Multi-cage systems and dynamic transformations
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Date
2024
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Abstract
Biological systems use multi-component low-symmetry receptors with dynamic structures operating in networks far from equilibrium to perform complex functions. While coordination cages have taken great inspiration from the structure of these receptors by harnessing their cavities, the use of their dynamic nature and function is still in its infancy. The introduction of heteroleptic cages has brought the possibility of forming low-symmetry assemblies but also of going towards multi-functional species. However, several of the previously described features are missing to this day in order to get closer to biological complexity and function.
Hence, this thesis aims to introduce a new concept in the field: multi-cage systems. The complexity does not only arise anymore from the number of components in a single assembly but from the number of discernable assemblies and their structure within a population of cages. A new type of self-sorting namely heteromeric completive self-sorting is coined and defines cage populations where multiple heteroleptic structures coexist orthogonally. This is further extended to the creation of cage systems in which heteroleptic and homoleptic hosts coexist. This concept allows a further step towards mimicking biological systems as it enables the possibility of both intramolecular and intermolecular transmission of information. Multiple coexisting assemblies can in theory perform orthogonal, cooperative or antagonist functions opening new avenues for the applications of cages in systems chemistry.
The proof of concept of multi-cage systems is introduced by taking advantage of a singular diketopyrrolopyrrole (DPP) ligand, which can adopt distinct conformations and thus offers the possibility of forming coexisting cages with different topologies. One of these topologies is the trans-figure-of-eight Pd2A2B2 heteroleptic structure and is central to most of the results of the thesis. The two DPP ligands form the eight by adopting an S shape conformation and interlacing in the middle of the assemblies, occupying the cavity. It allows a precise arrangement of ligands, giving also robust heteroleptic cages, and owns an inherent chirality as the “8” can adopt P or M chirality. Not only this ligand is fostered for its structure and variability of conformation but it is also a well-known dye with strong luminescence. Therefore, we demonstrate its use for the emergence of a new multi-cage self-sorting as well as its utilization for the formation of single multi-chromophore assemblies with outstanding photophysical properties thanks to intramolecular energy transfer.
A second system presenting the premises of functionality is then introduced. Taking advantage of guest affinity, we show that an increase in the number of components of a system results in the simplification of its self-sorting outcome. This can be seen as a case of “simplexity”. A heteroleptic cage can eventually coexist with a homoleptic host-guest complex and subsequently uptake another anionic guest orthogonally to the first binding event. This is defined as guest segregation in a multi-cage system and differs from the classical narcissistic self-sorting. Further harnessing the slight guest affinity for one of the homoleptic species finally allows the creation of a system where three host-guest complexes coexist orthogonally without shuffling their components or guests.
Eventually, the concept of cage population is extended to dynamic mixtures in which competition for resources between components allows the emergence of chiral memory and out-of-equilibrium phenomenon. We use a chiral ligand derived from the Tröger base structure to both induce chirality on pristine fullerene C60 and C70 and drive the formation of a unique chiral diastereoisomer of the figure-of-eight species. Because of the high affinity of this ligand to form a host-guest complex with fullerene, it is then possible to create a transformable multi-cage system in which the chiral figure-eight ‘releases’ its outer ligands and exchanges them for non-chiral ones. However, we still observe a chiral answer in circular dichroism witnessing the (partial) retention of chirality of the central “8” created by the DPP ligands. We then take advantage of the differences in both thermodynamic and kinetic behaviors to drive the transformative system out of equilibrium and demonstrate the transient formation of the chiral figure-eight before its decay to reform the thermodynamically preferred complex with fullerene.
This thesis introduces the concept of populations, networks, segregated functions, and transformative out-of-equilibrium behavior in the field of cages.
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Keywords
Coordination cages, System chemistry, Heteroleptic cage