Of the PCs typically employed in PEnT reactions, organic carbonyl-based photosensitizers have the highest ET. For instance, xanthone and acetophenone (Figure ) have similar ET values of 3.22 eV (74.2 kcal mol -1 , 310.5 kJ mol -1 ) and 3.21 eV (74.0 kcal mol -1 , 309.6 kJ mol -1 ). Their very high ET comes at the cost that they do not absorb light in the visible region (absorption maxima, at λabs of 337 and 275 nm for xanthone and acetophenone, respectively), which means that their absorption spectra may likely overlap with the absorption spectra of the substrate or other additives and thus the PC may not be selectively excited. Organometallic PCs are particularly popular for PEnT reactions in part because the presence of the heavy metal centre ensures an essentially quantitative triplet yield as a result of fast intersystem crossing (ISC) mediated by its large spin-orbit coupling (SOC). Some of the most commonly used organometallic PCs for PEnT reactions include [Ir(dF(CF3)ppy)2(dtbbpy)](PF6) (ET = 2.68 eV) for reactions requiring a relatively high ET and [Ru(bpy)3](PF6)2 (ET = 2.12 eV) for those that do not (Figure ). 1 MeOH/EtOH glass. ET for [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 taken from the room temperature emission maximum in MeCN. For the organic PCs ET is measured from the phosphorescence spectra at 77 K. Over the last decade or so organic thermally activated delayed fluorescence (TADF)