Molecular Electronic States Research Articles

Photophysical characterization of hydroxy and ethoxy phenalenone derivatives

Phenalenone has been extensively studied due to its photophysical behavior. However, fewer are the articles referring to the phenalenone derivatives as singlet oxygen generators. The incorporation of substituents on the phenalenone ring rearranges the molecular electronic states changing its photophysical behavior. In order to rationalize the effect of the presence of electron-donor substituents on the phenalenone ring, we studied two ethoxy derivatives and their corresponding hydroxyl precursors. All derivatives prepared present smaller singlet oxygen quantum yield values than phenalenone. These lower quantum yields can be rationalized by considering non-radiative decay of singlet excited state absent in unsubstituted phenalenone. The hydroxy and alkoxy derivatives substituted in position 6 of phenalenone have larger fluorescence quantum yields than the ones substituted in position 3. Interestingly, 3-hydroxy-phenalenone shows a low emission quantum yield with two emission bands, which can be related with equilibria between diketo and enol tautomers in the first excited singlet state. Flash photolysis spectra for all derivatives were measured and the extinction coefficients of triplet–triplet absorption were evaluated near 500nm.

Electronic structure and stability of the SiO2+ dications produced in tomographic atom probe experiments.

The molecular electronic states of the SiO2+ dication have been investigated in a joint theoretical and experimental analysis. The use of a tip-shaped sample for tomographic atom probe analysis offers the unique opportunity to produce and to analyze the lifetime of some excited states of this dication. The perturbation brought by the large electric field of the polarized tip along the ion trajectory is analyzed by means of molecular dynamics simulation. For the typical electric fields used in the experiment, the lowest energy triplet states spontaneously dissociate, while the lowest energy singlet states do not. We show that the emission process leads to the formation of some excited singlet state, which dissociates by means of spin-orbit coupling with lower-energy triplet states to produce specific patterns associated with Si+ + O+ and Si2+ + O dissociation channels. These patterns are recorded and observed experimentally in a correlated time-of-flight map.

Quantum Information Measures and Their Use in Chemistry

Background: The resultant measures of the information content in molecular electronic states are revisited. In these overall measures the current-related terms complement the probability functionals of classical Information Theory. The nonclassical Shannon entropy reflects the average magnitude of the state phase distribution, while the related current term in the complementary Fisher measure accounts for the average phase-gradient. Method: These generalized Fisher and Shannon descriptors of the information content in complex electronic states are applied to determine the phase-equilibria in molecules. The vertical and horizontal equilibria in molecules, which mark the extrema of the nonclassical and resultant entropy/information descriptors, respectively, are explored. The requirement, that the resultant measures have common solutions of the associated extreme entropy/information principles, calls for the negative sign of the nonclassical gradient measure. Conclusion: These extrema imply the phase-transformation of molecular electronic states. A separation of the density (modulus) and current (phase) factors in many-electron states is effected using the Harriman-Zumbach-Maschke construction of complex antisymmetric states yielding the specified electron density. A combined description of molecular systems, which accounts for the density and current degrees-of-freedom of electronic states is summarized and continuity equations for the state phase and resultant entropy are discussed. Keywords: Continuity equations, current-information, information continuity, molecular equilibria, phase-entropy/information, entropy.

Trichalcogenasumanene ortho-Quinones: Synthesis, Properties, and Transformation into Various Heteropolycycles.

The regioselective transformation of heterobuckybowl trichalcogenasumanenes 1 a,b at peripheral butoxy groups afforded trichalcogenasumanene ortho-quinones 2 a,b. Compounds 2 a,b are distinct from 1 a,b in terms of their molecular geometry and electronic state; that is, they have a shallower bowl depth and show absorbance in the NIR region. The reaction of 2 a,b with diamines resulted in a variety of heteropolycycles, including molecular spoon 3 a-6 a, planar π-systems 3 b-6 b, and highly twisted [7-6-6]-fused systems 7 a,b. These new heteropolycycles had different optical/electrical properties: 4 a,b showed hole mobility of approximately 0.002 cm2 V-1 s-1 , 6 a displayed red emission in both solution and the solid state, and 7 a,b formed tight stacks of the curved π-surface.

