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

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).