Functional Molecular Magnetic Materials Based on Transition Metal Complexes, Organocyanide Radicals and Metal Organic Frameworks

Abstract

Molecule-based functional magnetic materials are of great interest due to their promising potential applications in new generation of electronic devices. Single molecule magnets (SMMs) and spin-crossover (SCO) moieties are two important categories of molecular materials that exhibit magnetic bistability. Solid state assemblies of these molecules combined with other functional moieties, such as tetracyanoquinodimethane (TCNQ) and metal organic frameworks (MOFs) can result in an enhancement of the original properties and/or coexistence/synergistic interactions of more than one functionality. The goal of the research in this dissertation was to explore ways to manipulate solid state structures and physical properties of SMMs and SCO building blocks by (1) manipulating supramolecular interactions between TCNQ and SCO/SMM units and (2) using crystalline MOFs platforms for nanostructuring SMMs for potential applications in electronic devices.

Two types of molecular magnetic bistability, SMM properties (in [Coᶦᶦ(Fctp)₂](TCNQ)₂ (1)) and SCO (in [Coᶦᶦ(Fctp)₂](TCNQ)₂·MeCN (2) and [Coᶦᶦ(Fctp)₂](TCNQF)₂·MeCN (3)), have been achieved with the same transition metal complex, [Coᶦᶦ(Fctp)₂]²⁺, by using the two TCNQ radicals, TCNQ·⁻ and TCNQF·⁻. Single crystal structures and theoretical computational studies reveal that the supramolecular interactions play important roles in the tuning of the magnetic properties.

A mixed-stacked of partially charged TCNQ (electron acceptor) and the phenothiazinyl (PTZ, electron donor) group on the SCO molecule ([Co(PTZ-tpy)₂]²⁺) were achieved in the compound [Co(PTZ-tpy)₂]₂(TCNQ)₄.₅·2.5MeCN (6) by introducing charge transfer interactions between conducting and SCO building blocks. The temperature dependent solid state structures as well as the magnetic and conducting properties were compared to the non-SCO Zn analogues, [Zn(PTZtpy)₂]₂(TCNQ)₄.₅·2.5MeCN (7) which revealed synergistic behavior of SCO and electron transfer between different TCNQ stacks as well as the semiconducting properties. The results demonstrate that introducing supramolecular interactions is a promising way to manipulate solid state structure as well as to realize interesting synergistic effects between different functionalities.

Finally, the installation of octahedral Coᶦᶦsingle SMM sites in the mesoporous MOF, UMCM-1, was achieved via post synthetic ion exchange reactions. Detailed studies of magnetization relaxation processes of the installed Coᶦᶦ SMMs moieties were performed. The results indicate that the undesired Raman and direct relaxation processes were suppressed by using the mesoporous MOF as a platform for the SMMs assembly.