Dresden 2011 – scientific programme
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DS: Fachverband Dünne Schichten
DS 39: Organic Electronics and Photovoltaics CPP-IV (jointly with CPP, HL, and O)
DS 39.5: Talk
Wednesday, March 16, 2011, 15:15–15:30, ZEU 222
Triplet Excimer Emission in a Series of CBP-Derivatives — •Sebastian Hoffmann1, Pamela Schrögel2, Rodrigo Albuquerque1, Michael Rothmann2, Peter Strohriegl2, and Anna Köhler1 — 1Department of Physics, University of Bayreuth, 95440 Bayreuth — 2Macromolecular Chemistry I, Department of Chemistry, University of Bayreuth, Bayreuth 95440, Germany
Carbazole-based materials such as 4,4*-bis(N-carbazolyl)-2,2*-biphenyl (CBP) and its derivatives are frequently used as matrix materials for phosphorescent emitters in organic light emitting diodes. An essential requirement for such matrix materials is a high energy of their first triplet excited state. Here we present a detailed spectroscopic investigation supported by DFT calculations on two series of CBP derivatives, where CH3 and CF3 substituents introduce strong torsion into the molecular structure. The resulting poor coupling between the two halves of the molecules leads to an electronic structure similar to that of N-phenyl-3,6-dimethyl-carbazole, with high triplet state energy of 2.95 eV. However, we also observe a triplet excimer emission centred at about 2.5-2.6 eV in all compounds. We associate this triplet excimer with a sandwich geometry of neighboring carbazole moieties. For compounds with more polar CF3 substituents, the lifetime of the intermolecular triplet excited state extends into the millisecond range for neat films at room temperature. We attribute this to an increased charge-transfer character of the intermolecular excited state for the more polar substituents. [1] S.T. Hoffmann et al, J. Phys.Chem. B, in press.