Benzoheterocyclic [b]-fused BODIPYs: synthesis and effects of N, O, and S on structure, photophysical properties, and aggregation

Abstract

Next-generation optoelectronic devices require organic small-molecule light-emitters with high luminous efficiency and strong resistance to non-radiative decay. BODIPY dyes fulfill these criteria, particularly when fused with heteroaromatic rings, which enhances emission efficiency and induces redshift by suppressing conformational relaxation and non-radiative pathways. Herein, we reported the synthesis of six symmetric and asymmetric benzoheterocyclic [b]-fused BODIPYs incorporating nitrogen, oxygen, or sulfur atoms, addressing the underexplored impact of heteroatom type on BODIPY optoelectronic behavior. Nitrogen- and oxygen-containing derivatives were synthesized via a one-pot protocol combining nucleophilic aromatic substitution (S_NAr) and C–H activation. A new method using ¹⁹F NMR was developed for the structural identification of [b]-fused heteroaromatic BODIPYs. Single-crystal X-ray diffraction, electrostatic potential (ESP) mapping, and NICS(0) calculations demonstrated that heteroatoms, through electronegativity and intermolecular interactions, promote the formation of one-dimensional J-type aggregates, facilitating electron delocalization and charge transfer. Photophysical studies revealed significantly increased fluorescence quantum yield (75.7%–85.4%) and enhanced resistance to non-radiative decay. Water-dependent spectroscopic measurements confirmed J-aggregation and associated spectral changes. Electrochemical analysis highlighted the effect of asymmetric heteroatoms on HOMO/LUMO energy levels, supporting a proposal of a “half-sum rule”, consistent with DFT/TD-DFT calculations. These findings provide valuable insights for the rational design of advanced optoelectronic molecular materials.