Spin crossover, LIESST, and NIESST - Fascinating electronic games in iron complexes

Coordination compounds of transition metal ions with open-shell electron configurations may exhibit dynamic electronic structure phenomena depending on the nature of the coordinating ligand sphere. The change of spin state with temperature (''thermal spin crossover'') and light-induced electron transfer processes leading to long-lived metastable states are among the most fascinating electronic games encountered in transition metal compounds and are presently under intensive study by chemists and physicists. The first part of this lecture will survey briefly some highlights of previous work and present recent results on thermal spin crossover in iron(II) compounds. The second part of the lecture will focus on selected examples of ''Light-Induced Excited Spin State Trapping'' (LIESST), with special emphasis on the occurrence of bistability and cooperative interactions during LIESST relaxation. The last part of the lecture will be devoted to ''Nuclear Decay-Induced Excited Spin State Trapping'' (NIESST), the unique experiment which allows one to make use of the nuclear disintegration of Co-57 --> Fe-57 as an ''internal light source'' to generate such photophysical aftereffects as aliovalent charge and anomalous spin states, and to detect and characterize such metastable states simultaneously by time-integral and time-differential Mossbauer emission spectroscopy. Results from lifetime measurements by Mossbauer as well as optical techniques prove that the same relaxation mechanism applies to both phenomena, LIESST and NIESST.