Antimony(V) Complexes of an N 2 O 2 ‐Type Tetradentate Azadipyrromethene Ligand and Their Near‐Infrared Optical Properties
Tomohiro Agou, Ryo Moriyama, Ryo Inoue, Kazuya Kubo, Masato Morita, Yoshiyuki Mizuhata, Tatsuya NabeshimaABSTRACT
Antimony(V) azadipyrromethene complexes featuring a tetradentate N 2 O 2 ‐type ligand were synthesized to investigate the impact of axial ligands on their photophysical properties. Reaction of the azadipyrromethene ligand with SbCl 5 afforded the benchtop‐stable dichloro complex aza‐LSbCl 2 in high yield. Axial ligand substitution enabled the conversion of aza‐LSbCl 2 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‐LSbCl 2 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 (S 1 ) 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.