Toward High Intensity Forbidden EPR Transitions In Bimetallic Complexes

With support from the Chemical Structure, Dynamics & Mechanisms B Program of the Chemistry Division, Professor Joseph Zadrozny of the Department of Chemistry at Colorado State University is developing new classes of dinuclear transition metal complexes to probe forbidden electron paramagnetic resonance (EPR) spectroscopic transitions. The goal of this research is to exploit the characteristics of these dinuclear species to understand how molecular structure can assist in violating EPR selection rules to enable high intensity transitions. The specific transitions being explored may yield new capabilities for bioimaging techniques that use EPR transitions. The research project spans the interface of inorganic and physical chemistries with synthetic and spectroscopic analyses as a key part of the work. The broader outreach goals for the program will be to create an easy-to-access lesson for synthetic chemists to approach the selection rules of EPR. This plan will follow the pedagogical strategies commonly employed for more common techniques, e.g. UV-vis spectroscopy, and as such aims to be a cornerstone in bringing new chemists into the EPR arena. The longer-term goal for the work is to enable low-frequency EPR transitions at high magnetic field, which would permit the chemical sensitivity of EPR imaging to be integrated with MRI. The singlet-to-triplet transition of two-spin systems is a promising way to realize such an EPR transition, because of its unique field dependence. Yet, the transition is forbidden and therefore weak hence, chemical design strategies for molecules to override the EPR selection rule and increase signal intensity are needed. Dinuclear metal complexes have a plethora of magnetic interactions that could potentially violate the EPR selection rule, but experimental tests of the efficacy of these violations have yet to be performed. This work will study the singlet-to-triplet transition in a variety of dinuclear metal complexes to explore how exchange coupling, hyperfine coupling, and relative spin orientation all contribute to the intensity of the forbidden resonance. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.