Ultrafast Studies of the Light-Induced Spin Change in Fe(II)-Polypyridine Complexes

The phenomenon of spin-crossover (SCO), which implies a thermally induced transition from a low spin (LS) ground state to a high spin (HS) excited state (or the reverse) in metal-based molecular complexes has been demonstrated for derivatives of ions with d4, d5, d6 and d7 configurations and is observed for all these in complexes of the first transition series.1 SCO processes can also be triggered by pressure or irradiation by visible light. Research on the latter phenomenon witnessed a major development after McGarvey et al.2, 3 discovered that for a number of iron(II) and iron(III) polypyridine complexes in solution, the HS state could be populated efficiently at the expense of the LS state by pulsed laser excitation. Near ambient temperatures these light-induced HS states have lifetimes ranging from nanoseconds to microseconds, which are governed by the HS to LS relaxation kinetics. Following this discovery, Decurtins et al.4 showed that at cryogenic temperatures the HS/LS relaxation slows down such that under continuous irradiation, the iron(II) spin-crossover systems can be trapped in the HS state, for which the expression ‘light-induced excited state spin trapping (LIESST)’ was coined. The discovery of LIESST triggered several years of intense research not only with regard to the mechanism of trapping in the HS state of the system but also into the chemical and physical parameters governing the lifetimes of the low temperature metastable HS states.5, 6 The fact that LIESST can be observed in crystalline solids raised interest in the study of cooperative effects. The interest in spin trapping is not just academic but is also stirred by the potential applications in optical writing/magnetic reading for magnetic data storage.7, 8 Iron(II) polypyridine complexes are also considered as potential (and cheaper) candidates for dye-sensitized solar cells,9–12 which have thus far been most successful with ruthenium-based complexes.13 Finally, Fe(II) polypyridine complexes appear to have several photophysical properties in common with iron porphyrins, which play an important role in respiration and oxygen transport.