Mechanistic insights into two-photon-driven photocatalysis in organic synthesis

Abstract

A mechanism based on the sequential absorption of two photons by the components of a redox couple has been recently proposed for catalysis of the energetically demanding reduction of aryl halides. Here, we analyze the suggested photochemical mechanism of this reaction, which employs perylenediimide (PDI) as a photocatalyst, on the basis of spectroscopic, electrochemical and electron paramagnetic resonance data. Our results indicate that the photoexcited PDI radical anion (*PDI˙) cannot play the role of a photosensitizer in the aforementioned process. Instead, the reduction of 4'-bromoacetophenone likely involves *PDI˙ decomposition products. The extremely short lifetime of the photoexcited transient species, as *PDI˙, is a major general limitation for photocatalytic schemes based on sequential two-photon excitation. In order to better understand the potential of such schemes, we discuss them in the context of the Z-scheme in natural photosynthesis.

Multi‐Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

“…The quantification of product accumulation as a function of the continuous irradiation time (over seconds or minutes) can provide information about so‐called induction phases, which are initial periods during which no photochemical product is yet accumulated (lower trace in Figure b). During this time, pre‐catalysts can be converted to the real catalysts in a light‐dependent process . Lastly, the efficiency of photoproduct formation is expected to be wavelength‐dependent (Figure c) .…”

“…The lowest excited state of PDI .− has a lifetime of only 145 ps, the extinction coefficient of PDI .− at the employed irradiation wavelength of 455 nm is comparatively low, and C−Cl bond cleavage is typically the rate‐determining step in the reductive dehalogenation of aryl chlorides (Ar−Cl), making reverse electron transfer from [Ar−Cl] .− to PDI very competitive. Using the reduction of 4′‐bromoacetophenone as a model reaction, the mechanistic study found that the onset of acetophenone formation correlated with PDI .− degradation, and the conclusion was that this reaction takes place through an unidentified photo‐decomposition product of PDI . Using the less challenging substrate 4‐bromobenzaldehyde instead of 4′‐bromoacetophenone, a combined NMR and UV/Vis study with in‐situ photo‐irradiation found no clear correlation between PDI .− degradation and product formation, and the mechanism of Figure seems to operate …”

“…The light‐driven reduction of aryl halides using PDI (perylene diimide) as the catalyst and TEA (triethylamine) as the reductant (Table , entry 2) was interpreted in the same mechanistic framework of Figure , but spectroscopic and photochemical studies cast doubt on the initial mechanistic interpretation . The lowest excited state of PDI .− has a lifetime of only 145 ps, the extinction coefficient of PDI .− at the employed irradiation wavelength of 455 nm is comparatively low, and C−Cl bond cleavage is typically the rate‐determining step in the reductive dehalogenation of aryl chlorides (Ar−Cl), making reverse electron transfer from [Ar−Cl] .− to PDI very competitive.…”

“…Thermal electron transfer from the reduced metal complex (M ( n −1)+ ) to the oxidized photosensitizer (PS .+ ) subsequently regenerates the resting state of the overall system. This concept resembles the Z‐Scheme of natural photosynthesis more closely than most of the abovementioned two‐photon excitation strategies, because it relies on two distinct chromophores, not just on the same chromophore in different redox states …”

Multiphotonen‐Anregung in der Photoredoxkatalyse: Konzepte, Anwendungen und Methoden

“…Die lichtgetriebene Reduktion von Arylhalogeniden mit PDI (Perylendiimid) als Katalysator und TEA (Triethylamin) als Reduktionsmittel (Tabelle , Eintrag 2) wurde mit dem gleichen mechanistischen Modell (siehe Abbildung ) interpretiert, jedoch werfen spektroskopische und photochemische Untersuchungen Zweifel an der anfänglichen mechanistischen Interpretation auf . Der niedrigste angeregte Zustand von PDI .− hat eine Lebenszeit von lediglich 145 ps, der Extinktionskoeffizient von PDI .− bei der verwendeten Bestrahlungswellenlänge (455 nm) ist verhältnismäßig gering, und der C‐Cl‐Bindungsbruch ist typischerweise der geschwindigkeitsbestimmende Schritt bei einer reduktiven Dehalogenierung von Arylchloriden (Ar‐Cl), was Rückelektronentransfer von [Ar‐Cl] .− auf PDI sehr konkurrenzfähig machen dürfte.…”