Encapsulation of Phosphine-Terminated Rhenium(III) Chalcogenide Clusters in Silica Nanoparticles

Cationic phosphine-terminated rhenium(III) chalcogenide clusters-[Re6Se8(Et3P)(5)I]I, [Re6S8(Et3P)(5)Br]Br, [Re6Se8(Bu3P)(5)I]I, and [Re6S8(Bu3P)(5)Br]Br-were synthesized and encapsulated in silica nanospheres in a one-pot, base-catalyzed hydrolysis in acetonitrile. The cluster-doped silica nanoparticles have diameters of 10-20 nm, as observed by transmission electron microscopy (TEM). The diameter is dependent upon the volume of the solvent added to the system. Reactions conducted in > 10 mL of acetonitrile led to the isolation of particles similar to 200 nm in diameter. The absorption and emission properties of the clusters were maintained upon encapsulation. The H-1 resonance of the alkyl groups was not observed in the silica-cluster composites via nuclear magnetic resonance (NMR), and the emission blue-shifts, indicating that the clusters reside within the silica framework, rather than on the surface. Upon irradiation by light (lambda > 420 nm), both the clusters and their silica composites can generate singlet oxygen, demonstrating the oxygen permeability of silica. The smaller silica-cluster composites are potential candidates for photodynamic therapy and for other applications of singlet oxygen. The encapsulation is ineffective for neutral and anionic clusters. Electrostatic interaction between cationic clusters and the anionic, deprotonated silanol groups is proposed to drive the encapsulation.