title: Intense photoluminescence from mixed solutions of C70 and palladium octaethylporphyrin: A supramolecular heavy atom effect.
abstract: Unusually intense near-infrared (near-IR) photoluminescence has been observed from mixed solutions of C70 and palladium octaethylporphyrin (PdOEP). The novel emission has a spectrum similar to C70 phosphorescence and an intensity that is approximately 20 times greater than that of C70 fluorescence. The emitting species is identified as a noncovalently bound, short-lived triplet exciplex of C70 with PdOEP. The emission is essentially C70 phosphorescence intensified by spin-orbit coupling from the Pd atom in the nearby metalloporphyrin. This supramolecular heavy atom effect increases the C70 emissive quantum yield to approximately 1 x 10(-2) in degassed hexane solution at room temperature. The radiative rate constant is enhanced by a factor of 10(5), to approximately 7 x 10(4) s(-1), which is a value that exceeds the phosphorescence rate constant of PdOEP. Comparative studies in a rigid poly(methyl methacrylate) (PMMA) matrix show that the excited state of the static C70-PdOEP complex decays in approximately 150 ns. A Job's plot analysis shows that the complex has a 1:1 stoichiometry. It forms dynamically in solution and is relatively weakly bound, with an estimated equilibrium constant near 100 M(-1). Qualitatively similar supramolecular heavy atom effects were also observed for complexes of PdOEP with C60 and fullerene derivatives.
authors.affiliation: Department of Chemistry and R. E. Smalley Institute for Nanoscale Science and Technology, Rice University, 6100 Main Street, Houston, Texas 77005, USA.
authors.forename: Dmitri A
authors.initials: DA
authors.lastname: Tsyboulski
authors.forename: Sergei M
authors.initials: SM
authors.lastname: Bachilo
authors.forename: R Bruce
authors.initials: RB
authors.lastname: Weisman