Photophysical properties of bichromophoric Fe(II) complexes bearing an aromatic electron acceptor
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The replacement of heavy metals used by industry to produce optical devices would considerably reduce the environmental and economic cost of man-made technology. A possible strategy relies on the employment of lighter and more abundant metals like iron. The exploitability of the photophysics of Fe(II) complexes is, however, generally limited by their short excited-state lifetimes and poor emission properties. The present work studies the impact of appending an electron acceptor (anthracene) to N-heterocyclic carbene (NHC) iron complexes with the aim to trap the excited-state energy and, therefore, delay the excited-state decay of the considered iron compounds. Hence, the photophysical properties of six prototypes (built with different spacers between the NHC ligand and the anthracene moieties) have been studied by using time-dependent density functional theory and by determining the natural transition orbitals of the excited states. The computational results suggest that ethynyl bridges induce dual absorption properties, covering red and infrared wavelengths in addition to the violet–blue absorption of the metal-to-ligand charge transfer band, already reported for the parent compound. The nature of the lowest lying triplet states indicates that, for all the considered prototypes, the excitation involves π* orbitals localized over anthracene, confirming its electron acceptor capabilities and suggesting a possible equilibrium between different excited states that might lead to enhanced excited-state lifetimes and/or boosted luminescence properties.
KeywordsFe(II) complexes Excited states TD-DFT Electron acceptor N-heterocyclic carbene Anthracene
The authors thank Prof. Stefan Haacke (Université de Strasbourg) for his valuable suggestions and fruitful scientific discussions. This work has been funded by the French Agence Nationale de la Reserche (ANR) under the PhotIron project. A. F.-M. acknowledges the French Région Grand Est government and the ANR for postdoctoral contracts, and the financial support from the project CTQ2017-87054-C2-2-P of the Spanish MICINN. All calculations have been done using the local computing resources of the LPCT laboratory.
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