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

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-mPEG₅₀₀₀, 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.