Hierarchical Assemblies of Supramolecular Coordination Complexes

Abstract

Hierarchical self-assembly (HAS) is a multilevel organization process that first assembles elementary molecular units into ordered secondary structures via noncovalent interactions, which further act as the building blocks to form more complex multifunctional superstructures at the next level(s). The HAS strategy has been used as a versatile method for the preparation of soft-matter nanoarchitectures of defined size and morphologies, tunable luminescence, and biological importance. However, such preparation can be greatly simplified if well-defined dynamic structures are employed as the cores that upon linking form the desired nanoarchitectures. Discrete supramolecular coordination complexes (SCCs) with well-defined shapes, sizes, and internal cavities have been widely employed to construct hierarchical systems with functional diversity. This Account summarizes the prevailing strategies used in recent years in the preparation of SCC-based HASs and illustrates how the combination of dynamic metal-ligand coordination with other interactions was used to obtain hierarchical systems with interesting properties. HASs with dual orthogonal interactions involving coordination-driven self-assembly and hydrogen bonding/host-guest interaction generally result in robust and flexible supramolecular gels. Likewise, hybridization of SCCs with a suitable dynamic covalent network via a hierarchical strategy is useful to prepare materials with self-healing properties. The intrinsic positive charges of the SCCs also make them suitable precursors for the construction of HASs via electrostatic interactions with negatively charged biological/abiological molecules. Furthermore, the interplay between the hydrophilic and lipophilic characters of HASs by varying the number and spacial orientation of alkyl/oxyethylene chains of the SCC is a simple yet controllable approach to prepare ordered and tunable nanostructures. Certain SCC-cored hierarchical systems exhibit reversible polymorphism, typically between micellar, nanofiber, and vesicular phases, in response to various external perturbations: heat, photoirradiation, pH-variance, redox-active agents, etc. At the same time, multiple noncovalent interaction mediated HASs are growing in numbers and are promising candidates for obtaining functionally diverse materials. The photophysical properties of SCC-based HASs have been used in many analytical applications. For example, embedding tetraphenylethene (TPE)-based pyridyl ligands within metallo-supramolecular structures partially restricts the molecular rotations of its phenyl rings, endowing the resultant SCCs with weak emissions. Further aggregation of such HASs in suitable solvents results in a marked enhancement in emission intensity along with quantum yields. They act as sensitive sensors for different analytes, including pathogens, drugs, etc. HASs are also useful to develop multidrug systems with cooperative chemotherapeutic effects. Hence, the use of HASs with theranostic SCCs combining cell-imaging agents and chemotherapeutic scaffolds is a promising drug delivery strategy for cancer theranostics. At the same time, their responsiveness to stimuli, oftentimes due to the dynamic nature of the metal-ligand interactions, play an important role in drug release via a disassembly mechanism.

Figure 1.

Preparation of a hexagonal SCC from suitable precursors via coordination driven self-assembly and its HAS into well-defined nanostructures using multiple orthogonal interactions.

Figure 2.

Synthesis and supramolecular polymerization of UPy-functionalized rhomboid 8 and hexagons 9 and 10. Adapted with permission from ref 14. Copyright 2013 National Academy of Sciences (USA).

Figure 3.

(A) Synthesis and formation of DSPs of UPy-functionalized rhomboids 11–13. (B) DSP of 13 by PM6 semiempirical molecular orbital methods. TEM images of aggregates of 13 obtained from (C) CH₂Cl₂ and (D) DMSO. Adapted with permission from ref 15. Copyright 2013 American Chemical Society.

Figure 4.

HAS-mediated metallogelation by 16 and 17. Stimuli-responsive behavior of the metallogel obtained from 16. Panel B Adapted with permission from ref 16. Copyright 2016 Wiley-VCH; Panel C Adapted with permission from ref 17. Copyright 2018 American Chemical Society.

Figure 5.

Preparation of metallacycle 18 and its host–guest complexation with bisammonium salt 19 forming a supramolecular polymeric network. Adapted with permission from ref 18. Copyright 2014 American Chemical Society.

Figure 6.

Schematic illustration of supramolecular metallogelation via HAS of 20 with 21–23. Adapted with permission from ref 21. Copyright 2017 The Royal Society of Chemistry.

Figure 7.

(A) Preparation of 24. (B) Schematic illustration and (C, D) AFM images of various morphologies obtained during DNA compaction by 24. Adapted with permission from ref 25. Copyright 2014 The Royal Society of Chemistry.

Figure 8.

Schematic illustration of the formation of a LC polymer gel from 26. Adapted with permission from ref 28. Copyright 2017 Wiley-VCH.

Figure 9.

Various nanostructures formed by compounds 29–31. Adapted with permission from ref 29. Copyright 2014 Wiley-VCH.

Figure 10.

(A) Precursors reported by Yan et al. and Sun et al. for the preparation of metallacycles. Schematic illustration of the formation of various nanostructures by (B) 35 and 36 and (C) 37^D/37^L and 38^D/38^L depending upon concentration. Panel B Adapted with permission from ref 30. Copyright 2013 American Chemical Society. Panel C Adapted with permission from ref 31. Copyright 2017 American Chemical Society.

Figure 11.

HAS of 39, 40, and 42 via triply orthogonal noncovalent interactions. Adapted with permission from ref 32. Copyright 2016 American Chemical Society.

Figure 12.

Cartoon representations of formation of supramolecular copolymers by 43 and 44. Adapted with permission from ref 33. Copyright 2016 American Chemical Society.

Figure 13.

(A) Preparation of [2]catenanes 45–47. (B) The NO₃⁻ triggered transformation of 48–50 into 45. (C) Thermoresponsive behavior of 47. Adapted with permission from ref 35. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 14.

Synthesis of TMV/51 biohybrid. Adapted with permission from ref 37. Copyright 2016 American Chemical Society.

Figure 15.

(A) Molecular structures of 52 and 53 and (B) their supramolecular oligomerization. (C) Emission spectra of 1:1 mixture of 52 and 53 at different concentrations. Inset shows photographs of 52 (right), 53 (left), and mixture of equimolar 52 and 53 (middle) in acetone under 365 nm UV-light. Panels B and C Adapted with permission from ref 39. Copyright 2017 National Academy of Sciences (USA).

Figure 16.

(A) Schematic representation and (B) AFM image of the HAS of 54 and heparin. Adapted with permission from ref 41. Copyright 2015 American Chemical Society.

Figure 17.

CO₂-responsive morphological transformation of 25 in water. Adapted with permission from ref 42. Copyright 2017 American Chemical Society.

Figure 18.

Synthesis of a hexagonal metallodendrimer 56 from 3 and 55, its HAS into vesicles and micelles, and the subsequent halide-induced controlled release of guests. Adapted with permission from ref 43. Copyright 2014 American Chemical Society.

Figure 19.

(A) Synthesis and GSH-triggered cascade elimination reaction of 57. (B) Cross-linking of PhenPt with DNA. (C) Schematic representation of the cellular uptake of DOX-loaded nanostructures self-assembled from 57. Panel C adapted with permission from ref 44. Copyright 2017 American Chemical Society.

Figure 20.

Schematic illustration of promising future prospects of HASs of SCCs.

Chart 1.

Precursors 1–7 Reported by Yan et al. for the Preparation of Metallacycles