Synthesis, photophysical, theoretical and electroluminescence study of triphenylamine-imidazole based blue fluorophores for solution-processed organic light emitting diodes

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The deep blue organic light emitting diodes (OLEDs) are necessary for solid-state lighting (SSL) and flat panel displays. In the practical application of OLEDs, high efficiencies and luminance with simple device structure, are highly desirable. Hence, we have designed and synthesized three blue (Donor-Acceptor) fluorophores with linear, bent and star-shaped molecular geometry by incorporating triphenylamine (TPA) as a hole transporting unit and imidazole as an electron transporting unite with electron withdrawing Ph-CF3 moiety at the N1 position of imidazole unit. The synthesized fluorophores were structurally characterized by Nuclear Magnetic Resonance Spectroscopy (NMR) and Mass spectroscopy. The linear-shaped fluorophore structurally characterized by single crystal X-ray diffraction. Photophysical, electrochemical and electronic (theoretical) properties of the synthesized fluorophores were systematically investigated. All the fluorophores are showing deep blue emission in DCM solution. The synthesized fluorophores are shown good quantum yield (Φ) in solution and solid phase. The structurally simple OLEDs devices were fabricated by using these fluorophores as emitters. All the fluorophores showed deep blue electroluminescence (EL) emission with good device efficiency. Among all the devices, based on TBIMTPA with a 5 wt % dopant concentration exhibited better performance in the series with a maximum luminance of 994 cd/m2, a current efficiency of 2.6 cd/A, a power efficiency of 1.0 lm/W and an EQE of 3.2% at brightness of 100 cd/m2. These results indicate that the introduction of imidazole-based accepter on the TPA core is a favorable design strategy towards deep blue emitters.