Regulation of Multistep Spin Crossover Across Multiple Stimuli in a2-D Framework Material

We investigate the effects of a broad array of external stimuli on the structural,spin-crossover (SCO) properties and nature of the elastic interaction within the two-dimensional Hofmann framework material [Fe(cintrz)2Pd(CN)4]middotguest (cintrz =N-cinnamalidene 4-amino-1,2,4-triazole;Amiddotguest; guest = 3H2O, 2H2O, and O). This frameworkexhibits a delicate balance between ferro- and antiferro-elastic interaction characters; we showthat manipulation of the pore contents across guests = 3H2O, 2H2O, and O can be exploitedto regulate this balance. InAmiddot3H2O, the dominant antiferroelastic interaction characterbetween neighboring FeIIsites sees the low-temperature persistence of the mixed spin-statespecies {HS-LS} for {Fe1-Fe2} (HS = high spin, LS = low spin). Elastic interaction strain isresponsible for stabilizing the {HS-LS} state and can be overcome by three mechanisms: (1)partial (2H2O) or complete (O) guest removal, (2) irradiation via the reverse light-inducedexcited spin-state trapping (LIESST) effect (lambda= 830 nm), and (3) the application of externalhydrostatic pressure. Combining experimental data with elastic models presents a clearinterpretation that while guest molecules cause a negative chemical pressure, they also haveconsequences for the elastic interactions between metals beyond the simple chemical pressure picture typically proposed.