Three near-infrared emitters (2TPA-QBT, 2MeTPA-BT and TPA-QBT-MeTPA) were rationally designed and synthesized. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations showed that the introduction of mono- or di-methyl groups between the donors and acceptor could result in the spatial configuration changing greatly for 2MeTPA-QBT and TPA-QBT-MeTPA compared to their parent compound 2TPA-QBT. The emission of TPA-QBT-MeTPA had a more obvious hybridized local and charge transfer feature (HLCT) based on the influence of the steric hindrance of the methyl substituent. Attributed to their different spatial configurations and luminescence mechanisms, different emission wavelengths with photoluminescent quantum yields of 26%, 38% and 34% in toluene, as well as 24%, 27% and 31% in 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) doped film, were observed for 2TPA-QBT, 2MeTPA-QBT and TPA-QBT-MeTPA, respectively. The constructed organic light-emitting devices (OLEDs) displayed electroluminescence with emission peaks at 728, 693 and 710 nm, with maximum external quantum efficiencies of 1.58%, 1.33% and 3.02% for the 2TPA-QBT, 2MeTPA-QBT and TPA-QBT-MeTPA-doped OLEDs, respectively. This work illustrated the effect of spatial configuration changes on the luminescence properties of donor-acceptor-type organic emitters.
Keywords: electroluminescence; hybridized local and charge transfer; near-infrared emitters; organic light-emitting diodes.