Understanding and controlling the efficiency of Au(24)M(SR)(18) nanoclusters as singlet-oxygen photosensitizers

Singlet oxygen, (1)O(2), can be generated by molecules that upon photoexcitation enable the (3)O(2) → (1)O(2) transition. We used a series of atomically precise Au(24)M(SR)(18) clusters, with different R groups and doping metal atoms M. Upon nanosecond photoexcitation of the cluster, (1)O(2) was efficiently generated. Detection was carried out by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The resulting TREPR transient yielded the (1)O(2) lifetime as a function of the nature of the cluster. We found that: these clusters indeed generate (1)O(2) by forming a triplet state; a more positive oxidation potential of the molecular cluster corresponds to a longer (1)O(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 (1)O(2) by gold nanoclusters. Understanding on a molecular basis why these molecules may perform so well in (1)O(2) photosensitization is instrumental to controlling their performance.