Regioselective construction of two isomeric BN-fused aromatic frameworks enabling the synthesis of ultralong room-temperature phosphorescence materials

Abstract

BN-fused aromatic compounds have garnered significant attention due to their unique electronic structures and exceptional photophysical properties, positioning them as highly promising candidates for applications in organic optoelectronics. However, the regioselective synthesis of BN isomers remains a formidable challenge, primarily stemming from the difficulty in precisely controlling reaction sites, limiting structural diversity and property tunability. Herein, we propose a regioselective synthetic strategy that employs 2,1-BN-naphthalene derivatives, wherein selective activation of N–H and C–H bonds is achieved in conjunction with ortho-halogenated phenylboronic acids. Under uniform reaction conditions, two distinct boron–nitrogen fused ring isomers were successfully synthesized. A computational analysis of C–X bond dissociation energies indicates that the observed regioselectivity is most likely governed by the interplay of halogen electronegativity, atomic radius, and bond dissociation energy parameters. Interestingly, the two isomers exhibit markedly distinct room-temperature phosphorescence (RTP) in polyvinyl alcohol (PVA). Specifically, 3a@PVA demonstrates ultralong RTP characteristics, featuring an exceptionally long phosphorescence lifetime of up to 2388.2 ms and an afterglow persisting for more than 30 seconds, significantly longer than the 286.1 ms observed for 4a@PVA. Theoretical investigations reveal that 3a possesses a higher spin–orbit coupling constant and more intersystem crossing channels than 4a. Additionally, the dual-sided fixation of the 3a@PVA system imposes significant constraints on intramolecular motions, effectively suppressing non-radiative decay pathways, which accounts for its distinct afterglow behaviors. These divergent photophysical characteristics significantly enhance their potential applications in advanced anti-counterfeiting technologies. This work not only establishes a versatile synthetic strategy for the regioselective construction of BN-fused isomers but also provides fundamental insights into the rational design of BN-incorporating organic RTP systems.