Two-Dimensional Sheet-like Supramolecular Polymers from a Pt(II)-Coordinated Bipyridine Complex in Aqueous Solution

Abstract

In recent years, considerable attention has been devoted to supramolecular architectures such as helical fibers, tubules, and nanotoroids (nanorings) owing to their unique physical properties. In this study, we synthesized a bipyridine-based Pt(II) complex bearing two chloride ions that act as a bidentate ligand. Interestingly, the resulting mononuclear bipyridine-based Pt(II) complex (PtCl2-L1) self-assembles into two-dimensional sheet-like supramolecular polymers. The monomeric PtCl2-L1 exhibits distinct spectroscopic behaviors depending on the composition of DMSO/H2O mixed solvents. When the water fraction exceeds 30%, PtCl2-L1 undergoes supramolecular polymerization. In particular, well-defined two-dimensional sheet-like structures are formed in a DMSO/H2O mixture (7:3, v/v). The resulting supramolecular polymer readily dissociates into monomeric species upon the addition of acetonitrile, demonstrating the reversible nature of the assembly process. Furthermore, the 2D sheet-like structures exhibit photoluminescence at 440 nm, which can be attributed to metal-to-ligand charge transfer (MLCT). The non-sigmoidal transition observed in the absorbance changes as a function of solvent composition was analyzed using a global fitting method, allowing determination of the intrinsic Gibbs free energy of the supramolecular polymerization. In addition, the conformers of the monomeric species and tetramers were investigated by density functional theory (DFT) calculations. The optimized geometries of a trimeric assembly of PtCl2-L1 were further simulated at the semi-empirical QM/PM6 level.