Trichalcogenasumanene ortho‐Quinones: Synthesis, Properties, and Transformation into Various Heteropolycycles

AbstractThe regioselective transformation of heterobuckybowl trichalcogenasumanenes 1 a,b at peripheral butoxy groups afforded trichalcogenasumanene ortho‐quinones 2 a,b. Compounds 2 a,b are distinct from 1 a,b in terms of their molecular geometry and electronic state; that is, they have a shallower bowl depth and show absorbance in the NIR region. The reaction of 2 a,b with diamines resulted in a variety of heteropolycycles, including molecular spoon 3 a–6 a, planar π‐systems 3 b–6 b, and highly twisted [7‐6‐6]‐fused systems 7 a,b. These new heteropolycycles had different optical/electrical properties: 4 a,b showed hole mobility of approximately 0.002 cm2 V−1 s−1, 6 a displayed red emission in both solution and the solid state, and 7 a,b formed tight stacks of the curved π‐surface.

Extended Space Charge Region and Unoccupied Molecular Band Formation in Epitaxial Tetrafluorotetracyanoquinodimethane Films

Generating well-defined molecular structures at inorganic/organic interfaces and within molecular films is fundamental for charge carrier transport and thus the performance of organic molecule-based (opto)electronic devices. Here we show by means of low-energy electron diffraction that tetrafluorotetracyanoquinodimethane (F4TCNQ) grows in an epitaxial fashion on the Au(111) surface, resulting in a unit cell which consists of one molecule. In this well-ordered crystalline films we found the formation of an extended space charge region and a dispersing unoccupied electronic molecular state using energy- and angle-resolved two-photon photoemission. The latter finding is a clear proof for band formation in the crystalline molecular structure. We suggest that the high electron affinity of F4TCNQ and a bandlike electron transport are responsible for the formation of the space charge region. Using F4TCNQ as a hole injection layer may open the opportunity to manipulate the hole injection barrier in a controlled w...

Dynamics of photoionization from molecular electronic wavepacket states in intense pulse laser fields: A nonadiabatic electron wavepacket study.

A theory for dynamics of molecular photoionization from nonadiabatic electron wavepackets driven by intense pulse lasers is proposed. Time evolution of photoelectron distribution is evaluated in terms of out-going electron flux (current of the probability density of electrons) that has kinetic energy high enough to recede from the molecular system. The relevant electron flux is in turn evaluated with the complex-valued electronic wavefunctions that are time evolved in nonadiabatic electron wavepacket dynamics in laser fields. To uniquely rebuild such wavefunctions with its electronic population being lost by ionization, we adopt the complex-valued natural orbitals emerging from the electron density as building blocks of the total wavefunction. The method has been implemented into a quantum chemistry code, which is based on configuration state mixing for polyatomic molecules. Some of the practical aspects needed for its application will be presented. As a first illustrative example, we show the results of hydrogen molecule and its isotope substitutes (HD and DD), which are photoionized by a two-cycle pulse laser. Photon emission spectrum associated with above threshold ionization is also shown. Another example is taken from photoionization dynamics from an excited state of a water molecule. Qualitatively significant effects of nonadiabatic interaction on the photoelectron spectrum are demonstrated.

Formation and stimulated photodissociation of metastable molecules with emission of photon at the collision of two atoms in a laser radiation field

The formation of metastable molecules (Feshbach resonances) at the collision of two atoms and subsequent stimulated transition to a lower unbound electronic molecular state, with emission of a photon of the laser radiation has been investigated. This can develop, in particular, for Rb2 molecules due to resonance scattering of two Rb atoms. This process is a basis for the creation of excimer lasers. Expressions have been obtained for the cross sections of elastic and inelastic resonance scattering and the intensity of the stimulated emission of the photons.

The effect of field modulation on the vibrational population of the photoassociated NaK and its dynamics**Project supported by the National Natural Science Foundation of China (Grant No. 11674198) and the Natural Science Foundations of Shandong Province, China (Grant Nos. ZR2014AM022 and ZR2015AL023). Partial financial support from Taishan Scholar Project of Shandong Province is also acknowledged.

This paper presents calculation results for the photoassociation of a NaK molecule with a two-color modulated laser and gives a detailed analysis about them. For the two-step photoassociation process in intense fields, the effect of two-color modulated laser parameters, such as relative phase, envelope period, and laser intensity, on the population of the molecular electronic state can be obtained by solving the time-dependent Schrödinger equation through the quantum wave packet method. The numerical simulation shows not only that the influence of laser parameters on the vibrational distribution presents some regularity, but also that a higher population in the ground electronic state can be realized through adjusting these laser parameters.

