Biradicaloid Character of Thiophene‐Based Heterophenoquinones: The Role of Electron–Phonon Coupling

The quinoidal versus biradicaloid character of the ground state of a series of thiophene‐based heterophenoquinones is investigated with quantum‐chemical calculations. The role of the ground‐state electronic character on molecular structure and vibrational properties is emphasized. The vibrational activities are experimentally determined and their analysis is performed by taking advantage of the definition of a collective vibrational coordinate (the

     

Single‐Molecule Spectroscopy on a Ladder‐Type Conjugated Polymer: Electron–Phonon Coupling and Spectral Diffusion

We employ low‐temperature single‐molecule spectroscopy combined with pattern recognition techniques for data analysis on a methyl‐substituted ladder‐type poly(para‐phenylene) (MeLPPP) to investigate the electron–phonon coupling to low‐energy vibrational modes as well as the origin of the strong spectral diffusion processes observed for this conjugated polymer. The results indicate weak electron–phonon coupling to low‐frequency vibrations of the surrounding matrix of the chromophores, and that low‐energy intrachain vibrations of the conjugated backbone do not couple to the electronic transitions of MeLPPP at low temperatures. Furthermore, these findings suggest that the main line‐broadening mechanism of the zero‐phonon lines of MeLPPP is fast, unresolved spectral diffusion, which arises from conformational fluctuations of the side groups attached to the MeLPPP backbone as well as of the surrounding host material.     

Role of electron–electron coalescence density in density functional theory

An expression for the evaluation of electron–electron coalescence density as a functional of the density for any electron system is proposed. The formula, clarifies previously advanced upper bounds for this quantity and provides a method to independently estimate the system‐averaged on‐top exchange–correlation hole. The relationship with the on‐top pair density shows that producing the true electron–electron coalescense should be considered as a leading physical requirement for trial wave functions in any energy minimization scheme. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Soft Lattice and Defect Covalency Rationalize Tolerance of β‐CsPbI3 Perovskite Solar Cells to Native Defects

Although all‐inorganic metal halide perovskites (MHPs) have shown tremendous improvement, they are still inferior to the hybrid organic–inorganic MHPs in efficiency. Recently, a conceptually new β‐CsPbI₃ perovskite reached 18.4 % efficiency combined with good thermodynamic stability at ambient conditions. We use ab initio non‐adiabatic molecular dynamics to show that native point defects in β‐CsPbI₃ are generally benign for nonradiative charge recombination, regardless of whether they introduce shallow or deep trap states. These results indicate that MHPs do not follow the simple models used to explain defect‐mediated charge recombination in the conventional semiconductors. The strong tolerance is due to the softness of the perovskite lattice, which permits separation of electrons and holes upon defect formation, and only allows carriers to couple to the low‐frequency vibrations. Both factors decrease notably the non‐adiabatic coupling and slow down the dissipation of energy to heat.     

Electronic coupling in electron transfer and the influence of nuclear modes: theoretical and computational probes

Long-range electronic coupling of local donor and acceptor sites is formulated in the context of thermal and optical electron transfer and then illustrated with examples based on electronic structure calculations. The relationship of the calculated results to available experimental kinetic and optical data is discussed in detail. The influence of nuclear modes on the magnitude of the coupling (i.e., departures from the Condon approximation) is investigated in terms of both discrete molecular modes and solvent modes, and a general expression is presented for the modulation of the superexchange tunneling gap by motion along the electron transfer reaction coordinate. AcknowledgementsThe author is grateful to R.J. Cave and M. Rust for making available molecular coordinates for acridinium derivatives, and to R.J. Cave for several valuable discussions. This work was supported by the Division of Chemical Sciences, US Department of Energy, under grant DE-AC02-98CH10886.     

Universal Scaling of Intrinsic Resistivity in Two‐Dimensional Metallic Borophene

Two‐dimensional boron sheets (borophenes) have been successfully synthesized in experiments and are expected to exhibit intriguing transport properties. A comprehensive first‐principles study is reported of the intrinsic electrical resistivity of emerging borophene structures. The resistivity is highly dependent on different polymorphs and electron densities of borophene. Interestingly, a universal behavior of the intrinsic resistivity is well‐described using the Bloch–Grüneisen model. In contrast to graphene and conventional metals, the intrinsic resistivity of borophenes can be easily tuned by adjusting carrier densities, while the Bloch–Grüneisen temperature is nearly fixed at 100 K. This work suggests that monolayer boron can serve as intriguing platform for realizing tunable two‐dimensional electronic devices.     

