Red-to-NIR Emissive Radical Cations Derived from Simple Pyrroles for Bio-Imaging

Red-to-near-infrared (NIR) fluorophores are highly desirable for bio-imaging with advantages of excellent tissue penetration ability and less interference from auto-fluorescence, but their synthesis usually require tedious procedures and it’s thus highly desirable for red-to-NIR fluorophores directly generated from easily available substrates. Compared with the conventional closed-shell fluorophores, radical cations are featured with a large red-shift absorption, but most of them are not fluorescent due to the fast internal conversion between excited and ground state with a small energy gap. Moreover, radical cations suffer from instability because they can easily undergo radical coupling or nucleophilic addition reactions. Herein, we found that 2,5-dimethylpyrroles can rapidly generate red-to-NIR emissive radical cations, which can be stabilized by adsorption on thin layer chromatography (TLC) plate or being encapsulated in cucurbit[7]uril (CB[7]). The NIR-emissive radical cations derived from pyrroles were verified by electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations. Importantly, the pyrrole-derived radical cations encapsulated in CB[7] can be used for mitochondrial imaging in living cells and tumor imaging in vivo with a high signal-to-noise ratio. The easily available and red-to-NIR emissive radical cations derived from simple pyrroles are promising for applications in biomedical study.