[No abstract available]

The electrochemical and charge storage properties of different lignins inside biopolymer electrodes were studied and correlated with the chemical variations of the lignins as indicated from the nuclear magnetic resonance (NMR) spectroscopic data. The varying fractions of monolignols were found to correlate with charge storage properties. It was found that as the sinapyl to guaiacyl (S/G) ratio increased both the specific capacitance and charge capacity increased considerably. This indicates that quinones generated on S-units can contribute more to charge storage in the biopolymer electrodes.

A semi-empirical Holstein-Peierls model is used to study the temperature effects on the polaron stability in organic semiconductors at a molecular scale. The approach takes into account both intra- and intermolecular electron-lattice interactions and is aimed at describing charge transport in the system. Particularly, we present a systematic numerical investigation to characterize the influence of both temperature and electric field on the stability as well as mobility of the polaron. It is found that the parameter space for which the polaron is dynamically stable is quite limited and the variations in some of these parameters strongly depend on the temperature. The electric field can play a role in further localizing the charge causing a compression of the lattice distortions associated with the polaron, increasing thereby its stability, up to a field strength of approximately 2.0 mV angstrom(-1). Considering field strengths higher than this critical value, the polaron is annihilated spreading charge through the lattice. Furthermore, we have studied the polaron mobility as a function of the anisotropy of the system, going from a one-dimensional system via a highly anisotropic two-dimensional system to a uniform two-dimensional system. There is a clearly observed mobility edge for the polaron; it exhibits a high mobility in the one-dimensional system but as the coupling in the second dimension is turned on the polaron slows down and becomes immobile in the uniform system. The results provided by this transport mechanism are in good agreement with experimental observations and may provide guidance to improve the charge transport in organic optoelectronic devices.

Excited-state aromaticity (ESA) and antiaromaticity (ESAA) are by now well-established concepts for explaining photophysical properties and photochemical reactivities of cyclic, conjugated molecules. However, their application is less straightforward than the corresponding process by which the thermal chemistry of such systems is rationalized in terms of ground-state aromaticity (GSA) and antiaromaticity (GSAA). Recognizing that the harmonic oscillator model of aromaticity (HOMA) provides an easy way to measure aromaticity on geometric grounds, it is therefore notable that this model is yet to be parameterized for excited states. Against this background, we here present a new parameterization of HOMA - termed HOMER - for the T-1 state of both carbocyclic and heterocyclic compounds based on high-level quantum-chemical calculations. Considering CC, CN, NN and CO bonds and testing the parametrization using calculated magnetic data as reference, we find that the description of ESA and ESAA by HOMER is superior to that afforded by the original HOMA scheme, and that it reaches the same overall quality as HOMA does for GSA and GSAA. Furthermore, we demonstrate that the derived HOMER parameters can be used for predictive modeling of ESA and ESAA at very different levels of theory. Altogether, the results highlight the potential of HOMER to facilitate future studies of ESA and ESAA.

Photochemical reactions enabling efficient transformation of aromatic systems into energetic but stable non-aromatic isomers have a long history in organic chemistry. One recently discovered reaction in this realm is that where derivatives of 1,2-azaborine, a compound isoelectronic with benzene in which two adjacent C atoms are replaced by B and N atoms, form the non-hexagon Dewar isomer. Here, we report quantum-chemical calculations that explain both why 1,2-azaborine is intrinsically more reactive toward Dewar formation than benzene, and how suitable substitutions at the B and N atoms are able to increase the corresponding quantum yield. We find that Dewar formation from 1,2-azaborine is favored by a pronounced driving force that benzene lacks, and that a large improvement in quantum yield arises when the reaction of substituted 1,2-azaborines proceeds without involvement of an intermediary ground-state species. Overall, we report new insights into making photochemical use of the Dewar isomers of aromatic compounds. Quantum-chemical calculations combined with molecular-dynamics simulations reveal mechanisms for improving the quantum yields by which aromatic compounds form their non-aromatic Dewar isomers, with potential implications in solar-energy storage.

The common approach to investigate the impact of aromaticity on excited-state proton transfer by probing the (anti)aromatic character of reactants and products alone is scrutinized by modelling such reactions involving 2-pyridone. Thereby, it is found that energy barriers can be strongly influenced by transient changes in aromaticity unaccounted for by this approach, particularly when the photoexcited state interacts with a second excited state. Overall, the modelling identifies a pronounced effect overlooked by most studies on this topic.

