Stable, deep-red emitting trityl radical probes with polarity-insensitive fluorescence for selective detection of...

The development of stable and efficient luminescent organic radicals for sensing applications remains a significant challenge. Herein, we report the design and synthesis of two novel fluorescent probes, TTM-BPA and TTM-Phen, based on a tris(2,4,6-trichlorophenyl)methyl (TTM) radical core functionalized with N,N-bis(pyridin-2-ylmethyl)aniline (BPA) and 1,10-phenanthroline (Phen) moieties, respectively. Both radical probes exhibit intense, deep-red emission that is remarkably insensitive to solvent polarity, a desirable feature for robust sensing applications. The introduction of extended π-conjugated systems dramatically enhanced their photostability by over 300-fold compared to the parent TTM radical. Leveraging the coordination capabilities of the pyridine-based ligands, these probes demonstrate excellent performance as selective chemosensors. TTM-BPA acts as a specific "turn-off" fluorescent probe for ferric ions (Fe³⁺) and protons (H⁺), with detection limits of 4.4 μM and 0.65 μM, respectively. TTM-Phen selectively detects both Fe³⁺ and ferrous ions (Fe²⁺) with high sensitivity, achieving detection limits of 1.3 μM and 0.92 μM. The sensing mechanism is attributed to fluorescence quenching via an electron transfer process upon coordination with the target analytes. Practical applications have shown that TTM-BPA and TTM-Phen can be used for fluorescence imaging of Fe³⁺ and Fe²⁺, as well as for the detection of iron ions in tap water. This work not only presents a new class of highly stable, deep-red emitting radical probes but also provides a versatile platform for the development of advanced sensors for biologically and environmentally important species.