The effect of increasing molecular complexity on the dynamics of biologically relevant chromophores: from dynamic competition to wavepacket evolution

Abstract

The work undertaken in this thesis focuses on the effect of increasing molecular complexity on the excited state dynamics in biologically relevant chromophores which have been isolated in the gas phase. The prototypical species phenol is used as a foundation upon which the remainder of the thesis is built. By sequentially adding functionality to the phenol archetype, we gradually develop an understanding of the effect such modifications can have on the excited state landscape, and hence the observed dynamics. The results obtained provide some important first steps towards understanding the excited state dynamics exhibited by larger, more complex, biologically relevant systems. The first part of this thesis presents H atom elimination dynamics from resorcinol (1,3-dihydroxybenzene) following excitation with ultraviolet light. This investigation utilises time-resolved velocity map imaging and ion yield techniques. Building on previous experiments on phenol-type species, this chapter provides detailed insight into the profound effect that a seemingly small modification can have on the excited state landscape in heteroaromatic, phenol-like systems. Increasing the excitation energy allows the competition between tunnelling and internal conversion to be observed; highlighting that as the absorbed energy increases, internal conversion quickly becomes the dominant relaxation pathway. The results observed in the latter part of the thesis elegantly highlight the sensitivity of time-resolved ion yield spectroscopy for the study of vibrational motion in important biological motifs. For all three species that we investigate: catechol (1,2-dihydroxybenzene), guaiacol (2-methoxyphenol) and syringol (2,6-dimethoxyphenol), exquisite insight into the early-time vibrational motions is garnered. This is achieved by virtue of the varying ionisation cross-section afforded by the dramatic geometry change following photoexcitation (and photoionisation). The highly complementary techniques of time-resolved ion yield and velocity map imaging utilised throughout this thesis provide unprecedented insight into the excited state dynamics of the target species, and by coupling with high level abinitio calculations, we are able to rationalise the dynamics of increasingly complex biologically relevant molecules