Tailoring Excited State Properties and Energy Levels Arrangement via Subtle Structural Design on D-π-A Materials

The donor-pi-conjugated-acceptor (D-pi-A) structure is an important design for the luminescent materials because of its diversity in the selections of donor, pi-bridge and acceptor groups. Herein, we demonstrate two examples of D-pi-A structures capable to finely modulate the excited state properties and arrangement of energy levels, TPA-AN-BP and CZP-AN-BP, which possess the same acceptor and -bridge but different donor. The investigation of their photophysical properties and DFT calculation revealed that the D-pi-A structure with proper donor, pi-bridge and acceptor can result in separation of frontier molecular orbitals on the corresponding donor and acceptor with an obvious overlap on the pi-bridge, resulting in a hybridized local and charge-transfer (HLCT) excited state with high photoluminescent (PL) efficiencies. Meanwhile, their singlet and triplet states are arranged on corresponding moieties with large energy gap between T-2 and T-1, and a small energy gap between S-1 and T-2, which favor the reverse intersystem crossing (RISC) from high-lying triplet levels to singlet levels. As a result, the sky-blue emission non-doped OLED based on the TPA-AN-BP reached maximum external quantum efficiency (EQE) of 4.39% and a high exciton utilization efficiency (EUE) of 77%. This study demonstrates a new strategy to construct highly efficient OLED materials.