Absence of delayed fluorescence and triplet–triplet annihilation in organic light emitting diodes with spatially orthogonal bianthracenes

Abstract

Two compounds, 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN), which has a single anthracene unit, and 10,10′-diphenyl-9,9'-bianthracene (PPBA), which has two spatially orthogonal anthracene units, were compared and investigated in terms of photoelectric characteristics and the reverse intersystem crossing (RISC) process in organic light emitting diodes (OLEDs). Transient electroluminescence (EL) measurements indicated large contributions of triplet–triplet annihilation (TTA) for MADN but almost no contribution of TTA for PPBA. The magnetic field dependence of EL for the two anthracene compounds was also different. The EL of MADN was sensitive to the magnetic field at high current density, but PPBA showed less dependence, which indicated the absence of the TTA process for PPBA. TD-DFT calculations revealed that PPBA has doubly degenerate lowest triplet states (T₁ and T₂) with a much lower energy than S₁, which is unfavorable for thermally activated delayed fluorescence (TADF). The near-zero ΔE_ST between highly excited states S_m≥1 and T_n>2 is favorable to RISC at a highly excited state. Oxygen quenching of photoluminescence only for PPBA and decreasing EL intensity with decreasing temperature only for PPBA support the existence of the RISC path from T_n>2 to S_m≥1. A high external quantum efficiency of 11% in blue OLEDs with PPBA was obtained, indicating that this orthogonal anthracene type of molecular design for RISC at a highly excited state would expand the material development of compounds emitting blue fluorescence for OLEDs.