Over the last decade molecular containers have been increasingly studied within the context of complex chemical systems. Herein we discuss selected examples from the literature concerning three aspects of this field: complex host-guest behaviour, adaptive transformations of molecular containers and reactivity modulation within them.

A growing variety of covalent reactions have been employed to achieve the post-assembly modification (PAM) of self-assembled metallosupramolecular complexes. Covalent PAM enables the late-stage derivatisation of pre-assembled parent complexes in a modular fashion, thus expanding the chemical space available for supramolecular synthesis. The oldest and most widespread implementation of covalent PAM is in metal-preorganised covalent synthesis. Recent work, however, has broadened the scope of covalent PAM to include: protocols for efficiently grafting new functionalities onto supramolecular architectures, reactions that permanently 'lock-down' metastable complexes, and covalent bond-forming stimuli that trigger controlled structural transformations between distinct supramolecular species. This review highlights key examples of each of these distinct kinds of covalent PAM in metallosupramolecular chemistry, before providing a perspective upon future challenges and opportunities.

Natural reaction cascades control the movement of biomolecules between cellular compartments. Inspired by these systems, we report a synthetic reaction cascade employing post-assembly modification reactions to direct the partitioning of supramolecular complexes between phases. The system is comprised of a self-assembled tetrazine-edged Fe II 8L12 cube and a maleimide-functionalized Fe II 4L6 tetrahedron. Norbornadiene (NBD) functions as the stimulus that triggers the cascade, beginning with the inverse electron-demand Diels-Alder reaction of NBD with the tetrazine moieties of the cube. This reaction generates cyclopentadiene as a transient by-product, acting as a relay signal that subsequently undergoes a Diels-Alder reaction with the maleimide-functionalized tetrahedron. Cyclooctyne can selectively inhibit the cascade by outcompeting NBD as the initial trigger. Triggering the cascade with 2-octadecylNBD leads to selective alkylation of the tetrahedron upon cascade completion. The increased lipophilicity of the C18-tagged tetrahedron drives this complex into a nonpolar phase, allowing its isolation from the initially inseparable mixture of complexes.Reaction cascades allow information to be relayed 1 from an initial stimulus to an eventual output across relatively long distances within biological systems. 2 Such cascades frequently involve sequences of selective covalent bond forming reactions that act on self-assembled biomolecules, such as the posttranslational modification (PTM) of proteins 3 or epigenetic modification of DNA. 4 These cascades, which can be mediated by the action of fleeting signal molecules such as nitric oxide, 5 enable cells to respond dynamically to their environment in response to changing biochemical demands. Inspired by these natural cascades, organic chemists have developed cascade reactions for performing multiple bond-forming processes within a single reaction flask, thereby simplifying synthetic routes to complicated molecules. Recently, some of these cascades have featured supramolecular elements such as container molecules, which act as 'nanoreactors' 6, 7 to facilitate particular steps in the cascade. [8][9][10][11] There have also been efforts to include features critical to biological functions, such as inhibition or feedback loops, 12 to develop more complex supramolecular systems that exhibit nuanced responses to transient chemical species. To date, however, none of these supramolecular cascades have harnessed covalent post-assembly modification (PAM) of the supramolecular components as a mechanism for signal transduction.Covalent PAM reactions must satisfy several prerequisites before they can be successfully employed for the structural alteration of a discrete supramolecular complex: they must be chemoselective, afford nearquantitative yields and proceed under mild conditions to avoid damaging the complex. Due to these stringent requirements, 'click' reactions, such as the strain-promoted Huisgen alkyne-azide cycloaddition 13,14 or inverse electron-demand Diels-Alder (IEDDA...

S inglet oxygen is a common reactive oxygen species that is formed as a byproduct of many photosensitized processes. Due to its high reactivity, singlet oxygen irreversibly damages a range of unsaturated organic compounds. 1À4 The 1 Δ g r 3 Σ g

We report the covalent post-assembly modification of kinetically metastable amine-bearing Fe(II)2L3 triple helicates via acylation and azidation. Covalent modification of the metastable helicates prevented their reorganization to the thermodynamically favored Fe(II)4L4 tetrahedral cages, thus trapping the system at the non-equilibrium helicate structure. This functionalization strategy also conveniently provides access to a higher-order tris(porphyrinatoruthenium)-helicate complex that would be difficult to prepare by de novo ligand synthesis.

Meridional (mer) coordination promotes the generation of larger and lower-symmetry prismatic metallosupramolecular structures, in contrast with the facial (fac) coordination common to smaller and higher-symmetry polyhedra. Here, we describe a general route to the selective formation of large metallosupramolecular prisms that contain exclusively mer-coordinated metal vertices. The use of 2-formylpyridine subcomponents that contain perfluorophenylene substituents at their 5-positions resulted in stereoselective formation of the iron(II) complexes from these subcomponents. Only mer vertices were observed, as opposed to the statistical fac/mer mixture otherwise generated. This mer-selective self-assembly could be used to prepare tetragonal (M8L12), pentagonal (M10L15), and hexagonal (M12L18) prisms by taking advantage of the subtle selectivities imposed by the different anilines and counterions employed. The equilibrium between the tetragonal and pentagonal prism followed a linear free-energy relationship, with the ratio between structures correlating with the Hammett σp(+) parameter of the incorporated aniline. The contrasting preferences of the fluorinated and nonfluorinated ligands to generate prisms and tetrahedra, respectively, were quantified energetically, with the destabilization increasing linearly for each "incorrect ligand" incorporated into either structure.

We report the synthesis, characterisation and polymerisation of two novel asymmetric perylene diimide acrylate monomers. The novel monomers form a sensitiser-acceptor pair capable of undergoing Förster resonance energy transfer, and were incorporated as copolymers with tert-butyl acrylate. The tert-butyl acrylate units act as spacers along the polymer chain allowing high concentrations of dye while mitigating aggregate quenching, leading to persistent fluorescence in the solid state at high concentrations of up to 0.3 M. Analysis of fluorescence kinetics showed efficient energy transfer between the optically dense sensitiser and the lower concentration acceptor luminophores within the polymer. This reduced reabsorption within the material demonstrates that the copolymer-scaffold energy transfer system has potential for use in luminescent solar concentrators.

Post-assembly modification (PAM) is a powerful tool for the modular functionalization of self-assembled structures. We report a new family of tetrazine-edged Fe(II)4L6 tetrahedral cages, prepared using different aniline subcomponents, which undergo rapid and efficient PAM by inverse electron-demand Diels-Alder (IEDDA) reactions. Remarkably, the electron-donating or -withdrawing ability of the para-substituent on the aniline moiety influences the IEDDA reactivity of the tetrazine ring 11 bonds away. This effect manifests as a linear free energy relationship, quantified using the Hammett equation, between σ(para) and the rate of the IEDDA reaction. The rate of PAM can thus be adjusted by varying the aniline subcomponent.