Antimony(V) Complexes of an N2O2-Type Tetradentate Azadipyrromethene Ligand and Their Near-Infrared Optical Properties.

Antimony(V) azadipyrromethene complexes featuring a tetradentate N2O2-type ligand were synthesized to investigate the impact of axial ligands on their photophysical properties. Reaction of the azadipyrromethene ligand with SbCl5 afforded the benchtop-stable dichloro complex aza-LSbCl2 in high yield. Axial ligand substitution enabled the conversion of aza-LSbCl2 to its dihydroxy, chloromethoxy, and dimethoxy derivatives aza-LSb(OH)2, aza-LSbCl(OMe), and aza-LSb(OMe)2, respectively. Photophysical studies revealed a ligand-dependent fluorescence turn-on effect; while the dichloro complex aza-LSbCl2 and the chloromethoxy complex aza-LSbCl(OMe) are nearly nonemissive, the aza-LSb(OH)2 and aza-LSb(OMe)2 exhibit near-infrared (NIR) emission with photoluminescence quantum yields (PLQYs) of 8 and 6%, respectively. Time-dependent density functional theory (TD-DFT) calculations indicate that the fluorescence quenching in the dichloro and chloromethoxy complexes stems from a dissociative excited state characterized by Sb─Cl bond elongation, whereas oxygen-based ligands provide a structurally stable singlet excited state (S1) manifold. These findings demonstrate that post-complexation axial-ligand engineering at the antimony center is an effective strategy for modulating the optical properties of antimony(V) azadipyrromethene complexes, highlighting a distinctive feature not accessible in conventional coordinatively saturated aza-BODIPY systems.