1 research outputs found
Photochemical generation of a viable oxidizing agent and application to dye photobleaching
PhD ThesisMolecular 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.Umm Al-Qura University as represented via Saudi Embass