H₂PO₄^-- and Solvent-Induced Polymorphism of an Amide-Functionalized [Pt(N^C^N)Cl] Complex

A simple [Pt(N^C^N)Cl] complex functionalized with an amide group was prepared, and its absorption and emission properties were examined in different solvents in response to various anions. On the one hand, in the presence of H₂PO₄^-, the solution of the complex shows distinct color changes in CH₃CN, together with a ratiometric emission change from a green emission band at 537 nm to a deep red emission band at 680 nm. On the other hand, two-step spectral changes were observed in response to H₂PO₄^- in CH₂Cl₂, with the green emission being attenuated first followed by the appearance of enhanced and yellow-green emissions at a lower-energy region. These recognition processes are highly selective for H₂PO₄^- against other common anions including F^-, Cl^-, Br^-, I^-, OAc^-, NO₃^-, and HSO₄^-. In addition, the platinum complex displays multistage emission polymorphism in mixed CH₃CN/H₂O solvent of various ratios. The hydrogen-bonding interaction between H₂PO₄^- and the amide unit was confirmed by NMR analysis. In the solid state, this platinum complex emits red light. However, the composite material of the platinum complex with H₂PO₄^- shows purely monomeric yellow emissions. The solid-state materials were further analyzed by single-crystal X-ray and Fourier-transform IR analysis. These studies suggest that this simple platinum complex is useful for the selective recognition of H₂PO₄^- and as solid-state emitting materials with tunable emission colors.