Variable temperature X-ray crystal structures for the spin coupled [Fe2(μ-OH)(μ-O2CCH3)2(HBpz3)2]+ complex were acquired and studied computationally, where it was found there was observable structural effects attributed to increasing the thermal occupation of higher spin states. These effects were manifest as structural changes in the exchange coupled dimer that were not present in a structural analogue. It was determined through computational studies that these structural changes act to reduce the exchange coupling between the FeIII centers which previous studies have shown could affect the potential reactivity of this system. Detailed mechanistic studies on the spin coupling have shown this change in coupling is mediated primarily by a change in the (μ-OH) bond distance, which may or may not be due to the thermal occupation of higher spin states or could be due to other external effects.A covalently linked intramolecular donor-acceptor assembly consisting of a ruthenium polypyridyl bound to a MnIIZnII bearing macrocycle was characterized and the variable temperature time resolved emission of this compound was investigated where the presence of a thermally activated quenching process was discovered. This quenching of the emissive excited state of the Ru donor by the macrocyclic acceptor was determined to have a thermal barrier of 80 ± 20 cm-1 and was found to be proceeding via a Dexter energy transfer mechanism. The origin of this barrier wasdetermined to be due to a reorganization process that raised the energy of the acceptor due to the rigid medium in which these compounds were studied. This assignment was confirmed through supporting density functional theory calculations.Related to the MnIIZnII donor-acceptor system, computational studies on the exchange coupled MnII2 Schiff-base macrocycle that was previously studied as the acceptor in a donor-acceptor assembly were performed to provide insight as to the observed increased quenching rate in an analogous MnII2 donor-acceptor system compared to MnIIZnII system. The spin coupled states were investigated via the broken symmetry formalism, and the electronic structure of ligand field excited spin coupled states was also studied. The orbital mechanisms of the exchange interaction were studied and it was determined that the linking ligand in the donor-acceptor assembly has minimal impact on the spin coupling of this system, so excited states of the energy donor would have little impact on the thermal occupation spin state in this system. Interestingly, it was also found through the broken symmetry electronic structure investigations that there were substantial thermodynamic differences in the ligand field based excited states of the two systems such that the observed thermal barrier to quenching in the MnIIZnII system is deduced to be non-existent based on the computational results. In this way there is potential that the spin coupling interaction has affected the dynamics of this system.
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