Oxa-phenalene [b]-fused BODIPY dyes: Synthesis, structures, and photophysical properties

Conventional BODIPY dyes exhibit narrow absorption and emission bands in the visible spectrum, limiting their utility in near-infrared (NIR) applications. Addressing this challenge—designing BODIPY-based NIR fluorophores with broad spectral features—requires strategic π-conjugation extension and heteroatom fusion. In this work, we synthesized four novel oxa-phenalene [b]-fused BODIPY dyes (OFB-1–OFB-4) via regioselective nucleophilic aromatic substitution and palladium-catalyzed intramolecular Heck coupling, starting from mono- or dichloro-BODIPY precursors and 1-hydroxy-8-bromonaphthalene. The fused oxa-phenalene unit enforces exceptional coplanarity within the BODIPY core, driving substantial bathochromic shifts into the NIR region. The bis-fused derivative, OFB-2, achieves absorption and emission maxima at 707 nm and 725 nm, respectively. Remarkably, the mono-fused derivative OFB-1 exhibits dual absorption bands with a full-width at half-maximum (FWHM) of 102 nm, a hallmark of intramolecular charge transfer (ICT) effects. Electronic structure analysis reveals that oxa-phenalene [b] fusion uniquely modulates the frontier molecular orbitals: it minimally impacts the LUMO energy level but significantly elevates the HOMO energy level, thereby narrowing the HOMO–LUMO gap. This contrasts sharply with traditional [b]-fused BODIPYs, where π-extension typically reduces LUMO levels. The selective HOMO elevation in oxa-phenalene-fused systems highlights its potential for tailoring optoelectronic properties while retaining desirable electron-transport characteristics.