Density matrix-based time-dependent configuration interaction approach to ultrafast spin-flip dynamics

<p>Recent developments in attosecond spectroscopy yield access to the correlated motion of electrons on their intrinsic timescales. Spin-flip dynamics is usually considered in the context of valence electronic states, where spin–orbit coupling is weak and processes related to the electron spin are usually driven by nuclear motion. However, for core-excited states, where the core-hole has a nonzero angular momentum, spin–orbit coupling is strong enough to drive spin-flips on a much shorter timescale. Using density matrix-based time-dependent restricted active space configuration interaction including spin–orbit coupling, we address an unprecedentedly short spin-crossover for the example of L-edge (2p→3d) excited states of a prototypical Fe(II) complex. This process occurs on a timescale, which is faster than that of Auger decay (∼4 fs) treated here explicitly. Modest variations of carrier frequency and pulse duration can lead to substantial changes in the spin-state yield, suggesting its control by soft X-ray light.</p

Time dependent – density functional theory characterization of organic dyes for dye-sensitized solar cells

<p>We aim at providing better insight into the parameters that govern the intramolecular charge transfer (ICT) and photo-injection processes in dyes for dye-sensitised solar cells (DSSC). Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are utilized to study the geometry, electronic structure, electrostatic potential (ESP) and absorption spectrum, for a representative donor-π bridge-acceptor (D–π–A) dye for DSSC. The coplanar geometry of the dye (D1) facilitates strong conjugation and considerable delocalization originating the π CT interaction from donor to acceptor orbitals and the hyper-conjugative interactions involving Rydberg states. A model simulating the adsorption of the dye on the TiO₂ surface is utilized to estimate binding energies. The effect of fluorine substituents in the π-spacer on the quantum efficiency of DSSCs was investigated. Gibb’s free energy values, redox potentials, excited state lifetime, non-linear optical properties (NLO) and driving forces for D1 and its fluorinated derivatives were computed.</p

Protonation and deprotonation enthalpies of alloxan and implications for the structure and energy of its complexes with water: a computational study

<div><p>The optimized geometries, harmonic vibrational frequencies, and energies of the structures of monohydrated alloxan were computed at the DFT/ωB97X-D and B3LYP/6–311++G** level of theory. Results confirm that the monohydrate exists as a dipolar alloxan–water complex which represents a global minimum on the potential energy surface (PES). Trajectory dynamics simulations show that attempt to reorient this monohydrate, to a more favorable orientation for H-bonding, is opposed by an energy barrier of 25.07 kJ/mol. Alloxan seems to prefer acting as proton donor than proton acceptor. A marked stabilization due to the formation of N–H–OH₂ bond is observed. The concerted proton donor–acceptor interaction of alloxan with one H₂O molecule does not increase the stability of the alloxan–water complex. The proton affinity of the O and N atoms and the deprotonation enthalpy of the NH bond of alloxan are computed at the same level of theory. Results are compared with recent data on uracil, thymine, and cytosine. The intrinsic acidities and basicities of the four pyrimidines were discussed. Results of the present study reveal that alloxan is capable of forming stronger H-bonds and more stable cyclic complex with water; yet it is of much lower basicity than other pyrimidines.</p></div

Electronic structure and acid–base properties of Kojic acid and its dimers. A DFT and quantum topology study

<p>Kojic acid is a polyfunctional heterocyclic compound, with several important reaction centres; it has a wide range of applications in the cosmetic, medicine, food, agriculture and chemical industries. The present study aims at better insight into its electronic structure and bonding characteristics. Thus, density functional theory at the M06-2x /6-311++G^** level of theory is used to investigate its ground state electronic and acid–base properties. Protonation and deprotonation enthalpies are computed and analysed. The ability of Kojic acid to form both water complexes and dimers is explored. Several different complexes and dimer structures were examined. Natural bond order and quantum topology features of the charge density were analysed. The origin of the stability of the studied complexes and dimer structures can be traced to hydrogen bonding, π-conjugative and non-covalent dispersive interactions.</p

Magnetite Fe₃O₄ (111) Surfaces: Impact of Defects on Structure, Stability, and Electronic Properties

We present a comprehensive investigation, via first-principles density functional theory (DFT) calculations, of various surface terminations of magnetite, Fe₃O₄ (111), a major iron oxide that also has a number of applications in electronics and spintronics. We compare the thermodynamic stability and electronic structure among the different surfaces terminations. Interestingly, we find that surfaces modified with point defects and adatoms are close in surface energy and that they can be more stable than bulk-like terminations in the oxygen-rich and -poor regimes. These surfaces show different surface chemistry and electronic structures as well as distinctive spin polarization features near the Fermi level with regard to those previously considered in the literature. Our studies provide an atomic level insight for magnetite surfaces, which is a necessary step to understanding their interfaces with organic layers in OLEDs and spintronic devices

