Photochemical generation of a viable oxidizing agent and application to dye photobleaching

Abstract

Molecular photochemistry is a well-established branch of science that owes its origins to the pioneering studies of Alexander Schönberg in Egypt during the 1940’s and 50’s. The main advantage of photochemistry relies on the use of sunlight to drive “difficult” reactions, the most important being the production of molecular oxygen and chemical fuel by way of natural photosynthesis. While photovoltaic cells and systems based on inorganic semiconductors have developed enormously during the past few decades, there have been few comparable advances in molecular photochemistry. This is despite the ready availability of powerful spectroscopic and computational tools. Indeed, the mantle of “artificial photosynthesis” has been transferred from chlorophyll derivatives to silicon and its allies. In this thesis, we explore the concept of developing simple photochemical means to drive oxidative processes that might be both practical and costeffective. Several systems are considered and subjected to experimental study. Chapter 1 presents a general introduction to the field and illustrates both the potential and the frustration offered by molecular photochemistry. Typical excited state reactions are described in terms of simple molecular orbital diagrams. The discussion is directed towards dye photobleaching; a topic of considerable contemporary significance given the recent advances in single-molecule fluorescence and super-resolution microscopy. Ways to protect dyes against the deleterious effects of exposure to laser light are reviewed and common reactive intermediates are identified. The introduction provides background information for the subsequent research work. Experimental protocols for monitoring the course of dye photofading are introduced. This chapter is followed by an account of the experimental practices followed during our work. Chapter 2 includes a description of the methods used for data analysis. Chapter 3 recognises the need to generate a relatively stable intermediate oxidant if a viable photobleaching strategy is to be devised. Most of the intermediates generated under illumination survive for periods of a few microseconds or less and therefore require high concentrations of substrate to ensure chemical trapping before deactivation. The approach suggested involves the photochemical production of an organic hydroperoxide that is sufficiently stable to be characterised by NMR spectroscopy. The bleaching capability of this species is assessed by 1 specific reference to the decolouration of indigo. The latter is a popular dye that displays exceptional stability towards sunlight. Chapter 4 continues this theme by examining the photobleaching of methylene blue (MB) in water. This cationic dye is known to bleach quickly on exposure to visible light and we have studied the mechanism and efficacy of the bleaching reaction. The importance of light intensity and dissolved oxygen are stressed. The key discovery made here relates to the ability of high concentrations of urea to inhibit photochemical bleaching of MB. This is a surprising result that offers promise for providing protection for oxidative damage of certain dyes. The mechanism by which urea operates has been deduced by way of kinetic measurements. Chapter 5 looks at the catalysed bleaching of a novel strapped boron dipyrromethene (BODIPY) derivative. This dye represents a new type of BODIPY dye that displays circularly polarised luminescence. Direct and catalysed photobleaching processes are compared and the importance of dissolved oxygen is assessed. The dye has a highly strained geometry imposed by the straps attached to the boron atom. Oxidation can help relieve this steric strain. Chapter 6 takes a different perspective and enquires if chlorine dioxide can be generated by photochemical protocols. Chlorine dioxide is an important bleach but is not in common use because it is difficult to manufacture in bulk and has limited stability on storage. In principle, it should be possible to generate chlorine dioxide by photochemical means. This could provide access to a powerful antiseptic reagent in remote locations using sunlight as the only energy input. Several putative ways to produce chlorine dioxide are considered and an analytical protocol is devised for quantitative determination. It is concluded that a practical set-up could be engineered for the in-situ production of chlorine dioxide in aqueous solution.