A catalytically active nanoassembly comprising Cu-nanoparticles grown on integrated and active supports (large pore Zr-doped mesoporous SBA-15 silica) has been synthesized and used to promote CO2 hydrogenation. The doped mesoporous material was synthesized using a sal-gel method, in which the pore size was tuned between 11 and 15 nm while maintaining a specific surface area of about 700 m(2) g (1). The subsequent Cu nanoparticle growth was achieved by an infiltration process involving attachment of different functional groups on the external and internal surfaces of the mesoporous structure such that 7-10 nm sized Cu nanoparticles grew preferentially inside the pores. Chemisorption showed improved absorption of both CO2 and H-2 for the assembly compared to pure SBA-15 and 15% of the total CO2 was converted to methanol and dimethyl ether at 250 degrees C and 33 bar.

Through a combination of density functional theory calculations and cluster-expansion formalism, the effect of the configuration of the transition metal atoms and spin-orbit coupling on the thermodynamic stability and electronic bandgap of monolayer 2H-Mo1-xWxS2 is investigated. Our investigation reveals that, in spite of exhibiting a weak ordering tendency of Mo and W atoms at 0 K, monolayer 2H-Mo1-xWxS2 is thermodynamically stable as a single-phase random solid solution across the entire composition range at temperatures higher than 45 K. The spin-orbit coupling effect, induced mainly by W atoms, is found to have a minimal impact on the mixing thermodynamics of Mo and W atoms in monolayer 2H-Mo1-xWxS2; however, it significantly induces change in the electronic bandgap of the monolayer solid solution. We find that the band-gap energies of ordered and disordered solid solutions of monolayer 2H-Mo1-xWxS2 do not follow Vegards law. In addition, the degree of the SOC-induced change in band-gap energy of monolayer 2H-Mo1-xWxS2 solid solutions not only depends on the Mo and W contents, but for a given alloy composition it is also affected by the configuration of the Mo and W atoms. This poses a challenge of fine-tuning the bandgap of monolayer 2H-Mo1-xWxS2 in practice just by varying the contents of Mo and W.

Driven by the fabrication of bulk and monolayer FeTe2 (ACS Nano, 2020, 14, 11473-11481), we explore the lattice, dynamic stability, electronic and magnetic properties of FeTeS and FeSeS Janus monolayers using density functional theory calculations. The obtained results validate the dynamic and thermal stability of the FeTeS and FeSeS Janus monolayers examined. The electronic structure shows that the FeTe2 bulk yields a total magnetization higher than the FeTe2 monolayer. FeTeS and FeSeS are categorized as ferromagnetic metals due to their bands crossing the Fermi level. So, they can be a good candidate material for spin filter applications. The biaxial compressive strain on the FeTe2 monolayer tunes the bandgap of the spin-down channel in the half-metal phase. By contrast, for FeTeS, the biaxial strain transforms the ferromagnetic metal into a half-metal. The electric field applied to the FeSeS monolayer in a parallel direction transforms the half-metal to a ferromagnetic metal by closing the gap in the spin-down channel.

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Recent exciting developments in synthesis and properties study of the Germanane (GeH) monolayer have inspired us to investigate the structural and electronic properties of the van der Waals GeH/Graphene (Gr) heterostructure by the first-principle approach. The stability of the GeH/Gr heterostructure is verified by calculating the phonon dispersion curves as well as by thermodynamic binding energy calculations. According to the band structure calculation, the GeH/Gr interface is n-type Ohmic. The effects of different interlayer distances and strains between the layers and the applied electric field on the interface have been investigated to gain insight into the van der Waals heterostructure modifications. An interlayer distance of 2.11 angstrom and compressive strain of 6% alter the contact from Ohmic to Schottky status, while the electric field can tune the GeH/Gr contact as p- or n-type, Ohmic, or Schottky. The average electrostatic potential of GeH/Gr and the Bader charge analysis have been used to explain the results obtained. Our theoretical study could provide a promising approach for improving the electronic performance of GeH/Gr-based nano-rectifiers.

