N-Heterocyclic Carbene Complex Explained - American Chemical

Letter pubs.acs.org/JPCL

Exceptional Excited-State Lifetime of an Iron(II)−N‑Heterocyclic Carbene Complex Explained Lisa A. Fredin,† Mátyás Pápai,‡ Emese Rozsályi,‡ György Vankó,‡ Kenneth War̈ nmark,§ Villy Sundström,∥ and Petter Persson*,† †

Theoretical Chemistry Division, §Centre for Analysis and Synthesis, and ∥Department of Chemical Physics, Chemical Center, Lund University, Box 124, SE-22100 Lund, Sweden ‡ Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary S Supporting Information *

ABSTRACT: Earth-abundant transition-metal complexes are desirable for sensitizers in dye-sensitized solar cells or photocatalysts. Iron is an obvious choice, but the energy level structure of its typical polypyridyl complexes, featuring low-lying metal-centered states, has made such complexes useless as energy converters. Recently, we synthesized a novel iron−N-heterocyclic carbene complex exhibiting a remarkable 100-fold increase of the lifetime compared to previously known iron(II) complexes. Here, we rationalize the measured excited-state dynamics with DFT and TD-DFT calculations. The calculations show that the exceptionally long excited-state lifetime (∼9 ps) is achieved for this Fe complex through a significant destabilization of both triplet and quintet metal-centered scavenger states compared to other FeII complexes. In addition, a shallow 3MLCT potential energy surface with a low-energy transition path from the 3MLCT to 3MC and facile crossing from the 3MC state to the ground state are identified as key features for the excited-state deactivation. SECTION: Spectroscopy, Photochemistry, and Excited States

M

Chart 1. Structure of [Fe(CNC)2]2+ (CNC = 2,6-Bis(3methylimidazole-1-ylidine)pyridine) and Geometrical Structural Parametersa

etal polypyridyl complexes have long been of interest as light harvesters for photochemical and photophysical devices. Most studies have been performed on ruthenium(II) complexes that have favorable excited-state properties for many applications.1−4 Using iron instead of ruthenium in such complexes would be an important step to promote lightharvesting applications on a large scale as iron is earthabundant, inexpensive, and environmentally benign. However, its intense metal-to-ligand charge transfer (MLCT) absorption has been considered unexploitable in energy conversion applications5 due to the low-lying metal-centered (MC) quintet (Q) high-spin state that typically deactivates the 1,3MLCT manifolds on a subpicosecond time scale.6 Studies of the prototype [Fe(bpy)3]2+ (bpy = 2,2-bipyridine) complex have revealed an excited-state decay mechanism that involves ultrafast intersystem crossing (ISC) from the first populated 1 MLCT state to the 3MLCT state, followed by an ultrafast (subpicosecond) cascade of ISCs to the 3MC and then to the 5 MC state.7,8 Thus, destabilizing these MC states should result in a longer-lived MLCT state. The 3MC state in Rubis(tridentate)9 and tris(bidentate)10 polypyridine complexes has been effectively destabilized through the introduction of a strongly σ-donating ligand in the first coordination sphere, likely resulting in an increased t2g/eg-like orbital splitting.11 We recently successfully used this approach by synthesizing an Fe− NHC complex, [Fe(CNC)2](PF6)2 (CNC = 2,6-bis(3-methylimidazole-1-ylidine)pyridine) (Chart 1) that displayed a 100fold extended excited-state lifetime of 9 ps in acetonitrile © 2014 American Chemical Society

a

q1 is the average of blue bonds, q2 is the average of red bonds, R is the average of all Fe−ligand bonds, O is the average deviation of all possible angles between Fe−ligand bonds indicated by the dashed gray lines, and P is the average deviation of all of the ligand twist angles (green) (Table 1).

compared to that from previously known FeII complexes. Moreover, no significant 5MC population was observed in the transient absorption measurements, which is a strong indication that the deactivation processes are dramatically different in this compound.12 Received: April 28, 2014 Accepted: May 27, 2014 Published: May 27, 2014 2066

