Jordan Herder & Ben Walusiak, Graduate Students, Cahill Lab, Department of Chemistry
Photoinduced charge transfer of solid-state uranyl-viologen-bearing materials:
A structural, optical, and kinetic study
Jordan Herder | Graduate Student, Cahill Group, Department of Chemistry
Uranium photoreactivity is an active area of study aimed at understanding actinide properties, informing nuclear forensic applications, and evaluating the potential for use of uranium in catalysis. However, the dynamics of solid-state uranium-bearing material photochemistry is often predicated on a mechanism derived from solution state chemistry. A systematic study of a series of simple uranyl-containing compounds indicates that this mechanism, while correct in some cases, may be inappropriate for the interpretation of solid-state uranyl reactivity. This work details the synthesis and characterization of three new uranyl compounds featuring the uranyl-tetrachloride [UO2Cl4]2- dianion paired with various 1,1’-dialkyl-4,4’-bipyridinum dications (i.e. viologens). A simple uranyl species paired with the redox-active cation may support charge transfer in the solid state providing a unique perspective when probing the photochemistry of uranyl-bearing materials. Irradiation with UV light causes a marked reduction of uranyl emission intensity over time, which can be partially recovered by placing the sample in the dark. Kinetic treatment of the time-dependent data reveals a quenching efficiency and rate constant dependence on viologen identity. We propose a photoinduced electron transfer mechanism for this reaction which is support by experimental and computational data. Our findings support the formation of a notable oxidized [UO2Cl4]1- species bearing a U(VI) bound to a radicalized chloride ligand. These efforts will inform studies of uranyl as a photocatalyst or in radiation detection, yet perhaps more importantly, may contribute to our fundamental understanding of energy transfer within actinide bearing hybrid materials.
Exploring Structural and Optical Properties of Hybrid Tellurium Halide Perovskite Derivatives: An Experimental and Computational Approach
Ben Walusiak | Graduate Student, Cahill Lab, Department of Chemistry
Perovskite materials have received significant attention over the last decade as the light absorbing component in solar cells, whose power conversion efficiencies are capable of rivaling current silicon based cells. Of the known perovskite compositions, tellurium halides are under-characterized and poorly understood, with only a handful of reported examples. We have synthesized and characterized a family of hybrid tellurium perovskites that consist of low dimensional anionic tellurium halide units separated and charge balanced by halogenated organic cations. These materials exhibit low energy band gaps, desirable for photovoltaic applications, which can be tuned by varying halogen identity and non-covalent interactions between inorganic and organic ions. Computational density of state (DOS), natural bonding orbital (NBO), and quantum theory of atoms in molecules (QTAIM) methods were utilized to determine absorption mechanisms, quantify non-covalent interaction strength, and evaluate tellurium halide bond character as a function of second sphere interactions. Our findings demonstrate that second sphere interactions directly influence structural and optical properties of these materials by altering inner sphere metal halide bonding and orbital hybridization, informing experimental efforts to expand the under-developed area of tellurium halide perovskites.