Capturing the generation and structural transformations of molecular ions.

Molecular ions are ubiquitous and play pivotal roles1-3 in many reactions, particularly in the context of atmospheric and interstellar chemistry4-6. However, their structures and conformational transitions7,8, particularly in the gas phase, are less explored than those of neutral molecules owing to experimental difficulties. A case in point is the halonium ions9-11, whose highly reactive nature and ring strain make them short-lived intermediates that are readily attacked even by weak nucleophiles and thus challenging to isolate or capture before they undergo further reaction. Here we show that mega-electronvolt ultrafast electron diffraction (MeV-UED)12-14, used in conjunction with resonance-enhanced multiphoton ionization, can monitor the formation of 1,3-dibromopropane (DBP) cations and their subsequent structural dynamics forming a halonium ion. We find that the DBP+ cation remains for a substantial duration of 3.6 ps in aptly named 'dark states' that are structurally indistinguishable from the DBP electronic ground state. The structural data, supported by surface-hopping simulations15 and ab initio calculations16, reveal that the cation subsequently decays to iso-DBP+, an unusual intermediate with a four-membered ring containing a loosely bound17,18 bromine atom, and eventually loses the bromine atom and forms a bromonium ion with a three-membered-ring structure19. We anticipate that the approach used here can also be applied to examine the structural dynamics of other molecular ions and thereby deepen our understanding of ion chemistry.

How the Interplay among Conformational Disorder, Solvation, Local, and Charge-Transfer Excitations Affects the Absorption Spectrum and Photoinduced Dynamics of Perylene Diimide Dimers: A Molecular Dynamics/Quantum Vibronic Approach.

In this contribution we present a mixed quantum-classical dynamical approach for the computation of vibronic absorption spectra of molecular aggregates and their nonadiabatic dynamics, taking into account the coupling between local excitations (LE) and charge-transfer (CT) states. The approach is based on an adiabatic (Ad) separation between the soft degrees of freedom (DoFs) of the system and the stiff vibrations, which are described by the quantum dynamics (QD) of wave packets (WPs) moving on the coupled potential energy surfaces (PESs) of the LE and CT states. These PESs are described with a linear vibronic coupling (LVC) Hamiltonian, parameterized by an overlap-based diabatization on the grounds of time-dependent density functional theory computations. The WPs time evolution is computed with the multiconfiguration time-dependent Hartree method, using effective modes defined through a hierarchical representation of the LVC Hamiltonian. The soft DoFs are sampled with classical molecular dynamics (MD), and the coupling between the slow and fast DoFs is included by recomputing the key parameters of the LVC Hamiltonians, specifically for each MD configuration. This method, named Ad-MD|gLVC, is applied to a perylene diimide (PDI) dimer in acetonitrile and water solutions, and it is shown to accurately reproduce the change in the vibronic features of the absorption spectrum upon aggregation. Moreover, the microscopic insight offered by the MD trajectories allows for a detailed understanding of the role played by the fluctuation of the aggregate structure on the shape of the vibronic spectrum and on the population of LE and CT states. The nonadiabatic QD predicts an extremely fast (∼50 fs) energy transfer between the two LEs. CT states have only a moderate effect on the absorption spectrum, despite the fact that after photoexcitation they are shown to acquire a fast and non-negligible population, highlighting their relevance in dictating the charge separation and transport in PDI-based optical devices.

Structure and Energetics of Dye-Sensitized NiO Interfaces in Water from Ab Initio MD and Large-Scale GW Calculations.

The energy-level alignment across solvated molecule/semiconductor interfaces is a crucial property for the correct functioning of dye-sensitized photoelectrodes, where, following the absorption of solar light, a cascade of interfacial hole/electron transfer processes has to efficiently take place. In light of the difficulty of performing X-ray photoelectron spectroscopy measurements at the molecule/solvent/metal-oxide interface, being able to accurately predict the level alignment by first-principles calculations on realistic structural models would represent an important step toward the optimization of the device. In this respect, dye/NiO surfaces, employed in p-type dye-sensitized solar cells, are undoubtedly challenging for ab initio methods and, also for this reason, much less investigated than the n-type dye/TiO2 counterpart. Here, we consider the C343-sensitized NiO surface in water and combine ab initio molecular dynamics (AIMD) simulations with GW (G0W0) calculations, performed along the MD trajectory to reliably describe the structure and energetics of the interface when explicit solvation and finite temperature effects are accounted for. We show that the differential perturbative correction on the NiO and molecule states obtained at the GW level is mandatory to recover the correct (physical) interfacial energetics, allowing hole transfer from the semiconductor valence band to the highest occupied molecular orbital (HOMO) of the dye. Moreover, the calculated average driving force quantitatively agrees with the experimental estimate.

