Valence one-electron and shake-up ionization bands of polycyclic aromatic hydrocarbons. II. Azulene, phenanthrene, pyrene, chrysene, triphenylene, and perylene

Spin polarized current in chiral organic radical monolayers

The chirality-induced spin selectivity (CISS) effect is the capability of chiral molecules to act as spin filters, i.e. to selectively sort flowing electrons based on their spin states. The application of this captivating phenomenon holds great promise in the realm of molecular spintronics, where the primary focus lies in advancing technologies based on chiral molecules to regulate the injection and coherence of spin-polarized currents. In this context, we conducted a study to explore the spin filtering capabilities of a monolayer of the thia-bridged triarylamine hetero[4]helicene radical cation chemisorbed on a metallic surface. Magnetic-conductive atomic force microscopy revealed efficient electron spin filtering at exceptionally low potentials. Furthermore, we constructed a spintronic device by incorporating a monolayer of these molecules in between two electrodes, obtaining an asymmetric magnetoresistance trend with signal inversion in accordance with the handedness of the enantiomer involved, indicative of the presence of the CISS effect. Our findings underscore the significance of thia[4]azahelicene organic radicals as promising candidates for the development of quantum information operations based on the CISS effect as a tool to control the molecular spin states.

Cyanocyclopentadiene-Annulated Polycyclic Aromatic Radical Anions: Predicted Negative Ion Photoelectron Spectra and Singlet-Triplet Energies of Cyanoindene and Cyanofluorene Radical Anions

Isomer-specific negative ion photoelectron spectra (NIPES) of cyanoindene (C9H7CN) and cyanofluorene (C14H9N), acquired through the computation of Franck-Condon (FC) factors that utilize harmonic vibrational frequencies and normal mode vectors derived from density functional theory (DFT) at the B3LYP/aug-cc-pVQZ and 6-311++G(2d,2p) basis sets, are reported. The adiabatic electron affinity (EA) values of the ground singlet (S0) and the lowest lying triplet (T1) states are used to predict site-specific S0-T1 energies (ΔEST). The vibrational spectra of the S0 and T1 states are typified by ring distortion and ring C-C stretching vibrational progressions. Among all the S0 isomers in C9H7CN, the 2-cyanoindene (2-C9H7CN) is found to be the most stable at an EA of 0.716 eV, with the least stable isomer being the 1-C9H7CN at an EA of 0.208 eV. In C14H9N, the most stable S0 isomer, 2-cyanofluorene (2-C14H9N), has an EA of 0.781 eV. The least stable S0 isomer in C14H9N is the 9-C14H9N, with an EA of 0.364 eV. The FC calculations are designed to mimic simulations that would be performed to aid in the analysis of experimental spectra obtained in NIPE spectroscopic techniques. The vibrational spectra, adiabatic EAs, and ΔEST values reported in this study are intended to act as a guide for future gas-phase ion spectroscopic experiments and astronomical searches, especially with regard to the hitherto largely unexplored C14H9N isomers.

Efficient Spin-Selective Electron Transport at Low Voltages of Thia-Bridged Triarylamine Hetero[4]helicenes Chemisorbed Monolayer

The Chirality Induced Spin Selectivity (CISS) effect describes the capability of chiral molecules to act as spin filters discriminating flowing electrons according to their spin state. Within molecular spintronics, efforts are focused on developing chiral-molecule-based technologies to control the injection and coherence of spin-polarized currents. Herein, for this purpose, we study spin selectivity properties of a monolayer of a thioalkyl derivative of a thia-bridged triarylamine hetero[4]helicene chemisorbed on a gold surface. A stacked device assembled by embedding a monolayer of these molecules between ferromagnetic and diamagnetic electrodes exhibits asymmetric magnetoresistance with inversion of the signal according to the handedness of molecules, in line with the presence of the CISS effect. In addition, magnetically conductive atomic force microscopy reveals efficient electron spin filtering even at unusually low potentials. Our results demonstrate that thia[4]heterohelicenes represent key candidates for the development of chiral spintronic devices.

