Synthesis, photophysical and electrochemical properties of novel carbazole-triazine based high triplet energy, solution-processable materials

A series of molecules; tBuCz1SiTrz, tBuCz2SiTrz and tBuCz3SiTrz, which contain carbazole unit as hole-transporting group (donor-D) and triazine unit as electron transporting group (acceptor-A) were synthesized and characterized as high-triplet energy (> 2.9 eV), solution-processable bipolar emitting materials. The conjugation between the D-A groups was interrupted by using bulky tetraphenylsilane groups as spacer aiming to obtain large bandgap and high-triplet energy. The photophysical behaviors of the molecules were investigated by UV-Vis absorption, photoluminescence, phosphorescence, photoluminescence quantum yield and lifetime measurements. Solvent polarity effects were investigated on the intramolecular charge transfer (ICT) behaviour and large solvatochromic effect was observed with the increasing solvent polarity. Electrochemical properties were determined by cyclic voltammetry. All molecules showed oxidation bands arise from the carbazole groups. Reduction bands were originated from the triazine groups and the intramolecular charge transfer between D-A groups. Photophysical, electrochemical and computational characterizations addressed that tBuCz2SiTrz has the weakest ICT character, highest photoluminescence quantum yield (PLQY) and charge balance.