Tethered Systems for the Study of Higher-Order Charge-Transfer Complexes

This research has resulted in the first direct solution of the equilibrium constants of higher-order charge-transfer complexes. Chapter One provides a general background of charge-transfer (CT) complexes, both the 1:1 and higher-order complexes. A historical discussion about the nature of the charge-transfer complex is presented as well as application possibilities. In Chapter Two the synthesis and characteristics of unique unsymmetrical conveniently tethered compounds are discussed. We use proton NMR spectroscopy, ultraviolet-visible (UV-vis) absorption spectroscopy, and cyclic voltammetry to test whether the compound AE1 is a pre-associated intramolecular charge-transfer complex. When comparing proton NMR spectra, the shifts of chemical shift for the compound AE1 indicate a stacked conformation based on consideration of magnetic anisotropy. Comparisons of the UV-vis spectra for all the investigated tethered compounds and model compounds reveal that the molar absorptivity is greatest for AE1 which indicates the formation of the intramolecular CT complex. The results from cyclic voltammetry were not as clear; while giving clear indications of an increase in the energy of the lowest unoccupied molecular orbital (LUMO), there was not the expected decrease in the energy of the highest occupied molecular orbital (HOMO). Chapter Three contains experimental analyses of equilibrium constants for 1:1 and higher-order CT complexes. Prior to the analysis of a higher-order CT complex--specifically, AE1 with external acceptor tetracyanoethylene (TCNE), a new methodology using UV-vis spectroscopy is discussed for the 1:1 CT complex formed between hexamethylbenzene (HMB) and TCNE. This literature-adapted approach provides more accurate results than the Benesi-Hildebrand method that many groups utilize. The results of our analysis find that the equilibrium constants for higher-order complexes with excess acceptor are larger than their 1:1 counterpart while the molar absorptivity is decreased. The molar absorptivity for complexes with excess donor is larger than their 1:1 counterpart. In Chapter Four we use computational simulations to examine the energy profiles of our tethered molecules from the perspective of the electron-donor and electron-acceptor moieties ranging from being from nearer to farther from each other. These results are consistent with evidence presented in Chapter Two. The simulations also allowed us to obtain the equilibrium constants of several complexes in order to compare with the methods described in Chapter Three and to examine the formations of CT complexes that eluded analyses experimentally. The additional complexes supported and expanded the findings described in Chapter Three. Chapter Five contains overall conclusions based on all the evidence gathered. While the results are consistent with previous literature, these are the first detailed analyses of equilibria associated heterogeneous DAD′ and ADA′ higher-order CT complexes.