Nuclear Dynamical Correlation Effects in X‑ray Spectroscopy from a Theoretical Time-Domain Perspective

To date X-ray spectroscopy has become a routine tool that can reveal highly local and element-specific information on the electronic structure of atoms in complex environments. Here, we focus on nuclear dynamical correlation effects in X-ray spectra and develop a rigorous time-correlation function method employing ground state classical molecular dynamics simulations. The importance of nuclear correlation phenomena is demonstrated by comparison against the results from the conventional sampling approach performed on the same data set for gas phase water. In contrast to the first-order absorption, second-order resonant inelastic scattering spectra exhibit pronounced fingerprints of nuclear motions. The developed methodology is not biased to a particular electronic structure method and, owing to its generality, can be applied to, e.g., X-ray photoelectron and Auger spectroscopies

Influence of Molecular Shape on Solid-State Packing in Disordered PC₆₁BM and PC₇₁BM Fullerenes

Molecular and polymer packings in pure and mixed domains and at interfacial regions play an important role in the photoconversion processes occurring within bulk heterojunction organic solar cells (OSCs). Here, molecular dynamics simulations are used to investigate molecular packing in disordered (amorphous) phenyl-C₇₀–butyric acid–methyl ester (PC₇₁BM) and its C₆₀ analogue (PC₆₁BM), the two most widely used molecular-based electron-accepting materials in OSCs. The more ellipsoidal character of PC₇₁BM leads to different molecular packings and phase transitions when compared to the more spherical PC₆₁BM. Though electronic structure calculations indicate that the average intermolecular electronic couplings are comparable for the two systems, the electronic couplings as a function of orientation reveal important variations. Overall, this work highlights a series of intrinsic differences between PC₇₁BM and PC₆₁BM that should be considered for a detailed interpretation and modeling of the photoconversion process in OSCs where these materials are used

Nonadiabatic Dynamics of Cycloparaphenylenes with TD-DFTB Surface Hopping

We implemented a version of the decoherence-corrected fewest switches surface hopping based on linear-response time-dependent density functional tight binding (TD-DFTB), enhanced by transition density analysis. The method has been tested for the gas-phase relaxation dynamics of two cycloparaphenylene molecules, [8]­CPP and [10]­CPP, explaining some important features of their nonadiabatic dynamics, such as the origin of their long fluorescence lifetimes (related to the slow radiative emission from the S₁ state) and the trend of increasing the fluorescence rate with the molecular size (related to an increase in the S₁–S₀ energy gaps and oscillator strengths in the larger molecule). The quality of the TD-DFTB electronic structure information was assessed through four quantities: excitation energies; charge-transfer (CT) numbers, which estimate the charge transfer character of states; participation ratio (PR), which describes delocalization of electronic density; and participation ratio of natural transition orbitals (PRNTO), which describes the multiconfigurational character of states. These quantities were computed during dynamics and recomputed for the same geometries with the higher-level long-range-corrected TD-LC-DFTB and a lower-level single-determinant approximation for the excited states, SD-(LC)-DFTB. Taking TD-LC-DFTB as the standard, TD-DFTB underestimates the excitation energies by ∼0.5 eV and overestimates CT and PR. SD-DFTB underestimates excitation energies and overestimates CT to the same extent that TD-DFTB does, but it predicts reasonable PR distributions. SD-LC-DFTB leads to an extreme overestimation of the excitation energies by ∼3 eV, overestimates the charge transfer character of the state, but predicts the PR values very close to those obtained with TD-LC-DFTB

Evidence for the Formation of Pyrimidine Cations from the Sequential Reactions of Hydrogen Cyanide with the Acetylene Radical Cation

Herein, we report the first direct evidence for the formation of pyrimidine ion isomers by sequential reactions of HCN with the acetylene radical cation in the gas phase at ambient temperature using the mass-selected variable temperature and pressure ion mobility technique. The formation and structures of the pyrimidine ion isomers are theoretically predicted via coupled cluster and density functional theory calculations. This ion–molecule synthesis may indicate that pyrimidine is produced in the gas phase in space environments before being incorporated into condensed-phase ices and transformed into nucleic acid bases such as uracil