Functional behaviour from controlled self-assembly: challenges and prospects

Abstract

Many naturally occurring systems show us how multi-component supramolecular assemblies can generate useful functional behaviour. In this article the problems and limitations associated with achieving such behaviour in artificial multi-component assemblies is discussed, together with two examples of functions in artificial supramolecular assemblies based on (i) host-guest chemistry in cavities of cages, and (ii) light-harvesting in multi-chromophore arrays. Important challenges for the future are summarised.

Highly efficient catalysis of the Kemp elimination in the cavity of a cubic coordination cage

“…It was immediately apparent from 1 H NMR experiments that conversion of benzisoxazole to 2-cyanophenolate in aqueous base was accelerated by the presence of cage. To measure the rate constant for the reaction in the cage (kcat) with little interference from the background reaction (kuncat), we first monitored the reaction in aqueous solution at pD 10.2 under conditions where there was excess catalyst (1 mM cage, 0.85 mM benzisoxazole).…”

“…Since the realisation that hollow molecular container molecules can accommodate guest molecules in their central cavities, [1][2][3] their ability to modify the reactivity of their bound guests has been of great interest. [4][5][6][7][8] Well known examples include Cram's stabilisation of highly-reactive cyclobutadiene; 4 Fujita's 'ship-in-a-bottle' synthesis of cyclic silanol oligomers; 5 Nitschke's stabilisation of P4 in a tetrahedral cage; 6 and the demonstration of unusual regioselectivity in a Diels-Alder reaction when the two reacting molecules are co-confined in a host cavity.…”

“…10 Coordination cages offer particular promise in this field because of the ease with which they can be formed by a self-assembly process from very simple component parts, using the predictable coordination geometries of metal ions to provide the three-dimensional ordering of the components which generates the necessary cavity. [1][2][3]8,11,12 In order for a container molecule to act as an efficient catalyst it needs to (i) recognise and bind the guest(s); (ii) accelerate the reaction by increasing the local concentration of reactants and/or stabilising the transition state; and (iii) expel the product to allow catalytic turnover. 8 Guest binding in cage cavities has been very well studied and is becoming a mature field, 1 to the extent that a modeling tool for quantitative prediction of guest binding has recently been reported by us.…”

“…[1][2][3]8,11,12 In order for a container molecule to act as an efficient catalyst it needs to (i) recognise and bind the guest(s); (ii) accelerate the reaction by increasing the local concentration of reactants and/or stabilising the transition state; and (iii) expel the product to allow catalytic turnover. 8 Guest binding in cage cavities has been very well studied and is becoming a mature field, 1 to the extent that a modeling tool for quantitative prediction of guest binding has recently been reported by us. 13 For a reaction of the bound guest to be accelerated by the host, the transition state must be bound more tightly than the ground state.…”

Supramolecular gel phase crystallization: orthogonal self-assembly under non-equilibrium conditions

“…60 Beautiful examples include metallocages and hydrogen bonded capsules with internal dimensions sometimes markedly in excess of a cubic nanometre, capable of including guest molecules and controlling their reactivity. 3 Thermodynamic self-assembly has also been used to construct complex self-assembled architectures 65 such as helicates, racks, ladders, grids and catenanes. 4 It has truly opened up a whole world of complex topological synthesis, given access to nanoscale molecular containers and, in some cases, functional molecular devices such as shuttles, rotors and switches.…”