Influence of ligand substituents on the chirality of metallosupramolecular architectures

Abstract

In biological systems, steric hindrance plays a critical role in supramolecular assembly by controlling intermolecular packing, interaction geometry, and the resulting structural specificity. Accordingly, introducing small substituents on ligands provides an artificial model to finely regulate the helicity and assembly pathway of supramolecular polymers. In this study, we report the self-assembly behavior of a terpyridine-based Pt(II) complex, R-Pt-Ph-C3, bearing a propylphenylacetylene ligand. Remarkably, R-Pt-Ph-C3 forms right- or left-handed helical supramolecular architectures in mixed DMSO/H2O media, with the handedness being solvent-composition dependent. The self-assembled structures exhibit intense photoluminescence at 570-620 nm, arising from metal-to-ligand charge transfer (MLCT) and/or metal-metal-to-ligand charge transfer (MMLCT) transitions. Circular dichroism (CD) analysis revealed opposite Cotton effects in DMSO/H2O (6:4, v/v) versus (4:6, v/v), indicating solvent-controlled inversion of supramolecular helicity. Spectroscopic (UV-vis, CD, FT-IR, 1H NMR) analyses demonstrated that assembly is driven by pi-pi stacking, intermolecular hydrogen bonding, and Pt center dot center dot center dot Pt interactions. Variable-temperature UV-vis studies and curve fitting using the equilibrium (EQ) model revealed that the assemblies follow either an isodesmic or cooperative nucleation-elongation growth mechanism, depending on solvent composition. The DMSO/H2O (6:4, v/v) system exhibited lower thermodynamic stability (Delta Ge = -23.3 kJ.mol-1) consistent with isodesmic growth, whereas the (4:6, v/v) system showed higher stability (Delta Ge = -27.0 kJ.mol-1) and strong cooperativity (sigma = 4.7 x 10-4). These findings highlight the ability to control supramolecular helicity, morphology, and growth mechanism of Pt(II)-based assemblies through subtle changes in solvent environment.