Understanding and Controlling the Efficiency of Au24 m(SR)18 Nanoclusters As Singlet-Oxygen Photosensitizers

Abstract

Singlet oxygen, 1O2, is an excited oxygen species [1] that can be efficiently generated by photosensitizers. These are molecules that upon photoexcitation eventually generate a triplet excited state that can be quenched by the ground state oxygen triplet, 3O2, to form 1O2 [1]. The latter finds uses in many applications, especially organic synthesis, photocatalysis, and photodynamic therapy [2-4]. On the other hand, its possible use depends on its effective lifetime, as 1O2 is a very reactive species that decays via quenching by the solvent and/or interactions with other molecules in solution [1]. This is the case of the photosensitizers themselves, and this may result in a serious drawback for efficient applications of 1O2. In this area, the use of monolayer-protected metal nanoclusters as photosensitizers is limited. We carried out a systematic study on the photosensitizer behavior of a series of molecular Au24M(SR)18 clusters [5,6], where the R group and the doping metal M were varied. Electrochemical characterization allowed us to select the best R/M combinations to test a sufficiently large oxidation-potential range. 1O2 detection was carried out by time-resolved electron paramagnetic resonance [7], which allowed us to determine 1O2 lifetimes spanning more than one order of magnitude (on the microsecond time scale), depending on the specific cluster employed. Among the main results, we found that: (i) a more positive potential for the oxidation of the cluster is matched by a (beneficial) longer 1O2 lifetime; (ii) proper design of the electrochemical properties of the cluster yields the same results as those of reference photosensitizers, despite the unprecedented low oxidation potentials exhibited by these clusters; (iii) kinetic analysis based on the Marcus theory provides important insights into the mechanism of the quenching of 1O2 by gold nanoclusters, how to control it, and why properly doped gold nanoclusters may perform very well in 1O2 photosensitization [8]. Further effects, detected by electrochemistry, are under investigation.