6.2
Outlook and Future Work
The work in this thesis has demonstrated not only the versatility of time-resolved ion yield and velocity map imaging techniques, but we have also explored the pro- found effect that even simple modifications to a molecule’s complexity can have on the ensuing excited state dynamics. Looking to the future there is huge potential for the techniques utilised in this thesis to be used in the study of larger and more biologically relevant systems. There are a number of avenues for future research that the group is currently exploring and a brief selection of possible investiga- tions is outlined below. Additionally, further collaboration between our group with other laboratories should result in extremely exciting and detailed insights into more complex molecular systems.
6.2.1 Vibrational Motion in Related Systems
In Chapters 4 and 5 we used time-resolved ion yield as an effective probe for vibra- tional motion following excitation to an electronic excited state. The varying ion- isation cross-section that we utilised as our sensitive probing method was afforded by a structural distortion in the excited state. In theory, provided a molecule ex- hibits an analogous geometry change upon photoexcitation (coupled with strong FC activity in the low-lying vibrational modes), vibrational motions can be inves- tigated in that system. This can hopefully lead to further details regarding how vibrational modes couple with one another in the electronic excited states of other important biological chromophores, which could provide invaluable insight into the relaxation mechanisms that are in operation.
6.2.2 Extension to the Solution Phase
Future experiments could extend our “bottom-up” approach towards an under- standing of the effect solvent has on the excited state dynamics of biological chro- mophores. There are two main approaches available: firstly direct collaboration with solution phase experimentalists allow comparative studies to be performed. In our laboratory, solution phase femtosecond experiments are now routinely per- formed using time-resolved electronic (UV/Vis) absorption spectroscopy. A fine example of a recent intra-group, dual-phase experiment can be seen in Reference 1.
Secondly, solvent effects can be measured in the gas phase, by performing clus- tering experiments. In experiments such as these, the molecule or ion of interest is clustered with a small number of solvent molecules (e.g. water, ammonia, methanol
etc.), allowing the effect of solvation to be investigated in a step-wise fashion (i.e.
small-to-large clusters). Another avenue of research would be to explore the ef- fect of clustering on the early time vibrational motions in catechol. These experi- ments have the potential to enable the observation of intermolecular energy transfer through the dampening of vibrational motions. Preliminary TR-IY measurements
Chapter 6 References
on the catechol dimer and catechol.(H2O)n wheren= 1 or 2, have indicated that even small clusters have a dramatic effect on the observed coherent wavepackets and the excited state lifetime.
6.2.3 Sunscreen Molecules
Despite the widespread use of sunscreens, very little is known regarding the pho- tochemistry/photophysics of the molecules incorporated in the many commercially available lotions. While they are designed to protect human skin from the harmful effects of the UV radiation from the Sun, the photochemistry of large biomolecules may result, for example, in the formation of free radicals responsible for DNA dam- age, and, consequently, cancer.2,3 Studying these compounds is essential to evalu- ating their safety and provides vital information that may lead to the development of more efficient sunscreens in the future.
Recent theoretical work by Karsili et al. suggests OH/OMe bond extensions, ring centred out-of-plane deformations and E/Z photoisomerism are all potentially operative internal conversion pathways for ferulic acid, a common sunscreen compo- nent.4 The1ππ∗ and1nπ∗ states of ferulic acid are found to be close in energy and possess multiple conical intersections, suggesting possible deactivation pathways ex- ist between these states. We have recently taken steps within the group (utilising solution- and gas-phase methodologies) to explore these deactivation mechanisms in a number of naturally occuring (plant) and commercial (synthesised) sunscreen molecules.5,6 The knowledge garnered by performing gas-phase measurements have thus far proven vital in the effort towards unravelling the complex solution phase dynamics.
References
1. Horbury, M. D., Baker, L. A., Quan, W.-D., Young, J. D., Staniforth, M., Greenough, S. E., and Stavros, V. G. J. Phys. Chem. A (2015).
2. Stavros, V. G. Nat. Chem.6(11), 955–956 (2014).
3. Sambandan, D. R. and Ratner, D. J. Am. Acad. Dermatol. 64(4), 748–758 (2011).
4. Karsili, T. N. V., Marchetti, B., Ashfold, M. N. R., and Domcke, W. J. Phys. Chem. A118(51), 11999–12010 (2014).
5. Baker, L. A., Horbury, M. D., Greenough, S. E., Ashfold, M. N., and Stavros, V. G. Photochem. Photobiol. Sci.14(10), 1814–1820 (2015).
6. Baker, L. A., Horbury, M. D., Greenough, S. E., Coulter, P. M., Karsili, T. N., Roberts, G. M., Orr-Ewing, A. J., Ashfold, M. N., and Stavros, V. G. J Phys. Chem. Lett.6(8), 1363–1368 (2015).
Appendix A
Appendix: Fit Functions
A.1
Fitting Parent and H atom Transients
The fitting of time resolved ion yield and velocity map imaging transients requires the use of multiple fitting functions; in particular a Gaussian distribution, an ex- ponential decay and an exponential rise. The latter two are convoluted with a Gaussian which represents our instrument response function (IRF). Where more than one exponential function (rise or decay) is required to accurately fit the data, combinations of the following functions can be utilised.
The Gaussian distribution in terms of pump probe delay (∆t) has the form:
g(∆t) =y0+A exp (t−t0)2 2σ2 (A.1) where y0 is the baseline offset, A is the magnitude, t0 is time zero (the centre of
the Gaussian) and σ is the peak width which can be related to the full width at half maximum (FWHM) by the following:
σ = FWHM
2√2ln2 (A.2)
The IRF convoluted exponential decay has the form:
f(∆t) =g(∆t)⊗A exp −(t−t0) τ (A.3) where A is the function’s magnitude, t is the offset from time zero, τ is the rise time and g(∆t) is the convoluted Gaussian, from Equation A.1, that models the IRF.
The closely related exponential rise, which is also convoluted with the IRF, has the form: f(∆t) =g(∆t)⊗A 1−exp −(t−t0) τ (A.4) where all paramaters are as described above.