Synthesis, photophysical characterisation and testing of a new type of photobleaching agent

Abstract

The purpose of this work, funded by Procter and Gamble (P&G), was the synthesis, photophysical characterization and testing of thioxanthone (TX) derivatives as photobleaching agents (PBA) for use in laundry products (Figure 1).


Figure 1 Photobleaching agent process: sunlight can promote the activation of PBA adsorbed on a not clean cloth, thanks to an efficient intersystem crossing and high rate of oxygen quenching it is possible to create singlet oxygen and continue the cleaning process during the drying

Procter and Gamble selected thioxanthone for its photophysical characteristics and high potential to be the next PBA generation. Firstly the main electronic absorption transition occurs in the near ultraviolet region; the solid compound is pale yellow and solution colourless, a distinct advantage compared to the PBA currently in use. Another important property is the efficient formation by photosensitisation of singlet oxygen. Thioxanthone is a good photosensitiser and has been used for almost a century in photodynamic therapy. Finally, the overall cost for TX production and storage is drastically lower compared to the available commercial PBA.
Unfortunately, TX has a poor solubility in water which limits the concentration in solution. In this work, ten new thioxanthone derivatives have been synthesised with the intent to maintain TX photophysical properties and increase water solubility. The synthetic pathways proposed in this work aim to be cheap, simple and highly reproducible, an advantage compared to current PBA.
P&G started a collaboration with Durham University aiming to overcome these disadvantages and to have a full understanding of the photophysical behaviour of the potential new PBA.
The achievement of this project is articulated and explained in this manuscripts as follow:
1. Introduction: historical role and evolution of detergents, state of the art on available commercial PBA, literature review on thioxanthone (TX)
2. Experimental: overview of equipment used for the photophysical characterization of derivatives (Chapter 4) and during tests performed at P&G (Chapter 5)
3. Synthesis: description of the synthetic route used to produce thioxanthone derivatives
4. Photophysical characterization of thioxanthone derivatives: a full analysis of photophysical behaviour of thioxanthone derivatives, interaction with crocin as stain simulator and detergents
5. Thioxanthone derivatives as Photobleaching Agents: a series of tests which simulate, in a small scale, the washing and drying process in the presence of current PBA and a selection of thioxanthone derivatives
6. The behaviour of thioxanthone upon acidification: an extensive analysis on the photophysical property on thioxanthone in the presence of H2SO4 and TFA
7. Conclusions and future steps