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

The experimental and theoretical results in neutral aqueous solutions reported here indicate that a proton-coupled electron transfer (PCET) from an alcohol C-H bond to the para-carbonyl is the initial and crucial process for the photoredox reaction of 2-(1-hydroxyethyl)-anthraquinone (HEAQ) to occur while the counterpart 3-(hydroxymethyl)-benzophenone (3-BPOH) compound displays a different PCET from an alcohol O-H bond to the carbonyl as the first step, followed by an intersystem crossing process that does not lead to the analogous photoredox, which is caused by a subtle charge-radical coupled effect between HEAQ and 3-BPOH. This can account for experimental results in the literature that HEAQ can undergo efficient photoredox but 3-BPOH does not under neutral aqueous conditions. These results have implications for the pH-dependent photochemical behavior of aromatic carbonyl compounds in aqueous media.