Elucidating the Mechanism of a Two-Step Spin Transition in a Nanoporous Metal−Organic Framework

The nanoporous metal-organic framework, Fe(bpe)2(NCS)2 x 3(acetone), SCOF-4(Ac) (where bpe is 1,2-bis(4'-pyridyl)ethane), displays a two-step spin crossover (SCO) transition (65-155 K) that is sensitive to the presence of acetone guest molecules. Structural analyses have revealed a structural phase transition, from tetragonal (P42(1)c) to orthorhombic (P2(1)2(1)2), associated with the spin transition that defines a checkerboard-like ordering of spin sites at the high-spin:low-spin plateau. The reversible desorption of the acetone guest species is accompanied by a complex series of structural phase transitions that describe a dramatic flexing of the structure. The thermal trapping of a metastable state with approximately 20-25% high-spin character was observed both magnetically and structurally upon rapid quenching of SCOF-4(Ac) to 10 and 25 K, respectively. The light-induced excited spin state trapping (LIESST) effects for SCOF-4(Ac) show a 55% excitation of a metastable HS state at 10 K and a characteristic T(LIESST) value of 52 K.