Simple phenyl bridge between cyano and pyridine units to weaken the electron-withdrawing property for blue-shifted emission in efficient blue TADF OLEDs

Abstract

Abstract A blue-emitting D-A1-Ph-A2 type compound 4-(2,3,5,6-tetra(9H-carbazol-9-yl)pyridin-4-yl)benzonitrile (4CzCNPhPy) is designed and synthesized by introducing a phenyl bridge to green emissive 2,3,5,6-tetracarbazole-4-cyano-pyridine (4CzCNPy). The phenyl extends the backbone of acceptor unit and employed as the conjugated bridge to share the electron distribution between strong electron-withdrawing cyano and pyridine moieties. As a result, the peak of intramolecular charge transfer (ICT) absorbance in CH2Cl2 and film is blue-shifted from 452 and 471 nm of 4CzCNPy to 391 and 382 nm of 4CzCNPhPy, which directly contributes to hyperchromatic-shifted emission from 535 and 555 nm of 4CzCNPy to 491 and 497 nm for the phenyl bridged 4CzCNPhPy in CH2Cl2 solution and neat film state respectively. 4CzCNPhPy with phenyl bridge exhibited higher glass transition temperatures (188 °C), shorter transient delayed decay time of 6.9 μs, higher triplet energy level of 2.57 eV and higher lowest unoccupied molecular orbitals (LUMO) of −2.78 eV compared to 159 °C, 10.2 μs, 2.21 eV and −3.26 eV for 4CzCNPy. In addition, the relatively high photoluminescence quantum yield of 51/61% at air/N2 conditions and low singlet-triplet bandgap of 0.04 eV permits 4CzCNPhPy as a potentially favorable blue-emissive delayed fluorescent emitter. By using 4CzCNPhPy as blue delayed fluorescence dopant in vacuum deposited device, a maximum external quantum efficiency of 13.7% is achieved at a satisfactory Commission International de I'Eclairage (CIE) coordinate of (0.15, 0.20).