Understanding and controlling the efficiency of Au24M(SR)18 nanoclusters as singlet-oxygen photosensitizers

Singlet oxygen, O-1(2), can be generated by molecules that upon photoexcitation enable the O-3(2) -> O-1(2) transition. We used a series of atomically precise Au24M(SR)(18) clusters, with different R groups and doping metal atoms M. Upon nanosecond photoexcitation of the cluster, O-1(2) was efficiently generated. Detection was carried out by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The resulting TREPR transient yielded the O-1(2) lifetime as a function of the nature of the cluster. We found that: these clusters indeed generate O-1(2) by forming a triplet state; a more positive oxidation potential of the molecular cluster corresponds to a longer O-1(2) lifetime; proper design of the cluster yields results analogous to those of a well-known reference photosensitizer, although more effectively. Comprehensive kinetic analysis provided important insights into the mechanism and driving-force dependence of the quenching of O-1(2) by gold nanoclusters. Understanding on a molecular basis why these molecules may perform so well in O-1(2) photosensitization is instrumental to controlling their performance.