Aza-BODIPY Platform: Towards an Efficient Water-Soluble Bimodal Imaging Probe for MRI and Near-Infrared Fluorescence

In this study, an original aza-BODIPY system comprising two Gd³⁺ complexes has been designed and synthesized for magnetic resonance imaging/optical imaging application, by functionalization of the boron center. This strategy enabled to obtain a positively-charged bimodal probe, which displays an increased water-solubility, optimized photophysical properties in the near-infrared region, and very promising relaxometric properties. The absorption and emission wavelengths are 705 and 741 nm respectively, with a quantum yield of around 10 % in aqueous media. Moreover, the system does not produce singlet oxygen upon excitation, which would be toxic for tissues. The relaxivity obtained is high at intermediate fields (16.1 mM^-1.s^-1 at 20 MHz and 310 K) and competes with that of bigger or more rigid systems. A full relaxometric and ¹⁷O NMR study and fitting of the data using the Lipari-Szabo approach showed that this high relaxivity can be explained by the size of the system and the presence of some small aggregates. These optimized photophysical and relaxometric properties highlight the potential use of such systems for future bimodal imaging studies.