Elucidation of the key role of Pt***Pt interactions in the directional self-assembly of platinum(II) complexes

Here, we report the use of an amphiphilic Pt(II) complex, K[Pt{(O3SCH2CH2 CH2)2bzimpy}Cl] (PtB), as a model to elucidate the key role of Pt-Pt interactions in directing self-assembly by combining temperature-dependent ultraviolet-visible (UV-Vis) spectroscopy, stopped-flow kinetic experiments, quantum mechanics (QM) calculations, and molecular dynamics (MD) simulations. Interestingly, we found that the self-assembly mechanism of PtB in aqueous solution follows a nucleation-free isodesmic model, as revealed by the temperature-dependent UV-Vis experiments. In contrast, a cooperative growth is found for the self-assembly of PtB in acetone--water (7:1, vol/vol) solution, which is further verified by the stopped-flow experiments, which clearly indicates the existence of a nucleation phase in the acetone--water (7:1, vol/vol) solution. To reveal the underlying reasons and driving forces for these self-assembly processes, we performed QM calculations and show that the Pt-Pt interactions arising from the interaction between the [p.sub.z] and dz orbitals play a crucial role in determining the formation of ordered self-assembled structures. In subsequent oligomer MD simulations, we demonstrate that this directional Pt-Pt interaction can indeed facilitate the formation of linear structures packed in a helix-like fashion. Our results suggest that the self-assembly of PtB in acetone--water (7:1, vol/vol) solution is predominantly driven by the directional noncovalent Pt-Pt interaction, leading to the cooperative growth and the formation of fibrous nanostructures. On the contrary, the self-assembly in aqueous solution forms spherical nanostructures of PtB, which is primarily due to the predominant contribution from the less directional hydrophobic interactions over the directional Pt-Pt and [tau]i--[tau]i interactions that result in an isodesmic growth. platinum amphiphile | directional assembly | molecular dynamics simulations | nucleation-growth model