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

Abstract

Following a computational approach, the use of strongly electron-donating cyclometallating ligands has allowed us to increase the (3)MC-(3)MLCT gap dramatically in Fe(ii) bis(tridentate) polypyridine complexes, and eventually to reverse the ordering between these states, yielding a (3)MLCT state that is clearly more stable than the (3)MC state. Simultaneously, the quintet excited states ((5)MC and (5)MLCT) 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).

Excited state decay of cyclometalated polypyridine ruthenium complexes: insight from theory and experiment

“…Similarly, Dixon and coworkers studied mono-and bis(cyclometalated) iron(II) complexes using DFT calculations. [159][160][161] The 3 MC state in [Fe( pbpy)(tpy)] + with peripheral cyclometalation is substantially higher in energy than in [Fe(dpb)(tpy)] + with central cyclometalation, very similar to the results presented here for the ruthenium homologues. Furthermore, they highlighted, that bis(cyclometalated) iron(II) complexes such as [Fe(dpb)( pbpy)] and [Fe( pbpy) 2 ] have very low-lying 3 MLCT states that are, in the case of [Fe(dpb)( pbpy)], only marginally distorted compared to the ground state geometry.…”

Excited‐State Kinetics of an Air‐Stable Cyclometalated Iron(II) Complex

“…In principle, the 5 MC state could still be lower in energy than the 3 MLCT state, but would remain unpopulated due to a high activation barrier caused by strong distortions. Still, our time‐resolved measurements confirm the theoretical calculations of Dixon et al . and Jakubikova et al .…”

“…This possibility would require direct relaxation pathways from both 3 MLCT states into the 1 A 1 ground state bypassing the typically involved 3 MC state. This is very much at odds with theoretical predictions for such complexes …”

“…In contrast, quantum chemical predictions provided by Jakubikova et al. and Dixon et al . for the effects of exchanging an N‐donor by a cyclometalating ligand could not be confirmed experimentally so far.…”

“…[6,13] In contrast, quantumc hemicalp redictions providedb yJ akubikovae tal. and Dixon et al [14,15,16] for the effects of exchanging an N-donor by ac yclometalating ligand could not be confirmede xperimentally so far.A lthough aryl carbanion ligands in the form of 2-phenylpyridine, 1,3-di(2-pyridinyl)benzene, and 6-phenyl-2,2'-bipyridine (Hpbpy already found applications in the context of noble metal complexes, [17,18] air-and waterstablec yclometalatedF e II compounds, allowing for photochemicala pplications, are lacking up to date. [19] With this work we will fill this gap and present a) as ynthetic access to heteroleptic,c yclometalated [Fe(N^N^N)(N^N^C cm )] + complexes, here in the form of [Fe(pbpy)(tpy)] + (tpy = 2,2':6',2''-terpyri-dine) and b) thoroughly investigate the ground-and excitedstate characteristics in comparison to the prototypical complex [Fe(tpy) 2 ] 2 + to prove the high potential of cyclometalating ligands forincreasing MLCT lifetimes.…”