Regioselective –NO₂ Substitution Enables Tunable Photophysics and Molecular Packing in a Polycyclic 1,2-BN Heteroarene Framework

Abstract

Heteroarenes containing boron-nitrogen (BN) bonds have attracted significant interest due to their enhanced optoelectronic properties. The introduction of a nitro (-NO₂) group into heteroarenes is particularly appealing because its strong electron- withdrawing character lowers the LUMO energy, reducing the HOMO-LUMO gap and inducing bathochromic shifts in both absorbance and emission spectra. However, NO₂-substituted heteroarenes rarely fluoresce and typically suffer from aggregation-caused quenching (ACQ) of emission, limiting their practical applications. While strategies have been developed to counteract ACQ in NO₂-functionalized systems, little attention has been directed toward NO₂-substituted BN- heteroarenes, particularly three-coordinate boron species such as pyrrolidinone-fused-1,2-BN-heteroarenes (PBNHs). In this study, we address the ACQ issue in NO₂-substituted 1,2-BN-heteroarenes by synthesizing four PBNHs, denoted as NO₂-PBNHs 8, 9, 10, and 11, each featuring a -NO₂ group at a distinct position (C8-C11) on the common scaffold. Using comprehensive photophysical analyses and time-dependent density functional theory (TD-DFT) calculations, we systematically evaluated how the -NO₂ substituent’s position influences photophysical properties and molecular packing. This work presents the first examples of electron-poor PBNHs with an identical framework exhibiting multifunctional, stimuli-responsive fluorescence properties. Modest positional changes of the -NO₂ group produced unexpected behaviors. NO₂-PBNHs 8 and 10 display both major aggregation-induced emission enhancement (AIEE) and solvatochromism within a single BN-aromatic backbone. NO₂-PBNH 9, although prone to ACQ, exhibited positive solvatochromism, robust reversible thermochromism, and a unique pitched π-stacking motif, highlighting its potential for temperature-sensing and charge- transport applications. NO₂-PBNH 11 uniquely exhibited both minor ACQ and major AIE properties in one BN-aromatic backbone. Together, these findings demonstrate that regioselective nitration is an effective strategy to tune molecular packing and fluorescence behavior in BN-heteroarenes. Furthermore, cytotoxicity assays confirmed that NO₂-PBNHs 8-11 are biocompatible, indicating potential for biological imaging. Overall, this study advances the understanding of aggregate formation in NO₂-substituted PBNHs and provides design principles for next generation NO₂-functionalized luminophores.