Efficient and Metal-Free Synthesis of Functionalized Unsymmetrical Pyrrolyl–Pyridine Boron Difluoride Fluorophores

Abstract

The rapid and sustainable construction of highly functionalized fluorophores remains a key challenge in organic synthesis. We detail a concise, metal-free synthetic platform for the preparation of unsymmetrical pyrrolyl–pyridine boron difluoride (BOPP) complexes from simple chalcone precursors. Central to this methodology is the optimization of a thiamine-mediated Stetter reaction under mild sonication, which efficiently generates key 1,4-diketone intermediates. Subsequent Paal-Knorr cyclization and BF3 complexation afford the desired BOPP dyes in good to excellent overall yields. This modular synthetic approach enabled facile expansion of the BOPP library, overcoming the inherent limitations of conventional cross-coupling methods. The synthesized BOPP fluorophores emit brightly in both solution and the solid state, featuring highly tunable emission profiles and large Stokes shifts of up to 88 nm. Supported by density functional theory (DFT) calculations, this study highlights the robustness of the synthetic protocol and establishes the unsymmetrical BOPP scaffold as a versatile platform for the development of next-generation fluorescent probes and functional optical materials.