Experimental and theoretical studies on charge transfer complex formed by TCTA and TPBi

The studies on charge transfer (CT) complex formed by 4,4,4″-tris(N-carbazolyl)-triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBi) were carried out by experiment and simulation calculations. Experimentally, exciplex-forming in TCTA:TPBi mixture is observed and electroplex is formed for TCTA/TPBi bi-layer structure. The density functional theory via DMol3 package was performed to explore the geometrical structures, stability, and electronic structures of the complexes. The calculation results demonstrate that geometrically stable structure of the complex is assumed to enable electroplex-forming because the molecular planes of donor and acceptor are separated by 5.5 Å. However, the formation of exciplex requires a small intermolecular separation of below 4 Å, which is relatively unstable for TCTA−TPBi complex and has to be driven by an external force. The requisition can be fulfilled in TCTA:TPBi mixture due to a reduction of intermolecular interval caused by tight molecular packing and strong intermolecular interaction deriving from mixing. The emission of exciplex species is thereof observed in TCTA:TPBi mixture and analogue electroplex in TCTA/TPBi bi-layer system. The calculated frontier molecular orbitals of the complexes exhibit the lowest unoccupied molecular orbital (LUMO) is positioned on TPBi and the highest occupied molecular orbital (HOMO) on TCTA, inferring that the emission of the CT complexes is originated from electron cross transition between the LUMO of TPBi and HOMO of TCTA. Meanwhile, the greater overlap of the molecular orbitals and larger oscillator strength of lowest-lying singlet excited states manifest that the CT complex in TCTA:TPBi mixture can be more efficiently produced, resulting in better electroluminescent performance, compared to TCTA/TPBi system.