Pathways for the photochemical hydrogen abstraction by n, π^*‐excited states

Abstract

As a result of recent experimental and theoretical investigations, new aspects of the photochemical hydrogen abstraction by n, π*‐excited states have emerged. The absolute reactivity of singlet‐excited n, π* states exceeds that of the corresponding triplet states, but the singlet reaction turns out to be chemically much less efficient than the triplet reaction. Hence, radiationless deactivation of the singlet states represents the dominant pathway for interaction with hydrogen donors. The fraction of singlet encounters undergoing deactivation appears to increase with decreasing reaction exothermicity. Hence, when relatively inert solvents like methanol or chloroform act as hydrogen donors towards singlet‐excited azoalkanes, the reaction becomes entirely inefficient. This constitutes an example of a novel fluorescence quenching mechanism, which is referred to as an “aborted” hydrogen abstraction. The inefficiency of the singlet reaction can be rationalized, making use of the most recent theoretical findings, in terms of the occurrence of a conical intersection, i.e., a real surface crossing, rather than the previously implicated avoided crossing. The conical intersection provides an efficient channel for nonadiabatic return to the ground‐state reactants and, thus, serves to account for the observed inefficiency of product formation from the singlet state. In general, the conical intersection appears to follow the transition state for hydrogen abstraction, but the consequences of a situation, where the conical intersection precedes the transition state are also discussed. The latter becomes reasonable for endothermic hydrogen abstractions, for which one would then expect a unit efficiency of singlet deactivation. The absolute reactivities in hydrogen abstractions, the involvement of CT interactions and tunneling, and the behavior of different n, π* chromophores (azoalkanes versus ketones) are also discussed in view of the most recent experimental data. General rules for photochemical hydrogen abstractions by n, π*‐excited states are expressed.

Investigation of Polar and Stereoelectronic Effects on Pure Excited-state Hydrogen Atom Abstractions from Phenols and Alkylbenzenes†

“…This results in the striking observation that the fluorescence quenching of DBO is generally chemically unproductive, i.e. quantum yields for photo‐product formation are low (<5%) (9, 17, 30), e.g. for toluene (<3% in neat solvent) and α‐tocopherol (<0.1%), to name an example of a reactive phenol.…”

“…In an effort to investigate polar and stereoelectronic effects on the transition state for hydrogen atom abstraction by DBO we therefore reasoned that the fluorescence quenching of DBO by phenols and alkylbenzenes could provide a model reaction for a "pure" photoinduced hydrogen abstraction devoid of complications caused by competitive exciplex formation as for excited ketones. Although numerous experimental and theoretical studies on the fluorescence quenching of DBO have been reported (9,(16)(17)(18)29,(31)(32)(33)(34)(35), none of them dealt with polar effects or a series of aromatic quenchers (except for some dihydroxyindoles [36] and electron-donating anilines [ 171) to allow a comparison with the large amount of data available for excited ketones. It should be stressed that the understanding of the electronic requirements of hydrogen transfer reactions from aromatic molecules is most relevant to biological problems.…”

“…Polar and stereoelectronic effects in photoinduced hydrogen abstractions have been the subject of considerable interest in molecular photochemistry (1–7). Hydrogen atom abstraction reactions leading to photoreduction products on a preparative scale are characteristic for chromophores with an n,π* electronic configuration (8, 9), often represented by triplet‐excited ketones, prominently benzophenone. The investigation of polar and stereoelectronic effects in hydrogen abstractions by triplet ketones is of particular interest in view of the reactivity of (ground state) alkoxyl radicals.…”

Structure-reactivity relationships in the photoreduction of n,π*-excited ketones and azoalkanes: the effect of reaction thermodynamics, excited-state electrophilicity, and antibonding character in the transition state

“…Hydrogen atom abstraction by n,π*-excited states, in particular triplet carbonyl compounds, has been intensively investigated. 1, 2 The radical-like nature of the interaction was recognised early. [3][4][5] Consequently, several models based on the thermochemistry of the cleavage and formation of covalent bonds were proposed to predict the activation barriers and rates of these photoreactions.…”

Structural Causes of Singlet/triplet Preferences of Norrish Type II Reactions in Carbonyls

“…35 To explain these phenomena, the presence of a S 1 /S 0 conical intersection (CI) along the NTII reaction pathway was proposed in the 1990s. 9,36 The possibility of a concerted S 0 NTII photolysis pathway has also been proposed, 16 and recent wavelength-dependent FT-IR photolysis data shows the formation of NTII photoproducts in pentan-2-one at energies below the predicted T 1 NTII threshold. 32…”