Exploring potential energy surfaces in ground- and excited states

Chemical reactivity of atoms, molecules and ions is governed by their underlying potential energy surface. Calculating the whole potential energy surface within reasonable bounds, is impossible for all but the smallest molecules. Usually, only parts of the full potential energy surface can be studied, namely stationary points and the minimum energy paths connecting them. By comparing energies of stationary points and their separating barriers, conclusions regarding possible reactions mechanism, or their infeasibility, can be drawn. Taking excited states into account leads to further complications, as now multiple potential energy surfaces have to be considered and root flips between different excited states may occur, requiring effective state-tracking. Part II of this thesis describes the required methods to locate stationary points and minimum energy paths on potential energy surfaces, by using surface-walking, chain-of-states optimization and intrinsic reaction coordinate integration. Several approaches to state-tracking are presented in chapter 4. Results of this thesis are presented in Part III, containing two contributions to the field of photochemistry: chapter 12 provides a possible excited-state reaction mechanism for a biaryl cross-coupling reaction and offers a plausible explanation for its high regioselectivity. The second contribution is the development pysisyphus (chapter 13), an external optimizer implemented in python, aware of excited states and thus the core of this thesis. By implementing the state-tracking algorithms outlined in chapter 4 it allows effective and efficient optimizations of stationary points in ground- and excited-states. The performance of pysisyphus is verified for several established benchmark sets. Results for several excited-state optimizations are presented in section 13.3, where pysisyphus shows good performance for the optimization of sizeable transition-metal complexes

Laser-induced electron diffraction of the ultrafast umbrella motion in ammonia

Visualizing molecular transformations in real-time requires a structural retrieval method with Ångström spatial and femtosecond temporal atomic resolution. Imaging of hydrogen-containing molecules additionally requires an imaging method that is sensitive to the atomic positions of hydrogen nuclei, with most methods possessing relatively low sensitivity to hydrogen scattering. Laser-induced electron diffraction (LIED) is a table top technique that can image ultrafast structural changes of gas-phase polyatomic molecules with sub-Ångström and femtosecond spatiotemporal resolution together with relatively high sensitivity to hydrogen scattering. Here, we image the umbrella motion of an isolated ammonia molecule (NH3) following its strong field ionization. Upon ionization of a neutral ammonia molecule, the ammonia cation (NH+3) undergoes an ultrafast geometrical transformation from a pyramidal (FHNH=107°) to planar (FHNH=120°) structure in approximately 8 femtoseconds. Using LIED, we retrieve a near-planar (FHNH=117±5°) field-dressed NH+3 molecular structure 7.8-9.8 femtoseconds after ionization. Our measured field-dressed NH+3 structure is in excellent agreement with our calculated equilibrium field dressed structure using quantum chemical ab initio calculations.J.B. and group acknowledge financial support from the European Research Council for ERC Advanced Grant “TRANSFORMER” (788218), ERC Proof of Concept Grant “miniX” (840010), FET-OPEN “PETACom” (829153), FET-OPEN “OPTOlogic” (899794), Laserlab- Europe (EU-H2020 654148), MINECO for Plan Nacional FIS2017-89536-P; AGAUR for 2017 SGR 1639, MINECO for “Severo Ochoa” (SEV- 2015-0522), Fundació Cellex Barcelona, CERCA Programme / Generalitat de Catalunya, and the Alexander von Humboldt Foundation for the Friedrich Wilhelm Bessel Prize. J.B., K.A. and R.Moszynski. acknowledge the Polish National Science Center within the project Symfonia, 2016/20/W/ST4/00314. J.B and B.B. acknowledge Severo Ochoa” (SEV- 2015-0522). J.B. and A.S. acknowledge funding from the Marie Sklodowska-Curie grant agreement No. 641272. C.D.L is supported in part by Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy under Grant No. DE-FG02-86ER13491. J.S. and S.G. highly acknowledges support from the European Research Council (ERC) for the ERC Consolidator Grant QUEM-CHEM (772676). The authors thank Alejandro Saenz for helpful discussions.Peer ReviewedPostprint (author's final draft

