π‑Conjugated Polymers Composed of BODIPY or Aza-BODIPY Derivatives Exhibiting High Electron Mobility and Low Threshold Voltage in Electron-Only Devices

Abstract

We present efficient electron-transport materials based of polymers. π-Conjugated copolymers composed of boron dipyrromethene (BODIPY) or Aza-BODIPY were synthesized via the efficient Suzuki–Miyaura cross-coupling reaction of (2,5-bis­(2-(2-(2-(pyridin-2-yloxy)­ethoxy)­ethoxy)­ethoxy)-1,4-phenylene)­diboronic acid with each of the diiodo-substituted BODIPY and Aza-BODIPY. Synthesized polymers exhibited high solubility even in polar solvents such as acetic acid. Their electronic and optical properties were studied by cyclic voltammetry, UV–vis absorption, and photoluminescence spectroscopies. The absorption and photoluminescence spectra of the obtained polymers were red-shifted in comparison with the corresponding monomers due to the increase in the HOMO level by the formation of donor–acceptor interactions and the expansion of main-chain conjugations, explained by their cyclic voltammograms and theoretical calculations of the model compounds using the density-functional theory method. Finally, the electron mobilities of the polymers were determined from the space-charge-limited current with electron-only device structure of ITO/Ca/polymer/BCP/LiF/Al. As a result, owing to their high electron acceptability and strong stacking interaction among the BODIPY or Aza-BODIPY units, it was found that the mobilities for the polymers ((1.5–3.6) × 10^–4 [cm² V^–1 s^–1]) were much higher than the value of Alq₃ (5.8 × 10^–5 [cm² V^–1 s^–1]), and their threshold voltages (5–7 V) were much smaller than that of the Alq₃ device (12 V)