Formation of triplet excimers and dimers in amorphous organic thin films and light emitting devices

We examine triplet excimers in neat films of the phosphorescent molecule: platinum (II) (2-(4',6'-difluorophenyl)pyridinato-N, C-2) acetyl acetate (FPt1). This molecule was chosen because of its square planar structure, allowing it to facially pack in a crystal with an intermolecular separation of only 3.4 +/- 0.1 Angstrom, thereby facilitating excimer formation between adjacent molecules. The transient and steady-state photoluminescent and electroluminescent properties of FPt1 are used to understand the formation of triplet excimers (E-3(0)*) and dimers (D-3(*)) in amorphous films grown by thermal evaporation. We find that E-3(0)* and D-3(*) have peak emission intensities at 2.07 and 1.83 eV, and have lifetimes of (1.6 +/- 0.3) mus and (800 80) ns at room temperature, respectively. When films are optically pumped, triplet excitons form on monomers and then diffuse via thermally activated hopping to nearby excimer sites with an activation energy of (37 +/- 14) meV. We find that the energy transfer efficiency from the monomer to the excimer state approaches 100% at room temperature, and that the photoluminescence efficiency of the excimer is three times less than that of the monomer. Further, excitation of triplet aggregate states, or metal-metal dimers via the association of cations and anions of electrically pumped FPt1 is achieved in an organic light-emitting device, which achieves a maximum internal quantum efficiency of similar to14%. Electrical pumping also reveals dimer states unresolved in photoluminescent studies. The high efficiency of the triplet excimers of FPt1 is significant since dimeric species, especially triplets, are often associated with concentration quenching and low quantum yields. (C) 2002 Elsevier Science B.V. All rights reserved.