The ‘π-Trap’ as an Unrestricted Crystal Sponge for Inherently Amorphous Cluster Compounds

Single crystal diffraction is one of the most common and powerful tools for structural elucidation in science. However, obtaining single crystals of adequate size and quality is not always trivial, with some chemicals inherently resisting all attempts. The ‘crystalline sponge’ method has attracted a lot of attention for crystallizing otherwise intrinsically amorphous compounds inside a metal organic framework (MOF).1,2 However, its application is limited by the size and stability of the pores within the networks. In this study, we propose a novel ‘unrestricted crystalline sponge’ method, which we denominate as the ‘π-trap’. It makes use of π-π interactions between C60 and nm-sized molecules that by themselves do not form crystalline compounds. Using this technique, we successfully crystallized adamantane-like organic-inorganic hybrid clusters, which exhibit extreme nonlinear optical properties only within the amorphous habitus, and resist any attempt for crystallization. As the clusters' low tendency to order in the long range was successfully overcome by the strong C60···cluster interactions in the ‘π-trap’, we were able to precisely determine their molecular structures. As we could show by optical spectroscopy and quantum chemical calculations, both the clusters and C60 behave like being dissolved in the other component, including the formation of cluster pairs previously proposed by theoretical studies and low-angle scattering experiments on amorphous samples. We propose that the described method is applicable to all kinds of amorphous compounds that allow for π-π interactions, without the size restrictions facing the ‘crystalline sponge’ method, especially when considering the usage of larger fullerenes.