Changes in physical, chemical and functional properties of whey protein isolate (WPI) and sugar beet pectin (SBP) conjugates formed by controlled dry-heating

A Maillard type reaction in the dry state was utilized to create conjugates between whey protein isolate (WPI) and sugar beet pectin (SBP) to achieve improved functional properties including solubility, colloidal stability and oil-in-water emulsion stability. To optimize the reaction conditions, mixtures of WPI and SBP at varying weight ratios, 3:1, 2:1 and 1:1, were heated at 60 °C, 79% RH for 72 h. Changes in the physical properties, i.e. solubility, and chemical compositions, i.e. free sulfhydryl and amine contents were assessed using chemical assays. Bradford assay results demonstrated that the protein solubility of WPI increased significantly, ∼20% by reacting with lower levels of SBP (3:1 and 2:1) whereas the total powder solubility was not affected until an equal ratio of SBP was used, and the reduction was only ∼15%. The free sulfhydryl (SH) and primary (NH2) and secondary amine (NH) contents of WPI were also mitigated by conjugating with SBP. The formation of the covalent conjugates between WPI and SBP was confirmed by SDS-PAGE gel electrophoresis through staining for both proteins and glycoproteins. The UV–VIS spectroscopy showed significant changes in the molecular electronic transition states of both WPI and the feruloyl moieties of SBP upon interaction and conjugation. Measurements of ζ-potential, particles size distribution and average particle size demonstrated that the WPI-SBP conjugates prepared at the weight ratio of 3:1 were most effective at stabilizing oil-in-water emulsions than other ratios used in this work.

Coherent control of the formation of cold heteronuclear molecules by photoassociation

We consider the formation of cold diatomic molecules in the electronic ground state by photoassociation of atoms of dissimilar species. A combination of two transition pathways from the free colliding pair of atoms to a bound vibrational level of the electronic molecular ground state is envisioned. The first pathway consists of a pump-dump scheme with two time-delayed laser pulses in the near-infrared frequency domain. The pump pulse drives the transition to a bound vibrational level of an excited electronic state, while the dump pulse transfers the population to a bound vibrational level of the electronic ground state. The second pathway takes advantage of the existing permanent dipole moment and employs a single pulse in the far-infrared domain to drive the transition from the unbound atoms directly to a bound vibrational level in the electronic ground state. We show that this scheme offers the possibility to coherently control the photoassociation yield by manipulating the relative phase and timing of the pulses. The photoassociation mechanism is illustrated for the formation of cold LiCs molecules.

Hybrid-Basis Close-Coupling Interface to Quantum Chemistry Packages for the Treatment of Ionization Problems.

The theoretical description of observables in attosecond pump-probe experiments requires a good representation of the system's ionization continuum. For polyelectronic molecules, however, this is still a challenge, due to the complicated short-range structure of correlated electronic wave functions. Whereas quantum chemistry packages (QCP) implementing sophisticated methods to compute bound electronic molecular states are well-established, comparable tools for the continuum are not widely available yet. To tackle this problem, we have developed a new approach that, by means of a hybrid Gaussian-B-spline basis, interfaces existing QCPs with close-coupling scattering methods. To illustrate the viability of this approach, we report results for the multichannel ionization of the helium atom and of the hydrogen molecule that are in excellent agreement with existing accurate benchmarks. These findings, together with the versatility of QCPs to describe a broad range of chemical systems, indicate that this is a valid approach to study the ionization of polyelectronic systems in which correlation and exchange symmetry play a major role.

