Gianluca Levi

Solvation plays a key role in many photochemical and biological processes. Yet, direct atomic-scale observation of solvent reorganization around photoexcited molecules containing only light atoms has so far remained elusive. Here, we use time-resolved X-ray scattering at an X-ray free-electron laser to track with a ~ 120 fs time resolution the photoinduced ultrafast rearrangement of polar acetonitrile solvent molecules around an organic hemithioindigo chromophore. The experiments reveal that the solvation shell reorganizes with a ~ 0.3 ps time constant driven by photoinduced charge transfer in the chromophore, and then reequilibrates over ~ 3 ps as the excited state population returns to the ground state. These results demonstrate the viability of using ultrashort X-ray pulses to directly visualize the motion of light atoms in solution, paving the way for atomic-scale understanding and control of photoinduced processes in biological and synthetic photoactive organic molecules.

Rydberg excited states of molecules pose a challenge for electronic structure calculations because of their highly diffuse electron distribution. Even large and elaborate atomic basis sets tend to underrepresent the long-range tail, overly confining the Rydberg state. An approach is presented here where the molecular orbitals are variationally optimized for the excited state using a plane wave basis set in a Hartree-Fock calculation, followed by a configuration interaction calculation. The use of excited state optimized orbitals greatly enhances the convergence of the many-body calculation, as illustrated by a full configuration interaction calculation of the 2s Rydberg state of H₂. A neural-network-based selective configuration interaction approach is then applied to calculations of 3s and 3p states of H₂O and NH₃. The obtained values of excitation energy are in close agreement with experimental measurements as well as previous many-body calculations where sufficiently diffuse atomic basis sets were used. Calculations using atomic basis sets lacking extra diffuse functions, such as aug-cc-pVTZ, give significantly higher estimates due to confinement of the Rydberg states.

Variational optimization of orbitals in time-independent density functional calculations of excited electronic states presents a significant challenge, as excited states typically correspond to saddle points on the electronic energy landscape. The optimization can be particularly difficult if the excitation involves significant rearrangement of the electron density, as for charge-transfer excitations. A simple strategy for variational orbital optimization of excited states is presented. The approach involves minimizing the energy while freezing the orbitals directly involved in the excitation, followed by a fully unconstrained saddle-point optimization. Both steps of this freeze-and-release strategy are carried out using direct optimization algorithms with the same computational scaling as ground-state calculations. The performance of the method is extensively assessed in calculations of intramolecular and intermolecular charge-transfer excited states of organic molecules and molecular dimers using a generalized gradient approximation functional. It is found that the freeze-and-release direct optimization approach can avoid variational collapse to spurious, charge-delocalized solutions for cases where conventional algorithms based on the maximum overlap method fail. For intermolecular charge transfer, the orbital-optimized calculations are found to provide the correct dependency of the energy on the donor–acceptor separation without requiring long-range exact exchange, something common time-dependent density functional theory approaches fail to achieve.

With the start of the user program at the European XFEL in 2017, and more recently with the LCLS-II upgrade, the X-ray repetition rate at X-ray free-electron lasers (XFELs) has been pushed into the kilo- and megahertz regimes. These high X-ray repetition rates provide an increase in the integrated flux at these facilities by orders of magnitude, potentially facilitating measurements that were previously infeasible due to signal-to-noise constraints. However, the high repetition rates lead to new challenges for sample delivery and a shorter time for the sample to recover between X-ray pulses. For solution-phase techniques, the X-ray–sample interactions will strongly perturb or even vaporize the sample jet. Although the sample can be replenished, up to X-ray repetition rates of ∼100 kHz, by flowing the jet at high speeds, this does not completely mitigate the jet perturbations. In this work, we present a characterization of the jet perturbations induced by the high X-ray repetition rates at the European XFEL. We show how these can introduce background signals in time-resolved X-ray solution scattering data measured at the Femtosecond X-ray Experiments (FXE) instrument. We show that it is possible to mitigate these experimental artifacts by employing an alternating excitation scheme combined with careful background subtraction and that implementing this approach in the experimental design outperforms more simple background subtraction schemes. The methodology, the observations and analysis results are discussed in relation to the evolving landscape of XFEL sources.

