"N2O2" Metal-azaDIPY complexes: a new class of NIR-fluorophores

Aza-BODIPYs are an increasingly studied class of fluorophores. They can be seen as an "aza-DIPY" ligand rigidified by a metalloid, a boron atom. Based on this idea, a series of complexes of group 13 metals (aluminum and gallium) have been synthesized and characterized. Impact of the metal and of the nature of the substituents of aza-DIPY core were investigated. The photophysical and electrochemical properties were determined and an X-ray structure of an azaGaDIPY was obtained. These data reveal that azaGaDIPY and azaAlDIPY exhibit significant red-shifted fluorescence compared to their analogue aza-BODIPY. Their emission can go up to 800 nm for the maximum emission length and up to NIR-II for the emission tail. This, associated with their electrochemical stability (no metal release whether oxidized or reduced) make them a promising class of fluorophores for optical medical imaging. Moreover, X-ray structure and molecular modeling studies have shown that this red shift seems to be more due to the geometry around the boron/metal than to the nature of the metal.