Cooperative thermal and optical switching of spin states in a new two-dimensional coordination polymer.

(Fe(pmd)2[Cu(CN)2]2) (pmd = pyrimidine) displays a rigid two-dimensional structure and undergoes thermal- and optical-driven spin crossover behaviour; cooperative elastic coupling between iron(II) ions in the framework induces thermal hysteresis in the HS <--> LS conversion and sigmoidal HS --> LS relaxation of the photo-induced HS state at low temperatures.