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

Steube, Jakob; Burkhardt, Lukas; Päpcke, Ayla; Moll, J.L.; Zimmer, Peter; Schoch, Roland; Wölper, Christoph; Heinze, Katja; Lochbrunner, Stefan

doi:10.1002/chem.201902488

Cited by 37 publications

(57 citation statements)

References 59 publications

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“…The use of NHC containing ligands has led to significant progress in this research field, however, other approaches have also made big steps forward recently. Cyclometallating ligands have been proposed to have very beneficial properties due to their strong σ-donating character [63][64][65][66][67]. Recently, the first synthesis of an air-stable cyclometallated iron complex has been presented along with investigations of the excited state properties of the compound and a 0.8 ps MLCT lifetime was found (a 5.5-fold increase compared to the 140 fs in the parent [Fe(tpy) 2 ] 2+ ) [68].…”

Section: Discussionmentioning

confidence: 99%

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

Kaufhold

Wärnmark

2020

Catalysts

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The use of iron in photoactive metal complexes has been investigated for decades. In this respect, the charge transfer (CT) states are of particular interest, since they are usually responsible for the photofunctionality of such compounds. However, only recently breakthroughs have been made in extending CT excited state lifetimes that are notoriously short-lived in classical polypyridine iron coordination compounds. This success is in large parts owed to the use of strongly σ-donating N-heterocyclic carbene (NHC) ligands that help manipulating the photophysical and photochemical properties of iron complexes. In this review we aim to map out the basic design principles for the generation of photofunctional iron NHC complexes, summarize the progress made so far and recapitulate on the synthetic methods used. Further, we want to highlight the challenges still existing and give inspiration for future generations of photoactive iron complexes.

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Section: Discussionmentioning

confidence: 99%

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

Kaufhold

Wärnmark

2020

Catalysts

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“…Compared to the value of 0.31 V for the analogous iron complex with two C^N^C ligands (2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine), a cathodic shift of 1.47 V is observed. 26 This corresponds to the behavior of the [Fe(tpy) 2 ] 2+ / [Fe(C^N^N) (tpy)] + pair with a cathodic shift of 0.83 V. 22 A quasi-reversible wave at 0.08 V is assigned to the Fe III/IV couple, while the irreversible wave at E p = 1.23 V is attributed to ligand oxidation (Extended Data Fig. 4a).…”

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confidence: 94%

“…[18][19][20][21] The quantum chemical predictions were recently supported by an experimental study from our groups. 22 The Fe II complex, derived from [Fe(tpy) 2 ] 2+ (tpy = terpyridin) by exchange of one tpy through a deprotonated phenylbipyridine, showed an extension of the MLCT lifetime by a factor of 5 and a decrease of the MC lifetime. Here we explore the application of cyclometalating ligands in iron(III) complexes to extend CT lifetimes, which nally results in a two-color luminescence that has never been observed for iron complexes before.…”

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confidence: 99%

Janus-type dual emission of a Cyclometalated Iron(III) complex

Steube¹,

Päpcke²,

Bokareva³

et al. 2020

Preprint

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Photoactive compounds are essential for photocatalytic and luminescent applications, such as photoredox catalysis or light emitting diodes. However, the substitution of noble metals, which are almost exclusively used, by base metals remains a major challenge on the way to a more sustainable world.1 Iron is a dream candidate for this ambitious aim.2 But compared to noble metal complexes that show long-lived metal-to-ligand charge-transfer (MLCT) states, realization of emissive and photoactive iron complexes is demanding, due to the fast deactivation of charge transfer states into non-emissive inactive states. No MLCT emission has been observed for monometallic iron complexes before. Consequently, dual emission could also not yet be realized with iron complexes, as it is a very rare property even of noble metal compounds. Here we report the FeIII complex [Fe(ImP)2][PF6] (HImP = 1,1’-(1,3-phenylene)bis(3-methyl-1-imidazol-2-ylidene)), showing Janus-type dual emission by combining LMCT (ligand-to-metal charge transfer) with MLCT luminescence. The respective excited states are characterized by a record lifetime of τMLCT = 4.2 ns, and a moderate τLMCT = 0.2 ns. Only two emissive FeIII compounds are known so far and they show LMCT luminescence only.3,4 The unique properties of the presented complex are caused by the specific ligand design combining four N-heterocyclic carbenes with two cyclometalating groups, using the σ-donor strength of six carbon atoms and the acceptor capabilities of the central phenyl rings. Spectroscopically, doublet manifolds could be identified in the deactivation process, while (TD)DFT analysis revealed the presence of quartets as well. With three key advancements of realizing the first iron complex showing dual luminescence, a MLCT luminescence and a world record MLCT lifetime, the results constitute a basis for future application of iron complexes as white light emitters and new photocatalytic reactions making use of the Janus-type properties of the developed complex.

