Triplet excimer emission in a series of 4,4'-bis(N-carbazolyl)-2,2'-biphenyl derivatives.

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 (OLED)s. 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 density functional theory (DFT) calculations on two series of CBP derivatives, where CH3 and CF3 substituents on the 2- and 2′-position of the biphenyl introduce strong torsion into the molecular structure. We find that the resulting poor coupling between the two halves of the molecules leads to an electronic structure similar to that of N-phenyl-3,6-dimethylcarbazole, with a high triplet-state energy of 2.95 eV. However, we also observe a triplet excimer emission centered 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 the more p...