Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers

Article pubs.acs.org/accounts

Fe N‑Heterocyclic Carbene Complexes as Promising Photosensitizers Yizhu Liu,†,‡,⊥ Petter Persson,§ Villy Sundström,‡ and Kenneth War̈ nmark*,† †

Centre for Analysis and Synthesis, Lund University, Box 124, 22100 Lund, Sweden Department of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden § Theoretical Chemistry Division, Lund University, Box 124, 22100 Lund, Sweden ‡

CONSPECTUS: The 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 considered as highly desirable alternatives. However, not much success has been achieved due to the extremely short-lived triplet metal-to-ligand charge transfer (3MLCT) excited state that is deactivated by low-lying metal-centered (MC) states on a 100 fs time scale. A fundamental strategy to design useful Fe-based photosensitizers is thus to destabilize the MC states relative to the 3 MLCT state by increasing the ligand field strength, with special focus on making eg σ* orbitals on the Fe center energetically less accessible. Previous efforts to directly transplant successful strategies from RuIIL6 complexes unfortunately met with limited success in this regard, despite their close chemical kinship. In this Account, we summarize recent promising results from our and other groups in utilizing strongly σ-donating N-heterocyclic carbene (NHC) ligands to make strong-field FeIIL6 complexes with significantly extended 3MLCT lifetimes. Already some of the first homoleptic bis(tridentate) complexes incorporating (CNHC^Npyridine^CNHC)-type ligands gratifyingly resulted in extension of the 3MLCT lifetime by more than 2 orders of magnitude compared to the parental [Fe(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine) complex. Quantum chemical (QC) studies also revealed that the 3MC instead of the 5MC state likely dictates the deactivation of the 3MLCT state, a behavior distinct from traditional FeIIL6 complexes but rather resembling Ru analogues. A heteroleptic FeII NHC complex featuring mesoionic bis(1,2,3-triazol-5-ylidene) (btz) ligands also delivered a 100-fold elongation of the 3MLCT lifetime relative to its parental [Fe(bpy)3]2+ (bpy = 2,2′bipyridine) complex. Again, a Ru-like deactivation mechanism of the 3MLCT state was indicated by QC studies. With a COOHfunctionalized homoleptic complex, a record 3MLCT lifetime of 37 ps was recently observed on an Al2O3 nanofilm. As a proof of concept, it was further demonstrated that the significant improvement in the 3MLCT lifetime indeed benefits efficient light harvesting with FeII NHC complexes. For the first time, close-to-unity electron injection from the lowest-energy 3MLCT state to a TiO2 nanofilm was achieved by a stable FeII complex. This is in complete contrast to conventional FeIIL6-derived photosensitizers that could only make use of high-energy photons. These exciting results significantly broaden the understanding of the fundamental photophysics and photochemistry of d6 FeII complexes. They also open up new possibilities to develop solar energy-converting materials based on this abundant, inexpensive, and intrinsically nontoxic element. functionality. In this regard, d6 TMCs such as RuII, OsII, and IrIII complexes represent the most extensively investigated classes of molecular photosensitizers thanks to their strongly absorbing and long-lived metal-to-ligand charge transfer (MLCT) excited states.1 However, continuous search for inexpensive and abundant alternatives is seen in the scientific community.6 While TMCs based on earth-abundant first-row transition metal

1. INTRODUCTION Efficient utilization of solar energy is a strategic topic for the longterm well-being of mankind. Molecular approaches based on transition metal complexes (TMCs) are highly interesting in this regard and have been widely explored thanks to their versatile electronic properties and structural motifs.1 Common in applications such as photocatalytic organic synthesis,2 photoelectrochemical cells,3,4 light-driven molecular devices,5 etc., a TMC-based photosensitizer absorbs sunlight and transfers the excited-state energy or electron to acceptors, fulfilling various © 2016 American Chemical Society

Received: April 15, 2016 Published: July 25, 2016 1477

DOI: 10.1021/acs.accounts.6b00186 Acc. Chem. Res. 2016, 49, 1477−1485

Article

Accounts of Chemical Research (TM) elements, e.g., copper,7 also emerged recently for such a purpose, the low-spin octahedral iron(II) complexes (FeIIL6) as lighter counterparts of RuII present a natural yet challenging option. These complexes share similarly intense MLCT transitions. However, the MLCT manifold is traditionally very short-lived. This has precluded both practical applications and a thorough understanding of their fundamental light-induced behaviors. Chart 1 shows some prototypical FeIIL6 complexes whose photophysical properties have been extensively investigated.

stabilizing the MLCT states or destabilizing the MC states, or the combination of the two.21 More specific strategies are summarized in Chart 2, taking the bis(tridentate) structure as an example. Since remarkable improvement has been achieved on the photophysical properties of relevant RuIIL6 complexes based on these strategies,21 one might naturally think of transferring them to the chemically akin FeIIL6 complexes. However, so far much less has been explored and proved to work for the latter. For example, although a benchmark 3 μs roomtemperature 3MLCT lifetime was achieved for [Ru(dqp)2]2+ (dqp = 2,6-di(quinolin-8-yl)pyridine) thanks to the close-toperfect octahedral geometry and thus the strong ligand field strength,22 the same strategy was, unfortunately, not immediately successful to an FeII counterpart.23 The same happened with the dcpp (dcpp = 2,6-bis(2-carboxypyridyl)pyridine) ligand designed for both destabilizing the MC states by the close-toperfect octahedral structure and stabilizing the MLCT states by enhancing the π-accepting ability of the ligand. Despite the 3.3 μs room-temperature 3MLCT lifetime obtained for its Ru II complex,24 it failed to significantly retard the 3MLCT deactivation in its FeII complexes.25,26 It is not until very recently that a 23 ps 3MLCT lifetime was obtained for a [FeII(tpy)2] chromophore incorporated in a trinuclear RuFeRu supramolecule featuring a highly conjugated π-system, attributed to the stabilization of the MLCT energy level or the equilibrium of the MLCT manifold with the triplet ligand-centered states.27 All these historical efforts underline the need to develop deactivation strategies to tackle the barrierless 3MLCT → 5MC deactivation of FeIIL6 complexes, which is more challenging compared to the thermally activated 3MLCT → 3MC transition for RuIIL6 complexes,28 in order to make the iron complexes useful for potential photochemical purposes.

