Transition-metal complexes are used as photosensitizers, in light-emitting diodes, for biosensing and in photocatalysis. A key feature in these applications is excitation from the ground state to a charge-transfer state; the long charge-transfer-state lifetimes typical for complexes of ruthenium and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron and copper being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs, it remains a formidable scientific challenge to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers. Here we present the iron complex [Fe(btz)3]3+ (where btz is 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)), and show that the superior σ-donor and π-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(iii) d5 complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer (2LMCT) state that is rarely seen for transition-metal complexes. The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.

A low-spin Fe( iii ) complex with 100-ps ligand-to-metal charge transfer photoluminescence

Recent exciting developments in the area of mononuclear photoactive complexes with Earth-abundant metal ions (Cu, Zr, Fe, Cr) for potential eco-friendly applications in (phosphorescent) organic light emitting diodes, in imaging and sensing systems, in dye-sensitized solar cells and as photocatalysts are presented. Challenges, in particular the extension of excited state lifetimes, and recent conceptual breakthroughs in substituting precious and rare-Earth metal ions (e.g. Ru, Ir, Pt, Au, Eu) in these applications by abundant ions are outlined with selected examples. Relevant fundamentals of photophysics and photochemistry are discussed first, followed by conceptual and instructive case studies.

Photophysics and photochemistry with Earth-abundant metals – fundamentals and concepts

ConspectusThe photophysics and photochemistry of transition metal complexes (TMCs) has long been a hot field of interdisciplinary research. Rich metal-based redox processes, together with a high variety in electronic configurations and excited-state dynamics, have rendered TMCs excellent candidates for interconversion between light, chemical, and electrical energies in intramolecular, supramolecular, and interfacial arrangements. In specific applications such as photocatalytic organic synthesis, photoelectrochemical cells, and light-driven supramolecular motors, light absorption by a TMC-based photosensitizer and subsequent excited-state energy or electron transfer constitute essential steps. In this context, TMCs based on rare and expensive metals, such as ruthenium and iridium, are frequently employed as photosensitizers, which is obviously not ideal for large-scale implementation. In the search for abundant and environmentally benign solutions, six-coordinate FeII complexes (FeIIL6) have been widely co...

Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers

Ruthenium complexes with polypyridine ligands are very popular choices for applications in photophysics and photochemistry, for example, in lighting, sensing, solar cells, and photoredox catalysis. There is a long-standing interest in replacing ruthenium with iron because ruthenium is rare and expensive, whereas iron is comparatively abundant and cheap. However, it is very difficult to obtain iron complexes with an electronic structure similar to that of ruthenium(II) polypyridines. The latter typically have a long-lived excited state with pronounced charge-transfer character between the ruthenium metal and ligands. These metal-to-ligand charge-transfer (MLCT) excited states can be luminescent, with typical lifetimes in the range of 100 to 1000 ns, and the electrochemical properties are drastically altered during this time. These properties make ruthenium(II) polypyridine complexes so well suited for the abovementioned applications. In iron(II) complexes, the MLCT states can be deactivated extremely rapidly (ca. 50 fs) by energetically lower lying metal-centered excited states. Luminescence is then no longer emitted, and the MLCT lifetimes become much too short for most applications. Recently, there has been substantial progress on extending the lifetimes of MLCT states in iron(II) complexes, and the first examples of luminescent iron complexes have been reported. Interestingly, these are iron(III) complexes with a completely different electronic structure than that of commonly targeted iron(II) compounds, and this could mark the beginning of a paradigm change in research into photoactive earth-abundant metal complexes. After outlining some of the fundamental challenges, key strategies used so far to enhance the photophysical and photochemical properties of iron complexes are discussed and recent conceptual breakthroughs are highlighted in this invited Concept article.

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Is Iron the New Ruthenium

Bright, long-lived emission from first-row transition-metal complexes is very challenging to achieve. Herein, we present a new strategy relying on the rational tuning of energy levels. With the aid of the large N-Cr-N bite angle of the tridentate ligand ddpd (N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine) and its strong σ-donating capabilities, a very large ligand-field splitting could be introduced in the chromium(III) complex [Cr(ddpd)2](3+), that shifts the deactivating and photoreactive (4)T2 state well above the emitting (2)E state. Prevention of back-intersystem crossing from the (2)E to the (4)T2 state enables exceptionally high near-infrared phosphorescence quantum yields and lifetimes for this 3d metal complex. The complex [Cr(ddpd)2](BF4)3 is highly water-soluble and very stable towards thermal and photo-induced substitution reactions and can be used for fluorescence intensity- and lifetime-based oxygen sensing in the NIR.

[Cr(ddpd)2]3+: A Molecular, Water‐Soluble, Highly NIR‐Emissive Ruby Analogue

A heteroleptic iron(II) complex [Fe(dcpp)(ddpd)](2+) with a strongly electron-withdrawing ligand (dcpp, 2,6-bis(2-carboxypyridyl)pyridine) and a strongly electron-donating tridentate tripyridine ligand (ddpd, N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine) is reported. Both ligands form six-membered chelate rings with the iron center, inducing a strong ligand field. This results in a high-energy, high-spin state ((5) T2 , (t2g )(4) (eg *)(2) ) and a low-spin ground state ((1) A1 , (t2g )(6) (eg *)(0) ). The intermediate triplet spin state ((3) T1 , (t2g )(5) (eg *)(1) ) is suggested to be between these states on the basis of the rapid dynamics after photoexcitation. The low-energy π(*) orbitals of dcpp allow low-energy MLCT absorption plus additional low-energy LL'CT absorptions from ddpd to dcpp. The directional charge-transfer character is probed by electrochemical and optical analyses, Mosbauer spectroscopy, and EPR spectroscopy of the adjacent redox states [Fe(dcpp)(ddpd)](3+) and [Fe(dcpp)(ddpd)](+) , augmented by density functional calculations. The combined effect of push-pull substitution and the strong ligand field paves the way for long-lived charge-transfer states in iron(II) complexes.

