Ultrabright NIR-II fluorophores enabled by noncovalent conformational locking for in vivo inflammation imaging

Ultrabright fluorophores emitting in the second near-infrared (NIR-II, 1000-1700 nm) window are highly desirable for biomedical and optoelectronic applications but remain rare, because fluorescence brightness requires the simultaneous realization of a large absorption coefficient (ε) and a high photoluminescence quantum yield (Φ_PL), two parameters that are inherently difficult to balance. Here, we develop an ultrabright semiconducting polymer fluorophore (P1) via a simple noncovalent conformational locking strategy that enforces a rigid and planar backbone with small dihedral angles. This design extends π-conjugation to boost ε and Φ_PL while effectively suppressing aggregation-caused quenching in P1 nanoparticles (P1 NPs). Compared to its unlocked analogue, P1 NPs exhibits a notable 360 nm redshift towards NIR-II emission, together with intensified ε and elevated Φ_PL. These properties afford P1 NPs 10-fold enhancement in NIR-II brightness relative to reported polymer fluorophores, enabling high-resolution vascular imaging and precise inflammation localization. These findings represent a significant design advance in creating ultrabright fluorophores for bioimaging and optoelectronic applications.