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 (MC)-M-3-(MLCT)-M-3 gap dramatically in Fe(II) bis(tridentate) polypyridine complexes, and eventually to reverse the ordering between these states, yielding a (MLCT)-M-3 state that is clearly more stable than the (MC)-M-3 state. Simultaneously, the quintet excited states ((MC)-M-5 and (MLCT)-M-5) 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).