"Crossbreeding" Small-Molecular Weight NIR-II Flavchromenes Endows Activatable Multiplexed In Vivo Imaging

High-performance near-infrared-II (NIR-II) fluorophores have attracted tremendous attention for in vivo dynamic bioimaging. However, the lack of stable, bright, and biocompatible molecular fluorophores becomes a major barrier on the way to the widespread endorsement of NIR-II bioimaging for clinic diagnosis. Here we propose a pi-conjugated "crossbreeding" dyad strategy to develop novel desirable NIR-II dyes, flavchromenes (flav+chromene) integrated from two individual fluorophores of flavylium and chromene via a short methine bridge, affording valuable features of extending spectra into the NIR-II region and conferring a substantive leap in the photophysical properties. Notably, these crossbreeding NIR-II dyes have small molecular weights, high molar extinction coefficients, and outstanding chemical stability. As demonstrated, the elaborated probe Flavchrom-4 endows activatable dual-modal in vivo imaging of endogenous ss-galactosidase activity, along with lighting-up NIR-II fluorescence signals and dramatically enhanced photoacoustic signals (16.3-fold). This concise pi-conjugated crossbreeding dyad strategy brings forth high-performance NIR-II fluorophore scaffolds with small molecular weights, high stability, and good brightness, thus greatly expanding high-resolution activatable in vivo imaging tools for intact tissues and living animals.