Stabilization of whey protein isolate (WPI) through interactions with sugar beet pectin (SBP) induced by controlled dry-heating

It has been shown previously that reacting whey protein isolate (WPI) with sugar beet pectin (SBP) through a Maillard type reaction in a dry-state led to noticeable improvement in the physical, chemical and functional properties of the end products. Studies on the changes in the molecular structure of whey protein as a result of the reaction were carried out in this work to provide a molecular understanding of these observed improvements. The effect of interacting and conjugating with SBP on the apparent secondary structure of WPI was examined by far-UV circular dichroism (CD) spectroscopy and followed by spectral deconvolution. Results showed that the structure of WPI, especially the antiparallel β-sheet element, was preserved by reacting with SBP at the weight ratios of WPI:SBP = 3:1, 2:1, and 1:1. The analysis of difference UV-VIS spectra indicated significant changes occurred in the molecular electronic states of both WPI and the feruloyl moieties of SBP. Initial Maillard reaction products were detected in the form of the FRET complexes with WPI as the donor, and SBP, the acceptor by the studies of the steady-state fluorescence and the lifetime of the excited states. The tertiary structural contacts in WPI, probed by the intrinsic Trp fluorescence, were disrupted significantly upon reacting with SBP at an equal ratio (1:1). The thermal stability of WPI, however, at both secondary and tertiary structural levels, was greatly improved by the dry-heating induced reaction with SBP at all levels. The thermal stability of the feruloyl groups of SBP was also enhanced by reacting with WPI. Furthermore, the fluorescence lifetime studies definitively identified the distinctive advanced Maillard reaction end products (AMREPs) formed between WPI and SBP at varying ratios.

Simulation of the photodetachment spectrum of HHfO- using coupled-cluster calculations.

The photodetachment spectrum of HHfO- was simulated using restricted-spin coupled-cluster single-double plus perturbative triple {RCCSD(T)} calculations performed on the ground electronic states of HHfO and HHfO-, employing basis sets of up to quintuple-zeta quality. The computed RCCSD(T) electron affinity of 1.67 ± 0.02 eV at the complete basis set limit, including Hf 5s25p6 core correlation and zero-point energy corrections, agrees well with the experimental value of 1.70 ± 0.05 eV from a recent photodetachment study [X. Li et al., J. Chem. Phys. 136, 154306 (2012)]. For the simulation, Franck-Condon factors were computed which included allowances for anharmonicity and Duschinsky rotation. Comparisons between simulated and experimental spectra confirm the assignments of the molecular carrier and electronic states involved but suggest that the experimental vibrational structure has suffered from poor signal-to-noise ratio. An alternative assignment of the vibrational structure to that suggested in the experimental work is presented.

ChemInform Abstract: Excited States Behavior of Nucleobases in Solution: Insights from Computational Studies

AbstractReview: 139 refs.

A spectroelectrochemical investigation of nanoparticle and molecular resonances in surface enhanced Raman scattering from crystal violet and malachite green

The interaction between nanoparticle plasmonic states and molecular electronic states is central to the large enhancements observed in surface‐enhanced Raman scattering (SERS). In this work, we use a model previously used in a fully quantum mechanical description of SERS to explain the enhancement of nontotally symmetric modes for crystal violet (CV) and malachite green (MG) adsorbed on silver nanoparticles. Our explanation is consistent with recent observations of the absorption spectra of dyes on silver nanoparticles that show the absence of charge‐transfer in these systems. Resonance with plasmon and molecular states explains the enhancement of nontotally symmetric modes, with enhancement dominated by the plasmon states. Spectroelectrochemical SERS studies using the same silver nanoparticles that are used to generate SERS in solution show increased SERS intensity for CV with positive potential up to +0.4 V (vs Ag/AgCl), consistent with a red‐shifted plasmon resonance, and decreasing intensity with more positive potentials, consistent with oxidation of the silver nanoparticles. Both crystal violet and malachite green show similar spectroelectrochemical behaviour with 514.5 nm excitation. Our spectroelectrochemical data show that SERS intensities from silver nanoparticles can be optimised with appropriately applied potentials. This work has highlighted the need for more detailed studies of the spectroelectrochemistry of dyes adsorbed on silver nanoparticles to more fully understand how SERS intensities respond to applied potentials. Copyright © 2016 John Wiley & Sons, Ltd.

A many-body states picture of electronic friction: The case of multiple orbitals and multiple electronic states.

We present a very general form of electronic friction as present when a molecule with multiple orbitals hybridizes with a metal electrode. To develop this picture of friction, we embed the quantum-classical Liouville equation (QCLE) within a classical master equation (CME). Thus, this article extends our previous work analyzing the case of one electronic level, as we may now treat the case of multiple levels and many electronic molecular states. We show that, in the adiabatic limit, where electron transitions are much faster than nuclear motion, the QCLE-CME reduces to a Fokker-Planck equation, such that nuclei feel an average force as well as friction and a random force-as caused by their interaction with the metallic electrons. Finally, we show numerically and analytically that our frictional results agree with other published results calculated using non-equilibrium Green's functions. Numerical recipes for solving this QCLE-CME will be provided in a subsequent paper.

