Heterojunction interfaces formed between different crystalline materials often provide platforms for emergent functions; however, their construction in molecular crystals remains highly challenging. This difficulty arises because even slight mismatches in lattice parameters or packing geometry can destabilize interfaces, drastically narrowing the tolerance for heteroepitaxy between different molecular crystals. Here, we present a solid-solution-mediated approach that enables on-seed-surface crystallization to form heterostructured molecular crystals from combinations of metal complexes that do not form heterojunctions. Using FeII and CoII complexes bearing a common tridentate ligand, we show that on-seed-surface crystallization fails because of substantial interfacial lattice mismatch. In contrast, solid-solution crystals composed of FeII and CoII complexes can serve as secondary components that establish lattice matching with the FeII seed crystal, leading to the formation of core–shell crystals. Notably, systematic variation of the FeII/CoII feed ratio in the solid-solution composition reveals that the lattice mismatch at the heterojunction becomes increasingly anisotropic as the FeII content decreases, depending on the crystallographic direction. Under appropriate conditions, this anisotropy gives rise to facet-selective on-seed-surface crystallization of the solid-solution component from the FeII seed, resulting in dumbbell-like crystals in which secondary segments grow from specific facets of the seed crystal. Single-crystal X-ray analysis further shows that the secondary segments adopt the crystal structure of the seed despite their very low FeII content. This demonstrates the utility of solid-solution-mediated lattice matching as a strategy for constructing heterostructured molecular crystals.
Fukui et al. (Fri,) studied this question.