Recently, research groups led by Prof. Tianfei Liu from Nankai University, in collaboration with Profs. Zhiyong Jiang of Henan University and Baokun Qiao of Henan Normal University, reported new insights into the ConPET mechanism (Figure 1). They conducted rigorous experimental tests, literature reviews, and theoretical analyses on the driving forces of electron transfer processes. Under strictly controlled dark, anhydrous, and oxygen-free conditions with highly purified reagents, a series of new experiments provided evidence that the proton-coupled electron transfer (PCET) process following ConPET cannot be ruled out. By carefully maintaining reaction conditions, the authors electrochemically synthesized a stable solution of PC•−. They then experimentally measured the fluorescence emission spectrum and emission lifetime of the excited-state PC*•−, observing fluorescence quenching upon substrate addition. Time-resolved EPR spectroscopic analysis of the PC•− solution under blue light irradiation revealed rapid disappearance of radical intermediates under continuous 450 nm laser irradiation. The complex kinetic profile did not align with the zero-order reaction kinetics expected from the solvent-mediated electron transfer pathway proposed by an Italian research team. Based on these experimental and theoretical results, combined with literature research, the authors demonstrated that the solvated electron pathway suggested by the Italian team in pure acetonitrile is thermodynamically unfeasible. The observations from the Italian team likely stemmed from inherent flaws in their model system, including decomposition of tetrabutylammonium cations and contamination from water or acetone in their reagents. The authors conducted an in-depth discussion on the application of time-resolved absorption spectroscopy in mechanistic studies of organic reactions. They emphasized the persistent challenges in such research: “Whenever we make an observation, we rely on theories and assumptions beyond those that we are testing. A negative finding could result from flawed assumptions or methodologies rather than reflecting the truth of the theory itself.” Thus, it remains critically important to strictly control the purity of test compounds, meticulously design experiments, and rigorously maintain reaction conditions. Subsequently, the authors propose four key recommendations for studying the mechanisms of ConPET and other photochemical reactions involving high-energy excited-state intermediates:
1. All the compounds, including 4DPAPN, other reagents, and all the solvents, must be strictly purified to eliminate the possible impact of impurities; the experimental environment must be strictly light-proof; and the experimental conditions must be strictly controlled to ensure anhydrous and oxygen-free conditions.
2. At present, the experimental reduction potential values of the solvated electrons in pure acetonitrile, DCM, and tBuBenzene remain unresolved. To verify the accuracy of the theoretically predicted values of the pure organic solvents, it is necessary to design suitable experiments to determine their precise values.
3. It should be noted that the experiments in Ceroni's recent Correspondence, as well as the experiments in this article, are all approximate. All experiments are apart from the actual experimental temperature and other environmental conditions and are merely model systems for the mechanism studies. Approximations include: a) introducing substances that are not present in the actual reaction system, such as tetrabutylammonium salts, and b) the comparison of thermodynamic driving force and kinetic analysis is only an approximate judgement based on data under standard conditions.Future mechanism studies utilizing time-resolved absorption spectroscopy will require careful consideration as to how to design experiments excited by 450 nm in order to further approximate the actual reaction conditions.
4. Due to the high energy of the excited-state radical anion 4DPAPN*•−, under actual conditions, our catalytic reactions may involve more than one pathway. Therefore, when studying the reaction intermediates through experimental methods, including time-resolved spectroscopy and intermediate-capture technology, statistical thermodynamics and probability distribution must be combined in order to comprehensively evaluate the reaction mechanism from the perspective of theoretical chemistry.
This second point holds particular significance, as it forms the thermodynamic foundation for evaluating reaction mechanisms involving solvated electrons. Yet, to the best of the authors' knowledge, experimentally verified energetic data for solvated electrons in rigorously purified organic solvents remain lacking. Addressing this gap requires designing precise measurement techniques—an endeavor demanding collaborative efforts among experimental physical chemists, physicists, and theoretical chemists. Concluding, the authors highlight transformative advances enabled by time-resolved EPR spectroscopy and infrared absorption spectroscopy in elucidating the fate and structural characteristics of reactive intermediates within complex catalytic systems. They emphasize that these techniques will serve as guiding beacons for future breakthroughs in photocatalytic research. Relevant achievements were published in Angew. Chem. Int. Ed., 2025. DOI: 10.1002/anie.202514007.
Figure 2. Three proposed reaction mechanisms.
