6533b823fe1ef96bd127f636

RESEARCH PRODUCT

Boosting Vis/NIR Charge-Transfer Absorptions of Iron(II) Complexes by N-Alkylation and N-Deprotonation in the Ligand Backbone.

subject

description

Reversing the 3MLCT / 3MC excited state order in iron(II) complexes is a challenging objective, yet would finally result in longsought luminescent transition metal complexes with an earthabundant 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 affects the optical properties. On the other hand, N-deprotonation in 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.