A new substitution pattern of BF azadipyrromethene (azaBODIPY) dyes was obtained by phenanthrene fusion through a key palladium-catalyzed intramolecular C-H activation reaction. These [a]-phenanthrene-fused azaBODIPYs have a near planar structure of the phenanthrene-fused azadipyrromethene core in the crystalline state. The chromophore absorbs (log ε > 5) and fluoresces (ϕ = 0.32-0.38) strongly above 700 nm with excellent photostability and may be used as an attractive bright NIR bioimaging agent.

A new class of phenanthrene-fused BF azadipyrromethene (azaBODIPY) dyes have been synthesized through a tandem Suzuki reaction and oxidative ring-fusion reaction, or a palladium-catalyzed intramolecular C-H activation reaction. These phenanthrene-fused azaBODIPY dyes are highly photostable and display markedly redshifted absorption (up to λ=771 nm) and emission bands (λ≈800 nm) in the near-infrared region. DFT calculations and cyclic voltammetry studies indicate that, upon annulation, more pronounced stabilization of the LUMO is the origin of the bathochromic shift of the absorption and high photostability.

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