It has been widely recognized in the scientific community that scarce elements such as ruthenium are not ideal for sustainable technology. During the past 10 years, progress has been made in exploring first-row transition metals as replacements for scarce metals in many solar cell and photocatalysis applications.[1] Iron analogues to well-performing ruthenium-complexes were early found to not yield nearly the same solar cell performance, despite Ru and Fe being congeners.[2] Prior to our efforts, by means of ultrafast spectroscopy it was found that the relevant excited state deactivates in less than a ps, a timescale not accessible for most electron-transfer reactions.[3]

In the work described here, a class of complexes called the iron-carbenes has by clever ligand design been able to enter a much wider range of timescales. Since the first iron-carbene published in 2013 (with a lifetime of 9 ps), excited state lifetimes are now reaching up to 2 ns with potential applications going towards the photocatalysis field.[4,5,6] Fe-carbenes was for the first time successfully used in photocatalysis in 2017,[7] and since then a bi-metallic dyad connecting an iron photosensitizer with a cobalt catalytic centre has been synthesized.[8] Also, the record efficiency for iron-based dye-sensitized solar cells is held by an Fe(II)-carbene complex.[9]

In the study from 2017, the lifetime of the photochemically relevant 3MLCT is still a big limiting factor.[7] To further prolong the lifetime, a key strategy is to facilitate the charge-separation by using sensitizers made as push-pull complexes. In this work, a new set of push-pull iron-carbenes have been characterized by ultrafast transient absorption spectroscopy both in solution and after sensitization of titania nanoparticles. In this way it is possible to study also interfacial electron-transfer processes. In particular, our results show how the newest set of push-pull molecules, with novel ligand design, yields better photochemical properties promising for solar cell applications and heterogeneous catalysis.

[1] O.S. Wenger, Photoactive Complexes with Earth-Abundant Metals, J. Am. Chem. Soc. 140 (2018) 13522–13533. https://doi.org/10.1021/jacs.8b08822.
[2] S. Ferrere, B.A. Gregg, Photosensitization of TiO2 by [FeII(2,2‘-bipyridine-4,4‘-dicarboxylic acid)2(CN)2]: Band Selective Electron Injection from Ultra-Short-Lived Excited States, J. Am. Chem. Soc. 120 (1998) 843-844. https://doi.org/10.1021/ja973504e
[3] J. E. Monat, J. K. McCusker, Femtosecond Excited-State Dynamics of an Iron(II) Polypyridyl Solar Cell Sensitizer Model, J. Am. Chem. Soc. 122 (2000) 4092–4097. https://doi.org/10.1021/ja992436o
[4] Y. Liu et al., Towards longer-lived metal-to-ligand charge transfer states of iron(II) complexes: an N-heterocyclic carbene approach, Chem. Commun. 49 (2013) 6412–6414. https://doi.org/10.1039/c3cc43833c.
[5] K.S. Kjær et al., Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime, Science (80-. ). 363 (2019) 249–253. https://doi.org/10.1126/science.aau7160.
[6] L. Lindh et al., Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis, Catalysts 10(3) (2020) 315. https://doi.org/10.3390/catal10030315
[7] P. Zimmer et al., N-Heterocyclic Carbene Complexes of Iron as Photosensitizers for Light-Induced Water Reduction, European Journal of Inorganic Chemistry 11 (2017) 1504-1509 https://doi.org/10.1002/ejic.201700064
[8] P. Zimmer et al., Towards Noble-Metal-Free Dyads: Ground and Excited State Tuning by a Cobalt Dimethylglyoxime Motif Connected to an Iron N-Heterocyclic Carbene Photosensitizer, European Journal of Inorganic Chemistry 48 (2018) 5203-5214 https://doi.org/ 10.1002/ejic.201800946
[9] E. Marchini et al., Recombination and regeneration dynamics in FeNHC(II)-sensitized solar cells, Chem. Commun. (Camb). 56 (2020) 543–546. https://doi.org/10.1039/c9cc07794d.