Chart 1. Prototypical FeIIL6 Complexes

Already in 1969, however, Fink and Ohnesorge reported the absence of luminescence of [Fe(bpy)3]2+ (bpy = 2,2′-bipyridine, complex 1), [Fe(phen)3]2+ (phen = 1,10-phenanthroline), [Fe(2-Me-phen)3]2+ (2-Me-phen = 2-methyl-1,10-phenanthroline), and [Fe(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine, complex 2) even down to 80 K.8 Since then, elucidation of the photophysical mechanism upon excitation into the MLCT band has been extensively pursued,9−12 with controversies still remaining to date.13,14 As shown in Figure 1, FeIIL6 complexes suffer from low

2. EXTENDING THE 3MLCT LIFETIME WITH Fe NHC CHEMISTRY In this Account, we summarize the advances in this field from our and other groups along the line of destabilizing the MC states of FeIIL6 complexes by raising the eg orbital energy with strongly σdonating ligands. One such class of ligands is N-heterocyclic carbenes (NHCs) featuring a divalent six-electron carbon atom stabilized by adjacent nitrogen atom(s).29 Due to the lower electronegativity of carbon compared to nitrogen, the in-plane lone electron pair on the carbene carbon atom as its HOMO level is significantly more energetic than that in common N-donors such as pyridyl ligands. Figure 2 summarizes the bonding interactions between NHCs and TMs. The major contribution is the σ-donation that effectively destabilizes the TM eg σ* orbitals, with π-interactions constituting relatively minor effects.30 Thanks to such electronic properties, NHCs have previously proved highly successful in extending the 3MLCT lifetimes of RuIIL6 complexes through destabilization of MC states.31−34 Such a strategy had nonetheless remained unexplored for FeIIL6 complexes, despite the sophisticated Fe NHC chemistry with almost exclusive application in Fe catalysis.35,36 Here, coordinatively unsaturated Fe NHC complexes facilitate oxidative addition of relatively unactivated substrates thanks to electronrich Fe centers.35,36 Conversely, for photosensitizers, since no substrate needs to be attached, a six-coordinate structure is preferred to maximize the MC destabilization by more σ-donor ligation. As our first attempt, we employed the bis(imidazole-2ylidene)pyridine (C^N^C) type tridentate ligands.37 The conjugative combination of pyridine and imidazolylidene took

Figure 1. Schematic potential energy surfaces (PESs) of RuIIL6 and FeIIL6 complexes. The inset compares their electronic structures.

energies of the eg orbitals and thus the low-lying metal-centered (MC) states due to the much smaller splitting of the octahedral ligand field compared to their RuII counterparts.15 Upon MLCT excitation, the 3MLCT state reached through intersystem crossing is generally deactivated into the high-spin 5MC state in the 100 fs time scale.16,17 The low energy level of the latter as well as the considerable structural distortion (Fe−L bond lengthening) therein due to the population of the eg σ* orbitals make them less interesting for photophysical or photochemical applications.18−20 Substantial research efforts have previously been dedicated to the extension of the MLCT lifetimes of octahedral d6 TMCs more commonly used for photochemical applications. In principle, for complexes featuring low-lying MC states, effective retardation of the 3MLCT deactivation would lie in either 1478


Article

Accounts of Chemical Research Chart 2. Representative Strategies for Extending the 3MLCT Lifetime of d6 MIIL6 Complexes

Figure 2. Schematic illustration of the bonding interactions between NHCs and TMs.

into consideration the poor π-accepting ability of NHC36 so that pyridine is present for accommodating the excited electron. With the aim to maximize the ligand-field strength, a bis(tridentate) configuration is preferred. However, such a structure found no precedence, although Danopoulos38 and Gibson39 had reported related complexes with Fe-embedded counterions, unsuitable for photochemical applications due to solubility issues. In our work, the FeII NHC complexes 4 and 5 (Figure 3) were successfully synthesized by reacting FeBr2 with in situ generated NHC ligands obtained through deprotonation of the azolium salts. The two complexes with N-tert-butyl and N-methyl substituents, respectively, together with [Fe(tpy)2]2+ (2) as the reference complex, constitute a nice series for comparisons. Compared to 5, the bulky tert-butyl groups in 4 cause steric repulsion between the ligands and thus expansion of the coordination sphere. Therefore, a gradual enhancement of the ligand-field strength was achieved from 2 to 4 to 5. Gratifyingly, this was matched by a gradual extension of their 3MLCT lifetimes according to femtosecond transient absorption spectroscopy (TAS) (Figure 3). Complex 4 showed a slightly increased 3MLCT lifetime of 300 fs compared to that of 2 as 145 fs. The dominating kinetics was still the sub-nanosecond ground-state bleach (GSB) nonetheless through the optically dark 5MC state, like for traditional FeIIL6 complexes. A distinct improvement was, in contrast, observed for 5 with intact Fe−NHC interactions featuring the 0.2 Å shorter Fe−CNHC bonds. Upon excitation into the low-energy MLCT band, this complex displayed a 9 ps 3 MLCT absorption decaying almost synchronously with GSB, a behavior more reminiscent of [Ru(tpy)2]2+.40 This represented a 60-fold improvement in 3MLCT lifetime compared to traditional FeIIL6 complexes. Quantum chemical (QC) calculations41 further revealed that, due to the strongly σ-donating nature of the NHC ligands, the 5MC state is not only significantly

