N → B Ladder Polymers Prepared by Postfunctionalization: Tuning of Electron Affinity and Evaluation as Acceptors in All-Polymer Solar Cells

A poly(biphenylene-pyrazinylene) (PPz, E-g(opt) = 3.10 eV) and a head-to-tail regioregular polypyridine (rr-P4Py, E-g(opt) = 3.25 eV) equipped with 1-alkenyl side chains have been prepared and postfunctionalized by hydroboration with different hydroboranes (9H-BBN, (C6F5)(2)B-H (BPF-H), C12B-H) to give the corresponding ladder polymers featuring intramolecular coordinative N -> B bonds. Characterization of the optical and electrochemical properties of the postfunctionalized polymers shows that the borylation strongly increases their electron affinity and lowers the optical gaps. Electron affinities between -3.7S eV (PPzBBN, E-g(opt) = 2.16 eV) and -4.35 eV (PPzBPF, E-g(opt) = 2.07 eV) can hydroborated PPz, while rr-P4Py-derivatives reach LUMO levels of -3.45 eV (P4PyBBN, E-g(opt) = 2.88 eV), -3.85 eV (P4PyBPF, E-g(opt) = 2.95 eV), and -4.15 eV (P4PyBCl(2), = 2.95 eV). The potential of this class of compounds as electron acceptors is demonstrated by the investigation of the semiconducting properties of PPzBBN and PPzBPF, which showed ambipolar charge transport with hole and electron mobilities in order of 2 X 10(-5) cm(2) s(-1). The polymers were tested as acceptors in all-polymer solar cells, which yielded functioning devices, with open-circuit voltages that directly reflect the electron affinity of the employed acceptor.