Methylated regioisomers of Re( <scp>i</scp> ) and Ir( <scp>iii</scp> ) tetrazole complexes: photophysical properties and optical imaging of brain tissue

Coordination of <i>N</i>1 and <i>N</i>2-methylated regioisomers of 2-(1<i>H</i>-tetrazol-5-yl)pyridine and 2-(1<i>H</i>-tetrazol-5-yl)quinoline to ReBr(CO)₃ and Ir(<b>ppy</b>)₂⁺ (<b>ppy</b> = cyclometalated 2-phenylpyridine) fragments resulted in the isolation of a small family of Re(I) and Ir(III) luminescent complexes. The complexes display phosphorescent emission from their triplet ligand-to-metal charge transfer excited states in degassed solution at room temperature. Notably, the position of the methyl substituent has a profound effect on the photophysical properties. The <i>N</i>1-methylated complexes in all cases display a significant redshift in the emission band. The shift is ascribed to a stabilisation of the π* orbitals of the tetrazole ligands, which is also supported by cyclic voltammetry and time-dependent density functional theory (TD-DFT) calculations. The complexes were used as luminescent labels for the staining of mouse brain tissue. The Re(I) complexes did not show any evident staining. On the other hand, the Ir(III) complexes - particularly those bound to the ligand containing the quinoline substituent - demonstrated affinity for lipid-rich myelinated regions in brain tissues and white matter in cerebellum tissues. The specificity of the Ir(III) complexes was further demonstrated by means of correlative optical microscopy and Fourier transform infrared (FTIR) microscopy.