Suppressing Non‐Radiative Decay via Cyanation: A Promising Design Strategy for Bright Organic NIR‐II Fluorophores

Abstract Achieving high fluorescence efficiency in organic fluorophores within the second near‐infrared window (NIR‐II, 1000∼1700 nm) remains challenging, as extended π‐conjugation and active intramolecular motions typically funnel excitation energy into non‐radiative decay. Here, we present peripheral cyanation as a molecular design strategy that directly modulates excited‐state dynamics and suppresses non‐radiative relaxation. Incorporation of cyano groups (A') into the D‐A‐D scaffold of BBTCz afforded BBTCzCN with an A’‐D‐A‐D‐A’ architecture, which significantly reduced vibronic coupling compared to the parent dye. Upon encapsulation with DSPE‐ m PEG 5000 , BBTCzCN nanoparticles (NPs) retained a high FLQY of 2.8% with a record‐high brightness of 565 M −1 cm −1 , representing a 10.4‐fold enhancement over BBTCz NPs and placing it among the brightest organic NIR‐II emitters reported to date. Mechanistic studies combining density functional theory and ultrafast spectroscopy revealed that cyanation synergistically suppressed vibrational relaxation and internal conversion, thereby prolonging radiative decay pathways. As a result, BBTCzCN NPs enabled high‐resolution vascular imaging, real‐time lymphatic tracking, and precise intraoperative delineation of tumors and peritoneal metastases. This work establishes peripheral cyanation as a broadly applicable molecular design strategy for tailoring excited‐state decay pathways, advancing the development of next‐generation NIR‐II fluorophores for deep‐tissue imaging and image‐guided surgery.