The π-trap approach for obtaining crystal structure data of inherently amorphous cluster compounds

Single crystal diffraction is one of the most common and powerful tools for structural elucidation. However, obtaining single crystals of adequate size and quality is not always trivial. The “crystalline sponge” method has been used for crystallizing intrinsically amorphous compounds inside a metal organic framework,1–4, but its application is limited by the size and stability of the pores within the networks. Here, we report the use of π–π interactions between C60 and nanometer-sized molecules that by themselves do not form crystalline compounds. Using this “π-trap” approach, we successfully crystallized adamantane-like clusters exhibiting extreme nonlinear optical properties, which so far resisted any attempt for crystallization. C60···cluster interactions enabled long-range order, so the clusters’ molecular structures could be precisely determined. Spectroscopy and quantum chemical studies showed that clusters and C60 behave like being dissolved in each other. This method should be applicable to all kinds of amorphous compounds that undergo π–π interactions.