PNNL

Abstract

Five bismuth(III)-organic compounds were synthesized using substituted 1,10-phenanthrolines (phens) locked in the outer coordination sphere through supramolecular interactions. All five compounds were photoluminescent in the solid state: two displaying solely phosphorescence, two solely fluorescence, and one a mix of both. It was found that the emission from the phosphorescent compounds arises from the triplet state emission (T1?S0) of the substituted phens. Excitation was proposed to occur through a triplet energy transfer mechanism, with the compound containing 2,9-dimethyl-1,10-phenanthrolinium displaying a sequential electron transfer and hole transfer, and the compound containing 2,9-dichloro-1,10-phenanthroline undergoing a concerted energy transfer mechanism. Bismuth induces spin-orbit coupling for triplet state population and additionally acts as a heavy metal attenuator for X-ray luminescence (radioluminescence). It is proposed that the strong p-p interactions in the phosphorescent compounds promote the electron transfer from the bismuth-organic moiety to the outer coordination phenanthroline derivatives; the purely fluorescent compounds lack any such p-p interactions, and thus show short-lived fluorescence and no radioluminescence. The effect of donor-acceptor charge, as well as hydrogen bond strength, on the luminescent lifetime is also discussed.