The chemical basis of the blue-black to pink-orange color change on cooking of lobster, due to thermal denaturation of an astaxanthin-protein complex, alpha-crustacyanin, in the lobster carapace, has so far been elusive. Here, we investigate the relaxation of the astaxanthin pigment from its bound enolate form to its neutral hydroxyketone form, as origin of the spectral shift, by analyzing the response of UV-vis spectra of a water-soluble 3-hydroxy-4-oxo-beta-ionone model of astaxanthin to increases in pH, and by performing extensive quantum chemical calculations over a wide range of chemical conditions. The enolization of astaxanthin is consistent with the X-ray diffraction data of beta-crustacyanin (PDB code: 1GKA) whose crystals possess the distinct blue color. We find that enolate formation is possible within the protein environment and associated with a large bathochromic shift, thus offering a cogent explanation for the blue-black color and the response to thermal denaturation and revealing the chemistry of astaxanthin upon complex formation.

We present a case study about inkjet printing as a tool for molecular patterning of silicon oxide surfaces with hydrophobic functionality, mediated by n-octadecyltrichlorosilane (OTS) molecules. In contrast to state-of-the-art techniques such as micro contact printing or chemical immersion with subsequent lithography processes, piezo drop-on-demand inkjet printing does not depend on physical masters, which allows an effective direct-write patterning of rigid or flexible substrates and enables short run-lengths of the individual pattern. In this paper, we used mesithylene-based OTS inks, jetted them in droplets of 10 pL on a silicon oxide surface, evaluated the water contact angle of the patterned areas and fitted the results with Cassie's law. For inks of 2.0 mM OTS concentration, we found that effective area coverages of 38% can be obtained. Our results hence show that contact times of the order of hundred milliseconds are sufficient to form a pattern of regions with OTS molecules adsorbed to the surface, representing at least a fragmented, inhomogeneous self-assembled OTS monolayer (OTS-SAM).

Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H₃PO₄, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au₂O₃ or Au(OH)₃. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.

The ultrafast dynamics of unsubstituted spironaphthopyran (SNP) were investigated using femtosecond transient UV and visible absorption spectroscopy in three different solvents and by semi-classical nuclear dynamics simulations. The primary ring-opening of the pyran unit was found to occur in 300 fs yielding a non-planar intermediate in the first singlet excited state (S-1). Subsequent planarisation and relaxation to the product ground state proceed through barrier crossing on the S-1 potential energy surface (PES) and take place within 1.1 ps after excitation. Simulations show that more than 90% of the trajectories involving C-O bond elongation lead to the planar, open-ring product, while relaxation back to the S-0 of the closed-ring form is accompanied by C-N elongation. All ensuing spectral dynamics are ascribed to vibrational relaxation and thermalisation of the product with a time constant of 13 ps. The latter shows dependency on characteristics of the solvent with solvent relaxation kinetics playing a role.

A novel approach of identifying metal atoms within a metal-organic surface coordination network using scanning tunnelling microscopy (STM) is presented. The Cu adatoms coordinated in the porous surface network of 1,3,8,10-tetraazaperopyrene (TAPP) molecules on a Cu(111) surface give rise to a characteristic electronic resonance in STM experiments. Using density functional theory calculations, we provide strong evidence that this resonance is a fingerprint of the interaction between the molecules and the Cu adatoms. We also show that the bonding of the Cu adatoms to the organic exodentate ligands is characterised by both the mixing of the nitrogen lone-pair orbitals of TAPP with states on the Cu adatoms and the partial filling of the lowest unoccupied molecular orbital (LUMO) of the TAPP molecule. Furthermore, the key interactions determining the surface unit cell of the network are discussed.

trans-Stilbene was adsorbed in the cavities of H-ZSM-5, H-MOR and H-BEA zeolites, originating monomeric and dimeric radical cations that were studied by EPR spectroscopy. The H-BEA samples were pretreated at three progressively higher temperatures. Simulation of the spectra allowed the evaluation of the relative amounts of monomers and dimers. The nature, quantity and relative amount of monomeric and dimeric radical species were correlated to chemical and dimensional modifications of zeolite channels caused by the dealumination of the zeolite structure and extra-framework aluminium species formation.

