Comparison of structural dynamics and coherence of d–d and MLCT light-induced spin state trapping

Chemical Science 2017, 8, 4978-4986

DOI: 10.1039/C6SC05624E

Light Induced Excited Spin State Trapping (LIESST) in FeII spin-crossover systems is a process switching molecules from low (LS, S=0) to high spin (HS, S=2) states. The direct LS-to-HS conversion being forbidden by selection rules, LIESST involves intermediate states such as 1,3MLCT or 1,3T. The intersystem crossing sequence results in a HS state, structurally trapped by metal-ligand bond elongation through the coherent activation and damping of molecular breathing. The ultrafast dynamics of this process was so far investigated in FeN6 ligand field systems, under MLCT excitation. Here we study LIESST in an FeIIN4O2 spin-crossover material of lower symmetry, which allows quite intense and low-energy shifted d-d bands. By combining ab-initio DFT and TD-DFT calculations and fs optical absorption measurements we demonstrate that, compared to MLCT, d-d excitation induces faster LS-to-HS switching and that the shorter intermediates enhance coherent structural dynamics.