Imaging the Renner-Teller effect using laser-induced electron diffraction

Structural information on electronically excited neutral molecules can be indirectly retrieved, largely through pump-probe and rotational spectroscopy measurements with the aid of calculations. Here, we demonstrate the direct structural retrieval of neutral carbonyl disulfide (CS$_2$) in the B$^1$B$_2$ excited electronic state using laser-induced electron diffraction (LIED). We unambiguously identify the ultrafast symmetric stretching and bending of the field-dressed neutral CS$_2$ molecule with combined picometer and attosecond resolution using intrapulse pump-probe excitation and measurement. We invoke the Renner-Teller effect to populate the B$^1$B$_2$ excited state in neutral CS$_2$, leading to bending and stretching of the molecule. Our results demonstrate the sensitivity of LIED in retrieving the geometric structure of CS$_2$, which is known to appear as a two-center scatterer

Imaging an isolated water molecule with an attosecond electron wave packet

We use laser-induced electron diffraction (LIED) to self-image the molecular structure of an isolated water molecular ion using its own retuning attosecond electron wave packet (EWP). Using LIED’s sub-femtosecond and picometre spatio-temporal resolution imaging capabilities, we observe the symmetric stretching of the O-H and H-H internuclear distances with increasing laser field strength.Postprint (published version

Ultrafast imaging of the Renner-Teller effect in a field-dressed molecule

We present experimental results of linear-to-bent transition of field-dressed molecules, mediated by Renner-Teller effect. Using the state-of-the-art laser-induced electron diffraction (LIED) technique, we image a bent and symmetrically stretched carbon disulfide (CS2) molecule populating an excited electronic state under the influence of strong laser field. Our findings are well-supported by ab initio quantum mechanical calculations.Peer ReviewedPostprint (published version

Metal-Free Aryl Cross-Coupling Directed by Traceless Linkers

The metal-free, highly selective synthesis of biaryls poses a major challenge in organic synthesis. We report the scope and mechanism of a promising new approach to (hetero)biaryls by the photochemical fusion of aryl substituents tethered to a traceless linker (photosplicing). Interrogating photosplicing with varying reaction conditions and comparison of diverse synthetic probes (40 examples, including a suite of heterocycles) showed that the reaction has a surprisingly broad scope and involves neither metals nor radicals. Quantum chemical calculations revealed that the C–C bond is formed by an intramolecular photochemical process that involves an excited singlet state and the traverse of a five-membered transition state, thus warranting consistent ipso‑ipso‑coupling fidelity. These results demonstrate that photosplicing is a unique aryl cross-coupling method in the excited state that can be applied to synthesize a broad range of biaryls. </div

Imaging an isolated water molecule using a single electron wave packet

Observing changes in molecular structure requires atomic-scale Ångstrom and femtosecond spatio-temporal resolution. We use the Fourier transform (FT) variant of laser-induced electron diffraction (LIED), FT-LIED, to directly retrieve the molecular structure of H2O + with picometer and femtosecond resolution without a priori knowledge of the molecular structure nor the use of retrieval algorithms or ab initio calculations. We identify a symmetrically stretched H2O + field-dressed structure that is most likely in the ground electronic state. We subsequently study the nuclear response of an isolated water molecule to an external laser field at four different field strengths.We acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (MINECO), through the “Severo Ochoa” Programme for Centres of Excellence in R&D (Grant No. SEV-2015-0522) Fundació Cellex Barcelona and the CERCA Programme/Generalitat de Catalunya. X.L., K.A., T.S., A.S., B.B., M.S., and J.B. acknowledge the European Research Council (ERC) for ERC Advanced Grant TRANSFORMER (Grant No. 788218), MINECO for Plan Nacional Grant No. FIS2017-89536-P, AGAUR for 2017 Grant No. SGR1639, and Laserlab-Europe (EU-H2020 654148). K.A., J.B., and R. Moszynski acknowledge the Polish National Science Center within the project Symfonia, 2016/20/W/ST4/00314. X.L. and J.B. acknowledge financial support from China Scholarship Council. A.S. and J.B. acknowledge Marie Sklodowska-Curie Grant Agreement No. 641272. J.S. and S.G. acknowledge the ERC Consolidator Grant QUEMCHEM (Grant No. 772676). C.D.L. is supported by the U.S. Department of Energy under Grant No. DE-FG02-86ER13491.Postprint (published version