Quantum chemical calculations aimed at identifying the factors controlling the acidity of phytochromobilin, the tetrapyrrole chromophore of the plant photoreceptor phytochrome, are reported. Phytochrome is converted from an inactive (Pr) to an active form (Pfr) through a series of events initiated by a Z  E photoisomerization of phytochromobilin, forming the Lumi-R intermediate, and much controversy exists as to whether the protonation state of the chromophore (cationic in Pr with all nitrogens protonated) changes during the photoactivation. Here, relative ground (S₀) and excited-state (S₁) pK_as of all four pyrrole moieties of phytochromobilin in all 64 possible configurations with respect to the three methine bridges are calculated in a protein-like environment, using a recently benchmarked level of theory. Accordingly, the relationships between acidity and chromophore geometry and charge distribution, hydrogen bonding, and light absorption are investigated in some detail, and discussed in terms of possible mechanisms making a proton transfer reaction more probable along the Pr  Pfr reaction than in the parent cationic Pr state. It is found that charge distribution in the cationic species, intra-molecular hydrogen bonding in the neutral, and hydrogen bonding with two highly conserved aspartate and histidine residues have a significant effect on the acidity, while overall chromophore geometry and electronic state are less important factors. Furthermore, based on the calculations, two processes that may facilitate a proton transfer by substantially lowering the pK_as relative to their Pr values are identified: (i) a thermal Z,anti  Z,syn isomerization at C5, occurring after formation of Lumi-R; (ii) a perturbation of the hydrogen bonding network which in Pr comprises the nitrogens of pyrroles A, B and C and the two aspartate and histidine residues.

We report the piezoelectric properties of ZnO nanowires (NWs) obtained by using a nanoindenter with a conductive boron-doped diamond tip. The direct piezoelectric effect was measured by performing nanoindentations under load control, and the generated piezoelectric voltage was characterized as a function of the applied loads in the range 0.2-6 mN. The converse piezoelectric effect was measured by applying a DC voltage to the sample while there was a low applied force to allow the tip being always in physical contact with the NWs. Vertically aligned ZnO NWs were grown on inexpensive, flexible, and disposable paper substrates using a template-free low temperature aqueous chemical growth method. When using the nanoindenter to measure the direct piezoelectric effect, piezopotential values of up to 26 mV were generated. Corresponding measurement of the converse piezoelectric effect gave an effective piezoelectric coefficient d(33)(eff) of similar to 9.2 pm V-1. The ZnO NWs were also characterized using scanning electron microscopy, X-ray diffraction, and high-resolution transmission electron microscopy. The new nanoindentation approach provides a straightforward method to characterize piezoelectric material deposited on flexible and disposable substrates for the next generation of nanodevices.

In the quest for finding novel thermodynamically stable, layered, MAB phases promising for synthesis, we herein explore the phase stability of ternary MAB phases by considering both orthorhombic and hexagonal crystal symmetries for various compositions (MAB, M₂AB₂, M₃AB₄, M₄AB₄, and M₄AB₆ where M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, and Co, A = Al, Ga, and In, and B is boron). The thermodynamic stability of seven previously synthesized MAB phases is confirmed, three additional phases are predicted to be stable, and 23 phases are found to be close to stable. Furthermore, the crystal symmetry preference for forming orthorhombic or hexagonal crystal structures is investigated where the considered Al-based MAB phases tend to favour orthorhombic structures whereas Ga- and In-based phases in general prefer hexagonal structures. The theoretically predicted stable MAB phases along with the structural preference is intended to both guide experimental efforts and to give an insight into the stability for different crystal symmetries of MAB phases.

A design strategy has been proposed to utilize structure-driven solution and solid-state fluorescence emission of polynitrogen atoms. The strategy uses benzimidazole as the electron donor and pyridine as the electron acceptor to construct D-A-type cyanopyridine ethylene molecules. Theoretical calculations reveal that compound 1 has energy-close isomers in dilute solutions, with planar conformation in S0 and S1 states, reducing molecular motion and thus enhancing radiation efficiency (quantum yield up to 42.7%). Conversely, the distorted cyanobenzene structure reduces the quenching effect of pi-pi stacking alignment, and hydrogen bonding between molecules limits molecular vibration and rotation, ultimately leading to strong emission in the solid state (quantum yield up to 27.4%). These dual-state luminescence systems have wide-ranging potential applications in information encryption and temperature sensors. Solution and solid-state fluorescence emission achieved through constructing D-A-type cyanopyridine ethylene molecules.

