Related Experiment Video
Updated: Jun 16, 2026
Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals
Published on: April 14, 2020
Optimized Octahedral Geometry and Hidden Rigidity of a mer-Tridentate N-Heterocyclic Carbene Ligand Enhance
Toshiki Mochimaru1, Tomohiro Ogawa1, Teruyuki Honda1
1Department of Chemistry, Faculty of Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
Abstract:
Fe(III) N-heterocyclic carbene (NHC) complexes are emerging photoactive first-row transition metal complexes. While optimizing ligand bite angles is a widely employed strategy to extend photoactive excited states, the lack of tridentate NHC ligands prevents exploration of better coordination geometries in the first-row transition metal complexes. Here, we demonstrate that a newly designed methylene-bridged mer-tridentate NHC ligand provides an improved coordination bite angle. Continuous Shape Measure (CShM) analysis reveals that the Fe(III) complex exhibits a significantly improved octahedral geometry (0.35) compared to existing phenylene-bridged mer-tridentate ligands (3.15). This optimized geometry results in a prolonged 2LMCT lifetime (τ = 670 ps), which is longer compared to the Fe(III) complexes with reported bidentate or mer-tridentate ligands. The Fe(III) complex retains a better octahedral geometry even in the distorted quartet metal-centered (4MC) state, indicating enhanced structural rigidity of the coordination environment. With this structural rigidity, the Fe(III) complex achieved an enhanced photoluminescence quantum yield (Φ = 0.6%). The methylene bridges seem inherently flexible; however, we revealed that the geometric constraints of the mer-tridentate arrangement and methylene moieties provide counterintuitive hidden structural rigidity. This work establishes the new mer-tridentate NHC ligand scaffold as a promising ligand platform for developing photoactive first-row transition metal complexes.
Related Concept Videos
02:58Crystal Field Theory - Octahedral Complexes
To explain the observed behavior of transition metal complexes (such as colors), a model involving electrostatic interactions between the electrons from the ligands and the electrons in the unhybridized d orbitals of the central metal atom has been developed. This electrostatic model is crystal field theory (CFT). It helps to understand, interpret, and predict the colors, magnetic behavior, and some structures of coordination compounds of transition metals.
CFT focuses on...
03:02Colors and Magnetism
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human eye.
02:42Valence Bond Theory
02:34Structural Isomerism
Isomers are different chemical species that have the same chemical formula. Structural isomerism of coordination compounds can be divided into two subcategories, the linkage isomers and coordination-sphere isomers.
Linkage isomers occur when the coordination compound contains a ligand that can bind to the transition metal center through two different atoms. For example, the CN− ligand can bind through the carbon atom or through the nitrogen atom. Similarly, SCN− can be...
01:26Variables Affecting Phosphorescence and Fluorescence
02:46Crystal Field Theory - Tetrahedral and Square Planar Complexes
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than the dxy,...