Structural and spectroscopic characterisation of the spin crossover in {Fe}(abpt)2({NCS})2 polymorph {A}

A crystallographic and solid state spectroscopic study of the spin crossover behaviour of Fe(abpt)2(NCS)2 (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) polymorph A is reported. Structural features including crystallographic cell parameters, bond lengths and distortion parameters are monitored between 375 K and 30 K and crystal structures are reported at seven temperatures across the spin transition. In addition, the light induced excited spin state trapping (LIESST) metastable high spin structure, HS*, is reported at 30 K by continuous irradiation with a 670 nm, 5 mW CW laser during the data collection. Relaxation of the HS* state at 30 K with the laser switched off is found to occur within �4000 s in accordance with the literature. High pressure single crystal datasets are also reported to examine the effect of pressure on the spin transition. Single crystal variable temperature UV-Vis spectroscopy and resonant ultrasound spectroscopy support the crystallographic evidence relating to the spin crossover transition presented herein. Strain analysis of the lattice parameters yields the temperature dependence of the spin order parameter, indicating strong spin�lattice coupling to give a volume strain of up to �4\% and a shear strain of up to �1.5\%. These, in turn, are responsible for changes in elastic constants by up to �35\%