Novel fluorine-containing X-branched oligophenylenes: Structure-hole blocking property relationships

Four fluorine-containing X-branched oligophenylenes, X-OPP(5)-Fn, n = 1,2,3 and X-OPP(5)-CF₃, have been synthesized and studied as potential hole-blocking materials. These molecular materials exhibit a low HOMO energy level (ca. 6.5 eV) and a large energy gap (ca. 3.5 eV), indicating that they can be employed as effective hole-blockers in blue-emitting organic EL devices. Four-layer organic EL devices consisting of CuPc as a hole injection layer, α-NPD as a hole-transporting and emitting material, X-OPP(5)-Fn or X-OPP(5)-CF₃ as a hole-blocker, and Alq₃ as an electron-transporter sandwiched between indium tin oxide (ITO) and LiF/Al electrodes were fabricated, and their performance was examined and compared with the widely used holeblocker bathocuproine (BCP), the non-fluorine-containing X-branched analogue X-OPP(5)-H and the X-branched analogue with one less phenyl ring, X-OPP(3)-F₂. The preliminary results show that these new molecular materials, especially X-OPP(5)-F₂, are excellent hole-blockers for blue-emitting organic EL devices.