The Fe II complex, derived from 2+ (tpy = 2,2′:6′,2′′-terpyridine) by exchanging one tpy with a deprotonated phenylbipyridine, showed an extension of the MLCT lifetime by a factor of five and a decrease of the metal-centred state lifetime, while cyclometalating phenylphenanthroline ligands deliver a luminescent Fe II complex with a 3MLCT lifetime of ~1 ns 14. These predictions were recently supported by experimental studies 14, 21. Fe II complexes with such ligand types were extensively studied in theoretical investigations by Jakubikova and Dixon 17, 18, 19, 20. More importantly, they show lower-energy and thus more easily accessible π* orbitals than NHC donors. Cyclometalating phenyl-containing ligands offer strong σ-donor but also π-donor properties. Due to the electron-rich nature of NHC ligands and the electron-poor nature of the d 5 electron configuration, MLCT emission could not yet be observed in emissive Fe III complexes. Rigid anionic tripodal carbenes achieve a 2LMCT lifetime of 2 ns, with a quantum yield of 2% in aerated MeCN solution 16. 15 exhibits fluorescence from a 2LMCT state with a lifetime of ~100 ps. The Fe III congener of the Fe II complex with an MLCT lifetime of 0.5 ns presented by Wärnmark et al. Only recently, one emissive mononuclear Fe II complex was reported 13, 14, while two emissive Fe III complexes possessing six electron-donating carbene donor units have been reported so far 15, 16. Exceptionally long MLCT lifetimes could be obtained: (1) with six N-heterocyclic carbene (NHC) donor groups ( τ = 0.5 ns) 10 and (2) using the HOMO inversion concept 11 ( τ = 2.7 ns) 12. In Fe II complexes, these strategies typically lead to MLCT lifetimes in the picosecond range. Attempts to invert the order of metal-centred and charge-transfer states focus on the destabilization of metal-centred levels by strong σ donors 3, 4, 5 or the stabilization of charge-transfer states by π acceptors 6, 7, 8, 9. Consequently, short lifetimes are observed for charge-transfer states 2. The low-energy metal-centred states act as dark excited-state traps, quenching potentially emissive metal-to-ligand charge-transfer (MLCT) or ligand-to-metal charge-transfer (LMCT) states. The greatest challenge in the search for photoactive iron complexes is posed by the weak ligand field splitting of t 2g- and e g*-based orbitals, which is smaller in 3 d transition metal complexes compared with their 4 d and 5 d counterparts 1. With a lifetime of 4.6 ns, the strongly reducing and oxidizing MLCT-dominated state can initiate electron transfer reactions, which could constitute a basis for future applications of iron in photoredox catalysis. The low-lying π* levels of the cyclometalating units lead to energetically accessible MLCT states that cannot evolve into LMCT states. This behaviour is achieved by a ligand design that combines four N-heterocyclic carbenes with two cyclometalating aryl units. Here we report the Fe III complex (HImP = 1,1′-(1,3-phenylene)bis(3-methyl-1-imidazol-2-ylidene)), showing a Janus-type dual emission from ligand-to-metal charge transfer (LMCT)- and metal-to-ligand charge transfer (MLCT)-dominated states. Emissive iron compounds are scarce and dual emission has not been observed before. Although iron is a dream candidate to substitute noble metals in photoactive complexes, realization of emissive and photoactive iron compounds is demanding due to the fast deactivation of their charge-transfer states.
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