Neutral Cyanine: Ultra‐Stable NIR‐II Merocyanines for Highly Efficient Bioimaging and Tumor‐Targeted Phototheranostics

Fluorescence imaging (FLI)‐guided phototheranostics using emission from the second near‐infrared (NIR‐II) window show significant potential for cancer diagnosis and treatment. Clinical imaging‐used polymethine ionic indocyanine green (ICG) dye is widely adopted for NIR fluorescence imaging‐guided photothermal therapy (PTT) research due to its exceptional photophysical properties. However, ICG has limitations such as poor photostability, low photothermal conversion efficiency (PCE), short‐wavelength emission peak, and liver‐targeting issues, which restrict its wider use. In this study, two ionic ICG derivatives are transformed into neutral merocyanines (mCy) to achieve much‐enhanced performance for NIR‐II cancer phototheranostics. Initial designs of two ionic dyes show similar drawbacks as ICG in terms of poor photostability and low photothermal performance. One of the modified neutral molecules, mCy890, shows significantly improved stability, an emission peak over 1000 nm, and a high photothermal PCE of 51%, all considerably outperform ICG. In vivo studies demonstrate that nanoparticles of the mCy890 can effectively accumulate at the tumor sites for cancer photothermal therapy guided by NIR‐II fluorescence imaging. This research provides valuable insights into the development of neutral merocyanines for enhanced cancer phototheranostics. A NIR‐II emissive neutral merocyanine dye is developed based on the clinically used ionic indocyanine green, exhibiting maintained strong near‐infrared absorption, improved photostability, enhanced photothermal performance, bright NIR‐II emission, and specific tumor accumulation. The mCy890 NPs are successfully utilized for efficient in vivo NIR‐II fluorescence bioimaging and cancer phototheranostics.