Excited‐State Conformational Dynamics Enables Visible‐Light‐Activated Near‐Infrared Phosphorescent Nano‐Assemblies for Bioimaging

Visible‐light‐excited near‐infrared ultralong organic phosphorescence (NIR‐UOP), has attracted increasing attention due to its unique advantages. However, the development of efficient NIR‐UOP systems remains challenging. In this study, we propose a rational molecular design strategy to achieve visible‐light‐excited ( λ ex = 450 nm) NIR‐UOP via excited‐state conformational dynamics. The optimized molecule, py‐2mNO 2 , exhibits NIR‐UOP emission at 635 and 700 nm, with a high phosphorescence intensity of 12 cd/m 2 and a lifetime of 268 ms. Mechanistic studies reveal that the dynamic conformational charge transfer facilitates intersystem crossing (ISC). Meanwhile, the relatively flat excited‐state potential energy surface enables the molecule to adopt a planar conformation, thereby suppressing non‐radiative decay through intramolecular attractive interactions. Leveraging these properties, py‐2mNO 2 ‐based nanoparticles were assembled using a polymethyl methacrylate (PMMA) isolation strategy. Consequently, high‐contrast phosphorescence imaging of hypoxic tumors with a signal‐to‐background ratio (SBR) of 39.47 ( λ e x = 365 nm) was achieved using this matrix‐isolated phosphorescent nano‐assembly, which could even be excited with a commercial mobile phone flashlight (SBR = 14.55). This work not only provides a rational design strategy for visible‐light‐excited NIR‐UOP systems but also establishes an efficient nano‐assembly approach for constructing NIR‐UOP nanoparticles suitable for in vivo imaging.