Ultrafast time-resolved photoelectron spectra are reported for the vacuum-ultraviolet (VUV) photoionization of acetylene following excitation to the Ã1Au state via UV absorption at 200 nm. The excitation energy lies above the lowest dissociation threshold to C2H X̃2Σ+ + H, as well as above the threshold for adiabatic dissociation of the Ã1Au state to form C2H (Ã2Π) + H. The time-dependent mass spectra and photoelectron spectra provide insight into the intramolecular decay processes of the Ã1Au state. In addition, photoelectron spectra of the Ã1Au state with VUV light access both the X̃2Πu and Ã2Σg+ states of the ion, as well as the predicted, but previously unobserved, 1 2Πg state, which corresponds to a two-hole, one-particle configuration that lies in close proximity to the Ã2Σg+ state. The 1 2Πg state is split into 2A2 + 2B2 and 2Ag + 2Bg states in the cis and trans configurations, respectively. Electronic structure calculations, along with trajectory calculations, reproduce the principal features of the experimental data and confirm the assignment of the 1 2Πg state.

Theoretical investigation of a wide range of photochemical and photophysical phenomena triggered by light absorption requires the use of nonadiabatic molecular dynamics methods. Among these techniques, surface hopping dynamics has emerged as the most widely used approach for modeling photochemical processes in experimentally relevant molecular systems. The Landau-Zener surface hopping method, a simpler alternative to the well-established Tully's fewest switches surface hopping algorithm, presents a compelling option because it does not require the evaluation of nonadiabatic coupling vectors or time derivative couplings. In this study, we present an adaptive time step version of the LZSH algorithm, that enhances its stabilitywhile maintaining computational efficiency. We assessed its performance by applying it to several benchmark systems, including the one-dimensional Tully models and the fully-dimensional DMABN molecule.

The calculation of electronic excited states in extended multichromophoric systems is computationally challenging. Here, we accelerate our recently introduced excitonic configuration interaction (ECI) method [T. Piteša et al. J. Chem. Theory Comput. 2024, 20, 5609] with the resolution-of-identity approximation for the two-site two-electron integrals in the calculation of the interchromophoric Coulomb and exchange terms. Additionally, a simple overlap-based scheme is introduced to prescreen the Cholesky-transformed tensor of the three-centric two-electron interchromophoric exchange integrals, significantly accelerating the expensive tensor contraction for the two-site exchange term. This reduces both cost and memory requirements, enabling large-scale calculations of systems with many chromophores. We demonstrate its efficiency and accuracy by calculating electronic excited states of chains of up to 32 BODIPY chromophores and networks of up to 100 peri-xanthenoxanthene units, with 12 320 and 43 600 basis functions, respectively. We achieve errors in the excitation energies below 30 meV, using site states calculated with time-dependent density functional theory.

We present the excitonic configuration interaction (ECI) method — a fragment-based analogue of the CI method for electronic-structure calculations of the multichromophoric systems. It can also be viewed as a generalization of the exciton approach which (i) allows embedding via point charges with arbitrary values in the site-state calculation, (ii) includes multi-local excitation (MLE) products of site states in the excitonic basis, in addition to the ground-state (GS) and local excitation (LE) products, and (iii) takes into account all contributions to the full-system Hamiltonian matrix elements within the strong-orthogonality assumption. Regarding (i), we present the excitonic analogue of the Hartree-Fock method — called the EHF approach — which finds the embedding charges that minimize the energy of the GS product. In (ii), one can restrict the excitation rank of the employed excitonic basis, which results in truncated-CI-like expansions (ECIS includes GS and LE products, ECISD additionally includes two-fragment excitation, etc.). The expressions for the matrix elements in (iii) are obtained within McWeeny’s group function theory, generalized to accommodate the flexible embedding in (i). We assess the performance of ECI by computing absorption spectra of two multichromophoric systems. The first system, a metal-free guanine quadruplex, has the chromophores connected via hydrogen bonds (a supramolecular complex). The second system, a guanine quadruplex with a central Mg-cation, additionally exhibits metal–ligand bonds between some chromophores. It is shown that the accuracy of ECI strongly depends on the chosen embedding charges and ECI expansion. The most accurate combinations — ECIS or ECISD with EHF embedding — yielded spectra that qualitatively and quantitatively agree with full-system direct calculations, with RMSDs of the excitation energies around 20 meV or 100 meV, respectively, for the first and second test system. We also show that ECISD based on CIS site-state calculations can predict states of dominant MLE character that would be inaccessible in a full-system CIS calculation.

