We measured a QYPC of 22.8 ± 0.4 %. This high quantum yield for OH -photogeneration, and thus CO2 capture, exceeds that afforded by photoacid approaches. For example, our previous investigation into merocyanine photoacids showed that productive phototriggered release of H + occurred with a quantum yield of only 1.3%. In experiments performed under aerobic conditions, radical recombination of the homolysis products appear to outcompete degradation pathways, such as solvent reaction, as the 380-550 nm feature returns to ground state with no other significant photoproducts observed. However, with rigorous exclusion of oxygen, nsTA experiments involving prolonged exposure to the excitation beam in 1 cm quartz cuvette produced one or more additional photoproducts (Figure ). As in the photoreduction of PBMeCl, we observe what appears to be a PBMe + bleach on top of another absorbing transient species. Therefore it appears that an environment free of oxygen may increase the lifetime of the PBMe• radical leading to activation of alternative photochemical pathways that compete with •OH recombination. An unusual practical consequence is that this photochemical CO2 system not only tolerates the presence of oxygen but may require it for stability, contrasting starkly with most known electrochemical systems for which minimizing oxygen exposure is a critical consideration. In contrast to the hydroxide release and CO2 capture process, the CO2 release portion of the cycle is photon-independent. Thus, its mechanistic possibilities are relatively more straightforward to analyze, and we proposed two possibilities. In mechanism 1, which is essentially the microscopic reverse of capture, the photobase is regenerated by nucleophilic attack of hydroxide, which is formed in minor equilibrium with bicarbonate, on the carbocation. The irreversible consumption of hydroxide continuously drives bicarbonate conversion to hydroxide (and free CO2), until all the photobase has been reconstituted and all the CO2 released. Alternatively, in mechanism 2, the carbocation is directly attacked by its counteranion bicarbonate, producing an unstable carbonic acid monoester that rapidly dissembles by deprotonation and C-O heterolysis into photobase and free CO2. If we assume the first step of each mechanism is involved in the rate-determining span, the predicted rate laws are, respectively: