Morphological stable spirobifluorene/oxadiazole hybrids as bipolar host materials for efficient green and red electrophosphorescence

Abstract

A series of 9,9'-spirobifluorene/oxadiazole hybrids with various linkages between two components, namely SBF-p-OXD (1), SBF-m-OXD (2), and SBF-o-OXD (3) are designed and synthesized through Suzuki cross-coupling reactions. The incorporation of a rigid and bulky spirobifluorene moiety greatly improves their thermal and morphological stability, with T(d) (decomposition temperature) and T(g) (glass transition temperature) in the ranges of 401-480 degrees C and 136-210 degrees C, respectively. 2 and 3 with meta- and ortho-linkage display higher triplet energy and blue-shifted absorption and emission than their para-linked analogue 1 owing to the decreasing pi-conjugation between the two components. Their HOMO and LUMO energy levels depend on the linkage modes within the range of 5.57-5.64 eV and 2.33-2.49 eV, respectively. Multilayer deep red electrophosphorescent devices with 1-3 as hosts were fabricated and their EL efficiencies follow the order of 3 (o)>2 (m)>1 (p), which correlates with their triplet energy and the separation of HOMO and LUMO distributions at molecular orbitals. The maximum external quantum efficiencies of 11.7% for green and 9.8% for deep red phosphorescent organic light-emitting diodes (OLEDs) are achieved by using 2 and 3 as host materials, respectively.