Rational Design and Application of an Indolium-Derived Heptamethine Cyanine with Record-Long Second Near-Infrared Emission

Heptamethine cyanine dyes, typified by indocyanine green, have been extensively employed as bioimaging indicators and theranostic agents. Significant efforts have been made to develop functional heptamethine cyanine dyes with outstanding bioimaging and theranostic utilities. In this work, we rationally designed and successfully developed a novel indolium-like heptamethine cyanine dye by installing indolium-derived polycyclic aromatic hydrocarbons on the terminal ends of a conjugated polyene backbone. This dye showed excellent photostability and showed bright fluorescent emission in the second near-infrared (NIR-II) window with a peak at approximately 1120 nm. Such long wavelength emission prompted a superior bioimaging resolution in vivo. In particular, this NIR-II dye had the remarkable capability of marking the blood vessels of the hindlimbs, abdomens, and brains of mice. More significantly, this dye involved a typical indolium-like heptamethine skeleton and exhibited two strong absorption bands in the 700–1300 nm NIR range, which endowed it with an intrinsic tumor-targeting capability and a high photothermal conversion efficiency (up to 68.2%), serving for the photothermal therapy of tumors under the guidance of NIR-II fluorescence imaging. This work provides an efficient design strategy for achieving indolium-like heptamethine cyanine dyes with further NIR-II emission.