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 (λex = 450 nm) NIR-UOP via excited-state conformational dynamics. The optimized molecule, py-2mNO2, exhibits NIR-UOP emission at 635 and 700 nm, with a high phosphorescence intensity of 12 cd/m2 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-2mNO2-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 (λe 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.