destabilized relative to the 3MC state, and energetically comparable to this state, but also pushed far away along the reaction coordinate. In this case, the 5MC state is no longer likely to be involved in the relaxation pathway. Instead, it is the 3MC state, which crosses the 3MLCT minimum in a rather flat region, that dictates the 9 ps decay of the 3MLCT state (Figure 4). Such a scenario is corroborated by the more recent quantum dynamics studies, ascribing the retarded 3MLCT deactivation to the significant displacement of the 3MLCT wavepacket relative to the 3MLCT−3MC crossing point.42 An issue with the C^N^C ligand in 5 is their high-lying ligand π* energy level43 due to the poor π-accepting ability of the NHCs.36 Therefore, while the MC states were successfully destabilized, the 3MLCT state was also raised. This resulted in a blue-shifted MLCT absorption maximum of 5 relative to 2 that makes it less ideal for many light-harvesting applications. In this context, Gros and co-workers reported a heteroleptic FeII NHC complex 6 comprising one CNHC^Npy^CNHC-derived ligand as in 5, together with a tpy-derived ligand to red-shift the absorption (Chart 3).44 The pending pyridyl moiety in 6 can be protonated to result in an even lower-energy MLCT absorption. However, the excited-state behavior of these complexes was not reported. Very recently, the same authors adopted another approach to stabilize the 3MLCT state.45 Starting from 5, the imidazolylidene moieties in the ligand were replaced with benzimidazolylidene to enhance the electronic delocalization. The resulting complex 7 displayed a 3MLCT lifetime of 16 ps. Further modification with carboxyl groups on the central pyridine delivered an impressive 3 MLCT lifetime of 26 ps for complex 8. QC calculations suggested an even lower energy of the 3MLCT state relative to the 3MC state. It is especially noteworthy that the long lifetimes of these complexes were achieved at comparable electrochemical potentials with those of parental complex 2 (see below). 1479


Article

Accounts of Chemical Research Chart 3. Chemical Structures of 6, 7, and 8

atom, btz is expected to be even more strongly σ-donating and better π-accepting compared to normal NHCs.47,48 9 was obtained as the first heteroleptic octahedral TMC incorporating the btz-type ligand. It is also the first heteroleptic FeII NHC complex combining four NHC sites with a bpy ligand that provides a valuable site for further modifications. The complex is likely stabilized by four pairs of π−π interactions between the spatially overlapping p-tolyl groups and adjacent ligand planes (Figure 5). This is supported by the fact that various attempts using N-alkyl substituents failed to deliver stable products. Studies of the electronic properties of complex 9 revealed surprisingly similar π-accepting behavior of the btz ligands compared to bpy despite their intended function as strong σdonors. QC calculations identified three almost isoenergetic LUMO levels with the orbital distribution dominatingly on bpy and each btz ligand, respectively. The lowest-energy MLCT band of 9 is also an almost equal mixture of Fe → bpy and Fe → btz transitions, supported by the obvious involvement of btz in ligand-based reduction processes in electrochemistry. The redshifted complex 9 displayed a 3MLCT lifetime of 13 ps in MeCN. Mechanistically, it is also the 3MC instead of 5MC state that dictates the photophysical cascade.46 Recently, another triazolylidene-bearing FeII complex 10 was reported49 (Chart 4). It is an interesting mingling between complexes 5 and 9. Its photophysical properties, unfortunately not reported alongside, are highly intriguing to be checked. While extending the 3MLCT lifetime is the main focus of this Account, other parameters, especially the redox potentials, are important in relation to the photoelectrochemical application of these complexes. Table 1 compares key optical and electrochemical properties of the FeII NHC complexes discussed in this Account. As revealed in Table 1, the FeIII/II redox potential already spans over 1 V among these complexes, and the poor πaccepting ability of NHC tends in general to result in significantly negative potentials for both Fe-based oxidation and ligand-based reduction processes. With the expansion of the library of FeII NHC complexes, a better understanding of how to fine-tune these parameters can be expected.

Figure 3. Chemical structures and kinetic trances at characteristic GSB (blue diamonds) and ESA (red triangles) wavelengths of complexes 5 (top), 4 (middle), and 2 (bottom) in MeCN excited at 485 nm.

Figure 4. Schematic PESs between the 3MLCT, 3MC, and 5MC states of complex 5 and their respective spin density plots at the minima. The red dots represent the positions of the minima and the black asterisks their projection on the ⟨q1,q2⟩ plane. q1 and q2 are the schematic average distances between Fe and the two ligands. The white dashed arrows represent the 3MLCT−3MC deactivation pathways.

3. EFFICIENT ELECTRON INJECTION TO TiO2 Photosensitization of TiO2 nanofilms is the basis for promising dye-sensitized solar cells (DSCs) and related photoelectrochemical systems, and provides an important ground for testing the viability of the FeIIL6 light-harvesting complexes in driving photoinduced electron transfer and associated charge transfer processes that are central to many solar energy conversion applications. In the late 1990s, Ferrere and Gregg first demonstrated the sensitization of TiO2 with Fe(dcbpy)2(CN)2

Apart from the bis(tridentate) structure, we also investigated a heteroleptic tris(bidentate) structure, maintaining the total numbers of NHC and pyridyl moieties but regrouping them as σ-donors and π-acceptors, respectively. Complex 9 was thus designed bearing two bidentate bis(1,2,3-triazolylidene) (btz) ligands and a bpy ligand (Figure 5).46 Due to the mesoionic nature and only one N atom adjacent to the carbene carbon 1480


Article

Accounts of Chemical Research

Figure 5. Design and crystal structure of complex 9.

