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Structural tuning of ligand-based two-electron intervalence charge transfer.
Julien Bachmann1, Daniel G Nocera
1Department of Chemistry, Massachusetts Institute of Technology, Cambridge, 02139-4207, USA.
Inorganic Chemistry
|September 27, 2005
Summary
The color of oxidized porphyrinogens shifts with metal ion size due to changes in molecular dipole moment. This impacts the energy of charge transfer transitions, affecting their optical properties.
Area of Science:
- Inorganic Chemistry
- Materials Science
- Photochemistry
Background:
- Porphyrinogens are macrocyclic compounds with unique electronic properties.
- The study focuses on two-electron-oxidized porphyrinogens, denoted as [L(Delta)M].
- The lowest-energy optical transition in these compounds is a ligand-based charge transfer.
Purpose of the Study:
- To investigate the effect of the central metal dication (M2+) on the optical properties of two-electron-oxidized porphyrinogens.
- To understand how the ionic radius of the metal dication influences the color and electronic transitions.
- To elucidate the relationship between molecular structure, dipole moment, and charge transfer characteristics.
Main Methods:
- Spectroscopic studies (UV-Vis absorption) to determine lambdamax values.
- Structural analysis to understand the positioning of the metal dication.
- Computational studies to analyze orbital energies and molecular dipole moments.
Main Results:
- The color of the mixed-valent oxidation state shifts from vermilion (lambdamax = 480 nm) to yellow (lambdamax = 270 nm) as the ionic radius of the central metal dication increases (Mg2+ to Zn2+ to Ca2+).
- Structural, spectroscopic, and computational data confirm that the metal dication's position relative to the dipyrrole unit modulates the molecular dipole moment.
- The molecular dipole moment, influenced by the metal dication's position, dictates the energy of the intervalence charge-transfer optical transition by affecting the relative energies of the highest occupied and lowest unoccupied orbitals.
Conclusions:
- The position of the central metal dication is a critical structural variable in [L(Delta)M] compounds.
- The metal dication's position directly influences the molecular dipole moment, thereby tuning the energy of the ligand-based charge transfer.
- This study provides insights into the structure-property relationships of oxidized porphyrinogens, relevant for designing materials with specific optical characteristics.