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“…The molecular and the electronic structures of these compounds are reminiscent of Fe II and Ru II polypyridine complexes, which have been investigated extensively in the past. Until now, no convincing case of steady-state MLCT luminescence from a Fe II complex has been reported despite significant advances in extending their 3 MLCT lifetimes in recent years [18][19][20][21][22][23]; hence, our Cr 0 complex currently seems to be the only example of a first-row d 6 -metal complex showing MLCT luminescence in solution at room temperature under steady-state photo-irradiation [24]. The Mo 0 diisocyanide complexes are not only emissive, but they can furthermore be employed in photoredox catalysis of thermodynamically challenging reductions, which cannot be performed with more widely known photoreductants such as fac-[Ir(ppy)3] (ppy = 2-phenylpyridine) [25].…”

Section: Introductionmentioning

confidence: 99%

Excited-State Relaxation in Luminescent Molybdenum(0) Complexes with Isocyanide Chelate Ligands

Herr

Wenger

2020

Inorganics

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Diisocyanide ligands with a m-terphenyl backbone provide access to Mo 0 complexes exhibiting the same type of metal-to-ligand charge transfer (MLCT) luminescence as the well-known class of isoelectronic Ru II polypyridines. The luminescence quantum yields and lifetimes of the homoleptic tris(diisocyanide) Mo 0 complexes depend strongly on whether methyl-or tert-butyl substituents are placed in α-position to the isocyanide groups. The bulkier tert-butyl substituents lead to a molecular structure in which the three individual diisocyanides ligated to one Mo 0 center are interlocked more strongly into one another than the ligands with the sterically less demanding methyl substituents. This rigidification limits the distortion of the complex in the emissive excited-state, causing a decrease of the nonradiative relaxation rate by one order of magnitude. Compared to Ru II polypyridines, the molecular distortions in the luminescent 3 MLCT state relative to the electronic ground state seem to be smaller in the Mo 0 complexes, presumably due to delocalization of the MLCT-excited electron over greater portions of the ligands. Temperature-dependent studies indicate that thermally activated nonradiative relaxation via metal-centered excited states is more significant in these homoleptic Mo 0 tris(diisocyanide) complexes than in [Ru(2,2 -bipyridine) 3 ] 2+ . complexes with monodentate arylisocyanide ligands are very strong photoreductants, capable, for example, of reducing anthracene to its radical anion form. Many different kinds of metal complexes with isocyanide ligands have been explored over the past few decades [11][12][13][14][15], but metals with the d 6 or d 10 electron configurations are unique in their ability to show luminescence from a 3 MLCT excited state.Whilst structurally more flexible, multidentate isocyanide chelate ligands had been known for some time [16], we found that chelating diisocyanide ligands based on a m-terphenyl backbone permit the synthesis of homoleptic tris(diisocyanide) complexes of Cr 0 and Mo 0 that luminesce from 3 MLCT excited states (Figure 1) [17]. The molecular and the electronic structures of these compounds are reminiscent of Fe II and Ru II polypyridine complexes, which have been investigated extensively in the past. Until now, no convincing case of steady-state MLCT luminescence from a Fe II complex has been reported despite significant advances in extending their 3 MLCT lifetimes in recent years [18][19][20][21][22][23]; hence, our Cr 0 complex currently seems to be the only example of a first-row d 6 -metal complex showing MLCT luminescence in solution at room temperature under steady-state photo-irradiation [24]. The Mo 0 diisocyanide complexes are not only emissive, but they can furthermore be employed in photoredox catalysis of thermodynamically challenging reductions, which cannot be performed with more widely known photoreductants such as fac-[Ir(ppy) 3 ] (ppy = 2-phenylpyridine) [25]. Thus, the Mo 0 diisocyanide complexes represent Earth-abundant alternatives to precious-...

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

Cited by 37 publications

References 59 publications

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

Janus-type dual emission of a Cyclometalated Iron(III) complex

Excited-State Relaxation in Luminescent Molybdenum(0) Complexes with Isocyanide Chelate Ligands

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