Powder Structure Analysis of Vapochromic Quinolone Antibacterial Agent Crystals

Vapochromic materials, or those that show a reversible color change induced by vapor, are expected to serve as valuable sensors for volatile organic compounds or humidity. Crystals of pipemidic acid (PPA), a quinolone antibacterial agent, were found to exhibit vapochromism, as they undergo a reversible color change in the presence of acetonitrile vapor. The colorless trihydrate phase transformed into a yellow anhydrous phase upon exposure to acetonitrile vapor and returned to the trihydrate phase under high humidity. Ab initio structure determination from powder diffraction and solid state 13C NMR measurements revealed that the molecule exists in its zwitterionic form in the colorless trihydrate phase, whereas it is non-zwitterionic in the anhydrous phase because of the rearrangement of hydrogen bonds, due to dehydration in the crystal state. Theoretical calculations revealed that the color change in PPA is due to the change in the molecular electronic state upon taking the non-zwitterionic form, which ge...

The relationship between magneto-optical properties and molecular chirality

The chiral nonanuclear Tb(III) clusters [Tb9(sal-(R)-Bt)16(μ-OH)10]+[NO3]− (Tb-(R)-Bt: sal-(R)-Bt=(R)-2-butyl salicylate) and [Tb9(sal-(S)-Bt)16(μ-OH)10]+[NO3]− (Tb-(S)-Bt: sal-(S)-Bt=(S)-2-butyl salicylate) were found to exhibit a unique magneto-optical property: the Faraday effect. The clusters were composed of 9 Tb(III) ions bridged by 10 μ-OHs and 16 chiral salicylic acid esters. The Faraday rotation angle of Tb-(R)-Bt was greater than that of Tb-(S)-Bt, indicating that the Faraday effect was affected by the chirality of the Tb(III) clusters. The chiroptical properties of the Tb(III) clusters were estimated using circular dichroism and circularly polarized luminescence. In this study, a new finding concerning chiral magneto-optical properties was investigated. By pairing chiral organic ligands with rare-earth clusters, scientists have gained surprising insights into magneto-optical material design. Many optical telecommunication devices use the Faraday effect — a means of rotating the polarization of light using magnetic fields — to prevent unwanted light transmissions. Now, Yasuchika Hasegawa from Hokkaido University in Japan and co-workers have observed the first-ever correlation between the Faraday effect and molecular chirality. The Faraday effect is traditionally viewed as being insensitive to natural optical activity such as chirality. But the team's previous investigations, which showed organic ligands influenced promising Faraday materials known as terbium glasses, inspired them to challenge long-held assumptions. They synthesized nine-membered terbium clusters and complexed them with chiral salicylate ligands. Different ligand chirality categorically affected the Faraday rotation angles — an effect that may arise from terbium's large transition magnetic dipole. Naturally occurring and magnetically induced optical activities (the Faraday effect) have contributed to our understanding of molecular electronic states, and have also had various applications in photonics. It has been generally considered that the Faraday effect is not affected by natural optical activity originating from chirality. Herein we describe for the first time a relationship between the Faraday rotation angles and chirality in a chiral lanthanide cluster. This finding provides new insights into the design of next-generation molecular Faraday materials and may lead to the development of a novel area of study within the field of chiral science.

Molecular electronic states near metal surfaces at equilibrium using potential of mean force and numerical renormalization group methods: Hysteresis revisited.

We investigate equilibrium observables for molecules near metals by employing a potential of mean force (PMF) that takes level broadening into account. Through comparison with exact data, we demonstrate that this PMF approach performs quite well, even for cases where molecule-electrode couplings depend on nuclear position. As an application, we reexamine the possibility of hysteresis effects within the Anderson-Holstein model (i.e., an impurity coupled both to a metal surface and a nuclear oscillator). As compared against the standard mean field approach by Galperin et al. [Nano Lett. 5, 125 (2005)], our PMF approach agrees much better with exact results for average electronic populations both at zero and finite temperature; we find, however, that mean field theory can be very useful for predicting the onset of dynamical instabilities, metastable states, and hysteresis.