INTERSYSTEM CROSSING IN FE(II) COORDINATION-COMPOUNDS

Due to the fact that for d6-systems there are a number of low-lying ligand field (LF) states the relaxation from excited states of Fe(II) coordination compounds is, in general, a very fast and radiationless process. In Fe(II) spin-crossover systems, however, the zero point energy difference between the two lowest states, namely the low-spin (LS) 1A1 and the high-spin (HS) 5T2 state, is of the order of k(B)T, and some systems can be converted quantitatively to the HS state well below the thermal transition temperature by irradiating either into MLCT or LF absorption bands of the LS species, with HS-->LS relaxation rates as small as 10(-6) s-1 at approximately 10 K. It is also possible to achieve a light-induced transient population of a HS state in Fe(II) LS compounds, but in this case the HS-->LS relaxation rates can be larger than 10(6) s-1 even at low temperatures. The HS-->LS relaxation rates show strong deviations from Arrhenius kinetics with nearly temperature independent tunnelling below approximately 70 K and a thermally activated behaviour above approximately 100 K. The range of 12 orders of magnitude in the low temperature tunnelling rate can be understood in terms of nonadiabatic multiphonon relaxation, where in the strong coupling limit, with the Huang-Rhys parameter S much larger than the reduced energy gap p, an inverse energy gap law holds.