Controlling Molecular Packing in Aqueous Metallosupramolecular Self-assembly by Ligand Geometry

Abstract

The coordination geometry of d⁸ transition metal complexes has been successfully exploited as a tool to tune photophysical properties and self-assembly pathways of supramolecular polymerization processes, with a focus being primarily placed on organic media. Expanding such controlled supramolecular and photophysical properties to assemblies in aqueous media by molecular design is, however, still challenging due to the difficulty in programming noncovalent interactions in water. Herein, we tackle this challenge by analyzing the aqueous self-assembly of amphiphilic Pt-(II) complexes of different molecular geometry in order to control self-assembly and metal-metal interactions in aqueous media. To this end, we have designed two Pt-(II) complexes, 1 and 2, containing an identical oligophenyleneethynylene (OPE)-based aromatic scaffold that differ in the molecular geometry (linear vs V-shaped) imposed by ligand substitution and studied their comparative self-assembly behavior in aqueous media. Even though both molecules follow the isodesmic mechanism of self-assembly, their structural difference strongly influences the molecular packing in aqueous media, which in turn impacts the photophysical properties (i.e. absence or presence of MMLCT) and the self-assembly outcome. While the molecular geometry for 2 enforces short Pt···Pt contacts driven by an efficient face-to-face stacking of the OPE backbone, the antiparallel packing of 1 with slight translational offset does not allow the formation of short Pt···Pt contacts. Such a distinct interplay of interactions for 1 and 2 in aqueous media leads to significant differences in photoluminescence.

Keywords: Pt(II) complexes; Self-assembly; amphiphilic systems; supramolecular polymerization; π-conjugated systems.

1. Chemical Structures of Pt­(II) Complexes 1 and 2 and Cartoon Representation of Their Aqueous Self-assembly Leading to Different Emission Properties Depending on the Pt···Pt Distances in the Assemblies

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(a, c) Solvent-dependent UV/vis spectra of 1 and 2, respectively (c = 10 μM, T = 298 K); (b, d) emission spectra of 1 and 2 in good (THF, red spectra) and aggregation-inducing solvents (water, blue spectra), respectively (c = 10 μM, T = 298 K, λ_ex = 430 nm). Inset: picture of solutions containing the aggregate of 1 and 2 in THF/water 1/99 (v/v) under ambient conditions under UV light.

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Denaturation UV/vis studies of 1 at c = 30 μM (a) and 2 at c = 10 μM (d) in THF/water at T = 298 K. The α_Agg vs THF volume fraction plots at various concentrations for 1 (b) and 2 (e) and corresponding fits to the denaturation model at T = 298 K. (c) and (f) Emission spectra of 1 and 2 at varying ratios of THF/water (c = 20 μM), respectively, at T = 298 K. The sharp peak around 500 nm is due to Raman scattering.

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(a) Partial ¹H NMR spectra of 1 (600 MHz, 298 K, c = 1 mM) in mixtures of THF-d ₈/D₂O along with proton assignment. The corresponding vol % of D₂O is mentioned for each ¹H NMR experiment. (b) Overlay of 2D-COSY (in red) and ROESY (in gray) spectra of 1 in 50/50 (v/v) THF-d ₈/D₂O (c = 8 mM). Colored boxes indicate the cross-signals from intermolecular interactions between adjacent protons in the supramolecular stack. For clarity, only the most relevant cross signals between aromatic protons are highlighted. (c) Corresponding intermolecular interactions are presented with an identical color code. (d) Top and side view of the optimized dimer structure of 1 using DFT (B3LYP/3-21g).

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(a) Partial ¹H NMR spectra of 2 (600 MHz, T = 298 K, c = 1 mM) in mixtures of THF-d ₈/D₂O along with proton assignment. The corresponding vol % of D₂O is mentioned for each ¹H NMR experiment. (b) Overlay of 2D-COSY (in red) and ROESY (in gray) spectra of 2 in 50/50 (v/v) THF-d ₈/D₂O (c = 8 mM). Colored boxes indicate the cross-signals from intermolecular interactions between adjacent protons in the supramolecular stack. For clarity, only the most relevant cross-signals between aromatic protons are highlighted. (c) Corresponding intermolecular interactions are presented with an identical color code. (d) Top and side view of the dimer of 2 optimized by DFT (B3LYP/3-21g).

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AFM height images of Agg1 (a) and Agg 2 (b) on a mica surface (spin-coated) and corresponding height profiles (inset). Both aggregates were freshly prepared in 1/99 (v/v) THF/water (c = 40 μM) at T = 298 K. Size distributions of Agg1 (c) and Agg2 (d) freshly prepared in 1/99 (v/v) THF/water (c = 40 μM) at T = 298 K, obtained from DLS experiments (scattering angle 90°).