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-trisTP3B and TTM-trisTP9B) 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-trisTP3B, 690 nm) and 1.1% (TTM-trisTP9B, 685 nm) in water. Their dual functionality as both fluorescent probes and MRI contrast agents was demonstrated. TTM-trisTP3B was taken up by living cells, localized near the cell nucleus, and showed fluorescence. The new radicals also provided a clear longitudinal relaxation time (T₁)-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.