Photoselective MLCT to d-d pathways for light-induced excited spin state trapping

We use femtosecond optical pump-probe spectroscopy to study the Light Induced Excited Spin State Trapping (LIESST) dynamics in an Fe^II spin-crossover material. In these systems, LIESST derives from fast molecular switching induced by light from low (LS, S = 0) to high spin (HS, S = 2) states, as reported for molecules in solution as well as in the solid state. Since the direct LS-to-HS conversion is forbidden by selection rules, the switching dynamics involves intermediate electronic states such as metal-to-ligand charge transfer (MLCT) or ligand-field excited states of singlet or triplet nature. In addition, the HS state is structurally trapped by the elongation of the metal-ligand bond, which is accompanied by the coherent activation and damping of the molecular breathing mode. The ultrafast LIESST dynamics was mainly investigated in FeN₆ ligand field systems with almost octahedral symmetry, under MLCT excitation. Our recent study on the Fe^II(pap-5NO₂)₂ spin-crossover material, with a Fe^IIN₄O₂ ligand field of C₂ symmetry, has shown that in addition to MLCT bands, optical excitation, through quite intense and low-energy shifted d-d bands, can also drive LIESST. Compared to MLCT, d-d excitation involves shorter-lived intermediates, drives faster LS-to-HS switching, and enhances the coherent structural dynamics. In this paper, we present an ultrafast study of the pump wavelength dependence of LIESST and we evidence a photoselective crossover from the MLCT to the d-d pathways.