A Heteroleptic Push-Pull Substituted Iron(II) Bis(tridentate) Complex with Low-Energy Charge-Transfer States

Reversing the 3MLCT / 3MC excited state order in iron(II) complexes is a challenging objective, yet would finally result in longsought luminescent transition metal complexes with an earthabundant central ion. One approach to achieve this goal is based on low-energy charge transfer absorptions in combination with a strong ligand field. Coordinating electron rich and electron poor tridentate oligopyridine ligands with large bite angles at iron(II) enables both low-energy MLCT absorption bands around 590 nm and a strong ligand field. Variations of the electron rich ligand by introducing longer alkyl substituents destabilizes the iron(II) complex towards ligand substitution reactions while hardly affects the optical properties. On the other hand, N-deprotonation in the ligand backbone is feasible and reversible yielding deep green complexes with charge-transfer bands extending into the near-IR region. Time-dependent Density Functional Theory calculations assign these absorption bands to transitions with dipole-allowed ligand-to-ligand charge transfer character. This unique geometric and electronic situation establishes a further regulating screw to increase the energy gap between potentially emitting charge transfer states and the non-radiative ligand field states of iron(II) dyes.

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201700959

Boosting Vis/NIR Charge-Transfer Absorptions of Iron(II) Complexes by N-Alkylation and N-Deprotonation in the Ligand Backbone.

A series of anchor-functionalized cyclometalated bis(tridentate) ruthenium(II) triarylamine hybrids [Ru(dbp-X)(tctpy)](2-) [2 a](2-) -[2 c](2-) (H3 tctpy=2,2';6',2''-terpyridine-4,4',4''-tricarboxylic acid; dpbH=1,3-dipyridylbenzene; X=N(4-C6 H4 OMe)2 ([2 a](2-) ), NPh2 ([2 b](2-) ), N-carbazolyl [2 c](2-) ) was synthesized and characterized. All complexes show broad absorption bands in the range 300-700 nm with a maximum at about 545 nm. Methyl esters [Ru(Me3 tctpy)(dpb-X)](+) [1 a](+) -[1 c](+) are oxidized to the strongly coupled mixed-valent species [1 a](2+) -[1 c](2+) and the Ru(III) (aminium) complexes [1 a](3+) -[1 c](3+) at comparably low oxidation potentials. Theoretical calculations suggest an increasing spin delocalization between the metal center and the triarylamine unit in the order [1 a](2+) <[1 b](2+) <[1 c](2+) . Solar cells were prepared with the saponified complexes [2 a](2-) -[2 c](2-) and the reference dye N719 as sensitizers using the I3 (-) /I(-) couple and [Co(bpy)3 ](3+/2+) and [Co(ddpd)2 ](3+/2+) couples as [B(C6 F5 )4 ](-) salts as electrolytes (bpy=2,2'-bipyridine; ddpd=N,N'-dimethyl-N,N'-dipyridin-2-yl-pyridine-2,6-diamine). Cells with [2 c](2-) and I3 (-) /I(-) electrolyte perform similarly to cells with N719. In the presence of cobalt electrolytes, all efficiencies are reduced, yet under these conditions [2 c](2-) outperforms N719.

Strongly Coupled Cyclometalated Ruthenium Triarylamine Chromophores as Sensitizers for DSSCs.

Dye-sensitized solar cells equipped with cationic and neutral RuII-based sensitizers [Ru(ddpd){tpy(COOH)3}]2+ [12+; ddpd = N,N′-dimethyl-N,N′-di(pyridin-2-yl)pyridin-2,6-diamine, tpy(COOH)3 = 2,2″6′,2″-terpyridine-4,4′,4″-tricarboxylic acid] and [Ru(ddpd){tpy(COOH)(COO)2}] (2) with and without the coadsorbent chenodeoxycholic acid were constructed with I3–/I– or the CoIII/II-based redox mediators [Co(bpy)3]3+/2+ (33+/2+; bpy = 2,2′-bipyridine) and [Co(ddpd)2]3+/2+ (43+/2+) in the presence of LiClO4 and 4-tert-butylpyridine. The best photovoltaic performance was achieved by using the 43+/2+ shuttle and the neutral sensitizer 2 without coadsorbent. The higher short-circuit photocurrent density and higher electron recombination lifetimes obtained with this combination suggest slow electron recombination kinetics at the TiO2 surface with the CoIII complex 43+. The slow electron transfer to 43+ is tentatively ascribed to the high-lying π* orbitals of the electron-rich ddpd ligands, which result in a weak electronic coupling. This contrasts with the faster recombination with 33+, which features the low-energy π* orbitals of the bpy ligands.

Andreas K. C. Mengel

Papers

A Heteroleptic Push-Pull Substituted Iron(II) Bis(tridentate) Complex with Low-Energy Charge-Transfer States

Boosting Vis/NIR Charge-Transfer Absorptions of Iron(II) Complexes by N-Alkylation and N-Deprotonation in the Ligand Backbone.

Strongly Coupled Cyclometalated Ruthenium Triarylamine Chromophores as Sensitizers for DSSCs.

A Bis(tridentate)cobalt Polypyridine Complex as Mediator in Dye‐Sensitized Solar Cells