Ferroelectrics find broad applications, e.g. in non-volatile memories, but the switching kinetics in real, disordered, materials is still incompletely understood. Here, we develop an electrostatic model to study ferroelectric switching using 3D Monte Carlo simulations. We apply this model to the prototypical small molecular ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA) and find good agreement between the Monte Carlo simulations, experiments, and molecular dynamics studies. Since the model lacks any explicit steric effects, we conclude that these are of minor importance. While the material is shown to have a frustrated antiferroelectric ground state, it behaves as a normal ferroelectric under practical conditions due to the large energy barrier for switching that prevents the material from reaching its ground state after poling. We find that field-driven polarization reversal and spontaneous depolarization have orders of magnitude different switching kinetics. For the former, which determines the coercive field and is relevant for data writing, nucleation occurs at the electrodes, whereas for the latter, which governs data retention, nucleation occurs at disorder-induced defects. As a result, by reducing the disorder in the system, the polarization retention time can be increased dramatically while the coercive field remains unchanged.

Responsive monolayers are key building blocks for future applications in organic and molecular electronics in particular because they hold potential for tuning the physico-chemical properties of interfaces, including their energetics. Here we study a photochromic SAM based on a conjugated azobenzene derivative and its influence on the gold work function (Phi(Au)) when chemisorbed on its surface. In particular we show that the Phi(Au) can be modulated with external stimuli by controlling the azobenzene trans/cis isomerization process. This phenomenon is characterized experimentally by four different techniques, kelvin probe, kelvin probe force microscopy, electroabsorption spectroscopy and ultraviolet photoelectron spectroscopy. The use of different techniques implies exposing the SAM to different measurement conditions and different preparation methods, which, remarkably, do not alter the observed work function change (Phi(trans)-Phi(cis)). Theoretical calculations provided a complementary insight crucial to attain a deeper knowledge on the origin of the work function photo-modulation.

Vertical excitation energies and transition dipole moments between excited electronic states have been calculated for the trans-polyenes series C4H6-C12H14 in order to study the formation of excited state absorption spectra of these species. Quadratic response theory is applied in conjunction with the self-consistent field method and a hierarchical set of coupled-cluster methods. The convergence of the excited state absorption, with respect to wavefunction and treatment of electron correlation and also the length of the oligomer unit, is studied, revealing a considerable demand on the computational effort in order to predict the excited state spectra with precision. The organization of the excited states is found to change in character along the polyene series. The inflexion point for the vertical excitation energies between the one-photon allowed 1(1)B(u) and the two-photon 2(1)A(g) state is predicted to occur between hexatriene and octatetraene. Good agreement with experiment is obtained for butadiene and hexatriene for which the most accurate calculations have been carried out.

Excitation energies and transition dipole moments between excited electronic states have been calculated using various theoretical methods to investigate the ability to describe excited state absorption. Quadratic response theory is used in combination with self-consistent field, multi-configurational self-consistent field, and coupled-cluster electronic structure methods. The results of these different methods are compared. The set of molecules considered includes lithium hydride, carbon monoxide, formaldehyde, formamide, and sym-tetrazine. For some of the molecules results are also compared with the method of applying linear response theory to an excited state wavefunction separately optimized by means of the multi-configurational self-consistent field method.

A computational protocol for magneto-chiral dichroism and magneto-chiral birefringence dispersion is presented within the framework of damped response theory, also known as complex polarization propagator theory, at the level of time-dependent Hartree-Fock and time-dependent density functional theory. Magneto-chiral dichroism and magneto-chiral birefringence spectra in the (resonant) frequency region below the first ionization threshold of R-methyloxirane and L-alanine are presented and compared with the corresponding results obtained for both the electronic circular dichroism and the magnetic circular dichroism. The additional information content yielded by the magneto-chiral phenomena, as well as their potential experimental detectability for the selected species, is discussed.

