From Nanocontainer to Nanocatalyst: Mechanistic Studies of [2+2] Photodimerization of Coumarin Derivatives within Cucubit[8]URIL

Controlling photoreactions remains a formidable challenge to chemists who have developed several approaches with varying degrees of success to achieve high reactivity/selectivity. Following nature's footprints, chemists have explored the use of confined media for controlling photoreactions. This thesis explores catalytic aspects of a water-soluble supramolecule known as a cucurbituril. Cucurbituril is a macrocyclic oligomer with a large enough cavity to sequester two guest molecules of appropriate size. The guest molecules explored in this thesis is coumarins. The model investigation involves host-guest complexes between cucurbit[8]uril (CB[8]) and coumarin to study the [2+2] photodimerization in water through various spectroscopic techniques. Our initial investigations explored the formation of host-guest complexes with coumarin guests that interacted with CB[8] host. This host guest complexation was used to explore and control photochemical reaction and photophysical properties of encapsulated coumarin guest molecules. The host-guest complexation was found to be dependent on the polarity of the coumarin and the volume constraints imparted by the CB[8] cavity. Observational insights from various coumarins provided insights into formation of host-guest complexes with CB[8]. Some coumarins do not form complexes but if they do they can form 1:1 and 1:2 host guest complexes as well as dynamic host-guest complexes (mixture of 1:1 and 1:2 host-guest complexes). Using dynamic host-guest complexes, we explored the use of CB[8] as a photocatalysts. Photodimerization of 6-methylcoumarin was explored as a model system to understand the supramolecular aspects of photocatalysis. The mechanism for photocatalysis was elucidated using various spectroscopic techniques. Both steady state and time-resolved experiments were carried to ascertain the thermodynamic and kinetic aspects of the supramolecular catalytic process. Spectroscopic investigations provided insights into vital role of dynamic complexes in the catalytic cycle as well as the extrusion of photoproduct from the cavity to enable turnover in the system. Thus this investigation provided an opportunity to build an overall picture of a novel supramolecular photocatalytic process in water. This will undoubtedly foster further development in the area of supramolecular photocatalysis.National Science Foundation (NSF CAREER CHE- 0748525 and CHE- 1213880)NSF ND-EPSCoR Seed GrantNSF ND-EPSCoR Doctoral Research Fellowshi