Wrapping a single crystal spin-crossover complex with a single crystal non-spin-crossover complex to modulate the spin-transition temperature

The formation of a heterojunction interface between different materials could lead to emergent functions that are not the simple sum of the properties of each component. Here we report the modulation of physical properties due to the lattice mismatch between two molecular crystals. Using metal complexes as a motif, we created a crystalline object that can be referred to as a core-shell crystal, in which a single crystal of one complex (Fe^II complex) is wrapped with a single crystal of a different complex (Co^II or Zn^II complex). X-ray analysis revealed that the Fe^II complex constituting the inner core exhibits spin-crossover behavior, while the Co^II (or Zn^II) complex constituting the outer shell does not. The crystal structure of the outer shell is different from that formed through spontaneous crystallization of the Co^II (or Zn^II) complex alone, but similar to that of the high-temperature phase of the spin-crossover Fe^II complex. Interestingly, the spin transition behavior of the Fe^II complex inside the core-shell crystals changes, demonstrating that in molecular materials, the formation of a heterojunction interface can modulate the properties of the entire bulk crystal. The fabrication of a ternary core-shell crystal using Fe^II, Co^II and Zn^II complexes is also presented.