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

We present efficient electron-transport materials based of polymers. pi-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) x 10(-4) [cm(2) V-1 s(-1)]) were much higher than the value of Alq(3) (5.8 x 10(-5) [cm(2) V-1 s(-1)]), and their threshold voltages (5-7 V) were much smaller than that of the Alq(3) device (12 V).