Guiglion, PFG; (2017) Modelling the photochemical properties of conjugated oligomers; understanding their application as photocatalysts. Doctoral thesis , UCL (University College London).

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Abstract

In this Ph.D. project, I introduce a new computational methodology, based on (timedependent) Density Functional Theory ((TD-)DFT), in order to determine if a molecule, here a conjugated oligomer, has the required photochemical properties to drive thermodynamically one or both water splitting half-reactions. This new approach takes electronic excitations into account rather than only relying on a static HOMO-LUMO description of the electronic structure, and therefore provides a more rigorous prediction of relevant thermodynamic potentials than ground-state DFT alone; it offers a relatively quick way of consistently screening for new photocatalysts for solar-driven water splitting. Using this computational framework, I investigate the optical properties of oligo(pphenylene), one of the simplest conjugated oligomers imaginable, as well as its thermodynamic potentials, relevant to the splitting of water into molecular hydrogen and oxygen. I then validate the methodology by confronting it to experimental data, before applying it to a wide range of conjugated oligomers, to determine whether or not they could be promising photocatalysts for water splitting, be it for the production of molecular hydrogen, oxygen gas, or both. In particular, I expose the reasons for the experimental lack of overall water splitting usually observed, and more particularly, the inability of many materials to oxidise water. Aside from purely photocatalytic considerations, I also discuss the optical properties of those oligomers and polymers, as they are tightly linked to their photocatalytic performance, with a particular emphasis on p-phenylene. I consistently study its three main isomers in order to shed some light into the relationship between their molecular structures and absorption/fluorescence spectra, and find the origin of the dramatic difference in the features exhibited by the latter, using a single computational approach, which, to the best of my knowledge, has never been done before.

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