Multi-H Shaped Macrocyclic Oligomers Consisting of Triphenylamine and Oligofluorene: Synthesis and Optoelectronic Properties

Novel multi-H shaped macrocyclic oligomers ((TPAF 3 ) 3 and (TPAF 5 ) 3 ) were designed and synthesized for use as hole-transporting emitters in blue-light-emitting diodes (LEDs). In the produced macrocyclics, three triphenylamine units are connected in series to form a macrocyclic core through the C-9's of the central fluorene units of three surrounding oligofluorenes, which, like rigid rods, stick out of the ring plane in both sides. This specific structure renders the macrocyclics better photoluminescence (PL) stabilities and lower crystallization tendencies than polyfluorenes and excellent optoelectronic properties. The macrocyclics emit deep blue fluorescence with high efficiencies both in solutions and thin films (Φ pl-film = 57% for (TPAF 3 ) 3 and 52% for (TPAF 5 ) 3 ). (TPAF 5 ) 3 films are stable against both electrochemical oxidation and reduction, with CV curves remaining unchanged in 10 successive potential scans. High-efficiency blue-light-emitting diodes were fabricated using the macrocyclics as the hole-transporting emitting layer. The device structure used in this study is ITO/(TPAF 3 ) 3 or (TPAF 5 ) 3 )/TPBI//LiF/Al, where TPBI, 1,3,5-tris( N -phenylbenzimidazol-2-yl) benzene, is a morphologically stable material functioning as a hole-blocking/electron-transporting layer. The (TPAF 5 ) 3 -based devices displayed a low turn-on voltage of 4 V and a maximum luminance of 1716 cd/m 2 at a voltage of 14.1 V; the (TPAF 3 ) 3 -based devices gave 901 cd/m 2 at 13.0 V. The maximum luminous efficiencies of the (TPAF 3 ) 3 - and (TPAF 5 ) 3 -based devices reached 0.63 cd/A at 187 cd/m 2 and 0.93 cd/A at 340 cd/m 2, respectively.