Ketyl Radical Formation via Proton-Coupled Electron Transfer in an Aqueous Solution versus Hydrogen Atom Transfer in Isopropanol after Photoexcitation of Aromatic Carbonyl Compounds

Abstract

The excited nπ* and ππ* triplets of two benzophenone (BP) and two anthraquinone (AQ) derivatives have been observed in acetonitrile, isopropanol, and mixed aqueous solutions using time-resolved resonance Raman spectroscopic and nanosecond transient absorption experiments. These experimental results, combined with results from density functional theory calculations, reveal the effects of solvent and substituents on the properties, relative energies, and chemical reactivities of the nπ* and ππ* triplets. The triplet nπ* configuration was found to act as the reactive species for a subsequent hydrogen atom transfer reaction to produce a ketyl radical intermediate in the isopropanol solvent, while the triplet ππ* undergoes a proton-coupled electron transfer (PCET) in aqueous solutions to produce a ketyl radical intermediate. This PCET reaction, which occurs via a concerted proton transfer (to the excited carbonyl group) and electron transfer (to the excited phenyl ring), can account for the experimental observation by several different research groups over the past 40 years of the formation of ketyl radicals after photolysis of a number of BP and AQ derivatives in aqueous solutions, although water is considered to be a relatively "inert" hydrogen-donor solvent.

Direct Observation of Photoinduced Ultrafast Generation of Singlet and Triplet Quinone Methides in Aqueous Solutions and Insight into the Roles of Acidic and Basic Sites in Quinone Methide Formation

“…Both the calculated UV–vis spectra of 2 (Tnπ*) and 2 (Tππ*) contribute to the absorbance in the fs-TA spectra at 53.7 ps, which suggests a mixture of 2 (Tnπ*) and 2 (Tππ*) in ACN. This assignment is consistent with DFT calculations that 2 (Tnπ*) and 2 (Tππ*) are energetically very close to each other (Δ G : 0.2 kcal/mol) and supported by recent publications. , 2 (S 1 ) goes through the ISC process to 2 (Tππ*) with a 10.6 kcal/mol energy gap because the 2 (Tππ*) appeared at 578 nm after 1.6 ps, equilibrated with 2 (Tnπ*) after 3.6 ps. It is noted that there is some species generated with an absorbance at 578 nm (see the line recorded at 1.7 ps) between 2 (S 1 ) and the mixture of the triplet state species.…”

Proton‐Coupled Electron Transfer in Photoredox Catalytic Reactions

“…Such observations clearly also indicate the relevance of PCET in HAT processes. Competition between a hydrogen transfer mechanism and a more general PCET process has been reported for aromatic ketones . This competition is determined by the character of the T 1 state (nπ* or ππ*).…”

Formylation of Fluoroalkyl Imines through Visible-Light-Enabled H-Atom Transfer Catalysis: Access to Fluorinated α-Amino Aldehydes

“…The excitation of an electron from a nonbonding orbital to a corresponding π*-orbital (CO bond, n , π*), followed by intersystem crossing (ISC), generates a triplet ketyl diradical . Through either an HAT or a proton-coupled electron transfer (PCET) event, the catalyst pair turns into a reducing radical II and an H-abstracting N -oxyl radical III . Radical III then plays a key role in the HAT step to facilitate the generation of a nucleophilic 1,3-dioxolan-2-yl radical IV .…”