sensitization behavior so that only UV light could be utilized. This was recently rationalized by Jakubikova and colleagues based on the computational studies comparing kinetic and thermodynamic parameters for injection processes from the lower- and higher-energy MLCT bands which had to compete with ultrafast sensitizer deactivation taking place on a ∼100 fs time scale.53,54 Almost at the same time, Meyer and colleagues studied Na2[Fe(bpy)(CN)4] featuring a direct metal-to-particle charge transfer transition from the FeII center to the TiIV acceptor site through the bridging cyano ligand.55,56 While such an approach can serve as an effective sensitization strategy, the generated FeIII cation may significantly attenuate the σ-donor strength of the bridging cyano ligand, labilizing the attachment of the sensitizer on TiO2.56 We envisaged that the significantly extended 3MLCT lifetime of the Fe NHC complexes would greatly improve the capabilities for photoinduced electron injection from the lowest-energy 3 MLCT state. A FeII NHC photosensitizer, complex 11, was thus synthesized by functionalizing complex 5 with carboxyl anchoring groups at the electron-accommodating pyridine moieties (Figure 6a).57 Such a functionalization also significantly stabilized the MLCT state, accounting for a favorable red-shift of the MLCT absorption maximum by 70 nm compared to 5. More importantly, a doubled 3MLCT lifetime of 18 ps was achieved in MeCN solution, which further increased to 37 ps upon immobilization on Al2O3 nanofilms, the longest 3MLCT lifetime reported to date of a FeII complex.57

Chart 4. Molecular Structure of 10

Table 1. Comparison of the Key Properties of FeII NHC Complexes complex

λmaxa (nm)

Eoxb (V)

Eredb (V)

τMLCTa (ps)

1 2 4 5 6 7 8 9 11

520 552 478 457 560 440 501 609 518

+0.68 +0.70 +0.40 +0.31 +0.54 +0.65 +0.74 −0.35 +0.45

−1.75 −1.66 −2.35 −2.39 −1.81 −2.19 −1.62 −2.28 −1.71

0.12c 0.15 0.3 9 16.4 26 13 18

a

In MeCN. bPotential versus ferrocenium/ferrocene. cIn aqueous solution, ref 12.

(dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid)50 and its derivatives.51,52 These sensitizers suffered from band-selective

Figure 6. (a) Chemical structure of 11−TiO2 assembly, (b) its TAS and TTS kinetics excited at 485 nm in comparison to RuN3, and (c) Jablonski diagram of the electronic states involved in photoinduced electron transfer between 11 and TiO2.57 1481


Article

Accounts of Chemical Research In parallel, Gros et al. also worked on complex 11.58 Apart from measuring its 3MLCT lifetime in solution, they tested the photovoltaic performance in a working DSC. Notwithstanding the improved photophysical property, however, the apparent light-to-electricity conversion property was not significantly different from that of the photosensitizers originally reported by Ferrere et al.50 On the other hand, we set to investigate the fundamental photoinduced electron injection mechanism employing a combination of electron paramagnetic resonance (EPR),59 TAS, and ultrafast time-resolved terahertz spectroscopy (TTS). These studies, especially the TTS kinetics in comparison with the benchmark RuN3 dye (Figure 6b), demonstrated an impressive interfacial electron injection yield of 92% for 11 (Figure 6c). Such surprisingly highly efficient electron injection from the lowest-energy MLCT band is in great contrast to the earlier Fe sensitizers discussed above.54 A detailed QC study on both the excited-state molecular properties and interfacial interactions of relevant FeII-based light harvesters corroborates the fundamentally favorable injection capabilities to nano-TiO2 in terms of driving force and interfacial electronic coupling.60 The 11−TiO2 assembly however suffers from facile charge recombination, leaving approximately only 15% of the charge-separated state at time scales longer than nanoseconds (Figure 6c). This may explain the still poor photovoltaic performance of this sensitizer in a working DSC device as reported by Gros et al.,58 highlighting the importance of further research to develop systems with improved interfacial control and excited-state properties.

proposed through theoretical studies,63−67 although photochemically interesting complexes have not been synthesized yet. From a ligand-field point of view, these point toward reversing the relative energies of 3MLCT and MC states, which manifests a gradual transition for the photochemistry of FeII complexes from the traditionally “weak-field” to the RuII-like stronger-field regimes. In this regard, much space for improvement can be foreseen in terms of precise control of the respective electronic states. Apart from the examples of low-spin FeIIL6 complexes discussed in this Account, recently, Damrauer et al. studied the MLCT transition of [Fe(dctpy)2](BF4)2 (dctpy = 6,6″-dichloro2,2′:6′,2″-terpyridine) featuring a high-spin quintet ground state.68 Light excitation likely populated the 5MLCT state and thereafter a thermalized 5/7MLCT state with a lifetime of 18 ps. The work is very interesting in that the MLCT transition of such weak-ligand-field complexes has long been ignored by the community. The long lifetime is also noteworthy in the context of fundamental photophysics. Although its significantly lower extinction coefficients present a challenge for immediate application as light harvesters, it is likely to present an area for further development. This further highlights the broad range of promising avenues to designing FeII-based photosensitizers in a wider sense. To conclude, innovative approaches based on first-row TM chemistry in general, and Fe NHC complexes in particular, have demonstrated great potential toward earth-abundant, inexpensive, and environmentally friendly photosensitizers. The integration of front-line organometallic chemistry, advanced spectroscopic and computational investigations has also brought about a complete conceptual change in the fundamental understanding of the metal−ligand interaction in Fe complexes. A rapid development of the field can thus be expected in the near future.

4. CONCLUSION AND OUTLOOK Recent progress in Fe NHC chemistry has resulted in a significant advance toward utilizing abundant and environmentally friendly Fe-based materials for solar energy conversion. This is quite unique and encouraging especially considering the extensively pursued yet otherwise unsuccessful approaches. Emerging capabilities of QC calculations to predict key aspects of the multidimensional excited-state potential energy landscapes highlight the importance of selective metal−ligand bond elongations for transitions from high-energy 1/3MLCT states to intermediate- and high-spin 3/5MC states.61 In terms of useful photophysics, it is, on the one hand, not enough that the ligandfield strength is rather high at the ground-state geometry if there is a facile downhill path toward structurally distorted low-energy MC states with reduced ligand-field strength. On the other hand, the structural difference between the MLCT and MC states, and in particular the strong dependence of the MC energy on the metal−ligand distances, means that the MLCT → MC deactivation can be retarded by an activation barrier even if there are low-energy MC states at significantly distorted metal coordination geometries. This highlights the uniqueness of the NHC approach presented in this Account for improving the photophysics of FeII complexes in terms of retaining considerable ligand-field strength in the relaxed MC states, and significant structural deviations of their PES minima from the 3MLCT state minimum. A combination of techniques has been employed to understand the Fe NHC complexes in the context of solar energy conversion. However, the reported relevant complexes are still scarce, and the field is definitely in its infancy. Apart from the possibility of varying the N-substituents based on present NHC motifs, the utilization of cyclic carbenes beyond normal NHCs is also highly desirable.62 Recently, cyclometalation approaches, utilizing carbanions as even stronger σ-donors, have also been