A study of the valence photoelectron spectrum of uracil and mixed water-uracil clusters

The valence ionization of uracil and mixed water-uracil clusters has been studied experimentally and by ab initio calculations. In both measurements, the spectrum onset shows a red shift with respect to the uracil molecule, with the mixed cluster characterized by peculiar features unexplained by the sum of independent contributions of the water or uracil aggregation. To interpret and assign all the contributions, we performed a series of multi-level calculations, starting from an exploration of several cluster structures using automated conformer-search algorithms based on a tight-binding approach. Ionization energies have been assessed on smaller clusters via a comparison between accurate wavefunction-based approaches and cost-effective DFT-based simulations, the latter of which were applied to clusters up to 12 uracil and 36 water molecules. The results confirm that (i) the bottom-up approach based on a multilevel method [Mattioli et al. Phys. Chem. Chem. Phys. 23, 1859 (2021)] to the structure of neutral clusters of unknown experimental composition converges to precise structure-property relationships and (ii) the coexistence of pure and mixed clusters in the water-uracil samples. A natural bond orbital (NBO) analysis performed on a subset of clusters highlighted the special role of H-bonds in the formation of the aggregates. The NBO analysis yields second-order perturbative energy between the H-bond donor and acceptor orbitals correlated with the calculated ionization energies. This sheds light on the role of the oxygen lone-pairs of the uracil CO group in the formation of strong H-bonds, with a stronger directionality in mixed clusters, giving a quantitative explanation for the formation of core-shell structures.

Investigation of a Tetrathiafulvalene-Based Fe2+ Thermal Spin Crossover Assembled on Gold Surface

A thick film and a monolayer of tetrathiafulvalene-based Fe2+ spin-crossover complex have been deposited by solution on a Au (111) substrate, attempting both self-assembling monolayer protocol and a simpler drop-casting procedure. The thermally induced spin transition has been investigated using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). Temperature-dependent investigations demonstrated the retention of the switching behavior between the two spin states in thick molecular films obtained by drop-casting, while in the monolayer sample, the loss of the spin-crossover properties appears as a possible consequence of the strong interaction between the sulfur atoms of the ligand and the gold substrate.

Physically inspired deep learning of molecular excitations and photoemission spectra

Modern functional materials consist of large molecular building blocks with significant chemical complexity which limits spectroscopic property prediction with accurate first-principles methods. Consequently, a targeted design of materials with tailored optoelectronic properties by high-throughput screening is bound to fail without efficient methods to predict molecular excited-state properties across chemical space. In this work, we present a deep neural network that predicts charged quasiparticle excitations for large and complex organic molecules with a rich elemental diversity and a size well out of reach of accurate many body perturbation theory calculations. The model exploits the fundamental underlying physics of molecular resonances as eigenvalues of a latent Hamiltonian matrix and is thus able to accurately describe multiple resonances simultaneously. The performance of this model is demonstrated for a range of organic molecules across chemical composition space and configuration space. We further showcase the model capabilities by predicting photoemission spectra at the level of the GW approximation for previously unseen conjugated molecules.

Rationalization of photo-detachment spectra of the indenyl anion (C9H7-) from the perspective of vibronic coupling theory

The nuclear dynamics of the low-lying first four electronic states of the prototypical indenyl radical is investigated based on first principles calculations to rationalize the experimental vibronic structure of the radical. The study is performed following both time-dependent and time-independent quantum-chemistry approaches using a model diabatic Hamiltonian. The construction of model Hamiltonians is based on the fits of the adiabatic energies calculated from the electronic structure method. The analyses of the static and dynamics results of the present study corroborate the experimental findings regarding the shape of the spectrum, vibrational progressions and the lifetime of the excited state. Finally, the present theoretical investigations suggest that the electronic non-adiabatic effect is extremely important for a detailed study of the vibronic structure and the electronic relaxation mechanism of the low-lying electronic states of the indenyl radical.

Ultrafast electronic relaxations from the S3 state of pyrene

The ultrafast relaxation occurring in pyrene upon excitation at 4.68 eV was studied in a supersonic gas-jet fs pump–probe experiment.

Electronic Structure of Biotin Conformers Studied with SAC-CI and OVGF Methods

In this work, the study was performed with 37 gas-phase conformers of biotin and two biologically active conformers of biotin in the ligand-receptor complexes with astavidin and streptavidin. The ionization energies and photoelectron spectra of conformers were calculated by two methods: the general-R symmetry-adapted cluster-configuration interaction (general-R-SAC-CI) method and the outer-valence Green's function (OVGF) method. The photoelectron spectrum of each conformer was calculated using basis set D95 (df,pd) for both methods. The simulated photoelectron spectra of free molecules and bioactive conformers calculated by the two methods were compared. Natural bonding orbital (NBO) calculations were also performed for the assignment of ionization bands of each conformer. NBO calculation indicated that the first to five ionization bands correspond to ionizations from orbitals localized in the two rings. The most important point about the ionization of all conformers is that the removal of an electron from the σ-bonding orbital (C-S) takes place above 10.0 eV.

