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Iron sensitizer converts light to electrons with 92% yield

Abstract

Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogues like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron–nitrogen–heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of electron paramagnetic resonance, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chemical calculations, we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the 3MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements.

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Figure 1: Chemical structures of the investigated complexes.
Figure 2: Steady-state UV–vis absorption spectra of the investigated compounds.
Figure 3: Energy-level alignment using quantum chemical calculations to assess electron transfer from the dyes to TiO2.
Figure 4: Difference EPR spectra for 2.
Figure 5: Transient absorption spectra of compound 2.
Figure 6: Photoconductivity spectra and photoconductivity kinetics calculated from transient terahertz spectroscopy data for 2 on TiO2.
Figure 7: Jablonski diagram of the electronic states involved in photo-induced electron transfer between 2 and TiO2, including time constants and quantum yields (in %).

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Acknowledgements

The authors thank E. Thyrhaug for help with steady-state absorption measurements, H. Němec for providing the THz photoconductivity kinetics of RuN3 on TiO2 and E. Unger for valuable discussions. 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).

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T.H., Y.L., P.C., K.K., V.S. and K.W. conceived and designed the experiments. Y.L. and O.G. performed the synthesis. Samples were prepared by Y.L., O.G., H.M. and P.C. R.L. carried out the electrochemical measurements. R.W. performed the electron microscopy and energy-dispersive X-ray spectroscopy. L.F. made the quantum chemical calculations and the data were analysed by L.F. and P.P. Y.L. and P.H. performed the electron paramagnetic resonance experiments and the data were analysed by P.H. and S.S. T.H., Y.L., P.C., K.K. and H.M. carried out the transient absorption spectroscopy experiments and the data were analysed by T.H. C.P. conducted the transient terahertz experiments and T.H. analysed the data. T.H., Y.L., V.S. and K.W. wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Kenneth Wärnmark.

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Harlang, T., Liu, Y., Gordivska, O. et al. Iron sensitizer converts light to electrons with 92% yield. Nature Chem 7, 883–889 (2015). https://doi.org/10.1038/nchem.2365

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