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. ⊥

Notes

The authors declare no competing financial interest. Biographies Yizhu Liu was born in Wuhan, China, in 1984. He finished his undergraduate study at Tsinghua University in 2005 and later obtained his PhD in 2011 at the same university under the direction of Prof. Hong Lin, working on photosensitizers for DSCs. He then moved to Lund University, Sweden, to conduct his postdoctoral research with Prof. Kenneth Wärnmark and Prof. Villy Sundström. Since 2015, he has been a postdoctoral research fellow at Swiss Federal Institute of Technology in Lausanne (EPFL), working with Prof. Kay Severin on the chemistry of nitrous oxide. Petter Persson was born in Geneva, Switzerland, in 1971, and grew up in Sweden and Germany. He received a B.A. in Natural Sciences from Cambridge University, England, in 1993, and a PhD in Quantum Chemistry from Uppsala University, Sweden, in 2000. He then conducted postdoctoral research both at Uppsala University, and with Prof. William A. Goddard III at the California Institute of Technology 1482


Article

Accounts of Chemical Research

light-driven molecular machines. Struct. Bonding (Berlin, Ger.) 2006, 123, 41−78. (6) Bozic-Weber, B.; Constable, E. C.; Housecroft, C. E. Light harvesting with earth abundant d-block metals: development of sensitizers in dye-sensitized solar cells. Coord. Chem. Rev. 2013, 257, 3089−3106. (7) Housecroft, C. E.; Constable, E. C. The emergence of copper(I)based dye sensitized solar cells. Chem. Soc. Rev. 2015, 44, 8386−8398. (8) Fink, D. W.; Ohnesorge, W. E. Temperature effects on chargetransfer luminescence intensity of some transition metal ion chelates. J. Am. Chem. Soc. 1969, 91, 4995−4998. (9) Creutz, C.; Chou, M.; Netzel, T. L.; Okumura, M.; Sutin, N. Lifetimes, spectra, and quenching of the excited states of polypyridine complexes of iron(II), ruthenium(II), and osmium(II). J. Am. Chem. Soc. 1980, 102, 1309−1319. (10) McCusker, J. K.; Walda, K. N.; Dunn, R. C.; Simon, J. D.; Magde, D.; Hendrickson, D. N. Subpicosecond 1MLCT→5T2 intersystem crossing of low-spin polypyridyl ferrous complexes. J. Am. Chem. Soc. 1993, 115, 298−307. (11) Monat, J. E.; McCusker, J. K. Femtosecond excited-state dynamics of an iron(II) polypyridyl solar cell sensitizer model. J. Am. Chem. Soc. 2000, 122, 4092−4097. (12) Gawelda, W.; Cannizzo, A.; Pham, V. T.; van Mourik, F.; Bressler, C.; Chergui, M. Ultrafast nonadiabatic dynamics of [FeII(bpy)3]2+ in solution. J. Am. Chem. Soc. 2007, 129, 8199−8206. (13) Zhang, W.; Alonso-Mori, R.; Bergmann, U.; Bressler, C.; Chollet, M.; Galler, A.; Gawelda, W.; Hadt, R. G.; Hartsock, R. W.; Kroll, T.; Kjaer, K. S.; Kubicek, K.; Lemke, H. T.; Liang, H. W.; Meyer, D. A.; Nielsen, M. M.; Purser, C.; Robinson, J. S.; Solomon, E. I.; Sun, Z.; Sokaras, D.; van Driel, T. B.; Vankó, G.; Weng, T. C.; Zhu, D.; Gaffney, K. J. Tracking excited-state charge and spin dynamics in iron coordination complexes. Nature 2014, 509, 345−348. (14) Auböck, G.; Chergui, M. Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+. Nat. Chem. 2015, 7, 629−633. (15) Figgis, B. N.; Hitchman, M. A. Ligand Field Theory and Its Applications; WILEY-VCH: New York, 2000. (16) Juban, E. A.; Smeigh, A. L.; Monat, J. E.; McCusker, J. K. Ultrafast dynamics of ligand-field excited states. Coord. Chem. Rev. 2006, 250, 1783−1791. (17) Cannizzo, A.; Milne, C. J.; Consani, C.; Gawelda, W.; Bressler, C.; van Mourik, F.; Chergui, M. Light-induced spin crossover in Fe(II)based complexes: The full photocycle unraveled by ultrafast optical and X-ray spectroscopies. Coord. Chem. Rev. 2010, 254, 2677−2686. (18) Hauser, A. Light-induced spin crossover and the high-spin→lowspin relaxation. Top. Curr. Chem. 2004, 234, 155−198. (19) Pápai, M.; Vankó, G.; de Graaf, C.; Rozgonyi, T. Theoretical investigation of the electronic structure of Fe(II) complexes at spin-state transitions. J. Chem. Theory Comput. 2013, 9, 509−519. (20) Sousa, C.; de Graaf, C.; Rudavskyi, A.; Broer, R.; Tatchen, J.; Etinski, M.; Marian, C. M. Ultrafast deactivation mechanism of the excited singlet in the light-induced spin crossover of [Fe(2,2′bipyridine)3]2+. Chem. - Eur. J. 2013, 19, 17541−17551. (21) Pal, A. K.; Hanan, G. S. Design, synthesis and excited-state properties of mononuclear Ru(II) complexes of tridentate heterocyclic ligands. Chem. Soc. Rev. 2014, 43, 6184−6197. (22) Abrahamsson, M.; Jäger, M.; Ö sterman, T.; Eriksson, L.; Persson, P.; Becker, H. C.; Johansson, O.; Hammarström, L. A 3.0 μs room temperature excited state lifetime of a bistridentate RuII-polypyridine complex for rod-like molecular arrays. J. Am. Chem. Soc. 2006, 128, 12616−12617. (23) Jäger, M. Beyond Classical Ruthenium(II) Polypyridyl Complexes; Uppsala University: 2009. (24) Schramm, F.; Meded, V.; Fliegl, H.; Fink, K.; Fuhr, O.; Qu, Z.; Klopper, W.; Finn, S.; Keyes, T. E.; Ruben, M. Expanding the coordination cage: a ruthenium(II)-polypyridine complex exhibiting high quantum yields under ambient conditions. Inorg. Chem. 2009, 48, 5677−5684. (25) Jamula, L. L.; Brown, A. M.; Guo, D.; McCusker, J. K. Synthesis and characterization of a high-symmetry ferrous polypyridyl complex:

(CALTECH), before moving to Lund University, Sweden, as an Assistant Professor in 2006. In 2010, he took up his current position as an Associate Professor in Theoretical Chemistry at Lund University. His research interests include quantum photoelectrochemistry and computational investigations of molecular and nanoscale solar energy conversion processes. Villy Sundström, born 1949 in the north of Sweden, obtained a PhD from Umeå University in 1977. From 1994 and to this date he has been a professor in Chemical Physics at Lund University. He has more than 40 years of experience in photochemistry, ultrafast spectroscopy, molecular structural dynamics, photosynthesis, and solar energy research. Early studies of chemical reaction dynamics and energy and electron transfer in photosynthesis more recently led to work on novel materials for solar energy conversion, DSCs, polymer solar cells, and now organo−metal− halide perovskite solar cells. Photocatalytic processes underlying production of solar fuels are studied using ultrafast X-ray methods. Time-resolved THz spectroscopy is extensively implemented for the study of carrier dynamics in nanostructured solar cell materials. Kenneth Wärnmark was born outside Stockholm, Sweden, in 1962. He received a Master in Chemical Engineering, specializing in chemistry, from the Royal Institute of Technology, Stockholm, in 1987. He continued his studies at the same university and obtained a PhD in organic chemistry in 1994, working with macrocyclic ligands under the supervision of Prof. Christina Moberg. He then moved to Strasbourg, France, to conduct postdoctoral research with Prof. Jean-Marie Lehn, working on diastereomerically pure ruthenium−polypyridyl complexes. In 1996 he moved to Lund University, Lund, Sweden, to take on a position as Assistant Professor of Organic Chemistry and in 1999 he continued at the same University on a position as Associate Professor. In 2010 he was promoted to full professor of Organic Chemistry. His research interests include Tröger’s base chemistry, supramolecular catalysis, self-assembly, molecular tubes, molecular receptors, and solar energy conversion processes.

■

ACKNOWLEDGMENTS The authors thank collaborators in P.P., V.S., and K.W.’s groups at Lund University and collaborators at Uppsala University (Dr. Ping Huang, Assoc. Prof. Reiner Lomoth and Prof. Stenbjörn Styring) for the experimental and theoretical investigations. This work was supported by the Crafoord Foundation, the Swedish Research Council (VR), the Knut and Alice Wallenberg (KAW) Foundation, and the Swedish Energy Agency. P.P. acknowledges support from the Swedish National Supercomputing Centre and the Lund University Intensive Computation Application Research Center supercomputing facilities. The European Research Council is acknowledged for an Advanced Investigator Grant to V.S. (226136-VISCHEM).

■

REFERENCES

(1) Balzani, V.; Bergamini, G.; Campagna, S.; Puntoriero, F. Photochemistry and photophysics of coordination compounds: overview and general concepts. Top. Curr. Chem. 2007, 280, 1−36. (2) Schultz, D. M.; Yoon, T. P. Solar synthesis: prospects in visible light photocatalysis. Science 2014, 343, 1239176. (3) Grätzel, M.; Janssen, R. A.; Mitzi, D. B.; Sargent, E. H. Materials interface engineering for solution-processed photovoltaics. Nature 2012, 488, 304−312. (4) Willkomm, J.; Orchard, K. L.; Reynal, A.; Pastor, E.; Durrant, J. R.; Reisner, E. Dye-sensitised semiconductors modified with molecular catalysts for light-driven H2 production. Chem. Soc. Rev. 2016, 45, 9−23. (5) Baranoff, E.; Barigelletti, F.; Bonnet, S.; Collin, J.-P.; Flamigni, L.; Mobian, P.; Sauvage, J.-P. From photoinduced charge separation to 1483


