Enhancement of Platinum Octaethyl Porphyrin Phosphorescence near Nanotextured Silver Surfaces

We observe more than a 200-fold increase in the photoexcited phosphorescent emission of PtOEP (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum II) in a polystyrene film on nanotextured silver surfaces, coincident with a reduction in the triplet state lifetime by a factor of 5. The large enhancement results in films with apparent luminescence quantum yields much greater than unity and can be understood in terms of increased radiative rates due to interactions between the molecules and the electron plasma in nearby silver nanoparticles. We study the photoluminescence efficiency, excitation spectrum, and decay dynamics as a function of film thickness and silver density. We use a model of the photophysics to decompose the phosphorescent enhancement into contributions from increases in absorption, emissive rate, and quenching. Quenching increases in importance for very thin films, and we conclude that ∼3 nm spacing between metal and chromophore leads to the largest photoluminescence enhancement.