Enhanced electron–phonon coupling at metal surfaces

Recent advances in experimental techniques and theoretical capabilities associated with the study of surfaces show promise for producing in unprecedented detail a picture of electron–phonon coupling. These investigations on surfaces of relatively simple metals can be the platform for understanding functionality in complex materials associated with the coupling between charge and the lattice. In this article, we present an introduction to electron–phonon coupling, especially in systems with reduced dimensionality, and the recent experimental and theoretical achievements. Then, we try to anticipate the exciting future created by advances in surface physics.     

Effects of electron–electron interaction on the band structure in polydiacetylenes

The ground‐state band structure of polydiacetylenes is theoretically studied with the extensional Su–Schriffer–Heeger model supplemented by electron–electron interactions. The results show the following. First, the interval of valence bands (conduction bands) increases because of the electron–electron interactions. Second, the effect of the on‐site Coulomb energy (U) is different from that of the nearest neighbor Coulomb repulsion (V); the competition between U and V shows that U makes the bandwidth narrower and the gap broader, whereas V makes the bandwidth broader and the gap narrower. There is a critical value of U/V. Third, the whole band width (E_w) decreases when the U/V ratio is less than 1.0 and increases when the U/V ratio is greater than 1.0 at V = 2.0 eV. Thus, the ground‐state band structure is sensitive to the U/V ratio. The results also show that electron–electron interactions can play an important role in the band structure of polydiacetylenes. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1656–1661, 2000     

Theoretical study of two‐dimensionally fused zinc porphyrins: DFT calculations

Electronic structures of D_4h square‐fused zinc porphyrin sheets of two types ( SA , SB ), where SA is a directly meso‐meso‐, β‐β‐, and β‐β‐linked array and SB is a directly β‐fused array, were compared using density functional theory (DFT). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of oligomeric SA _n are characteristically delocalized at the cyclooctatetraene‐like sites composed of β‐pyrrolic carbons and their nearest‐neighbor nitrogens. Those of oligomeric SB _n remain solitary monomeric features, reflecting weakly interacting porphyrin units. These two‐dimensionally (2D) square‐fused sheets, especially for SA _n, show effective reduction of both the HOMO–LUMO energy gaps (E_g) and the lowest Q‐like excitation energies because of LUMO's greater stabilization with increasing number of porphyrins than the corresponding one‐dimensionally (1D) linear‐fused tapes. To estimate the minimum value of E_g, the electronic band structures of the infinite‐fused SA _∞ and SB _∞ were examined in detail using modern periodic DFT. Results indicate a full metal for SA _∞, with HOMO and LUMO bands crossing the Fermi level, and a semiconductor with E_g ≈ 0.5 eV for SB _∞. Furthermore, the phonon modes and the electron–phonon coupling (EPC) constant of SA _∞ were calculated throughout the Brillouin zone using density functional perturbation theory (DFPT), yielding a weak EPC constant, λ = 0.35. Within the standard phonon‐mediated BCS mechanism, the superconducting transition temperature, T_c is demonstrated using the McMillan formula, predicting ≈0.5 K. Results show that SA _∞ will become a rare synthetic metal/superconductor without a metal‐insulator transition coming from Peierls lattice instability because it has no serious imaginary phonon modes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Nonadiabatic sudden increase of the cooperative kinetic effect at lattice energy stabilization—Microscopic mechanism of superconducting state transition: Model study of MgB2

