Quadruply B←N-Fused Dibenzo-azaacene with High Electron Affinity and High Electron Mobility

Abstract

For many years, organoboron compounds have been expected to show excellent electron-injecting and -transporting properties. However, lowest unoccupied molecular orbital (LUMO) energy levels (<i>E</i>_LUMO) of B-containing π-conjugated molecules are mostly higher than -4.0 eV and their electron mobilities are usually less than 10^-2 cm² V^-1 s^-1. In this work, we experimentally prove the remarkably high electron affinity and high electron mobility of organoboron compounds. Our strategy is to incorporate multiple boron-nitrogen coordination bonds (B←N) into azaacenes. We synthesized quadruply B←N-fused dibenzo-azaacene (<b>QBNA</b>) through one-pot multifold borylation cyclization reaction. The incorporation of four B←N units greatly changes the electronic structures and properties and significantly downshifts the electronic energy levels of <b>QBNA</b>. <b>QBNA</b> shows a <i>E</i>_LUMO of as low as -4.58 eV, which is among the lowest for n-type organic semiconductors. Single-crystal organic field-effect transistors of <b>QBNA</b> display unipolar n-type characteristic with an electron mobility of up to 1.60 cm² V^-1 s^-1 together with excellent ambient stability. This study thus provides a design strategy for high-performance n-type organic semiconductors and high electron-affinity π-systems based on organoboron chemistry.