To investigate influences on the topicity of perfluorinated halobenzenes as halogen bond (XB) donors in the solid state, we have conducted a database survey and prepared 18 novel cocrystals of potentially ditopic (13ditfb, 14ditfb) and tritopic (135titfb) XB donors with 15 monotopic pyridines. 135titfb shows high tendency to be mono- or ditopic, but with strong bases it can act as a tritopic XB donor. DFT calculations have shown that binding of a single acceptor molecule on one of the iodine atoms of the XB donor reduces the ESPmax on the remaining iodine atoms and dramatically decreases their potential for forming further halogen bonds, which explains both the high occurrence of crystal structures where the donors do not achieve their maximal topicity and the observed differences in halogen bond lengths. Despite the fact that this effect increases with the basicity of the acceptor, when the increase of halogen bond energy due to the basicity of the acceptor compensates its decrease due to the reduction of the acidity of the donor, it enables strong bases to form cocrystals in which a potentially polytopic XB donor achieves its maximal topicity.

The photochemically induced ring-opening isomeriza-tion reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene is a textbook example of a pericyclic reaction and has been amply investigated with advanced spectroscopic techniques. The main open question has been the identification of the single reactive state which drives the process. The generally accepted description of the isomerization pathway starts with a valence excitation to the lowest lying bright state, followed by a passage through a conical intersection to the lowest lying doubly excited state, and finally a branching between either the return to the ground state of the cyclic molecule or the actual ring-opening reaction leading to the open-chain isomer. Here, in a joint experimental and computational effort, we demonstrate that the evolution of the excitation-deexcitation process is much more complex than that usually described. In particular, we show that an initially high-lying electronic state smoothly decreasing in energy along the reaction path plays a key role in the ring-opening reaction.

The photochemically induced ring-opening isomerization reaction of 1,3-cyclohexadiene (CHD) to 1,3,5-hexatriene (HT) is a textbook example of a pericyclic reaction, and has been amply investigated with advanced spectroscopic techniques. The generally accepted description of the isomerization pathway starts with a valence excitation to the lowest-lying bright state, followed by a passage through a conical intersection to a dark doubly excited state, and finally a branching between either the return to the ground state of the cyclic molecule or the actual ring-opening reaction leading to the open-chain isomer. It was traditionally assumed that the dark reactive state corresponds to the second excited state of CHD at the Franck-Condon geometry. Here in a joint experimental and computational effort we demonstrate that the evolution of the excitation-deexcitation process is much more complex than usually described. In particular, we show that an initially high-lying electronic state smoothly decreasing in energy along the reaction path plays a key role in the ring-opening reaction. The conceptual basis of our work is that the dynamics to consider here is determined by diabatic states, whose populations are the ones closely related to the observed photoelectron signal.

The crystal structure of lithium 2,5-dihydroxyquinonate dihydrate bis(dimethyl sulfoxide) solvate (Li(2)DHQ(H2O)(2)(DMSO)(2)) reveals a close contact between the lone pair on the electropositive S atom of DMSO and the dianionic quinoid ring. This contact, resembling a p-hole interaction involving a dianion as an acceptor, was studied in detail by a combination of X-ray charge density and quantum chemical modeling. The topology of electron density reveals a bond path and a (3, -1) critical point between the S atom of the DMSO and a C atom of the quinoid ring, while an analysis of the electrostatic potential indicates that the interaction between these two atoms is repulsive. However, the global electrostatic interaction between the DMSO and the dihydroxyquinonate moieties is attractive (-19.1 kJ/mol), as well as the total in vacuo interaction energy (-42.8 kJ/mol).

A computational protocol for simulating time-resolved photoelectron signals of medium-sized molecules is presented. The procedure is based on a trajectory surface-hopping description of the excited-state dynamics and a combined Dyson orbital and multicenter B-spline approach for the computation of cross sections and asymmetry parameters. The accuracy of the procedure has been illustrated for the case of ultrafast internal conversion of gas-phase pyrazine excited to the 1B2u(ππ*) state. The simulated spectra and the asymmetry map are compared to the experimental data, and a very good agreement was obtained without applying any energy-dependent rescaling or broadening. An interesting side result of this work is the finding that the signature of the 1Au(nπ*) state is indistinguishable from that of the 1B3u(nπ*) state in the time-resolved photoelectron spectrum. By locating four symmetrically equivalent minima on the lowest-excited (S1) adiabatic potential energy surface of pyrazine, we revealed the strong vibronic coupling of the 1Au(nπ*) and 1B3u(nπ*) states near the S1 ← S0 band origin.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.