Photophysical studies on the dynamics of guest complexation with cyclodextrins
Murphy, Robert Scott
The main objective of this research is to investigate aspects responsible for the dynamics of guest molecules complexed with cyclodextrins (CD)s). We have shown with the use of a variety of photophysical techniques that the complexation. dynamics for guests with CDs are dependent on the structure of the guest molecule. An assortment of photophysical methods that included steady-state fluorescence, UV-Vis absorption, and laser-induced optoacoustic (LIOAS) spectroscopies, in combination with time-resolved techniques such as single photon counting fluorescence and triplet-triplet absorption (T-T abs) spectroscopies were employed to obtain a detailed understanding of the photophysics for fluorenones. From these photophysical investigations, we have demonstrated that several effects such as the nature and position of substituents, and the properties of the microenvironment are responsible for the photophysics observed for these aromatic ketones. The complexation of fluorenone and xanthone with CDs was investigated to obtain more information on how the structure of the guest molecule can affect the complexation dynamics of these host-guest systems. Induced circular dichroism (ICD) and picosecond fluorescence spectroscopy were employed to detail the structural differences observed for the CD complexes of these two ketones. Equilibrium constants were observed to be larger for xanthone with β-CD than with fluorenone. This result suggested that a more favorable complex is formed for xanthone than for fluorenone. However, in the presence of α-CD, fluorenone formed 2:1 host-guest complexes that were not observed for xanthone. These photophysical studies with additional support from theoretical calculations provided useful tools for understanding the structural intricacies of CD host-guest systems. These types of studies will be invaluable to the understanding of dynamics within supramolecular systems. To expand our knowledge on the structure-dynamics relationship that exists for CD complexes, we investigated the complexation dynamics of charged probes with CDs. Two styrene derivatives, trans-anethole (t-Ane) and 4-vinylanisole (4-VA), were chosen as precursors for the radical cations examined in these investigations. Quenching studies have demonstrated that the exit of the radical cations of t-Ane and 4-VA complexed with CD, even in the presence of alcohols, was faster than 20 ns. In addition, complexes with 2:1 host-guest stoichiometries were unsuccessful in reducing the dissociation rate constant of these charged species. From these studies we have shown that a structure-dynamics relationship does exist for CD host-guest systems. With the use of a variety of photophysical techniques and theoretical calculations, we have been able to better evaluate how the photophysics of probe molecules can be explored in the study of host-guest complexation. Small changes in structure have important consequences on the binding efficiencies of these probes to CDs. This information will aid in the understanding of the structure-dynamics relationship that occurs in supramolecular systems.