Tandem Auxochrome Design Enables Ready Access to Deep Shortwave Infrared Dyes

The shortwave infrared (SWIR, 1.0-2.0 μm) window is of growing importance for advanced photonic applications owing to its minimal attenuation in complex media. However, achieving full spectral coverage with compact organic molecules remains a formidable challenge, as conventional bathochromic strategies typically necessitate extensive π-extension or intricate fused-ring architectures. Here, we introduce a tandem auxochromic design wherein two simplified amino auxochromes are electronically coupled through a π-conjugated spacer. This configuration cooperatively lowers the effective ionization energy and amplifies conjugative coupling with the chromophore backbone, facilitating more efficient bandgap narrowing than traditional extension methods. When integrated onto a compact, charge-resonance-delocalized fluorenium scaffold, this approach enables continuous SWIR spectral tunability via a concise three-step synthesis, notably extending into the deep-SWIR region (>1.5 μm) at an exceptionally low molecular weight of 590 Da. The resulting dyes exhibit high spectral stability in polar solvents and strong resistance to high laser power and oxidative environments. We demonstrate the clear advantages of our approach through high-resolution in vivo and multiplexed SWIR imaging, while this work simultaneously provides a convenient entry for exploring SWIR photonic phenomena using organic materials.