Brominated trityl radical nanoparticles: metal-free ratiometric nanothermometer with near-infrared excimeric emission in water

The development of efficient, metal-free luminescent nanothermometers operating near the first near-infrared (NIR-I) biological window is crucial for advancing biomedical imaging and temperature sensing at the nanoscale. In this work, we report a new family of brominated organic radical nanoparticles (Br-ONPs), prepared by nanoprecipitation of a persistent tris(2,4,6-tribromophenyl)methyl (TTBrM) radical doped into its optically inert precursor matrix (TTBrM-αH). Aqueous dispersions of Br-ONPs exhibit high colloidal stability, sizes below 130 nm, and dual emission under a single excitation. Thus, in addition to the emission of isolated monomeric TTBrM radicals inside the matrix, an excimeric band emerges at doping ratios ≥15 wt% due to simultaneous formation of TTBrM dimers with short intermolecular contacts inside the matrix which are similar to those present in crystalline polymorphs. Compared to chlorinated analogues (Cl-ONPs), Br-ONPs show the desired red-shift in both monomer and excimer emissions, entering into the interesting NIR-I window. Theoretical calculations support the electronic origin of the excimer emission and the role of bromine in modulating the optical properties. Temperature-dependent fluorescence studies confirm the suitability of metal-free Br-ONPs as a novel and efficient ratiometric nanothermometer in aqueous environments, with tunable optical output. Altogether, these findings not only establish Br-ONPs as promising candidates for future in vivo bioimaging applications, but also provide valuable insights into the design principles and key parameters governing the performance of organic nanothermometers.