Water-assisted self-photoredox of 2-(1-hydroxyethyl)-9,10-anthraquinone through a triplet excited state intra-molecular proton transfer pathway

Abstract

Using multi-configurational perturbation theory (CASPT2//CASSCF), a novel self-photoredox reaction for 2-(1-hydroxyethyl)-9,10-anthraquinone was proposed to effectively occur through two steps of triplet excited state intra-molecular proton transfer (ESIPT) reaction aided by water wires without the introduction of an external oxidant or reductant. The photoinduced charge transfer along the desired direction was determined to be the major driving force for the occurrence of the energetically favorable ESIPT in the triplet state, in which the water wires function as an effective proton relay and photocatalyst to lower the reaction barrier. The computational results provide convincing evidence that the deprotonation of the hydroxyl group in the triplet state and connecting water molecule(s) between that hydroxyl group and the carbonyl group that is protonated by a nearby water molecule in the water wire is the initial reaction step that triggers the protonation of the carbonyl group seen in the previously reported time-resolved spectroscopy experiments that produces a protonated carbonyl triplet intermediate that then undergoes a subsequent deprotonation of the methylene C-H in the triplet and ground states to complete the self-photoredox reaction of anthraquinone. Comparison of the theoretical results with previously reported results from time-resolved spectroscopy experiments indicate the photoredox reactions can occur either via a concerted or non-concerted deprotonation-protonation of distal sites of the molecule assisted by the connecting water molecules. These new insights will help provide benchmarks to elucidate the photochemistry of the anthraquinone and benzophenone compounds in acidic and/or neutral aqueous solutions.

Time-Resolved Spectroscopic Study on the Photoredox Reaction of 2-(p-Hydroxymethyl)phenylAnthraquinone

“…This work revealed that HEAQ undergoes an efficient photoredox reaction in water containing solutions via an initial protonation on the carbonyl oxygen, followed by a deprotonation of the side methylene C-H bond. Recently, a computational investigation using CASSCF calculations examined the reaction route of the photoredox reaction of HEAQ 16 . This study found that in a neutral aqueous solution, the photoredox reaction of HEAQ appears to occur via an excited state intramolecular proton transfer (ESIPT) process 16 .…”

“…Recently, a computational investigation using CASSCF calculations examined the reaction route of the photoredox reaction of HEAQ 16 . This study found that in a neutral aqueous solution, the photoredox reaction of HEAQ appears to occur via an excited state intramolecular proton transfer (ESIPT) process 16 . The photoredox reaction can be rationalized by excited states that have substantial charge transfer character, in which the electron density of the benzene ring with the reactive CH 2 OH moiety is transferred to the central AQ ring.…”

To Photoredox or Not in Neutral Aqueous Solutions for Selected Benzophenone and Anthraquinone Derivatives

“…Organic photoredox reactions are one of the most rapidly expanding areas of radical chemistry in synthesis. − An efficient self-photoredox reaction, where the carbonyl is reduced to its alcohol while the alcohol at the side chain is oxidized to its carbonyl, was observed for benzophenones (BPs) and anthraquinones (AQs) in aqueous solutions. − Both 2-(1-hydroxyethyl)-anthraquinone (HEAQ) and 3-(hydroxymethyl)-benzophenone (3-BPOH) undergo efficient photoredox under moderately acidic aqueous conditions, which is proposed to take place via the carbonyl protonation process and then a subsequent deprotonation of the xylylene C–H bond. ,− Nevertheless, the mechanism has remained uncertain under nonacidic conditions, which needs to be addressed under neutral aqueous solutions. First, the origin for the experimental observations (Scheme ) that HEAQ can efficiently undergo the photoredox reaction while the related 3-BPOH and 4-BPOH compounds mainly generate the arylphenyl ketyl (ArPK) radical and do not exhibit an obvious photoredox , has not yet been clarified.…”

Advances in computational photochemistry and chemiluminescence of biological and nanotechnological molecules

“…ESPT occurring in an antraquinone via water molecules was also proposed as a plausible mechanism explaining the production of photoredox reactions without external oxidants or reductants. 120 In other theoretical works, Barbatti and co-workers 121,122 showed the potential use of the ESPT phenomenon to create two-color fluorescent markers for protein binding sites and the role of ESPT to provide photo-stability to peptide-bonded systems to UV irradiation.…”