Remarkable Suppression of Vibrational Relaxation in Organic Semiconducting Polymers by Introducing a Weak Electron Donor for Improved NIR‐II Phototheranostics

Abstract Exploration of high‐efficiency agents for near‐infrared‐II fluorescence imaging (NIR‐II FI) promotes the development of NIR‐II FI in life science. Despite the extensive use of organic semiconducting nanomaterials for NIR‐II FI, the fluorescence efficiency is barely satisfying, and the molecular guideline to improve the imaging quality has not been clarified yet. This contribution designs self‐brightened organic semiconducting polymers (OSPs) for improved NIR‐II phototheranostics of cancer. The amplification of NIR‐II brightness is realized by incorporating a weak electron‐donating unit (5,5′‐dibromo‐4,4′‐didodecyl‐2,2′‐bithiophene, DDB) into the semiconducting backbone with strong electron donor–acceptor alternated structure, which exhibits 6.3‐fold and 25‐fold fluorescence enhancement compared with the counterpart OSP at the same optical concentration and mass concentration, respectively. The broadband femtosecond transient absorption spectra experimentally elucidate the DDB doping‐induced suppression of vibrational relaxation as the underlying reason for the NIR‐II fluorescence amplification. Biocompatible nanoparticles fabricated from the optimal OSP₁₂ exhibit excellent NIR‐II phototheranostic performance both in vitro and in vivo. Our research not only reveals the mechanistic insights for fluorescence enhancement of the designed OSPs from the essential view but also highlights an effective molecular methodology to guide the rational design of imaging agents with enhanced NIR‐II brightness for improved phototheranostics in living subjects.