Annulative Suzuki–Miyaura Coupling for the Synthesis of Highly Fluorescent Near-Infrared PAH[ b ]-Fused BODIPYs

A one-pot annulative Suzuki-Miyaura strategy was developed for the synthesis of highly fluorescent, near-infrared (NIR)-emissive polycyclic aromatic hydrocarbon (PAH)-fused BODIPYs. Tetrahalogenated BODIPYs reacted with 1,2-bis(pinacolato)boryl arene or alkene via a fourfold cross-coupling/annulation cascade under Pd(PPh3)4 catalysis, furnishing phenanthro[b]- or naphtho[b]-fused products in 32-70% yields without isolation of intermediates. Single-crystal X-ray diffraction revealed slightly twisted frameworks (dihedral angles: 3.17-10.01°), and NICS(0) calculations confirmed strong local aromaticity. Photophysical studies displayed intense absorption at 658-675 nm and narrow NIR emission at 670-690 nm, together with small Stokes shifts (≤322 cm-1) and high fluorescence quantum yields (ΦF = 0.46-0.68). Their weak solvatochromism and substantial HOMO-LUMO overlap indicate that the emissive excited states are predominantly locally excited (LE) in character. Cyclic voltammetry demonstrated narrow electrochemical band gaps (1.51-1.70 eV) and low LUMO energy levels (-3.82 to -3.88 eV for 3a-c and 4e), while DFT and TD-DFT calculations showed that the lowest-energy excitation is dominated by a HOMO → LUMO π → π* transition. This modular annulation strategy provides concise access to rigid PAH-fused BODIPYs that combine efficient NIR emission with high fluorescence efficiency, offering a versatile platform for the development of advanced NIR fluorophores.