Stable, Bright, and Long-Fluorescence-Lifetime Dyes for Deep-Near-Infrared Bioimaging.

Near-infrared (NIR) fluorophores absorbing maximally in the region beyond 800 nm, i.e., deep-NIR spectral region, are actively sought for biomedical applications. Ideal dyes are bright, nontoxic, photostable, biocompatible, and easily derivatized to introduce functionalities (e.g., for bioconjugation or aqueous solubility). The rational design of such fluorophores remains a major challenge. Silicon-substituted rhodamines have been successful for bioimaging applications in the red spectral region. The longer-wavelength silicon-substituted congeners for the deep-NIR spectral region are unknown to date. We successfully prepared four silicon-substituted bis-benzannulated rhodamine dyes (), with an efficient five-step cascade on a gram-scale. Because of the extensive overlapping of their HOMO-LUMO orbitals, are highly absorbing (λ ≈ 865 nm and ε > 10 cm M). By restraining both the rotational freedom via annulation and the vibrational freedom via silicon-imparted strain, the fluorochromic scaffold of is highly rigid, resulting in an unusually long fluorescence lifetime (τ > 700 ps in CHCl) and a high fluorescence quantum yield (ϕ = 0.14 in CHCl). Their half-lives toward photobleaching are 2 orders of magnitude longer than the current standard (ICG in serum). They are stable in the presence of biorelevant concentration of nucleophiles or reactive oxygen species. They are minimally toxic and readily metabolized. Upon tail vein injection of (as an example), the vasculature of a nude mouse was imaged with a high signal-to-background ratio. dyes have broad potentials for bioimaging in the deep-NIR spectral region.