Physical and Photophysical Properties of Spin-Polarized Molecules

Michigan State University, East Lansing MI

Investigators

Abstract

This award in the Inorganic, Bioinorganic and Organometallic Chemistry program supports research by Professor James K. McCusker at Michigan State University to investigate the physical and photophysical properties of compounds subject to Heisenberg spin exchange. This phenomenon can occur whenever two (or more) paramagnetic centers are in close proximity so as to lift the degeneracy of spin-polarized electronic states. The research will elucidate the impact spin exchange on the photo-induced properties of molecules. Coupled to this work are studies geared toward gaining a more general understanding of the interplay between spin and charge density in open-shell molecules with the goal of developing protocols for the manipulation of the magnetic and photomagnetic properties of molecular systems. These research goals will be pursued along three distinct trajectories. First, a fundamental study of the interplay between spin and charge density in molecules will be undertaken. Computational work demonstrates that differential polarization of spin and charge density in phenoxy radicals can be achieved through judicious choice of substituents on the phenyl ring. The study will be expanded to semiquinones and phenanthrosemiquinones which can act as ligands in metalorganic hybrid compounds. Secpmd, the models developed through the computational work will then be tested through the preparation and characterization of Cr(III)- and Ni(II)-containing compounds so as to demonstrate the ability to tune intramolecular antiferromagnetic and ferromagnetic interactions, respectively, through synthetic means. A systematic examination of the effect of ground- and excitedstate spin exchange is then described through the study of a series of simple Cr(III) coordination compounds. The complexes contain redox-active quinoidal ligands that allow exchange to be turned on or off depending on the oxidation state of the ligand and/or the nature of the absorption feature targeted for photoexcitation. The third project begins an expansion into the realm of extended molecular magnetic systems. Dimetallic semiquinone complexes will allow for the systematic manipulation of spin coupling in a more complex system. Included in this project is a proposed photomagnetic experiment that will enable the direct detection of a ferrimagnetic interaction in a transient electronic excited state. In addition, this system provides a platform for the correlation of electrochemical, magnetic, and thermodynamic data that should yield the first experimental probe of the validity of the Heisenberg exchange model. The results of the study of differential spin and charge polarization leads toward a practical model that will allow researchers to think in terms of synthetically tuning exchange interactions, which would benefit those working in the field of single-molecule magnetism. The photophysical work on spin-coupled systems will provide insight into an aspect of the electronic structure of an large class of compounds for which there is currently little experimental information. Students involved in this research will receive broad-based training in synthetic chemistry, state-of-the-art spectroscopic methodology, magnetism, as well as the application of density functional theory to both organic and inorganic molecules.