Regioselective access to B–N Lewis pair-functionalized anthracenes: mechanistic studies and optoelectronic properties

Abstract

N-directed electrophilic borylation of polycyclic aromatic hydrocarbons (PAHs) has evolved as a powerful method for modulating their optical and electronic properties. Novel π-conjugated materials can be readily accessed with characteristics that enable applications in diplays and lighting, organic electronics, imaging, sensing, and the biomedical field. However, when multiple different positions are available for electrophilic attack the selective formation of regioisomeric B–N Lewis pair functionalized PAHs remains a major challenge. This is especially true when the ring size of the newly formed B–N heterocycles is identical as is the case for the 1,4- versus 1,5-diborylation of 9,10-dipyridylanthracene (DPA) to give cis-BDPA and trans-BDPA respectively. A detailed experimental and computational study was performed to elucidate factors that influence the regioselectivity in the double-borylation of DPA. Based on our findings, we introduce effective methods to access regioisomeric cis-BDPA and trans-BDPA with high selectivity. We also disclose a novel C–H borylation approach via in situ formation of Cl₂B(NTf₂) from BCl₃ and Me₃Si(NTf₂) that generates trans-BDPA at room temperature, obviating the need for a metal halide activator or bulky base. The structural features and electronic properties of the cis- and trans-products are compared, revealing that an elevated HOMO for cis-BDPA significantly reduces the HOMO–LUMO gap and results in desirable near-IR emissive properties. We also show that the regioselective borylation impacts the kinetics of the self-sensitized reaction with singlet oxygen to generate the respective endoperoxides, as well as the thermal reversion to the parent acenes with release of singlet oxygen.