Ladder-type organoboranes featuring intramolecular N→B coordination have been prepared through hydroboration of a 2-(ortho-styryl)pyridine (PhPy) with a series of hydroboranes, including 9H-9-borabicyclo[3.3.1]nonyl (9H-BBN), BH3 ⋅THF, HBCl2 ⋅SMe2 , HB(C6 F5 )2 , and a 9H-9-borafluorene derivative. The hydroboration reaction results in highly regioselective borylation under mild conditions and gives the products in good to excellent yields. The molecular structure and electronic properties of the obtained boranes have been experimentally investigated in detail, and complemented with DFT calculations to further elucidate the origin of differences in optical and electronic properties. The electron affinity of the conjugated system can be controlled through variation of the borane, while the optical properties are likewise directly linked to the type and molecular structure of the substituents on boron. The broad substrate range shows that this preparative approach is widely applicable to introduce chemically diverse boryl groups into conjugated systems.

An unusual reactivity of 2-(1-alkenyl)-pyridines towards hydroboration with 9H-borabicyclo[3.3.1]nonane (9H-BBN) has been employed to selectively introduce two borane groups into a conjugated quaterpyridine. Quantitative conversion of the substrate was observed with exclusive regioselectivity. A molecular structure that allows intramolecular N→B coordination was generated. The effect of the ladder formation on the molecular structure and the electronic properties of the conjugated system have been investigated. The synthetic strategy demonstrated herein offers a facile access to N→B ladder-type structures from readily available substrates, and allows to simultaneously introduce several boron centers under mild conditions.

In this work we report on a preparative strategy that allows the facile tuning of the electron affinities of π-conjugated organic n-type materials via intramolecular N→B coordination: 2fold hydroboration of a pyrazine-derived substrate with 9Hborabicyclo[3.3.1]nonane (9H-BBN) and Piers' borane ((C 6 F 5 ) 2 BH) furnishes the intramolecularly N→B-coordinated compounds PzBBN and PzBPF in high yield. For each borane racemic mixtures of the respective chiral derivatives and achiral meso compounds have been isolated, and their electrochemical, optical, and structural properties have been investigated and complemented by DFT calculations. The boranes exhibit high electron affinities with electrochemical LUMO energy levels of −3.69 and −4.30 eV, respectively. The corresponding radical anions PzBBN •− and PzBPF •− can be reversibly generated and are stable in solution at ambient temperature. Analysis of the radical species by EPR spectroscopy and single-crystal analysis of [PzBPF] •− [CoCp 2 ] + allowed further corroboration of the π-delocalized nature of these radicals.

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, (C 6 F 5) 2 B-H (BPF-H), Cl 2 B-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 −3.75 eV (PPzBBN, E g opt = 2.16 eV) and −4.35 eV (PPzBPF, E g opt = 2.07 eV) can be reached for 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 , E g opt = 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 × 10 −5 cm 2 V −1 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.

A series of electronically and structurally diverse N → B-ladder boranes has been prepared by hydroboration.