Through-Space vs Through-Bond Charge Transfer Fluorophores as Emitters for Efficient Blue Electroluminescent Devices

Herein, we report the design, synthesis, and properties of two donor-π-acceptor (D-π-A) charge transfer fluorophores pursuing an efficient deep blue emitter for organic light-emitting diode (OLED). TCyCN and TPhCN comprise triphenylamine as a donor and benzonitrile as an acceptor connecting by either [2.2]paracyclophane (Cy) as a through-space π-linker or phenyl ring (Ph) as a through-bond π-linker, respectively. In thin film, TCyCN shows deep blue emission with moderate fluorescence efficiency, while TPhCN displays cyan blue emission with a high fluorescence efficiency. The two molecules feature hybridized local and charge-transfer states with good thermal stability and balanced charge transport properties. They are successfully employed as non-doped blue emissive layers in OLEDs. The TCyCN-based device emits deep blue light peaked at 425 nm (CIE coordinates of (0.157, 0.076)) with a moderate electroluminescent (EL) performance (EQE_max=3.23 % and CE_max=2.06 cd A⁻¹), while the TPhCN-based device attains an excellent EL performance (EQE_max=6.24 % and CE_max=8.26 cd A⁻¹) with cyan blue emission peaked at 490 nm. This work provides insight into the relationship between molecular design and properties of D-π-A emitters, offering a guideline for tailoring new organic compounds for organic optoelectronics.