Brominated Trityl Radical Nanoparticles: Metal-Free Ratiomeric Nanothermometer with Near-Infrared Excimeric Emission in Water

Abstract

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(2,4,6tribromophenyl)methyl (TTBrM) radical doped into its optically inert precursor matrix (TTBrM-αH). Structural characterization of TTBrM revealed short intermolecular distances in its crystalline polymorphs, compatible with excimer formation. Aqueous dispersions of Br-ONPs exhibit high colloidal stability, sizes below 130 nm, and dual emission under a single excitation. A part of the the monomeric emission, an excimeric bands emerge at doping ratios ≥15 wt%, resulting in an intrinsic ratiometric thermometric response. Compared to chlorinated analogues (Cl-ONPs), Br-ONPs show a marked 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 Br-ONPs as ratiometric nanothermometer in aqueous environments, with tunable optical output and metal-free composition. 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.