D-n-A-n-D-type Fluorophores based on Pyridal[2,1,3]thiadiazole acceptor with hybridized local and charge-transfer excited-state for high-efficiency OLEDs

Hybridized local and charge-transfer (HLCT) excited-state fluorophores, which are characterized by high pho-toluminescence quantum yield (PLQY) and reverse intersystem crossing (RISC) from high-lying triplets to sin-glets, has emerged advantages in organic light-emitting diodes (OLEDs) application. Although blue and green HLCT fluorophores are well developed, the long wavelength ones (orange to red) are still behind. Here, based on more electron-deficient Pyridal[2,1,3]thiadiazole (PT) acceptor, a D-n-A-n-D-type HLCT emitter, namely 2CZP-PT, with orangered emission is designed and synthesized. Due to the less steric hindrance of pyridine nitrogen in PT, a small twist angle between the n bridge and acceptor on the nitrogen side is obtained, which endows the whole molecule with relative planar conformation. Thus 2CZP-PT exhibits HLCT characteristics with high PLQY (77% in THF solution and 44% in neat film), as well as energy levels arrangement favoring the RISC process from high-lying triplet levels to singlet levels. The doped devices based on 2CZP-PT reached a maximum external quantum efficiency (EQEs) of 9.57% with an electroluminescence (EL) peak at 568 nm. While the non-doped OLEDs based on 2CZP-PT emitter achieved a maximum EQEs of 1.34% with red EL peaked around 600 nm, whose efficiency roll-off is only 1.5% at 1000 cd m- 2. These device performances demonstrate PT based D-n-A-n-D-type fluorophores are promising for their long-wavelength OLEDs application.