Sheet-like 2D Manganese(IV) Complex with High Photothermal Conversion Efficiency

Abstract

We report a stable, water-soluble, mononuclear manganese(IV) complex [Mn^IV(H₂<b>L</b>)]·5H₂O (Mn-HDCL) that acts as an efficient photothermal material. This system is based on a hexahydrazide clathrochelate ligand (<b>L</b>/HDCL) and is obtained via an efficient one-pot templated synthesis that avoids the need for harsh reaction conditions. Scanning tunneling microscopy images reveal that Mn-HDCL exists as a 2D sheet-like structure. In Mn-HDCL, the manganese(IV) ion is trapped within the cavity of the cage-like ligand. This effectively shields the Mn(IV) ion from the external environment while providing adequate water solubility. As a result of orbital transitions involving the coordinated manganese(IV) ion, as well as metal-to-ligand charge transfer effects, Mn-HDCL possesses a large extinction coefficient and displays a photothermal performance comparable to single-wall carbon nanotubes in the solid state. A high photothermal conversion efficiency (ca. 71%) was achieved in aqueous solution when subjected to near-infrared 730 nm laser photo-irradiation. Mn-HDCL is paramagnetic and provides a modest increase in the <i>T</i>₁-weighted contrast of magnetic resonance images both <i>in vitro</i> and <i>in vivo</i>. Mn-HDCL was found to target tumors passively and allow tumor margins to be distinguished <i>in vivo</i> in a mouse model. In addition, it also exhibited an efficient laser-triggered photothermal therapy effect <i>in vitro</i> and <i>in vivo</i>. We thus propose that Mn-HDCL could have a role to play as a tumor-targeting photothermal sensitizer.