Trend in light-induced excited-state spin trapping in Fe(ii)-based spin crossover systems

A computational study of the light-induced excited spin-state trapping (LIESST) in a number of Fe(ii) spin crossover complexes, coordinated by monodentate, bidentate and multidentate ligands is carried out, with the goal to uncover the trend in the low temperature relaxation rate. A nine order of magnitude change in low temperature relaxation rate is observed among the complexes. The trend is rationalized in terms of the change in metal-ligand covalency, numerically estimated by the crystal orbital Hamiltonian population, thus influencing the back donation or delocalization of the electrons from the low-lying Fe(ii)-centered molecular orbital to the empty low-lying ligand-centered pi* antibonding molecular orbitals.