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, holetransporting 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 5V, a brightness of 3062 cd m(-2), and CIE color coordinates of ( 0.14, 0.22).