Bis‐Diimine Rhenium(I) Complexes for Biomedical Photoacoustic Tomography

A series of bis‐diimine rhenium(I) complexes Re(NN1‐OMe) − Re(NN3‐OMe) , containing neocuproine and methyl [2‐(pyridin‐2‐yl)quinoline‐4‐carboxylate (NN1), methyl [2,2′‐biquinoline]‐4‐carboxylate (NN2), dimethyl [2,2′‐biquinoline]−4,4′‐dicarboxylate (NN3) was synthesized and characterized. Utilization of the asymmetric NN1 and NN2 ligands affords two types of structural isomers, which were isolated and structurally studied by X‐ray diffraction analysis in the solid state. 1 H‐ 1 H COSY and NOESY NMR experiments confirmed preservation of the structural patterns in liquid media for the complexes under study. Alkaline hydrolysis of the ester groups in the NN# diimine ligands was performed to give the Re(NN1‐OK) − Re(NN3‐OK) complexes exhibiting higher water solubility that made possible to use them in biological experiments. In MeOH and aqueous media, the complexes display NIR absorption with a long wavelength band at ca. 715 nm extended up to 850 nm in the case of both forms of Re(NN3)‐OX . The Re(NN3‐OK) complex demonstrated stable photoacoustic signal in oxygenated blood phantoms and showed no significant toxicity with the cell viability above 80% even at concentrations of 1 mM in cell experiments with CHO‐K1 cell line.