The development of renewable energy sources is one of the biggest challenges in the 21st century. Within this context, great efforts are spent to develop new materials for cheaper and sustainable solar energy conversion schemes. Commercial dye-sensitized solar cells (DSSCs) are based on Ru(II) transition metal complexes as photo-sensitizers. But, ruthenium is rare and expensive, hence iron, abundant and cheap, is a good candidate to replace it. However, Fe(II) complexes are notorious for their ultrafast excited state spin crossover (SCO) into low-energy quintuplet states (5T2), cutting short on their use for light-harvesting applications relying on photo-sensitization. Very recently, it was shown that SCO can be avoided in Fe(II) complexes featuring N-heterocyclic carbene (NHC) ligands , and excited state lifetimes up to 26 ps were reported , making these complexes promising photo-sensitizers in DSSCs or photo-catalytic applications.
In this work, the effect of structural parameters and variations of the proto-typical octahedric Fe(II)-NHC complexes and up to ten different variants thereof were investigated by femtosecond transient absorption and picosecond fluorescence spectroscopy at room temperature in order to understand which structural and electronic factors contribute to increasing the excited state metal-to-ligand charge transfer state (3MLCT) lifetime.
From an energetic perspective, the aim of the chemical design is to increase the ligand field splitting so as to have the 5T2 state higher in energy than 3MLCT. The use of the strong -donating character of the carbene ligands led to a breakthrough in this respect. The experiments show that at minimum three carbene bonds are required to prevent SCO. Their hybridization with the metal-centered orbitals is optimal when the octahedral symmetry of the six coordinating Fe(II) bonds is respected. Bidentate ligands preserving the octahedral geometry are thus expected to induce a larger ligand field per carbene bond than tridentate ones, with smaller bite angles. We show indeed that three carbene bonds in bidentate ligands lead to the same 3MLCT lifetime as four carbene bonds in tridentate moieties.
An increased conjugation across the organic ligands is also beneficial since it lowers the 3MLCT energy. We made use of this effect in several complexes with increasing electron accepting character of the ligands, leading for the record lifetime complex (26 ps) to the theoretical prediction of the 3MLCT state being lower in energy than 5T2 . However, since the 5T2 requires a significant bond lengthening , a possible effect of the ligand substitutions on the structural rigidity of Fe-C bonds cannot be excluded.
Despite the successful development of these complexes displaying sufficiently long excited state lifetimes, DSSCs turn out to have very low power conversion efficiency (<0.5 %) . While charge recombination was identified as a potential drawback of the present chemical design , our latest experiments seem to indicate that the grafting and electronic coupling mechanisms to TiO2 surfaces is less effective than for comparable Ru complexes.
The project is funded by the French ANR programme (ANR-16-CE07-0013-02).
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