An acidic medium‐compatible deep‐near‐infrared dye for in vivo imaging

Abstract In vivo imaging in the deep near‐infrared (NIR) spectral region, that is, beyond 800 nm, has become popular due to its penetration depth. While imaging of the neutral medium/tissue has been repeatedly showcased, imaging of the high‐acidic medium remains challenging partly because of the high‐lying HOMO orbital and hence a high pKa of the electron‐donating group of the NIR fluorophores. We devised a novel electron‐donating group (D6) with which we further synthesized ECJ. ECJ exhibits an absorption wavelength beyond 900 nm and is fluorescent. Its pKa was found to be lower than zero, rendering it suitable for bioimaging of a highly‐acidic medium. Its potential for practical applications was showcased in proof‐of‐concept in vivo imaging with a mouse model.