Solvation plays a key role in photoinduced charge transfer, a fundamental process in many photochemical and biological systems. However, direct experimental observation of the atomic-scale solvent reorganization around an organic molecule following electronic excitation has so far remained elusive. We report time-resolved X-ray scattering measurements of a photoexcited organic hemithioindigo chromophore in the polar solvent acetonitrile, a system containing only light atoms. Exploiting an ~120 fs time resolution and the high X-ray photon flux offered by an X-ray free-electron laser, the experiments capture the ultrafast dipolar rearrangement of the solvent molecules surrounding the solute driven by photoinduced charge transfer. The transient solvation shell structure is found to be formed with a ~0.5 ps time constant and to reequilibrate over ~3 ps as the excited state population returns to the ground state. Transient absorption spectroscopy measurements and molecular dynamics simulations support the interpretation of the structural dynamics. These results demonstrate the viability of using ultrashort X-ray pulses to directly visualize the motion of light atoms in solution, paving the way for atomic-scale understanding and control of photoinduced processes in biological and synthetic photoactive organic molecules.

The elementary steps in the rotation of several second-generation molecular motors are analyzed by finding the minimum energy path between the metastable and stable states and evaluating the transition rate within harmonic transition state theory based on energetics obtained from density functional theory. Comparison with published experimental data shows remarkably good agreement and demonstrates the predictive capability of this approach. While previous measurements by Feringa and co-workers have shown that a replacement of the hydrogen atom at the stereogenic center by a fluorine atom can slow down the rate-limiting thermal helix inversion (THI) step by raising the energy of the transition state, even to the extent that the backreaction in the ground state becomes preferred in some cases, we find that a replacement of a CH₃ group by CF₃ at the same site accelerates the THI by elevating the energy of the metastable state without affecting the transition state significantly. Since these two fluorine substitutions have an opposite effect on the rate of the THI, the combination of both can provide ways to fine-tune the rotational speed of molecular motors.

By combining Hartree-Fock with a neural-network-supported quantum-cluster solver proposed recently in the context of solid-state lattice models, we formulate a scheme for selective neural-network configuration interaction (NNCI) calculations and implement it with various options for the type of basis set and boundary conditions. The method's performance is evaluated in studies of several small molecules as a step toward calculations of larger systems. In particular, the correlation energy in the N₂ molecule is compared with published full CI calculations that included nearly 10¹⁰ Slater determinants, and the results are reproduced with only 4 × 10⁵ determinants using NNCI. A clear advantage is seen from increasing the set of orbitals included rather than approaching full CI for a smaller set. The method's high efficiency and implementation in a condensed matter simulation software expands the applicability of CI calculations to a wider range of problems, even extended systems through an embedding approach.

Understanding the ultrafast vibrational relaxation following photoexcitation of molecules in a condensed phase is essential to predict the outcome and improve the efficiency of photoinduced molecular processes. Here, the vibrational decoherence and energy relaxation of a binuclear complex, [Pt2(P2O5H2)4]4− (PtPOP), upon electronic excitation in liquid water and acetonitrile are investigated through direct adiabatic dynamics simulations. A quantum mechanics/molecular mechanics (QM/MM) scheme is used where the excited state of the complex is modeled with orbital-optimized density functional calculations while solvent molecules are described using potential energy functions. The decoherence time of the Pt–Pt vibration dominating the photoinduced dynamics is found to be ∼1.6 ps in both solvents. This is in excellent agreement with experimental measurements in water, where intersystem crossing is slow (>10 ps). Pathways for the flow of excess energy are identified by monitoring the power of the solvent on vibrational modes. The latter are obtained as generalized normal modes from the velocity covariances, and the power is computed using QM/MM embedding forces. Excess vibrational energy is found to be predominantly released through short-range repulsive and attractive interactions between the ligand atoms and surrounding solvent molecules, whereas solute–solvent interactions involving the Pt atoms are less important. Since photoexcitation deposits most of the excess energy into Pt–Pt vibrations, energy dissipation to the solvent is inefficient. This study reveals the mechanism behind the exceptionally long vibrational coherence of the photoexcited PtPOP complex in solution and underscores the importance of short-range interactions for accurate simulations of vibrational energy relaxation of solvated molecules.