dx.doi.org/10.1021/jz500829w | J. Phys. Chem. Lett. 2014, 5, 2066−2071

The Journal of Physical Chemistry Letters

Letter

Quantum chemical calculations provide significant insights into the fundamental structural and electronic properties of metal complexes.13 The progress in computational modeling of excited-state properties in recent years includes extensive use of time-dependent (TD) density functional theory (DFT) calculations to study optical absorption spectra of many lightharvesting complexes with good accuracy.13 With greater computing power, it has become increasingly feasible to use quantum chemical calculations to investigate excited-state properties of metal complexes beyond the Franck−Condon region of vertical excitations from the ground state (GS). This provides important inroads to address complex issues of excited-state evolution, complementing ultrafast experimental investigations of excited-state kinetics and dynamics.3,4,14 Recent efforts to investigate excited-state effects beyond the Franck−Condon region included unrestricted DFT (uDFT) and TD-DFT calculations of potential energy surfaces (PESs) for the lowest triplet state (T1) of both short- and longlived RuII-bistridentate complexes.3,4 These calculations have, among other things, provided identification of 3MLCT and 3 MC stationary points,15 as well as information about activated decay pathways on this particular surface.3,4 Here, we study the promising FeII NHC system (Chart 1) computationally to better understand from where the longer lifetime arises and how to further improve FeII complexes for light harvesting.12 First, we address the energy level structure of the system using a previous reliable method of plotting constrained geometry potential energy curves (PECs) above the line that connects the optimized singlet GS and the lowest Q state (using the B3LYP*16/TZVP approach with ORCA 2.817,18), which allows for direct comparison to other FeII complexes.19 Then, we calculate the most relevant PES for the [FeII(CNC)2]2+ complex in greater detail (using PBE0/6311G(d,p) with the complete acetonitrile polarizable continuum model (PCM) with Gaussian G0920), with the aim of providing a more comprehensive picture of multidimensional 3 MLCT excited-state deactivation pathways. This extends previous work using similar methodology to locate stationary points for FeII complexes.15,19 In particular, DFT calculations provide a continuous adiabatic description of the lowest triplet (T) PES. Structural and electronic properties of the relaxed GS, 3 MLCT, 3MC, and 5MC states have been investigated for the [Fe(CNC)2]2+ complex. This allows us to propose a pathway for deactivation of the 1MLCT and 3MLCT states that accounts for the measured excited-state dynamics. Calculations of excited-state PESs provide an opportunity to investigate excited-state deactivations in significant detail.3,4 For several previously studied FeII systems, PECs in a single dimension above a coordinate connecting the GS and Q minima have provided clear indication of an excited-state deactivation cascade including the 5MC state.7,19,21 In Figure 1, the vertical excitation PECs, calculated in this manner,19 reveal a significant destabilization of the 3MC and 5MC states compared to other strong-field FeII complexes, such as [Fe(bpy)3]2+ or [Fe(tpy)2]2+.12 Moreover, the energy minimum of the lowest 5MC state exceeds that of the 3MC along this coordinate, providing a first indication of why the 5MC state is not observed experimentally. Recent calculations of RuII-bistridentate complexes3,4 have shown the importance of multidimensional excited-state relaxation pathways and full unrestricted geometry optimizations. We therefore studied fully optimized excited-state

Figure 1. Vertical excitation PECs of [Fe(CNC)2]2+ along the line connecting the optimized GS and lowest Q structures. Circular points are uDFT energies, and triangular points are TD-DFT calculated energies of each constrained geometry.19

minima and the resulting PECs (Figure 2) further from a more extensive two-dimensional coordinate perspective (em-

Figure 2. PECs versus q1. Red points are optimized minima, and black points are single-point energies calculated at the minimum geometries. The gray lines schematically show the PECs with curvature and crossing points consistent with those of the complete PESs. The inset indicates the differences between calculated minima geometries in both q1 and q2.

phasized in Figure 2 inset) with calculations carried out at the PBE0/6-311G(d,p) level of theory known to give accurate structural parameters. Selected structural results for the full unrestricted excited-state geometry optimizations are summarized in Table 1 and are presented as three central structural parameters (R, O, and P). The O value (octahedricity value) is a measure of the mean absolute deviation of the set of all metal ligand bond angles from their “ideal” octahedral values (ideal being O of 0).3 Thus, it is a measure of the angular distortion of the complex away from a perfectly octahedral geometry as discussed in standard ligand field theory. The O value identifies the amount of distortion in the complex due to twisting of the ligands themselves and with respect to the metal center. Also, 2067



Letter

Table 1. Calculated Structural Properties of [Fe(CNC)2]2+ for the Ground State (GS), 3MLCT, 3MC, and 5MC States Using PBE0|6-311G(d,p)|PCM(Acetonitrile).a exp.b E (eV) Mulliken spin on Fe q1c q2c Rd Oe Pe

