Molecular Photofragmentation Dynamics in the Gas and Condensed Phases

Abstract

This is the submitted manuscript. The final published version (version of record) is available online via Annual Review of Physical Chemistry at http://www.annualreviews.org/doi/10.1146/annurev-physchem-052516-050756 . Please refer to any applicable terms of use of the publisher University of Bristol -Explore Bristol Research General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms 1 Molecular Photofragmentation Dynamics in the Gas and Condensed PhasesMichael N.R. Ashfold, Daniel Murdock and Thomas A.A. Oliver School of Chemistry, University of Bristol, Bristol, U.K., BS8 1TS AbstractExciting a molecule with a UV photon often leads to bond fission, but the final outcome of the bond cleavage is typically both molecule and phase dependent. The photodissociation of an isolated gasphase molecule can be viewed as closed system: energy and momentum are conserved, and the fragmentation is irreversible. The same is not true in a solution-phase photodissociation process.Solvent interactions may dissipate some of the photoexcitation energy prior to bond fission, and will dissipate any excess energy partitioned into the dissociation products. Products that have no analogue in the corresponding gas-phase study may arise by, for example, geminate recombination. Here we illustrate the extents to which dynamical insights from gas-phase studies can inform our understanding of the corresponding solution-phase photochemistry and how, in the specific case of photoinduced ring-opening reactions, solution-phase studies can in some cases reveal dynamical insights more clearly than the corresponding gas-phase study.Keywords photochemistry, photodissociation, photoinduced ring opening, non-adiabatic dynamics, conical intersections. 2 HISTORICAL CONTEXTCiamician is recognised as a pioneer of (organic) photochemistry and an early advocate for fuel production by means of artificial photo'synthesis'. 1 His studies were performed in solution and usually driven by sunlight, at a time when the concept of the photon was yet to be fully accepted.Identifying the ultimate reaction products was in itself a major achievement, and any ideas regarding reaction mechanism were rudimentary. Even the concatenation of ideas like ground and excited electronic states, radiative and non-radiative transitions, spin multiplicity, etc, into what would later be termed a Jablonski diagram 2 still lay far in the future.A potentially more fruitful path at that time for those curious about the mechanisms of photochemical reactions was to focus on very simple molecular systems -diatomic and 'diatomic-like' molecules. generally polychromatic) light sources were now available, and the development of grating spectrometers enabled the recording of electronic absorption spectra for many small gas-phase molecules. Vibrational and even rotational fine structure was resolved and interpreted in many such ...

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…The recent literature contains numerous articles, theoretical and experimental, that highlight the central role of excited states formed by electron promotion to an antibonding σ* orbital in facilitating bond fission. [6][7][8][9] Such states, formed by exciting an electron from an occupied lone pair (n) or bonding ( or σ) orbital, are now recognised as pivotal in discussions of the UV photofragmentation dynamics in many broad families of molecules. Exemplars include: water, alcohols, phenols, ethers and their thioanalogues; ammonia, amines, azoles, etc.…”

The Role of Rydberg–Valence Coupling in the Ultrafast Relaxation Dynamics of Acetone

“…The influence of CIs on different reaction channels is demonstrated, for example, in molecular photofragmentation, as observed by photofragment translational spectroscopy. 28 , 29 The yields of reaction products and their kinetic energies were found to depend, for example, on the nature of the photoexcited state, 3 , 30 , 31 the vibrational excitation, 32 − 35 or dynamic resonances. 36 , 37 …”

Recent advances in multidimensional ultrafast spectroscopy

“…Coupling broadband ultraviolet light with vibrational probes is one logical extension of 2DEV for the study of non-adiabatic dynamics in ultraviolet chromophores such as nucleobases, cytochromes, photoswitches and smaller polyenes that are more amenable to ab initio studies. In the majority of these systems, CIs near to the vertical Franck-Condon region are far more commonplace than in visible chromophores and almost always inevitably lead to strongly coupled electronic-nuclear motion that 2DEV spectroscopy is well placed to interrogate [128,139,204,205]. The necessary technological components already exist: ultrashort broadband UV sources are readily available [184,206], 2DES has been performed with ultraviolet laser pulses and thus the significant technological challenge of creating phase-locked UV pulse pairs has been surmounted [95,[207][208][209][210][211].…”