Chemistry and Chemical Biology ETDs

Publication Date

Spring 4-15-2022

Abstract

Transition metal complexes ligated by Donor-Acceptor (D-A) biradicals are of interest for probing the degree of electronic coupling between chemically biased radical fragments. The unique nature of their electronic coupling has allowed these systems to serve as ground state analogues of charge separated excited states generated by photoexcitation. We have been interested in how the nature of the acceptor radical affects the magnitude of the electronic coupling matrix element in these biradicals. As such, we have synthesized a new series of D-A semiquinone-verdazyl (SQ-VD) biradical ligands and their transition metal complexes. We have characterized these new complexes by x-ray crystallography, electronic absorption and EPR spectroscopies, and detailed bonding and electronic structure computations. The results are discussed in terms of excited state D-A charge transfer processes, ground state magnetic exchange interactions, and a valence bond configuration interaction model that describes excited state contributions to the ground state magnetic exchange. Square-planar (bpy)Pt(bdt) (bdt = benzene-1,2-dithiol; bpy = 4-4’ bipyridine) D-A complexes are of interest due to the remarkable nature of their lowest energy bdt → bpy ligand-to-ligand charge transfer (LL’CT) excited states. The nature of this LL’CT transition results in the formation of a charge separated open shell singlet biradical excited state. These complexes readily intersystem cross (ISC) to the open shell triplet configuration with long-lived excited state lifetimes and observed photoluminescence. We have been interested in the effects of radical elaboration on the ISC rates and excited state lifetimes of LL’CT complexes. As such, we have synthesized a new verdazyl (VD) radical elaborated (bpy)Pt(BDT-VD) complex and performed spectroscopic and computational studies on this interesting system. Phosphorescent emission has been observed on this complex indicating the existence of a long-lived excited state with a different spin multiplicity (S = 3/2) than that of the electronic ground state (S = 1/2). The results are discussed in terms of the excited state electronic structure, nature of the excited state magnetic exchange interactions, and the potential for generating spin polarized excited states that will contribute to dynamic spin polarization effects in the ground state. Controlling excited state lifetimes are of key importance in designing photovoltaic and related devices. A primary mechanism for controlling T1→S0 decay is spin-orbit induced intersystem crossing coupled with long-distance charge separation. This process allows for an adequate reaction time for solar energy conversion or to introduce long-lived emission for various photonic applications. Here we have devised systems which possess two, close-lying excited states, the mixing of which can affect the lifetime. Without changing the spin multiplicity, lifetime modulation is achieved by hole migration between different excited states.

Project Sponsors

National Science Foundation

Language

English

Keywords

Donor-Acceptor Biradical, Exchange Coupling Interaction, Verdazyl Radical, Donor-Acceptor Platinum Complexes, Spin Dynamics, Hole Migration

Document Type

Dissertation

Degree Name

Chemistry

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Martin L. Kirk

Second Committee Member

Hua Guo

Third Committee Member

Brian Gold

Fourth Committee Member

Changjian Feng

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