Dynamics of spin state conversion processes in the solid state

Hyperfine Interactions - High spin (HS) ⇌ low spin (LS) conversions in transition metal complexes are nonradiative transitions between spin states. In this contribution, we present a study of...     

Intersystem crossing in Fe(II) coordination compounds

Fe(II) spin-crossover systems can be quantitatively converted from the low-spin (LS) to the high-spin (HS) state well below the thermal transition temperature by irradiating either into the metal-ligand charge transfer or d-d absorption bands, and even in low-spin systems a transient population of the HS state can be achieved. This fact can be made use of to determine HS LS relaxation rate constants for a wide variety of Fe(II) spin-crossover and low-spin systems. The HS LS relaxation shows strong deviations from an Arrhenius behaviour, with nearly temperature-independent tunnelling below 70 K and a thermally activated process above 100 K. The range of more than 12 orders of magnitude in the low temperature tunnelling rate constant can be understood in terms of a non-adiabatic multiphonon process, where in the strong vibronic coupling limit an inverse energy gap law holds.