Mechanism for 1 O2 Generation. Density functional theory (DFT) calculations were performed to estimate the ability of our selected molecules to produce 1 O2 upon photoirradiation. As illustrated in Figure , the energy transfer occurs from the triplet excited photosensitizer ( 3 AQ) to the acceptor triplet oxygen ( 3 O2) resulting in a singlet ground state photosensitizer ( 1 AQ) and a singlet excited state acceptor ( 1 O2). Hydroxyanthraquinones exhibit excited-state intramolecular hydrogen transfer (ESIHT) from hydroxyl groups towards the adjacent ketone functionality during light activation. Moreover, hydroxyanthraquinones with deuterium substituted hydroxyl groups are reported to have longer emission lifetimes compared to the non-substituted analogues. Therefore, we can hypothesize that the energy transfer (i.e., donor relaxation) is directly linked to reverse intramolecular hydrogen transfer (RIHT). In support of this notion, the DFT-calculated barrier (ΔG ‡ ) of the energy transfer for 9,10-AQ to produce 1 O2 is unrealistically high at 138.9