Modification of Luminescence from Dual-Emission Molecules by Plasmonic Surfaces

Phosphorescence from heavy-atom-free organic conjugated molecules is weak at room temperature due to the low probability of radiative recombination from the triplet state. In this study, the relative intensity of phosphorescence compared to fluorescence from a room-temperature, dual-emission organic molecule N,N′-bis(1-naphthalenyl)-N,N'-bis(phenyl)benzidine doped in a poly(9-vinylcarbazole) matrix on different plasmonic surfaces is investigated. A variety of different plasmonic surfaces are used to modify the ratio of phosphorescence to fluorescence: discrete Ag nanoparticle layers; discrete Au nanoparticle layers; porous Ag films; porous bimetallic Au/Ag films. The scattering of plasmonic surfaces is tuned to overlap with either the shorter wavelength fluorescence emission of the organic molecule or the longer wavelength phosphorescence emission of the organic molecule by employing metal layers with different compositions. An enhanced relative intensity of phosphorescence is experimentally observed from the organic molecular films on plasmonic surfaces compared to that on glass and planar metallic surfaces. Two mechanisms, radiative decay rate modification and excitation enhancement in the polymer host, are hypothesized to account for the relative phosphorescence intensity enhancement. The results indicate that plasmonic structures can not only enhance the absolute fluorescence/phosphorescence of luminescent materials, as reported by many previous studies, but also modify the relative weight of phosphorescence compared to fluorescence of a heavy-atom-free organic molecules at room temperature. The findings of this work demonstrate that plasmonic surfaces can be used as an external method to manipulate triplet emission from heavy-atom-free organic molecules.