Benchmarking semiempirical, Hartree–Fock, DFT, and MP2 methods against the ionization energies and electron affinities of short- through long-chain [n]acenes and [n]phenacenes

Vertical and adiabatic ionization energies (IEs) and electron affinities (EAs) were calculated for the n = 1–10 [n]acenes using a wide range of semiempirical, Hartree–Fock, density functional, and second-order Moller–Plesset perturbation theory model chemistries. None of the model chemistries examined was able to accurately predict the IEs or EAs for both short- through long-chain [n]acenes, as well as for extrapolations to the polymeric limit, when compared to available experimental and benchmark theoretical data. Except for 6-31G(d), the choice of the basis set does not affect B3LYP results significantly. Analogous calculations using a suite of eight modern and (or) popular density functionals for the n = 1–10 [n]phenacenes revealed similar problems in estimating the IEs and EAs of these compounds, with the sole exception of the M062X functional for adiabatic IEs and potentially the APFD, B3LYP, and MN12SX functionals for adiabatic EAs. The poor IE/EA prediction performance for the parent [n]acenes and [n]phenacenes may extend to their substituted derivatives and heteroatom-substituted analogs. Consequently, caution should be exercised in the application of non-high-level calculations for estimating the IE/EA of these important classes of materials.

Theoretical Study on Molecules of Interstellar Interest. II. Radical Cation of Compact Polycyclic Aromatic Hydrocarbons

Radical cations of polycyclic aromatic hydrocarbons have been postulated to be molecular carriers of diffuse spectroscopic features observed in the interstellar medium. Several important observations made by stellar and laboratory spectroscopists motivated us to undertake a detailed theoretical study attempting to validate the recorded data. In continuation of our work on this subject, we here focus on a detailed theoretical study of the doublet ground (X̃) and low-lying excited (Ã, B̃, and C̃) electronic states of the radical cation of phenanthrene, pyrene, and acenaphthene molecule. A multistate and multimode theoretical model in a diabatic electronic basis is developed here through extensive ab initio quantum chemistry calculations. Employing this model, first-principles nuclear dynamics calculations are carried out to unravel the spectral assignment, time-dependent dynamics, and photostability of the mentioned electronic states of the radical cations. The theoretical results compare well with the observed experimental data.

Electronic properties of FC(O)SCH2CH3. a combined helium(I) photoelectron spectroscopy and synchrotron radiation study

The valence electronic properties of S-ethyl flouromethanethioate (S-ethyl fluoromethsanethioate), FC(O)SCH2CH3, were investigated by means of He(I) photoelectron spectroscopy in conjunction with the analysis of the photofragmentation products determined by PEPICO (phtoelectron-photoion-coincidence) by using synchrotron radiation in the 11.1-21.6 eV photon energy range. The first band observed at 10.28 eV in the HeI photoelectron spectrum can be assigned with confidence to the ionization process from the HOMO [nπ(S) orbital], which is described as a lone pair formally localized on the sulfur atom, in agreement with quantum chemical calculations using the outer valence Green function method [OVGF/6-311++G (d,p)]. One of the most important fragmentation channels also observed in the valence region corresponds to the decarbonylation process yielding the [M-CO](·+) ion, which is clearly observed at m/z = 80. Moreover, S 2p and S 2s absorption edges have been examined by measuring the total ion yield spectra in the 160-240 eV region using variable synchrotron radiation. The dynamic of ionic fragmentation following the Auger electronic decay has been evaluated with the help of the PEPIPICO (photoion-photoion-photoelectron-coincidence spectra) technique.

Electron momentum spectroscopy of 1-butene: a theoretical analysis using molecular dynamics and molecular quantum similarity

The results of experimental studies of the valence electronic structure of 1-butene by means of electron momentum spectroscopy (EMS) have been reinterpreted on the basis of molecular dynamical simulations in conjunction with the classical MM3 force field. The computed atomic trajectories demonstrate the importance of thermally induced nuclear dynamics in the electronic neutral ground state, in the form of significant deviations from stationary points on the potential energy surface and considerable variations of the C-C-C-C dihedral angle. These motions are found to have a considerable influence on the computed spectral bands and outer-valence electron momentum distributions. Euclidean distances between spherically averaged electron momentum densities confirm that thermally induced nuclear motions need to be fully taken into account for a consistent interpretation of the results of EMS experiments on conformationally flexible molecules.

