Novel distyrylcarbazole derivatives as hole-transporting blue emitters for electroluminescent devices

We have synthesized three novel distyrylcarbazole derivatives for use as simultaneously hole-transporting and light-emitting layers in blue light-emitting diodes. Each compound, which contains a rigid carbazole core and two 2,2-diphenylvinyl end groups substituted through either the 3,6- or the 2,7- position, forms films satisfactorily and exhibits a blue emission with its PL maximum in the range 459-470 nm. Photophysical measurements indicate that twisting of the adjacent C-C bonds in the 3,6-position of the carbazole core in dilute solutions causes an efficient nonradiative relaxation to occur, yielding a much smaller quantum yield for fluorescence in 3,6-linked carbazoles. As intense emissions of 2,7-linked carbazoles are observed, such deactivation from an excited-state is inefficient therein. Electrochemical studies revealed that incorporation of the carbazole core increased the HOMO energy effectively; this feature facilitates hole injection. These distyrylcarbazole derivatives are promising bifunctional, blue-emitting, hole-transporting molecules for use in simple double-layer devices of a general structure ITO/emitting layer/TPBI/Mg: Ag, in which TPBI - 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene - serves as a hole-blocking and electron-transporting material. The devices prepared using 2,7-distyrylcarbazole as the emitter produced bright blue emissions having activating voltages below 3.0 V. A DPVTCz-based device attained a luminance efficiency of 3.11 cd A^-1 at 5 V, a brightness of 3062 cd m ^-2, and CIE color coordinates of (0.14, 0.22).