Water‐Soluble Luminescent Trityl Radical for Fluorescence/Magnetic Resonance Dual‐Modal Imaging Probe

Tris (2,4,6‐trichlorophenyl)methyl (TTM) radicals are promising scaffolds for fluorescence imaging (FI) and magnetic resonance imaging (MRI) owing to their long‐wavelength emission and intrinsic paramagnetism. Incorporating donor‐acceptor (D‐A) architectures into TTM radicals has been an established strategy to enhance the brightness and photostability of TTM radicals. However, conventional D‐A‐type TTM radicals exhibit strong charge‐transfer (CT)‐dominated emission that is easily quenched in water due to accelerated non‐radiative decay known as the energy‐gap law. Here, we report water‐soluble TTM radicals (TTM‐ tris TP3B and TTM‐ tris TP9B) which exhibit hybridized locally excited and CT character through rational modulation of the donor units and their substitution positions. New radicals exhibited high photostability and deep‐red emission with photoluminescence quantum yields of 1.4% (TTM‐ tris TP3B, 690 nm) and 1.1% (TTM‐ tris TP9B, 685 nm) in water. Their dual functionality as both fluorescent probes and MRI contrast agents was demonstrated. TTM‐ tris TP3B was taken up by living cells, localized near the cell nucleus, and showed fluorescence. The new radicals also provided a clear longitudinal relaxation time ( T 1 )‐weighted MRI contrast. This work represents the first demonstration of trityl radical functioning as practical FI/MRI dual‐modal imaging probes, establishing a molecular platform without any surfactants or metals in which the fluorophore and electron‐spin source are unified.