Although apparently forming without an explicit template, subsequent studies suggested that appropriately-sized solvent molecules could take on this role. An extended version of Rebek's glycouril monomer, for example, dimerised in benzene, but only did so in p-xylene in the presence of a suitable guest. Likewise, resorcinarene-based cavitands have been shown to cleanly dimerise in chloroform, benzene or toluene, as these were small enough to readily fit within the cavity of the host, whilst bulkier mesitylene as solvent required addition of suitable guests to template the assembly. Resocinarene tetraester 73, for example, existed in monomer form in chloroform, but assembled into a dimeric host in the presence of a tropylium cation (74) template. 149 Such host-guest interactions can be incredibly powerful in imparting stability to HB capsules: resorcinarene 72 did not spontaneously dimerise in acetone but could be induced to do so upon addition of + NEt4 as a suitable template. 146 Larger homomeric HB assemblies beyond dimers have also been investigated. Tetramers, 150 hexamers and octamers 151 have all been realised, with a large hexameric assembly 2.3 nm in diameter and a cavity volume of 2800 Å 3 recently reported. Seminally, MacGillivray and Atwood reported the solid-state structure of a hexamer formed from the self-assembly of a resorcinarene with 8 molecules of water through a concerted network of 60 hydrogen bonds. The solution-phase persistence of this structure, and its ability to encapsulate guests, in water-saturated chloroform, was subsequently demonstrated. In addition to these homomeric systems, heteromeric assemblies have been reported from interactions between complementary building blocks (Fig. ). Combining resorcinarenes functionalised with HB donors, such as carboxylic acids (e.g. 77) and alcohols (e.g. 76), and acceptors such as pyridines (e.g. 75), has been shown to lead to the formation of heterodimeric capsules (e.g. 75.76). Related architectures assembled from quite different building blocks, such as acid-functionalised resorcinarenes and tetrapyridyl porphyrins (including double-cavity systems) have also been reported. Charge-assisted HBs have been exploited to prepare heterodimeric capsules that have demonstrated enhanced stability in polar media including methanol and water. 157 Through careful design, multi-component heteromeric HB architectures have also been realised. [4+2]-type assemblies have been reported from cavitands assembled with complementary struts (e.g. 772784), 158 whilst a [6+3] architecture was shown to assemble from functionalised calixarene 79 and barbituric acid derivative 80. In summary, supramolecular capsules assembled through cooperative hydrogen bonds between subcomponents can be realised of varying topology, including both homomeric and heteromeric architectures. Perhaps unsurprisingly, these assemblies tend to only be stable in apolar, aprotic media, although there are exceptions. Systems held together by charge-assisted HBs, however, have demonstrated enhanced stability in highly competitive solvents, as have hosts that are additionally stabilised by interactions with guest molecules. A wide range of structures for the monomer units have been reported, in addition to the commonly explored calixarene and resorcinarene cavitands, with various HB units that are capable of interacting with themselves or complementary motifs.