title: Boosting Vis/NIR Charge-Transfer Absorptions of Iron(II) Complexes by N-Alkylation and N-Deprotonation in the Ligand Backbone.
abstract: Reversing the metal-to-ligand charge transfer (3MLCT)/metal-centered (3MC) excited state order in iron(II) complexes is a challenging objective, yet would finally result in long-sought luminescent transition-metal complexes with an earth-abundant central ion. One approach to achieve this goal is based on low-energy charge-transfer absorptions in combination with a strong ligand field. Coordinating electron-rich and electron-poor tridentate oligopyridine ligands with large bite angles at iron(II) enables both low-energy MLCT absorption bands around 590 nm and a strong ligand field. Variations of the electron-rich ligand by introducing longer alkyl substituents destabilizes the iron(II) complex towards ligand substitution reactions while hardly affecting the optical properties. On the other hand, N-deprotonation of the ligand backbone is feasible and reversible, yielding deep-green complexes with charge-transfer bands extending into the near-IR region. Time-dependent density functional theory calculations assign these absorption bands to transitions with dipole-allowed ligand-to-ligand charge transfer character. This unique geometric and electronic situation establishes a further regulating screw to increase the energy gap between potentially emitting charge-transfer states and the non-radiative ligand field states of iron(II) dyes.
authors.affiliation: Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, 55128, Mainz, Germany.
authors.forename: Andreas K C
authors.initials: AKC
authors.lastname: Mengel
authors.affiliation: Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, 55128, Mainz, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Bissinger
authors.affiliation: Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, 55128, Mainz, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Dorn
authors.affiliation: Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, 55128, Mainz, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Back
authors.affiliation: Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, 55128, Mainz, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Förster
authors.affiliation: Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, 55128, Mainz, Germany.
authors.forename: Katja
authors.identifier: http://orcid.org/0000-0003-1483-4156
authors.initials: K
authors.lastname: Heinze
keywords: charge transfer; excited states; iron complexes; ligand substitution; tridentate ligands