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Computational Estimate of the Photophysical Capabilities of Four Series of Organometallic Iron(II) Complexes

Abstract : In this study we examine a large range of organometallic Fe(II) complexes, aiming at identifying computationally the most promising ones in terms of photophysical properties. These complexes combine polypyridine, bis(phosphine) and carbon-bound ligands. DFT has allowed us to establish a comparative Jablonski diagram displaying the lowest singlet, triplet and quintet states. All the proposed FeN 5 C or FeN 3 P 2 C complexes unfavorably possess a lowest triplet state of MC nature. Among the FeN 4 C 2 and FeN 2 P 2 C 2 series, the carbene complexes display the least favorable excited state distribution, also having a low-lying 3 MC state. Validating our design strategy, we are now able to propose seven iron(II) complexes displaying a lowest excited state of 3 MLCT nature. Despite recent efforts and progress, both experimentally and computationally, the quest for luminescent iron(II) complexes is still highly challenging. Aiming at expanding the lifetime of their charge transfer excited states, two opposite strategies have been followed: (i) to increase the ligand field strength through geometric 1,2 or electronic 3-17 modifications, and (ii) to decrease the ligand field strength through steric bulk, obtaining a quintet MC ground state that allows the straightforward population of a 5 MLCT state. 18 Recent studies have brought significant advances, particularly a 100-fold increase in the MLCT lifetime, 3,6,9 reaching 26 ps, 10,18 and efficient TiO 2 sensitization. 5,7,9,14,15,17 Both experimentally-driven and theoretically-driven studies confirm the promising status of certain classes of iron(II) complexes, particularly for photoinduced electron transfer. Our contribution to the field is in the computational design of such complexes, using DFT to get a fast but reliable picture of the excited state distribution. Low-lying charge-tranfer states are desired to bring photophysical properties such as luminescence or electron transfer, while high-lying metal-centered excited states are desired to avoid spin crossover processes and non radiative deactivation. In this respect, we have previously reported a series of mono-and bis(cyclometallated) iron(II) compounds, among which the charge neutral bis(6-phenyl-2,2'-bipyridine)Fe(II) was the most promising
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Isabelle Dixon, Gauthier Boissard, Hannah Whyte, Fabienne Alary, Jean-Louis Heully. Computational Estimate of the Photophysical Capabilities of Four Series of Organometallic Iron(II) Complexes. Inorganic Chemistry, American Chemical Society, 2016, 55 (11), pp.5089-5091. ⟨10.1021/acs.inorgchem.6b00223⟩. ⟨hal-01331155⟩



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