Pyrene-centered cyanophenyl end-capped starbursts: Design, synthesis, stabilized blue electroluminescence and lasing properties

A novel class of monodisperse starburst molecules (T1-T3), which are comprised of a pyrene core, four short oligofluorene arms and cyanophenyl end-cappers, were designed and synthesized to explore the influence of electron-withdrawing moieties on their optoelectronic properties. The resulting materials demonstrated high fluorescence yields and excellent thermal stability with a high degradation temperature (T_d) of over 400 °C. Non-doped electroluminescent devices with the configuration of ITO/PEDOT:PSS/T1-T3/TPBI/LiF/Al exhibited highly efficient and stabilized blue electroluminescence (EL). Among all these solution-processed devices, T1 possessed the best performance with a maximum luminous efficiency (LE) of 3.52 cd A^-1 at 25 mA cm^-2 and a maximum brightness of 9194 cd m^-2 at 10 V, resulting from the improved electron affinity due to the large content of electron-withdrawing cyanophenyl moieties in the starbursts. Notably, the net gain coefficients reached 46-55 cm^-1. Moreover, the amplified spontaneous emission (ASE) threshold value remained detectable upon thermal annealing temperatures up to 230 °C for T1 and 280 °C for T2-T3. 1D DFB lasing (50% fill factor, pumped source at 375 nm) was thus obtained, demonstrating a low lasing threshold. The results confirm that the incorporation of cyanophenyl moieties as end cappers can enhance the electrical properties of pyrene-centered starbursts without substantially sacrificing their great optical gain properties, which makes this series of materials very promising for electrically pumped organic lasers.