The theory of the nonadiabatic electron–vibration interactions has been applied to the study of MgB₂ superconducting state transition. It has been shown that at nonadiabatic conditions in which the Born–Oppenheimer approximation is not valid and electronic motion is dependent not only on the nuclear coordinates but also on the nuclear momenta, the fermionic ground‐state energy of the studied system can be stabilized by nonadiabatic electron–phonon interactions at broken translation symmetry. Moreover, the new arising state is geometrically degenerate; i.e., there are an infinite number of different nuclear configurations with the same fermionic ground‐state energy. The model study of MgB₂ yields results that are in a good agreement with the experimental data. For distorted lattice, with 0.016 Å/atom of in‐plane out‐of‐phase B? B atoms displacements out of the equilibrium (E_2g phonon mode) when the nonadiabatic interactions are most effective, it has been calculated that the new arising state is 87 meV/unit cell more stable than the equilibrium–high symmetry clumped nuclear structure at the level of the Born–Oppenheimer approximation. The calculated T_c is 39.5 K. The resulting density of states exhibits two‐peak character, in full agreement with the tunneling spectra. The peaks are at ±4 meV, corresponding to the change of the π band density of states, and at ±7.6 meV, corresponding to the σ band. The superconducting state transition can be characterized as a nonadiabatic sudden increase of the cooperative kinetic effect at lattice energy stabilization (NASICKELES). © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Effect of magnetic exchange,double exchange,vibronic coupling,and asymmetry on magnetic properties in d2–d3 mixed‐valence dimers

The effect of magnetic exchange, double exchange, vibronic coupling, and asymmetry on magnetic properties of d²–d³ systems is discussed. The temperature‐dependent magnetic moment was calculated with the semiclassical adiabatic approach. The results show that the vibronic coupling from the out‐of‐phase breathing vibration on the metal sites (Piepho, Krausz, and Schatz [PKS] model) and the vibronic coupling from the stretching vibration between the metal sites (P model) favor the localization and delocalization of the “extra” electron in mixed‐valence dimers, respectively. The magnetic properties are determined by the interplay among magnetic exchange, double exchange, and vibronic coupling. The results obtained by analyzing d²–d³ systems can be generalized to other full delocalized dinuclear mixed valence systems with a unique transferable electron. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Influence of core–valence separation of electron localization function

The electron localization function (ELF) shows too-high values when computed from valence densities only (instead of using the total density). This effect is mainly found when d electrons are present in the outermost shell of the core. Although no pronounced qualitative differences could be noticed in the examples studied up to now, it is found that the quantitative differences between the values of ELF obtained from the valence densities only or from the total densities can be large. We also show, for the first time, an example (the Be atom) where ELF is obtained directly from the density. This exemplifies the possibility of computing ELF from highly accurate calculations (or from experimental data). © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1431–1439, 1997     

Spin–orbit coupling in oxygen containing diradicals

Intermediate diradicals which occur in the Paterno–Büchi photocycloaddition and in the Norrish type I photoreactions have been calculated taking into account the spin–orbit coupling (SOC) between the singlet (S) and triplet (T) states. Reaction paths for the photocycloaddition of formaldehyde to ethene and the diradical products of the α-cleavage of cyclohexanone have been optimized by the MNDO CI method for a number of different singlet and triplet states. SOC integrals are calculated by an effective one-electron approximation. Intermediate diradicals in the Paterno–Büchi reaction and the SOC effects are also studied ab initio with CAS SCF geometry optimization in a TZV basis set. Both methods predict a large SOC matrix element between the S and T states in the course of the C–C attack, while the SOC integral is two orders of magnitude smaller for the diradical produced in the C–O attack. In the Norrish type I photoreaction the oxygen atom also produces some nonzero contribution to the SOC integral which governs intersystem crossing in a ·C–C· diradical. For the diradicals produced by the α-cleavage of cyclohexanone a vibronic interaction is responsible for the SOC mixing between the lowest S and T states. The importance of one-center versus two-center SOC contributions in diradicals is briefly discussed.     

All‐electron LCAO calculations of the LiF crystal phonon spectrum: Influence of the basis set,the exchange‐correlation functional,and the supercell size