GS 0.00

1.96 1.96 1.95 7.86 2.01

1.95 1.95 1.94 7.75 0.51

3

MLCT 2.07 1.05 1.98 1.98 1.96 7.78 1.04

3

5

1.16 2.12 2.09 1.98 2.07 11.08 0.17

1.17 3.80 2.21 2.21 2.23 14.29 0.21

MC

results in a similar elongation of the Fe−N bonds as that of the 3 MC state, now occurring on both ligands. The 5MC geometry seems significantly removed from the easily accessible excitedstate region. Experimentally, [Fe(CNC)2]2+ was found to undergo ISC to a hypothesized 3MLCT state in 60% metal-to-metal character was considered MC). Then, fully optimized geometries of the GS, 3MLCT, 3MC, and 5MC excited states were found. These optimizations were performed using the Gaussian09 program,20 using the 6311G(d,p) basis set, the PBE024−26 hybrid functional, and a

Figure 4. Schematic excitation and deactivation pathway based on calculated PECs and PESs. Note the turn in q2 after the 3MC geometry explained by the 2D inset of optimized minima geometries.

be created by combining all information from the PECs and PESs. It is clear from both the initial PECs and PESs that between the GS, 3MLCT, and 3MC state, only one ligand is changing, but to reach the 5MC geometry, both ligands must expand. This can be seen from a sharp turn in q2 past the 3MC in Figure 4 (inset). From this view, it is clear that the 3MLCT to 3MC conversion is more favorable than a direct 3MLCT to 5 MC pathway, which has been suggested for other FeII complexes like [Fe(tpy)2]2+.7,15,19 After conversion to the 2069



■

PCM solvent description for acetonitrile. GS properties have been calculated using the spin-restricted S0 formalism, while spin-unrestricted DFT (UDFT) calculations have been performed for the lowest T (3MC, 3MLCT) and Q (5MC, 5 MLCT) state calculations. Each calculated excited state can be assigned as MLCT- or MC-like based on its Fe spin density (full analysis of the optimized minima is given in the Supporting Information). TD-DFT was used to probe the absorption properties of the complex. The calculated spectra clearly show a strong MLCT band, a few MC transitions at mid-energies, and then a π−π* ligand-centered band in the expected region. The main excitation features of the calculated spectrum (full details in the Supporting Information) match qualitatively well with the assigned bands found in the experimental spectra.12 From the fully optimized minima, additional PECs were drawn and used as a starting point to construct a lowest-energy T PES. T PES scans, with a spacing of 0.02 Å or multiples thereof as judged necessary by the curvature of the surface, have been performed with a view to investigate 3MLCT−3MC, 3 MC−5MC, and 3MLCT−5MC rearrangement pathways. Each point on the PES has been obtained using a full geometry PBE0/6-311G(d,p) relaxation of all structural parameters except those fixed as the probed coordinates in the PES scan. The resulting PES is referred to as a relaxed PES, which is comparable to previous Ru complex 2D surfaces.3 The accuracy of the PES in regions where there are nearly degenerate states may be limited by restrictions imposed by the use of a singledeterminant DFT description. After obtaining the T PES surface, single-point energies of the S0 and Q spin states were calculated for each point.