Accounting for Vibronic Features through a Mixed Quantum-Classical Scheme: Structure, Dynamics, and Absorption Spectra of a Perylene Diimide Dye in Solution.

The optical absorption spectrum of a perylene diimide (PDI) dye in acetonitrile solution is simulated using the recently developed (J. Chem. Theory Comput. 2020, 16, 1215-1231) Ad-MD|gVH method. This mixed quantum-classical (MQC) approach is based on an adiabatic (Ad) separation of soft(classical)/stiff(quantum) nuclear degrees of freedom and expresses the spectrum as a conformational average (over the soft coordinates) of vibronic spectra (for the stiff coordinates) obtained through the generalized vertical Hessian (gVH) vibronic approach. The average is performed over snapshots extracted from classical molecular dynamics (MD) runs, performed with a specifically parameterized quantum-mechanically derived force field (QMD-FF). A comprehensive assessment of the reliability of different approaches, designed to reproduce spectral shapes of flexible molecules, is here presented. First, the differences in the sampled configurational space and their consequences on the prediction of the absorption spectra are evaluated by comparing the results obtained by means of the specific QMD-FF and of a general-purpose transferable FF with those of a reference ab initio MD (AIMD) in the gas phase, in both a purely classical scheme (ensemble average) and in the Ad-MD|gVH framework. Next, classical ensemble average and MQC predictions are also obtained for the PDI dynamics in solution and compared with the results of a ″static″ approach, based on vibronic calculations carried out on a single optimized perylene diimide structure. In the classical ensemble average approach, the remarkably different samplings obtained with the two FFs lead to sizeable changes in both position and intensity of the predicted spectra, with the one computed along the QMD-FF trajectory closely matching its AIMD counterpart. Conversely, at the Ad-MD|gVH level of theory, the different samplings deliver very similar vibronic spectra, indicating that the error found in the absorption spectra obtained with the general-purpose FF mainly concerns the stiff modes. In fact, it can be effectively corrected by the quadratic extrapolation performed by gVH to locate the minima of the ground- and excited-state potential energy surfaces along such coordinates. Furthermore, in the perspective of studying the self-assembling process of PDI dyes and the vibronic spectra of large-size aggregates, the use of a molecule-specific QMD-FF also appears mandatory, considering the significant errors found in the GAFF trajectory in the flexible lateral chain populations, which dictate the supramolecular aggregation properties.

Dynamical and Environmental Effects on the Optical Properties of an Heteroleptic Ru(II)-Polypyridine Complex: A Multilevel Approach Combining Accurate Ground and Excited State QM-Derived Force Fields, MD and TD-DFT.

An integrated multilevel approach is here built by combining classical molecular dynamic (MD) simulations, time-dependent density functional theory (TD-DFT) calculations, and solvation dynamics linear response (LR) analysis, and successively employed to investigate the optical properties and solvation structure of a prototypical heteroleptic Ru(II)-polypyridyl complex, widely employed in dye sensitized solar cells. The MD simulations are performed with an accurately parametrized intramolecular force field (FF), specifically derived from the quantum chemical (DFT) description of the molecule, both for its singlet and triplet ground states. Solvent effects, in ethanol (EtOH) and dimethyl sulfoxide (DMSO), are taken into account at different levels of approximation, going from a totally implicit description (polarizable continuum) to an hybrid explicit/implicit scheme. Our results show that the developed FFs were able to accurately describe and preserve the octahedral coordination of the Ru(II) center along the MD trajectories, yielding an accurate picture of the solute dynamics. Noticeably, the dynamical effects and the inclusion of an explicit microsolvation shell were found to be crucial to get a good agreement with the experimental absorption spectrum in EtOH, in both shape and positions of the main bands. The significant experimental blue-shift of the two low-energy bands in DMSO, that is not reproduced by the simulated thermal-averaged spectra, is, instead, attributed to deprotonation phenomena of the carboxylic groups, induced by the strong nucleophilic character of the solvent. Finally, analysis of the solvent response shows that the structural changes in the first solvation shell, following the metal-ligand to ligand charge transfer excitation, cause, in the protic medium the breakdown of the linear response approximation, which, on the contrary, holds for DMSO.