Article

Accounts of Chemical Research approaching the 5T2/3T1 crossing point for FeII. Inorg. Chem. 2014, 53, 15−17. (26) Mengel, A. K.; Forster, C.; Breivogel, A.; Mack, K.; Ochsmann, J. R.; Laquai, F.; Ksenofontov, V.; Heinze, K. A heteroleptic push-pull substituted iron(II) bis(tridentate) complex with low-energy chargetransfer states. Chem. - Eur. J. 2015, 21, 704−714. (27) Wächtler, M.; Kübel, J.; Barthelmes, K.; Winter, A.; Schmiedel, A.; Pascher, T.; Lambert, C.; Schubert, U. S.; Dietzek, B. Energy transfer and formation of long-lived 3MLCT states in multimetallic complexes with extended highly conjugated bis-terpyridyl ligands. Phys. Chem. Chem. Phys. 2016, 18, 2350−2360. (28) Sun, Q.; Mosquera-Vazquez, S.; Suffren, Y.; Hankache, J.; Amstutz, N.; Lawson Daku, L. M.; Vauthey, E.; Hauser, A. On the role of ligand-field states for the photophysical properties of ruthenium(II) polypyridyl complexes. Coord. Chem. Rev. 2015, 282−283, 87−99. (29) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. An overview of N-heterocyclic carbenes. Nature 2014, 510, 485−496. (30) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L. Understanding the M(NHC) (NHC = N-heterocyclic carbene) bond. Coord. Chem. Rev. 2009, 253, 687−703. (31) Son, S. U.; Park, K. H.; Lee, Y. S.; Kim, B. Y.; Choi, C. H.; Lah, M. S.; Jang, Y. H.; Jang, D. J.; Chung, Y. K. Synthesis of Ru(II) complexes of N-heterocyclic carbenes and their promising photoluminescence properties in water. Inorg. Chem. 2004, 43, 6896−6898. (32) Schulze, B.; Escudero, D.; Friebe, C.; Siebert, R.; Gorls, H.; Kohn, U.; Altuntas, E.; Baumgaertel, A.; Hager, M. D.; Winter, A.; Dietzek, B.; Popp, J.; González, L.; Schubert, U. S. A heteroleptic bis(tridentate) ruthenium(II) complex of a click-derived abnormal carbene pincer ligand with potential for photosensitzer application. Chem. - Eur. J. 2011, 17, 5494−5498. (33) Brown, D. G.; Sanguantrakun, N.; Schulze, B.; Schubert, U. S.; Berlinguette, C. P. Bis(tridentate) ruthenium-terpyridine complexes featuring microsecond excited-state lifetimes. J. Am. Chem. Soc. 2012, 134, 12354−12357. (34) Brown, D. G.; Schauer, P. A.; Borau-Garcia, J.; Fancy, B. R.; Berlinguette, C. P. Stabilization of ruthenium sensitizers to TiO2 surfaces through cooperative anchoring groups. J. Am. Chem. Soc. 2013, 135, 1692−1695. (35) Ingleson, M. J.; Layfield, R. A. N-Heterocyclic carbene chemistry of iron: fundamentals and applications. Chem. Commun. 2012, 48, 3579−3589. (36) Riener, K.; Haslinger, S.; Raba, A.; Högerl, M. P.; Cokoja, M.; Herrmann, W. A.; Kühn, F. E. Chemistry of iron N-heterocyclic carbene complexes: syntheses, structures, reactivities, and catalytic applications. Chem. Rev. 2014, 114, 5215−5272. (37) Liu, Y.; Harlang, T.; Canton, S. E.; Chábera, P.; Suárez-Alcántara, K.; Fleckhaus, A.; Vithanage, D. A.; Göransson, E.; Corani, A.; Lomoth, R.; Sundström, V.; Wärnmark, K. Towards longer-lived metal-to-ligand charge transfer states of iron(II) complexes: an N-heterocyclic carbene approach. Chem. Commun. 2013, 49, 6412−6414. (38) Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. E. NHeterocyclic pincer dicarbene complexes of iron(II): C-2 and C-5 metalated carbenes on the same metal center. Organometallics 2004, 23, 166−168. (39) McGuinness, D. S.; Gibson, V. C.; Steed, J. W. Bis(carbene)pyridine complexes of the early to middle transition metals: survey of ethylene oligomerization and polymerization capability. Organometallics 2004, 23, 6288−6292. (40) Hewitt, J. T.; Vallett, P. J.; Damrauer, N. H. Dynamics of the 3 MLCT in Ru(II) terpyridyl complexes probed by ultrafast spectroscopy: evidence of excited-state equilibration and interligand electron transfer. J. Phys. Chem. A 2012, 116, 11536−11547. (41) Fredin, L. A.; Pápai, M.; Rozsályi, E.; Vankó, G.; Wärnmark, K.; Sundström, V.; Persson, P. Exceptional excited-state lifetime of an iron(II)−N-heterocyclic carbene complex explained. J. Phys. Chem. Lett. 2014, 5, 2066−2071. (42) Pápai, M.; Vankó, G.; Rozgonyi, T.; Penfold, T. J. High-efficiency iron photosensitizer explained with quantum wavepacket dynamics. J. Phys. Chem. Lett. 2016, 7, 2009−2014.

