Supramolecular processes that result in the formation of covalent bonds and occur in solution are expected to be difficult to achieve in the absence of solvent owing to the restricted conformational and translational degrees of freedom of molecules in solids. Diffusion rates of molecules in the crystalline state are in the order of 10 À15 m 2 s À1 , [1] which is at least six orders of magnitude less than in solution. In this context, organic molecules that act as supramolecular catalysts make up much of the current chemistry of molecular recognition. [2] Supramolecular catalysts are inspired by the dynamic mode of action of enzymes and take advantage of processes of molecular recognition to afford transformations that are highly stereoselective, environmentally friendly, and cost effective. Small-molecule supramolecular catalysts are typically endowed with hydrogen-bond-donor and -acceptor groups that assemble two simultaneously bound substrates (e.g. ditopic receptors) within a molecular complex for reaction. [3] Relatively high catalyst loadings are often required for these catalysts owing to effects of entropy, the reversibility and relative weakness of intermolecular forces in a liquid, as well as product inhibition. [4] All small-molecule supramolecular catalysts reported to date operate in the liquid phase, typically an organic solvent. [5] Here we report a ditopic supramolecular receptor, [6] in the form of the bifunctional hydrogen-bond donor 4,6-dichlororesorcinol (4,6-diCl-res) that operates as a supramolecular catalyst in the absence of solvent (Figure 1). The catalytic reaction is a [2+2] photodimerization of trans-1,2-bis(4pyridyl)ethylene (4,4'-bpe). [6b,c] The reaction directed by the catalyst occurs in a close-packed environment where molecular movement is at a minimum. The reaction results in the stereospecific formation of rctt-tetrakis(4-pyridyl)cyclobutane (4,4'-tpcb) in near quantitative yield. To achieve dynamic turnover between the catalyst, reactants, and product in the