Thermal Spin Crossover Behaviour of Two-Dimensional Hofmann-Type Coordination Polymers Incorporating Photoactive Ligands

Two spin crossover (SCO)-active 2D Hofmann-type framework materials, [Fe(3-PAP)(2)Pd(CN)(4)] (A) and [Fe(4-PAP)(2)Pd(CN)(4)] (B) containing the photoactive azo-benzene-type ligands 3-phenylazo-pyridine (3-PAP) and 4-phenylazo-pyridine (4-PAP) were prepared. These materials form non-porous Hofmann-type structures whereby 2D [(FePd)-Pd-II(CN)(4)] grids are separated by 3- or 4-PAP ligands. The iron(ii) sites of both materials (A and B) undergo abrupt and hysteretic spin transitions with characteristic transition temperatures T-1/2,: 178, 190K (T: 12K) and T-1/2,: 233, 250K (T: 17K), respectively. Photo-magnetic characterisations reveal light-induced excited spin state trapping (LIESST) activity in both A and B with characteristic T(LIESST) values of 45 and 40K. Although both free ligands show trans- to-cis isomerisation in solution under UV-irradiation, as evidenced via absorption spectroscopy, such photo-activity was not observed in the ligands or complexes A and B in the solid state. Structural analysis of a further non-SCO active isomer to B, [Fe(4-PAP)(2)Pd(CN)(4)]1/2(4-PAP) (B(4-PAP)), which contains free ligand in the pore space is reported.