(43) Chung, L. H.; Cho, K. S.; England, J.; Chan, S. C.; Wieghardt, K.; Wong, C. Y. Ruthenium(II) and osmium(II) complexes bearing bipyridine and the N-heterocyclic carbene-based C^N^C pincer ligand: an experimental and density functional theory study. Inorg. Chem. 2013, 52, 9885−9896. (44) Duchanois, T.; Etienne, T.; Beley, M.; Assfeld, X.; Perpète, E. A.; Monari, A.; Gros, P. C. Heteroleptic pyridyl-carbene iron complexes with tuneable electronic properties. Eur. J. Inorg. Chem. 2014, 2014, 3747−3753. (45) Liu, L.; Duchanois, T.; Etienne, T.; Monari, A.; Beley, M.; Assfeld, X.; Haacke, s.; Gros, P. C. A new record excited state 3MLCT lifetime for metalorganic iron(II) complexes. Phys. Chem. Chem. Phys. 2016, 18, 12550−12556. (46) Liu, Y.; Kjaer, K. S.; Fredin, L. A.; Chábera, P.; Harlang, T.; Canton, S. E.; Lidin, S.; Zhang, J.; Lomoth, R.; Bergquist, K. E.; Persson, P.; Wärnmark, K.; Sundström, V. A heteroleptic ferrous complex with mesoionic bis(1,2,3-triazol-5-ylidene) ligands: taming the MLCT excited state of iron(II). Chem. - Eur. J. 2015, 21, 3628−3639. (47) Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand, G. Crystalline 1H-1,2,3-triazol-5-ylidenes: new stable mesoionic carbenes (MICs). Angew. Chem., Int. Ed. 2010, 49, 4759−4762. (48) Schulze, B.; Schubert, U. S. Beyond click chemistry supramolecular interactions of 1,2,3-triazoles. Chem. Soc. Rev. 2014, 43, 2522−2571. (49) Iwasaki, H.; Yamada, Y.; Ishikawa, R.; Koga, Y.; Matsubara, K. Isolation and structures of 1,2,3-triazole-derived mesoionic biscarbenes with bulky aromatic groups. Eur. J. Org. Chem. 2016, 2016, 1651−1654. (50) Ferrere, S.; Gregg, B. A. Photosensitization of TiO2 by [FeII(2,2′bipyridine-4,4′-dicarboxylic acid)2(CN)2]: band selective e;ectron injection from ultra-short-lived excited states. J. Am. Chem. Soc. 1998, 120, 843−844. (51) Ferrere, S. New photosensitizers based upon [Fe(L)2(CN)2] and [Fe(L)3] (L = substituted 2,2′-bipyridine): yields for the photosensitization of TiO2 and effects on the band selectivity. Chem. Mater. 2000, 12, 1083−1089. (52) Ferrere, S. New photosensitizers based upon [FeII(L)2(CN)2] and [FeIIL3], where L is substituted 2,2′-bipyridine. Inorg. Chim. Acta 2002, 329, 79−92. (53) Yang, M.; Thompson, D. W.; Meyer, G. J. Dual pathways for TiO2 sensitization by Na2[Fe(bpy)(CN)4]. Inorg. Chem. 2000, 39, 3738− 3739. (54) Yang, M.; Thompson, D. W.; Meyer, G. J. Charge-transfer studies of iron cyano compounds bound to nanocrystalline TiO2 surfaces. Inorg. Chem. 2002, 41, 1254−1262. (55) Bowman, D. N.; Blew, J. H.; Tsuchiya, T.; Jakubikova, E. Elucidating band-selective sensitization in iron(II) polypyridine-TiO2 assemblies. Inorg. Chem. 2013, 52, 8621−8628. (56) Jakubikova, E.; Bowman, D. N. Fe(II)-polypyridines as chromophores in dye-sensitized solar cells: a computational perspective. Acc. Chem. Res. 2015, 48, 1441−1449. (57) Harlang, T. C.; Liu, Y.; Gordivska, O.; Fredin, L. A.; Ponseca, C. S., Jr.; Huang, P.; Chábera, P.; Kjaer, K. S.; Mateos, H.; Uhlig, J.; Lomoth, R.; Wallenberg, R.; Styring, S.; Persson, P.; Sundström, V.; Wärnmark, K. Iron sensitizer converts light to electrons with 92% yield. Nat. Chem. 2015, 7, 883−889. (58) Duchanois, T.; Etienne, T.; Cebrián, C.; Liu, L.; Monari, A.; Beley, M.; Assfeld, X.; Haacke, S.; Gros, P. C. An iron-based photosensitizer with extended excited-state lifetime: photophysical and photovoltaic properties. Eur. J. Inorg. Chem. 2015, 2015, 2469−2477. (59) Huang, J.; Buyukcakir, O.; Mara, M. W.; Coskun, A.; Dimitrijevic, N. M.; Barin, G.; Kokhan, O.; Stickrath, A. B.; Ruppert, R.; Tiede, D. M.; Stoddart, J. F.; Sauvage, J. P.; Chen, L. X. Highly efficient ultrafast electron injection from the singlet MLCT excited state of copper(I) diimine complexes to TiO2 nanoparticles. Angew. Chem., Int. Ed. 2012, 51, 12711−12715. (60) Fredin, L. A.; Wärnmark, K.; Sundström, V.; Persson, P. Molecular and interfacial calculations of iron(II) light harvesters. ChemSusChem 2016, 9, 667−675. 1484


Article

Accounts of Chemical Research (61) Ö sterman, T.; Abrahamsson, M.; Becker, H. C.; Hammarström, L.; Persson, P. Influence of triplet state multidimensionality on excited state lifetimes of bis-tridentate RuII complexes: a computational study. J. Phys. Chem. A 2012, 116, 1041−1050. (62) Melaimi, M.; Soleilhavoup, M.; Bertrand, G. Stable cyclic carbenes and related species beyond diaminocarbenes. Angew. Chem., Int. Ed. 2010, 49, 8810−8849. (63) Dixon, I. M.; Alary, F.; Boggio-Pasqua, M.; Heully, J. L. The (N4C2)2− donor set as promising motif for bis(tridentate) iron(II) photoactive compounds. Inorg. Chem. 2013, 52, 13369−13374. (64) Dixon, I. M.; Khan, S.; Alary, F.; Boggio-Pasqua, M.; Heully, J. L. Probing the photophysical capability of mono and bis(cyclometallated) Fe(II) polypyridine complexes using inexpensive ground state DFT. Dalton Trans. 2014, 43, 15898−15905. (65) Mukherjee, S.; Bowman, D. N.; Jakubikova, E. Cyclometalated Fe(II) complexes as sensitizers in dye-sensitized solar cells. Inorg. Chem. 2015, 54, 560−569. (66) Bowman, D. N.; Bondarev, A.; Mukherjee, S.; Jakubikova, E. Tuning the electronic structure of Fe(II) polypyridines via donor atom and ligand scaffold modifications: a computational study. Inorg. Chem. 2015, 54, 8786−8793. (67) Dixon, I. M.; Alary, F.; Boggio-Pasqua, M.; Heully, J. L. Reversing the relative 3MLCT−3MC order in Fe(II) complexes using cyclometallating ligands: a computational study aiming at luminescent Fe(II) complexes. Dalton Trans. 2015, 44, 13498−13503. (68) Shepard, S. G.; Fatur, S. M.; Rappé, A. K.; Damrauer, N. H. Highly strained iron(II) polypyridines: exploiting the quintet manifold to extend the lifetime of MLCT excited states. J. Am. Chem. Soc. 2016, 138, 2949−2952.

1485


Fe N-Heterocyclic Carbene Complexes as Promising Photosensitizers

Recommend Documents