Synthesis and Spectroscopic and Cellular Properties of Near-IR [a]Phenanthrene-Fused 4,4-Difluoro-4-bora-3a,4a-diaza‑s‑indacenes

A new synthetic method to build aryl-fused 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPYs) is reported. The intramolecular cyclization step was completed in a short time (1–2 h) and in high yields (>90%), due to the intrinsic rigid structural conformation of the precursor BODIPY and the high reactivity of its 1,7-bromo groups. The [a]­phenanthrene-fused BODIPYs 4a–c were characterized by NMR spectroscopy, HRMS, DFT calculations, and, in the case of 4a, by X-ray crystallography. Spectroscopic studies show that 4a–c strongly absorb and emit in the NIR spectral region, in the range 642–701 nm. In addition, BODIPYs 4b and 4c exhibit no toxicity in the light or dark in HEp2 cells and accumulate intracellularly in a time-dependent manner, mainly in the cell endoplasmic reticulum. These results suggest the potential use of [a]­phenanthrene-fused BODIPYs as NIR bioimaging probes.