An aromatic imine group enhances the EL efficiency and carrier transport properties of highly efficient blue emitter for OLEDs

This study not only describes the synthesis and characterization of a novel indenofluorene derivative, 6,6,12,12-tetraethyl-2,8-bis-[1,1′;3′,1′]terphenyl-4′-yl-6,12-dihydro-indeno[1,2-b]fluorine (TP-EIF), for use as an emitting material in blue organic light emitting diodes (OLEDs), but also the comparison with 6,6,12,12-tetraethyl-2,8-bis-[1,1′;3′,1′]terphenyl-4′-yl-6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine (TP-EPY). UV-visible and PL spectra showed that the TP-EPY absorption and emission bands were red-shifted with narrow full widths at half maxima (FWHMs) compared to TP-EIF, both in solution and as a film. Density functional theory electronic structure calculations suggested that the spectral differences arose from the electron withdrawing properties of the imine group in the indenopyrazine, which increased conjugation in the core group and decreased conjugation in the side group at the LUMO level. The lower energy of the LUMO in TP-EPY resulted in increased active electron injection in electron-only devices and cyclic voltammetry (CV) measurements. The hole mobilities, which were measured by time of flight (TOF) methods, were 1.8 × 10−3 and 1.1 × 10−4 cm2 V−1 s for TP-EPY and TP-EIF, respectively, which demonstrated that TP-EPY displayed excellent hole transport abilities. The electron mobility was also high in TP-EPY (8.3 × 10−5 cm2 V−1 s). TP-EPY was, therefore, shown to be bipolar, unlike TP-EIF. The electroluminescence (EL) performance of TP-EPY in OLED devices was higher than that of TP-EIF: the luminance efficiencies were 1.72 and 0.76 cd A−1, and the power efficiencies were 0.65 and 0.27 lm W−1, for TP-EPY and TP-EIF, respectively. In particular, the FWHMs of the EL spectral bands were 54 nm for TP-EPY and 73 nm for TP-EIF, which demonstrated enhanced emission spectral purity. The external quantum efficiency of TP-EPY was 5.1%, which was more than twice the value for TP-EIF (2.19%).