Nanosecond Metal-to-Ligand Charge-Transfer State in an Fe(II) Chromophore: Lifetime Enhancement via Nested Potentials

Area of Science:

• Inorganic Chemistry
• Photochemistry
• Materials Science

Background:

• Long-lived charge-transfer states in iron(II) complexes are typically limited to pseudo-octahedral geometries.
• Developing alternative strategies for such states using different coordination motifs and ligand properties is highly desirable.

Purpose of the Study:

• To synthesize and characterize a novel air-stable, tetragonal iron(II) complex with a macrocyclic ligand.
• To investigate the photophysical properties, specifically the metal-to-ligand charge-transfer (MLCT) lifetime, of the synthesized complex.
• To explore the influence of the ligand structure and solvent environment on the MLCT state.

Main Methods:

• Synthesis and structural determination of the iron(II) complex, Fe(HMTI)(CN)2.
• Photophysical characterization in various solvents.
• Density functional theory (DFT) calculations to understand electronic structure and potential energy surfaces.

Main Results:

• A 1.25 ns MLCT lifetime was achieved in the tetragonal iron(II) complex, Fe(HMTI)(CN)2.
• The HMTI ligand's π-acidity and the macrocycle's rigidity contribute to stabilizing the excited state.
• The MLCT state's lifetime and energy are strongly influenced by solvent interactions with the cyano ligands.

Conclusions:

• This study reports the first example of a long-lived charge-transfer state in an iron(II) macrocyclic complex.
• The findings demonstrate a new strategy for designing functional iron complexes by combining macrocyclic ligands with π-acidic properties.
• The solvent-dependent photophysics highlights the potential for external stimuli-responsive materials.