The influence of torsion on excimer formation in bipolar host materials for blue phosphorescent OLEDs.

We present a combined detailed spectroscopic and quantum chemical study on the bipolar host materials BPTRZ and MBPTRZ in solution and in neat film. In the two compounds, the hole transporting carbazole is separated from the electron transporting triazine moiety by a fully aromatic but non-conjugated meta-linked biphenyl unit. The two materials differ by an additional steric twist at the biphenyl in MBPTRZ, which is achieved by methyl-substitution in 2- and 2'-position of the biphenyl. We find that while the twist shifts the triplet state in MBPTRZ to higher energies (3.0 eV in solution) compared to BPTRZ (2.8 eV in solution), this also localizes electron density on the carbazole moiety, leading to excimer formation in neat films.