Photochemical reaction dynamics of organic chromophores in solution investigated with ultrafast transient absorption spectroscopy

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The photochemical dynamics of various organic molecules in solution are investigated using ultrafast transient electronic and vibrational absorption spectroscopy to explore the effects of molecular structure and solvent environment on the photochemical reaction pathways. First, the influence of ring strain on photoinduced ring opening of cyclic ketones is revealed by using different excitation energies to reach the first excited electronic singlet (S1) states of cyclobutanone, cyclopentanone and cyclohexanone. The fraction of the excited state population undergoing ultrafast ring opening in <1 ps increased with greater ring strain in the cyclic ketones and with higher excitation energies, both of which help to overcome the energy barrier to C-C bond breaking. Furthermore, the lifetimes of the S1 states were found to be shorter for smaller ring sizes. The excited-state dynamics of a photoexcited sunscreen molecule, diethylamino hydroxybenzoyl hexyl benzoate, were then studied in four different solvents. The competition between alternative relaxation pathways involving enol-keto tautomerization and central C-C bond twisting showed a clear solvent-dependence, with the former favoured in non-polar solvents, and the latter in polar and protic solvents. Moreover, the choice of solvent played a role in the ground-state recovery rate, and in the likelihood of populating the DHHB triplet states which could sensitise other molecules used in sunscreen formulations. Finally, progress is reported with an ongoing studying on phenyl cation generation from the photodissociation of chlorobenzene, motivated by new strategies for N2 activation photochemistry. The photoexcitation was performed in two solvents, cyclohexane and perfluorohexane. Transient absorption bands at wavelengths of 500 - 600 nm, prominent in the non-reactive perfluorohexane solvent, identified an intermediate species proposed to be a complex with charge-transfer character corresponding to a phenyl cation and a chloride anion. Future work will seek further evidence for the chemical nature of the intermediate from transient IR spectroscopy.
Date of Award28 Sep 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorAndrew J Orr-Ewing (Supervisor) & Michael N R Ashfold (Supervisor)

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