Applications of physical organic chemistry: Two examples of 'weakly-bound' dimers

Abstract

This thesis describes two projects involving novel applications of physical organic chemistry. The first project involved investigation into a series of nitrile-substituted benzyl radicals and their remarkable persistence and attenuated reactivity towards oxygen. Following the characterization of these species by time-resolved spectroscopic techniques, we anticipated that, through radical-radical recombination reactions, they could successfully employed in carbon-carbon bond formation via the application of the Persistent Free Radical Effect (PFRE). In general, radical-radical coupling reactions are inadequate for organic synthetic purposes as they tend to yield complex product mixtures. However, this restriction may be circumvented through use of the PFRE, a kinetic phenomenon that leads to cross-product formation in coupling reactions between persistent and transient radicals. It is possible to exploit this effect as a synthetic tool through the use of a "radical buffer", in which persistent radicals are generated thermally from dimeric derivatives of the aforementioned persistent carbon-centred radicals. If a transient radical is then introduced photochemically, it will recombine selectively with the persistent radical species. This process is demonstrated in the steady-state photolysis and laser flash photolysis studies of dibenzyl ketone with 2,2,3,3-tetraphenylsuccinonitrile to produce 2,3,3-triphenylproprionitrile. Therefore, by carefully choosing persistent/transient radical pairs, this method may be applied as a simple and effective alternative for more complex organic syntheses. In subsequent experiments, we found that, along with novel carbon-carbon bond formation, involvement of the persistent carbon-centred radicals could also be applied to other systems, such as the photodecomposition of a pesticide, and the trapping of peroxyl and hydroperoxyl radicals. The second project investigated the use of pyrene fluorescence quenching as a technique to detect electron-deficient compounds. The photophysical properties of pyrene in dilute and concentrated solutions have been a topic of intense research interest for many years and the dynamic equilibrium between the monomer and excimer forms has been largely elucidated. However, the effect of certain fluorescence quenchers on the ratio of emission from the two species remained largely unexplored. Time-resolved and steady-state fluorescence techniques were employed to study the quenching behaviour of concentrated pyrene solutions. It was found that certain quenchers, namely electron-poor compounds, were able to efficiently quench both the monomer and excimer species, with bimolecular quenching rate constants approaching diffusion control. As a consequence, by monitoring the monomer-to-excimer fluorescence ratio, it is possible to identify and quantify these compounds. Therefore, this characteristic allows for potential security-related applications, including the detection of explosives and for rapid screening of complex samples suspected of containing explosives. The concept of differential monomer-to-excimer fluorescence quenching was then extrapolated to other systems that we found intriguing. For example, it was found that, in addition to nitroaromatics, molecular oxygen also quenches both species with rate constants approaching diffusion-control; thus, this methodology can also feasibly be applied to sensing dissolved oxygen. As well, we explored the effect of constrained media on differential quenching, including the use of gas-permeable silicone films and supramolecular materials (zeolites), both heavily-loaded with pyrene, with the idea that these materials could potentially be developed as cheap and disposable sensor materials.