■

ASSOCIATED CONTENT

* Supporting Information Additional computational details, including complete TD-DFT modeling of absoption maxima and orbital analysis, energy gaps among singlets, triplets, and quintets, spin density analysis, and Cartesian coordinates of all minima. Full references for refs 2, 8, 12, and 20 are also given. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Graetzel, M.; Janssen, R. A. J.; Mitzi, D. B.; Sargent, E. H. Materials Interface Engineering for Solution-Processed Photovoltaics. Nature 2012, 488, 304−312. (2) Magnuson, A.; Anderlund, M.; Johansson, O.; Lindblad, P.; Lomoth, R.; Polivka, T.; Ott, S.; Stensjö, K.; Styring, S.; Sundström, V.; et al. Biomimetic and Microbial Approaches to Solar Fuel Generation. Acc. Chem. Res. 2009, 42, 1899−1909. (3) 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. (4) Ö sterman, T.; Persson, P. Excited State Potential Energy Surfaces of Bistridentate RuII Complexes  A TD-DFT Study. Chem. Phys. 2012, 407, 76−82. (5) Meyer, T. J. Photochemistry of Metal Coordination Complexes: Metal to Ligand Charge Transfer Excited States. Pure Appl. Chem. 1986, 58, 1193−1206. (6) Cho, H.; Strader, M. L.; Hong, K.; Jamula, L.; Gullikson, E. M.; Kim, T. K.; de Groot, F. M. F.; McCusker, J. K.; Schoenlein, R. W.; Huse, N. Ligand-Field Symmetry Effects in Fe(II) Polypyridyl Compounds Probed by Transient X-ray Absorption Spectroscopy. Faraday Discuss. 2012, 157, 463−474. (7) 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. (8) Zhang, W.; Alonso-Mori, R.; Bergmann, U.; Bressler, C.; Chollet, M.; Galler, A.; Gawelda, W.; Hadt, R. G.; Hartsock, R. W.; Kroll, T.; et al. Tracking Excited State Charge and Spin Dynamics in Iron Coordination Complexes. Nature 2014, 509, 345−348. (9) Bomben, P. G.; Robson, K. C. D.; Koivisto, B. D.; Berlinguette, C. P. Cyclometalated Ruthenium Chromophores for the DyeSensitized Solar Cell. Coord. Chem. Rev. 2012, 256, 1438−1450. (10) Bessho, T.; Yoneda, E.; Yum, J.-H.; Guglielmi, M.; Tavernelli, I.; Imai, H.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. New Paradigm in Molecular Engineering of Sensitizers for Solar Cell Applications. J. Am. Chem. Soc. 2009, 131, 5930−5934. (11) 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. (12) Liu, Y.; Harlang, T.; Canton, S. E.; Chabera, P.; SuarezAlcantara, K.; Fleckhaus, A.; Vithanage, D. A.; Goransson, E.; Corani, A.; Lomoth, R.; et al. Towards Longer-Lived Metal-to-Ligand Charge Transfer States of Iron(II) Complexes: An N-Heterocyclic Carbene Approach. Chem. Commun. 2013, 49, 6412−6414. (13) Vlček, A., Jr.; Záliš, S. Modeling of Charge-Transfer Transitions and Excited States in d6 Transition Metal Complexes by DFT Techniques. Coord. Chem. Rev. 2007, 251, 258−287. (14) Dixon, I. M.; Alary, F.; Heully, J.-L. Electronic Peculiarities of the Excited States of [RuN5C]+ vs. [RuN6]2+ Polypyridine Complexes: Insight from Theory. Dalton Trans. 2010, 39, 10959−10966. (15) 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. (16) Reiher, M.; Salomon, O.; Hess, B. A. Reparameterization of Hybrid Functionals Based on Energy Differences of States of Different Multiplicity. Theor. Chem. Acc. 2001, 107, 48−55. (17) Neese, F. ORCA, version 2.8; Max-Planck-Institut fü r Bioanorganische Chemie: Mülheim an der Ruhr, Germany, 2010. (18) Neese, F. The ORCA Program System. WIREs Compt. Mol. Sci. 2012, 2, 73−78. (19) Pápai, M.; Vankó, G.; de Graaf, C.; Rozgonyi, T. Theoretical Investigation of the Electronic Structure of Fe(II) Complexes at SpinState Transitions. J. Chem. Theory Comput. 2013, 9, 509−519. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,

S

■

Letter

ACKNOWLEDGMENTS

Thanks to Y. Liu, K. Skov Kjær, P. Chábera, T. Harlang, J. Uhlig, S. Canton, H. Mateos, and O. Gordivska for discussions. This work was supported by the Crafoord Foundation, the Swedish Research Council, the Knut and Alice Wallenberg Foundation, the European Research Council (ERC-StG259709), and the “Lendület” (Momentum) Programme of the Hungarian Academy of Sciences. P.P. acknowledges support from the NSC and LUNARC supercomputing facilities. V.S. acknowledges support from the Swedish Energy Agency and ERC (226136-VISCHEM). 2070



Letter

B.; Petersson, G. A.; et al. Gaussian 09, revision C.01; Gaussian, Inc.: Wallingford, CT, 2009. (21) Suaud, N.; Bonnet, M.-L.; Boilleau, C.; Labèguerie, P.; Guihéry, N. Light-Induced Excited Spin State Trapping: Ab Initio Study of the Physics at the Molecular Level. J. Am. Chem. Soc. 2008, 131, 715−722. (22) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Stable Carbenes. Chem. Rev. 1999, 100, 39−92. (23) Hecker, C. R.; Gushurst, A. K. I.; McMillin, D. R. Phenyl Substituents and Excited-State Lifetimes in Ruthenium(II) Terpyridyls. Inorg. Chem. 1991, 30, 538−541. (24) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods Without Adjustable Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110, 6158−6170. (25) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple [Phys. Rev. Lett. 77, 3865 (1996)]. Phys. Rev. Lett. 1997, 78, 1396−1396.

2071


N-Heterocyclic Carbene Complex Explained - American Chemical

Recommend Documents