Excited-State Switching Frustrates the Tuning of Properties in Triphenylamine-Donor-Ligand Rhenium(I) and Platinum(II) Complexes

The photophysical properties of a series of rhenium(I) tricarbonyl and platinum(II) bis(acetylide) complexes containing a triphenylamine (TPA)-substituted 1,10-phenanthroline ligand have been examined. The complexes possess both metal-to-ligand charge-transfer (MLCT) and intraligand charge-transfer (ILCT) transitions that absorb in the visible region. The relative energies and ordering of the absorbing CT states have been successfully controlled by changing the metal center and modulating the donating ability of the TPA group through the addition of electron-donating methoxy and electron-withdrawing cyano groups. The ground-state properties behave in a predictable manner as a function of the TPA substituent and are characterized with a suite of techniques including electronic absorption spectroscopy, resonance Raman spectroscopy, electrochemistry, and time-dependent density functional theory calculations. However, systematic control over the ground-state properties of the complexes does not extend to their excited-state behavior. Unexpectedly, despite variation of both the MLCT and ILCT state energies, all of the luminescent complexes displayed near-isoenergetic emission at 298 K, yet the emissive lifetimes of the complexes vary from 290 ns to 3.9 μs. Excited-state techniques including transient absorption and transient resonance Raman, combined with a suite of quantum-chemical calculations, including scalar relativistic effects to elucidate competitive excited-state relaxation pathways, have been utilized to aid in assignment of the long-lived state in the complexes, which was shown to possess differing 3MLCT and 3ILCT contributions across the series

Light-responsive paper strips as CO-releasing material with a colourimetric response

Carbon monoxide (CO) is known for its multifaceted role in human physiology, and molecules that release CO in a controlled way have been proposed as therapeutic drugs. In this work, a light-responsive CO-releasing molecule (CORM-Dabsyl) showed a strong colourimetric response upon photochemical CO-release, owing to the tight conjugation of a Mn(i) tricarbonyl centre to a dabsyl chromophoric ligand (L). Whereas the complex was very stable in the dark in nitrogen-purged aqueous media, CO-release was effectively triggered using 405 nm irradiation. CORM-Dabsyl, L and the inactive product iCORM-Dabsyl have been investigated by DFT and TD-DFT calculations. Only mild toxicity of CORM-Dabsyl was observed against LX-2 and HepaRG® human cell lines (IC50 ∼ 30 μM). Finally, to develop a CO storage and release material that is readily applicable to therapeutic situations, CORM-Dabsyl was loaded on low-cost and easily disposable paper strips, from which the light triggered CO-release was conveniently visible with the naked eye

Excited-State Switching in Rhenium(I) Bipyridyl Complexes with Donor–Donor and Donor–Acceptor Substituents

The optical properties of two Re(CO)3(bpy)Cl complexes in which the bpy is substituted with two donor (triphenylamine, TPA, ReTPA2) as well as both donor (TPA) and acceptor (benzothiadiazole, BTD, ReTPA-BTD) groups are presented. For ReTPA2 the absorption spectra show intense intraligand charge-transfer (ILCT) bands at 460 nm with small solvatochromic behavior; for ReTPA-BTD the ILCT transitions are weaker. These transitions are assigned as TPA → bpy transitions as supported by resonance Raman data and TDDFT calculations. The excited-state spectroscopy shows the presence of two emissive states for both complexes. The intensity of these emission signals is modulated by solvent. Time-resolved infrared spectroscopy definitively assigns the excited states present in CH2Cl2 to be MLCT in nature, and in MeCN the excited states are ILCT in nature. DFT calculations indicated this switching with solvent is governed by access to states controlled by spin–orbit coupling, which is sufficiently different in the two solvents, allowing to select out each of the charge-transfer states