Resonant X-ray photo-oxidation of light-harvesting iron (II/III) N-heterocyclic carbene complexes

Robert H Temperton,Jens Uhlig,Niclas Johansson,Giulio D’Acunto,Meiyuan Guo,Petter Persson,Om Prakash,Joachim Schnadt,Kenneth Wärnmark,Nils W Rosemann

doi:10.1038/s41598-021-01509-7

Abstract

Two photoactive iron N-heterocyclic carbene complexes {[hbox {Fe}^{{{rm{II}}}}(hbox {btz})_2(hbox {bpy})]^{2+}} and {[hbox {Fe}^{{rm{III}}}(hbox {btz})_3]^{3+}}, where btz is 3,3’-dimethyl-1,1’-bis(p-tolyl)-4,4’-bis(1,2,3-triazol-5-ylidene) and bpy is 2,2’-bipyridine, have been investigated by Resonant Photoelectron Spectroscopy (RPES). Tuning the incident X-ray photon energy to match core-valence excitations provides a site specific probe of the electronic structure properties and ligand-field interactions, as well as information about the resonantly photo-oxidised final states. Comparing measurements of the Fe centre and the surrounding ligands demonstrate strong mixing of the Fe {hbox {t}_{{rm{2g}}}} levels with occupied ligand pi orbitals but weak mixing with the corresponding unoccupied ligand orbitals. This highlights the importance of pi-accepting and -donating considerations in ligand design strategies for photofunctional iron carbene complexes. Spin-propensity is also observed as a final-state effect in the RPES measurements of the open-shell hbox {Fe}^{{rm{III}}} complex. Vibronic coupling is evident in both complexes, where the energy dispersion hints at a vibrationally hot final state. The results demonstrate the significant impact of the iron oxidation state on the frontier electronic structure and highlights the differences between the emerging class of hbox {Fe}^{{rm{III}}} photosensitizers from those of more traditional hbox {Fe}^{{rm{II}}} complexes.

Highlights

Iron-based transition metal complexes are attractive for the development of earth-abundant solar energy conversion technologies[1,2,3], but traditional FeII polypyridyl complexes suffer from rapid losses of the excited state energy from initially excited metal-to-ligand charge transfer (MLCT) states to low-energy metal-centred (MC) states[4]
The participant enhancement of the resonantly-oxidised final states proved to be a sensitive probe of spin-states, ligand-field interactions and ultra-fast vibrational effects that are directly relevant to the functionality of the complexes
The measurements enabled a comparison of the oxidation state and the closed- vs open- shell nature of the complexes

Summary

Introduction

Iron-based transition metal complexes are attractive for the development of earth-abundant solar energy conversion technologies[1,2,3], but traditional FeII polypyridyl complexes suffer from rapid losses of the excited state energy from initially excited metal-to-ligand charge transfer (MLCT) states to low-energy metal-centred (MC) states[4]. The photofunctionality of the FeIII complexes is noteworthy given that the photo-excited Ligand-to-Metal CT (LMCT) excited states have only rarely shown useful photochemical properties for d5 transition metal systems[8]. The electronic structure properties of transition metal complexes can be studied using X-ray spectroscopy methods including X-ray absorption spectroscopy (XAS)[22,23] and photoelectron spectroscopy (PES)[24,25,26]. In RPES, the incident photon energy is tuned to match a resonant transition allowing the resulting “participant” decay channel to be measured[36]. It is important to highlight that whilst the “particpant”/“spectator” nomenclature is useful when discussing spectral features in RPES, it has limitations and in some cases the distinction is not uniquely defined[39]

Methods

Discussion

Conclusion