Plasmon Enhancement of Triplet Exciton Diffusion Revealed by Nanoscale Imaging of Photochemical Fluorescence Upconversion

Photon upconversion based on the process of triplet triplet annihilation in a system of organic donor and acceptor molecules has been attracting increasing attention because it can potentially lead to improved efficiency of light energy conversion devices working under sunlight irradiation. Here we aim to gain insight into the effect of localized plasmons of metal nanostructures on the individual photophysical steps involved in the upconversion mechanism. We present an optical microscopy study of the photophysical properties of plasmonic hybrid nanostructures composed of silver nanowires combined with an upconversion system consisting of platinum octaethylporphyrin donor molecules and 9,10-diphenylanthracene acceptor molecules dispersed in poly(methyl methacrylate) thin film. Using an image-splitting technique, we simultaneously record upconversion and phosphorescence microscopy images and analyze the results to decouple the individual photophysical events. In addition to moderate intensity enhancement on the nanowires, the upconversion emission intensity can be enhanced up to 15-fold in the vicinity of hotspots formed by the silver nanowire junctions, compared with a 4-5-fold enhancement of phosphorescence in the same locations. Furthermore, whereas the phosphorescence enhancement is localized in the hotspots, the upconversion emission is enhanced along micrometer distances on the top of the nanowires. These findings are interpreted in terms of plasmon enhancement of Dexter-type energy transfer between the triplet states of the donor and acceptor as well as between the triplet states of the acceptor molecules. The latter gives rise to the apparent long-distance propagation of the triplet excitons along the nanowires.