Theoretical study on molecules of interstellar interest. I. Radical cation of noncompact polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs), in particular, their radical cation (PAH(+)), have long been postulated to be the important molecular species in connection with the spectroscopic observations in the interstellar medium. Motivated by numerous important observations by stellar as well as laboratory spectroscopists, we undertook detailed quantum mechanical studies of the structure and dynamics of electronically excited PAH(+) in an attempt to establish possible synergism with the recorded data. In this paper, we focus on the quantum chemistry and dynamics of the doublet ground (X̃) and low-lying excited (Ã, B̃, and C̃) electronic states of the radical cation of tetracene, pentacene, and hexacene molecule. This study is aimed to unravel photostability, spectroscopy, and time-dependent dynamics of their excited electronic states. In order to proceed with the theoretical investigations, we construct suitable multistate and multimode Hamiltonians for these systems with the aid of extensive ab initio calculations of their electronic energy surfaces. The diabatic coupling surfaces are derived from the calculated adiabatic electronic energies. First principles nuclear dynamics calculations are then carried out employing the constructed Hamiltonians and with the aid of time-independent and time-dependent quantum mechanical methods. The theoretical results obtained in this study are found to be in good accord with those recorded in experiments. The lifetime of excited electronic states is estimated from their time-dependent dynamics and compared with the available data.

Photoelectron and electron momentum spectroscopy of tetrahydrofuran from a molecular dynamical perspective

The results of experimental studies of the valence electronic structure of tetrahydrofuran employing He I photoelectron spectroscopy as well as Electron Momentum Spectroscopy (EMS) have been reinterpreted on the basis of Molecular Dynamical simulations employing the classical MM3 force field and large-scale quantum mechanical simulations employing Born-Oppenheimer Molecular Dynamics in conjunction with the dispersion corrected ωB97XD exchange-correlation functional. Analysis of the produced atomic trajectories demonstrates the importance of thermal deviations from the lowest energy path for pseudorotation, in the form of considerable variations of the ring-puckering amplitude. These deviations are found to have a significant influence on several outer-valence electron momentum distributions, as well as on the He I photoelectron spectrum.

The band 12 issue of norbornane: a study of higher shake-up states

In line with a recent study of the electronic structure of the cage compound norbornane (J. Chem. Phys. 121 (2004), 10525; J. Phys. Chem. A 109 (2005), 4267), symmetry adapted cluster expansion configuration interaction (SAC-CI) general R calculations have been performed and compared with results obtained by the third order algebraic diagrammatic construction scheme [ADC(3)]. Comparison has been made with previously performed electron momentum spectroscopy (EMS) and ultraviolet photo-electron measurements. The region around ~25 eV (band 12), characterized by an elaborated band in the EMS spectrum which is missing in previous Green's function and ADC calculations, is investigated. This study is completed with outer-valence Green's function (OVGF) and SAC-CI/SD-R calculations, and results are obtained by employing (single and double) ionization extended second order ADC [ADC(2)-x]. Since ADC(3) only includes 2h-1p shake-up states, while SAC-CI general-R also includes higher order states, the agreement between both methods assures that the higher order shake-up states do not play an important role in the ionization spectrum of norbornane. While the band-12 issue of norbornane is therefore still open for further discussion, a tentative description in terms of ultrafast nuclear dynamical effects and autoionization processes has become more plausible.

Outermost and inner-shell electronic properties of ClC(O)SCH2CH3 studied using HeI photoelectron spectroscopy and synchrotron radiation

A study of valence electronic properties of S-ethyl chlorothioformate (S-ethyl chloromethanethioate), ClC(O)SCH(2)CH(3), using HeI photoelectron spectra (PES) and synchrotron radiation is presented. Moreover, the photon impact excitation and dissociation dynamics of ClC(O)SCH(2)CH(3) excited at the S 2p and Cl 2p levels are elucidated by analyzing the total ion yield (TIY) spectra and time-of-flight mass spectra acquired in multicoincidence mode [photoelectron-photoion coincidence (PEPICO) and photoelectron-photoion-photoion coincidence (PEPIPICO)]. The HeI photoelectron spectrum is dominated by features associated with lone-pair electrons from the ClC(O)S- group, the HOMO at 9.84 eV being assigned to the n(π)(S) sulfur lone-pair orbital. Whereas the formation of C(2)H(5)(+) ion dominates the fragmentation in the valence energy region, the most abundant ion formed in both the S and Cl 2p energy ranges is C(2)H(3)(+). Comparison with related XC(O)SR (X = H, F, Cl and R = -CH(3), -C(2)H(5)) species reveals the impact of the alkyl chain on the photodissociation behavior of S-alkyl (halo)thioformates.

Density functional theory study on the electron spectra of naphthalene and azulene vapours

The ionization and excitation spectra of valence and core electrons of naphthalene and azulene in the gas phase are studied with density functional theory. The results are compared with available experimental data and previous calculations. New estimates are proposed for the ionization energies of both valence and core electrons and the calculated excitation energies are consistent with experiment.