For the first time the convergence of the phonon frequencies and dispersion curves in terms of the supercell size is studied in ab initio frozen phonon calculations on LiF crystal. Helmann–Feynman forces over atomic displacements are found in all‐electron calculations with the localized atomic functions (LCAO) basis using CRYSTAL06 program. The Parlinski–Li–Kawazoe method and FROPHO program are used to calculate the dynamical matrix and phonon frequencies of the supercells. For fcc lattice, it is demonstrated that use of the full supercell space group (including the supercell inner translations) enables to reduce essentially the number of the displacements under consideration. For Hartree–Fock (HF), PBE and hybrid PBE0, B3LYP, and B3PW exchange‐correlation functionals the atomic basis set optimization is performed. The supercells up to 216 atoms (3 × 3 × 3 conventional unit cells) are considered. The phonon frequencies using the supercells of different size and shape are compared. For the commensurate with supercell k ‐points the best agreement of the theoretical results with the experimental data is found for B3PW exchange‐correlation functional calculations with the optimized basis set. The phonon frequencies at the most non‐commensurate k ‐points converged for the supercell consisting of 4 × 4 × 4 primitive cells and ensures the accuracy 1–2% in the thermodynamic properties calculated (the Helmholtz free energy, entropy, and heat capacity at the room temperature). © 2009 Wiley Periodicals, Inc. J Comput Chem 2009     

Pd(II) complexes of Schiff bases and their application as catalysts in Mizoroki–Heck and Suzuki–Miyaura cross‐coupling reactions

Schiff bases of 2‐(phenylthio)aniline, (C₆H₅)SC₆H₄N?CR (R = (o‐CH₃)(C₆H₅), (o‐OCH₃)(C₆H₅) or (o‐CF₃)(C₆H₅)), and their palladium complexes (PdLCl₂) were synthesized. The compounds were characterized using ¹H NMR and ¹³C NMR spectroscopy and micro analysis. Also, electrochemical properties of the ligands and Pd(II) complexes were investigated in dimethylformamide–LiClO₄ solution with cyclic and square wave voltammetry techniques. The Pd(II) complexes showed both reversible and quasi‐reversible processes in the ?1.5 to 0.3 V potential range. The synthesized Pd(II) complexes were evaluated as catalysts in Mizoroki–Heck and Suzuki–Miyaura cross‐coupling reactions. Copyright © 2015 John Wiley & Sons, Ltd.     

Monte Carlo simulation of full energy spectrum of electrons emitted from silicon in Auger electron spectroscopy

A Monte Carlo simulation including surface excitation, Auger electron‐ and secondary electron production has been performed to calculate the energy spectrum of electrons emitted from silicon in Auger electron spectroscopy (AES), covering the full energy range from the elastic peak down to the true‐secondary‐electron peak. The work aims to provide a more comprehensive understanding of the experimental AES spectrum by integrating the up‐to‐date knowledge of electron scattering and electronic excitation near the solid surface region. The Monte Carlo simulation model of beam–sample interaction includes the atomic ionization and relaxation for Auger electron production with Casnati's ionization cross section, surface plasmon excitation and bulk plasmon excitation as well as other bulk electronic excitation for inelastic scattering of electrons (including primary electrons, Auger electrons and secondary electrons) through a dielectric functional approach, cascade secondary electron production in electron inelastic scattering events, and electron elastic scattering with use of Mott's cross section. The simulated energy spectrum for Si sample describes very well the experimental AES EN(E) spectrum measured with a cylindrical mirror analyzer for primary energies ranging from 500 eV to 3000 eV. Surface excitation is found to affect strongly the loss peak shape and the intensities of the elastic peak and Auger peak, and weakly the low energy backscattering background, but it has less effect to high energy backscattering background and the Auger electron peak shape. Copyright © 2014 John Wiley & Sons, Ltd.     

Colloidal Synthesis and Charge‐Carrier Dynamics of Cs2AgSb1−yBiyX6 (X: Br,Cl; 0 ≤y ≤1) Double Perovskite Nanocrystals

A series of lead‐free double perovskite nanocrystals (NCs) Cs₂AgSb_1?yBi_yX₆ (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs₂AgSbBr₆ NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag–Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag–Bi or Ag–Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge‐carrier relaxation. The two fast trapping processes are dominated by intrinsic self‐trapping (ca. 1–2 ps) arising from giant exciton–phonon coupling and surface‐defect trapping (ca. 50–100 ps). Slow hot‐carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot‐carrier relaxation are also discussed.     

Short communication: Carbon–Silicon coupling between fluorenyl Grignard reagents and dichlorosilanes for synthesis of novel copolymers and chemical properties of the copolymers

A series of novel silicon‐containing fluorene polymers has been prepared by carbon–silicon (C–Si) coupling between fluorenyl Grignard reagents and dichlorosilanes, and their optical and thermal properties have been studied. Copyright © 2003 John Wiley & Sons, Ltd.