Topological arrangement of fluorenyl-substituted carbazole triads and starbursts: Synthesis and optoelectronic properties

A series of fluorenyl-substituted carbazole linear triads and starbursts were synthesized by the Suzuki and Sonogashira coupling reactions. In these compounds, a fluorene unit is connected to the 1,8; 2,7; or 3,6 positions of the central carbazole ring to form linear triads or linked to the 1,3,6,8 positions to form starbursts. The optoelectronic properties of these different functionalized carbazole materials were investigated, and light-emitting devices based on starbursts were constructed and characterized. For the linear triads, we observed that the 2,7-position substitution is more efficient in extending the conjugation with the sacrifice of triplet energy. Moreover, the theoretical calculations predict that substitution at the 1,8 positions gives a high triplet energy of 2.71 eV, indicating that 1,8 substitution is an effective way to design high energy level host materials. Tetrasubstituted starbursts show a much lower HOMO and band gap than that of linear triads. It was interesting to observe that 4FCz shows a deep blue emission with high triplet energy. Moreover, 4FECz shows the lowest LUMO energy level and a very low hole reorganization energy, indicating its good hole transporting property.