Enhancement of phosphorescence by surface-plasmon resonances in colloidal metal nanoparticles: The role of aggregates

The spectroscopic and near-field scanning optical microscopy (NSOM) studies of phosphorescent films doped with colloidal gold nanoparticles (NPs) are presented. Films with a high concentration of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) dispersed in a neutral polymer poly[(methyl methacrylate)-co-(ethyl acrylate)] demonstrate a twofold increase of the phosphorescence quantum yield after the addition of aggregated NPs. In materials doped with unaggregated particles, a decrease of the emission yield is observed. Theoretical modeling of the phosphorescence transients suggests a minimization of the triplet-triplet quenching owing to the presence of fast process that decrease the concentration of chromophores in the excited state and may be both of radiative and non-radiative origin. NSOM examination of the films reveals increased light emission around large NP clusters. This observation demonstrates significant enhancement of the spontaneous emission rates by the large aggregates, although unaggregated NPs introduce mostly phosphorescence quenching sites.