Our Research
Many photochemical reactions in organic chemistry take place on an ultrafast timescale of femtoseconds and picoseconds, the natural timescale of nuclear motion. Examples are the initial steps of photosynthesis in plants, of the vision process in the eye or the photobiological synthesis of vitamin D. Such ultrafast reactions are enabled by conical intersections, degeneracies of electronic states in the reacting molecule. Such degeneracies provide photoexcited molecules efficient pathways to return to the potential energy surface of the ground state. Their positions on the ground state surfaces often give the relaxing photoexcited molecules access to minima on the ground state surface other than the reactant minimum, i.e. they enable photochemical reactions.
Quantum chemical treatment of conical intersections is notoriously difficult, since the most important approximation of quantum chemical methods, the Born-Oppenheimer approximation, is invalid due to correlated electronic and nuclear motion in their vicinity. To gain a comprehensive understanding of the role of conical intersections in ultrafast photochemical reaction mechanism and to follow the correlated motion of electrons and nuclei, we use experimental methods with complementary observables, selectively focusing either on electronic or nuclear motion. We use soft X-ray spectroscopy at the Linac Coherent Light Source (LCLS) to focus on electronic motion. To selectively follow structural dynamics of the nuclei in a molecule, we use hard X-ray and relativistic electron diffraction at LCLS and the MeV UED facility at SLAC. Furthermore, we closely work together with our colleagues from theory on the interpretation of our experimental findings. The following sections give a more detailed description of current projects within our group.
Mechanistic insight into electrocyclic photochemistry
Electrocyclic reactions have been subject of intense research since their discovery in the middle of the 20th century. Due to their stereo-specificity, they have become an important tool for organic synthesis. Moreover, they play an important role e.g. in the vitamin D production in human skin. Electrocyclic reactions can be initiated not only thermally but also photochemically, however, with inverted stereo-specificity. This observation triggered intense research and lead to the formulation of the famous Woodward-Hoffmann rules (Chemistry Nobel Prize 1981). These rules predict the stereo-specificity of both thermal and photochemical reactions based on simple orbital symmetry considerations and shaped the way chemists think about the quantum nature of chemical reactions. However, the Woodward-Hoffmann rules provide only limited insight into the mechanisms preserving the structural information during the evolution from a reactant into a reaction product structure. Novel time-resolved imaging methods like ultrafast electron and X-ray diffraction provide now the opportunity to gain insight into these mechanisms on the length scale of atomic bond distances and in real time. We are interested in the mechanistic details of electrocyclic ring-opening which we investigate in the prototypical molecule 1,3-cyclohexadiene and derivatives like α-phellandrene and α-terpinene.
In a recently published study of the ultrafast photochemical ring-opening of 1,3-cyclohexadiene, we used ultrafast electron diffraction to investigate how the nuclei rearrange during this reaction. We could follow the ring-opening in real time by observing transient changes of the atomic distances in the molecule. Furthermore, we observed a substantial acceleration of the ring-opening motion, when the molecule accessed the electronic ground state through a conical intersection. The ring-opening motion transforms into rotation of the ends of the carbon chain, which is generated as a reaction product. This rotation can be followed up to at least one picosecond.
In a follow-up study, we investigated an analog ring-opening reactions in the molecule α-phellandrene, a derivative of 1,3-cyclohexadiene. It is a natural flavor and produced by many plants. It exhibits an isopropyl substituent which is bonded to one of the two sp3 hybridized carbons directly involved in the bond dissociation. This substitution gives rise to two ground state conformers of the molecule with different orientations of the isopropyl group with respect to the six-membered ring. It can be either oriented quasi in the ring plane or perpendicular to the ring plane. The Woodward-Hoffmann rules predict different photoproducts for the two conformers. We could determine from the static electron diffraction signatures of the molecule, that our gas sample was dominated by only one of the two conformers. In our time-resolved diffraction study, we could for the first time image the evolution of a specific conformer into its Woodward-Hoffmann-predicted reaction product in real time and space.
Publications:
The role of nπ* states in ultrafast photophysics and photochemistry
When molecules absorb visible or UV light, this can in many cases be understood by excitation of a single electron of the molecule from its highest occupied molecular orbital (HOMO) with π character to its lowest unoccupied molecular orbital (LUMO) with similar character. Accordingly, the electronic character of the populated excited state is labeled as ππ*. Ultrafast dynamics involving ππ* states and the ground state can be simulated using state-of-the-art methods with high accuracy. However, organic molecules with biological relevance, e.g. the nucleobase (DNA building block) thymine, exhibit heteroatoms like oxygen or nitrogen. These atoms have lone pair (n) electrons, which give rise to low-lying exicted states with a different character, the excitation of a single electron from an n to a n empty π orbital (nπ*). Such states are typically optically dark, i.e. they have very low absorption cross-sections. However, they are known to be populated from ππ* states through conical intersections and play an important role for photochemistry as “gateway” states. For example, their electronic character allows for intersystem crossing to reactive triplet states on an ultrafast timescale. Ultrafast dynamics between ππ* and nπ* states and intersystem crossing processes still pose significant challenges for modern simulation methods. We are using time-resolved photoelectron and soft X-ray spectroscopy, in particular Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy with selective sensitivity to electronic structure changes, to achieve a better understanding of the role of nπ* excited states in ultrafast dynamics.
NEXAFS spectroscopy probes resonant transitions of 1s electrons to empty valence orbitals. Since 1s binding energies of different elements are several 10s to 100s of electronvolts apart in the soft X-ray spectral region, different elements in a molecule can be selectively interrogated by NEXAFS spectroscopy. The cross-sections of transitions between 1s and empty valence orbitals are strongly sensitive to the spatial overlap of the 1s and the valence orbital. Since the 1s orbitals are strongly localized at the element, NEXAFS spectroscopy is sensitive to the local valence orbital structure at the position of the interrogated element. With time-resolved NEXAFS spectroscopy, we can follow local valence electron structure changes. Since n-orbitals are much more localized around heteroatoms than π-orbitals, time-resolved NEXAFS spectroscopy is highly sensitive to e.g. the change of the valence electronic structure due to population of an nπ* state through a conical intersection with a ππ* state. For example, we could show with the help of time-resolved NEXAFS spectroscopy of the oxygen atoms that thymine undergoes internal conversion through a conical intersection between an electronic state with ππ* character to a state with nπ* character within 70 fs.