2‐Azaazulenium Hemicyanine Dyes for NIR‐II Multimodal Imaging and Combined Phototherapy

ABSTRACT Hemicyanine dyes are well‐defined donor–π–acceptor molecules whose photophysical and biological properties can be precisely tuned through rational molecular design. However, efficient emitters operating in the second near‐infrared (NIR‐II) region remain scarce, hindering their broader biomedical applications. Here, we report a strategically engineered series of 2‐azaazulenium hemicyanines that achieve bright NIR‐II emission together with multifunctionality. Among them, SWJT‐OCH 3 integrates intense NIR‐II fluorescence with outstanding photothermal and photodynamic performance, enabling multimodal imaging and combined phototherapy when encapsulated into biocompatible nanoparticles (SWJT NPs). Furthermore, an activatable probe (SWJT‐Probe) derived from this platform allows highly sensitive and selective detection of peroxynitrite (ONOO − ) both in vitro and in vivo. This work establishes 2‐azaazulenium hemicyanines as versatile molecular scaffolds and provides a promising design paradigm for multifunctional NIR‐II fluorophores with broad biomedical potential.