We report on the phase stability of chemically ordered and disordered quaternary MAX phases - TiMAlC, TiM2AlC2, MTi2AlC2, and Ti2M2AlC3 where M = Zr, Hf (group IV), M = V, Nb, Ta (group V), and M = Cr, Mo, W (group VI). At 0 K, layered chemically ordered structures are predicted to be stable for M from groups V and VI. By taking into account the configurational entropy, an order-disorder temperature T-disorder can be estimated. TiM2AlC2 (M = Cr, Mo, W) and Ti2M2AlC3 (M = Mo, W) are found with Tdisorder 4 1773 K and are hence predicted to be ordered at the typical bulk synthesis temperature of 1773 K. Other ordered phases, even though metastable at elevated temperatures, may be synthesized by non-equilibrium methods such as thin film growth. Furthermore, phases predicted not to be stable in any form at 0 K can be stabilized at higher temperatures in a disordered form, being the case for group IV, for MTi2AlC2 (M = V, Cr, Mo), and for Ti2M2AlC3 (M = V, Ta). The stability of the layered ordered structures with M from group VI can primarily be explained by Ti breaking the energetically unfavorable stacking of M and C where M is surrounded by C in a face-centered cubic configuration, and by M having a larger electro-negativity than Al resulting in a fewer electrons available for populating antibonding Al-Al orbitals. The results show that these chemically ordered quaternary MAX phases allow for new elemental combinations in MAX phases, which can be used to add new properties to this family of atomic laminates and in turn prospects for tuning these properties.

The formation of silver-ethylene complexes in dehydrated Ag-SAPO-11 molecular sieve have been observed by electron paramagnetic resonance spectroscopy (EPR) after gamma-irradiation at 77 K. Such reactive intermediate can play an important role in catalytic conversion of ethylene on silver loaded molecular sieves. The Ag(C2H4)(2) stabilized inside relatively small channels of SAPO-11 molecular sieve it is observed even at 290 K. The 'gas-phase' geometries of Ag(C2H4) and Ag(C2H4)(2) complexes, and respective hyperfine coupling constants were calculated applying DFT quantum chemical methods. The hyperfine coupling constants calculated for Ag(C2H4)(2) complex of D-2h symmetry are in very good agreement with those obtained experimentally.

Excitons play a critical role in light emission when it comes to organic semiconductors. In high exciton concentration regimes, monomolecular and bimolecular routes for exciton recombination can yield different products affecting significantly the materials optical properties. Here, the dynamical decay of excitons is theoretically investigated using a kinetic Monte Carlo approach that addresses singlet exciton diffusion. Our numerical protocol includes two distinct exciton-exciton interaction channels: exciton annihilation and biexciton cascade emission. Our findings reveal that these channels produce different consequences concerning diffusion and spectroscopic properties, being able to explain diverging experimental observations. Importantly, we estimate critical exciton densities for which bimolecular recombination becomes dominant and investigate its effect on average exciton lifetimes and diffusion lengths.

Phytochromes are widespread photoreceptors responsive to red and far-red light that exist in two photochromic forms Pr (inactive) and Pfr (active). The Pr  Pfr conversion proceeds through a series of events initiated by Z  E photoisomerization of the tetrapyrrole chromophore, believed to occur at C15 of the methine bridge between rings C and D. Recent crystal structures show that ring D in Pr is less tightly packed by the protein than rings A, B and C, which should favor the C15 reaction over reactions at C4 (AB methine bridge) and C10 (BC). In the present work, quantum chemical methods are used to establish the intrinsic reactivity of the chromophore towards all three possible Z  E isomerization events in the absence of steric effects and specific interactions with the protein. Using a level of theory that reproduces spectroscopic data with anaccuracy of 0.2 eV, it is demonstrated that isolated conditions allow the C10 photoreaction to substantially dominate. This finding suggests that the different degrees of ring-packing observed inthe protein are crucial not only to facilitate a reaction at C15, but also to prevent an intrinsically more favorable reaction at C10 from taking place.

The C15-Z,syn  C15-E,anti isomerization of phytochromobilin that underlies the photoactivation of phytochrome, the plant photoreceptor responsive to red and far-red light, is investigated by means of quantum chemical methods. By calculating ground and excited-state potential energy surfaces for a phytochromobilin model comprising the full tetrapyrrolic skeleton, and taking into consideration rotations around the C14–C15 and C15=C16 bonds constituting the methine bridge between pyrrole rings C and D, it is found that a stepwise Z  E, syn  anti mechanism is energetically preferable over a concerted Z,syn  E,anti mechanism. In particular, on the basis of the calculated potential energy surfaces, it is proposed that the primary photochemical reaction involves a Z  E isomerization only, and that the subsequent syn  anti isomerization proceeds thermally.

