Reversing the relative 3MLCT–3MC order in Fe(II) complexes using cyclometallating ligands: a computational study aiming at luminescent Fe(II) complexes

Following a computational approach, the use of strongly electron-donating cyclometallating ligands has allowed us to increase the 3MC–3MLCT gap dramatically in Fe(II) bis(tridentate) polypyridine complexes, and eventually to reverse the ordering between these states, yielding a 3MLCT state that is clearly more stable than the 3MC state. Simultaneously, the quintet excited states (5MC and 5MLCT) are displaced away from the region (in terms of geometry and energy) where classical photophysics occur, allowing us to avoid magnetism. The situation is thus similar to that of classical ruthenium polypyridine complexes. This opens the way towards luminescent iron(II) complexes, in particular Fe(II)bis(6-phenyl-2,2′-bipyridine) Fe(NNC)2.