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

Abstract

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.