The conformational dependence of the electronic absorption by astaxanthin is believed to be of relevance for the bathochromic shift that this carotenoid assumes upon binding to crustacyanin, the protein macromolecular complex responsible for the slate-blue colouration of lobster shell. Here, we report quantum chemical calculations suggesting that the bathochromic shift that can be attributed to changes in astaxanthin conformation brought about by binding to the protein in fact is rather small. In particular, by subjecting an exhaustive set of different astaxanthin conformations to time-dependent density functional theory (TD-DFT) and semiempirical configuration interaction singles (ZINDO/S) calculations, it is found that the bathochromic shift due to the protein-enforced coplanarity of the β-ionone rings with the polyene chain is considerably smaller (TD-DFT: 22–37 nm; ZINDO/S: 11–19 nm) than the >100 nm shift that recently [B. Durbeej and L. A. Eriksson, Chem. Phys. Lett., 2003, 375, 30] was predicted to arise from a hydrogen-bond mediated interaction involving one of the astaxanthin C4 keto groups and a histidine residue of the surrounding protein. Moreover, the calculations suggest that the protein-induced bowing of astaxanthin about the center of the polyene chain is of no relevance for the observed bathochromic shift.

We present an overview of excited state quantum chemical calculations aimed at elucidating controversial issues regarding the photochemistry of the protein-bound chromophores astaxanthin and phytochromobilin. In particular, we show how the application of time-dependent density functional theory and other single-reference quantum chemical excited state methods have contributed to shed new light on the origin of the >0.5 eV bathochromic shift of the electronic absorption by the carotenoid astaxanthin in the protein macromolecular complex crustacyanin, and the mechanism for C15-Z,syn  C15-E,anti isomerization of the tetrapyrrole phytochromobilin that underlies the photoactivation of the plant photoreceptor phytochrome. Within the approximation that exciton coupling is neglected, the calculations on astaxanthin provide support for the notion that the bathochromic shift, which is responsible for the slate-blue coloration of lobster shell, is due to polarization rather than a conformational change of the chromophore in the protein-bound state. Furthermore, the polarization is attributed to a hydrogen-bonded protonated histidine residue. The calculations on phytochromobilin, in turn, suggest that a stepwise C15-Z,syn  C15-E,syn (photochemical), C15-E,syn  C15-E,anti (thermal) mechanism is much more favorable than a concerted, fully photochemical mechanism, and that neutral forms of the chromophore are much less likely to photoisomerize than the parent, protonated form. Accordingly, the calculations indirectly support the view that the photoactivation of phytochrome does not involve a proton transfer from the chromophore to the surrounding protein.

Glutarimide single crystals X-irradiated at room temperature were reinvestigated at 150 K with the purpose to obtain information about possible ring opening and other fragmentation processes involving free radicals in this compound, previously only observed to take place in aqueous solutions. In previous work, using EPR,ENDOR and EIE spectroscopy, two H-abstraction radicals present in a 3:1 relative ratio in the irradiated crystals were identified. In the present work the detection of a third radical species, III, is reported. The g- and hyperfine coupling tensors for all three radicals at 150 K were obtained. Based on simulations of the EPR spectra the relative abundance of the three radicals was estimated to be 60, 25 and 15% for radicals I, II and III, respectively. Radical III is proposed to be of the allyl radical type -CH=CH-1CH- formed formally by a concerted H2 elimination from C2 and C3 of radical I and/or from C3 and C4 of radical II. This proposal and structure is at variance with observations from aqueous solution studies of succinimide, where the CO-CH2 bond was susceptible for rupture. However, the assignment is consistent with density functional theory (DFT) calculations, predicting equivalent hyperfine interactions to the two a-type protons at C2 and C4 in excellent. agreement with the experimentally determined hyperfine coupling tensors. EIE results confirmed that both couplings originate from the same radical species. Possible mechanisms for the formation of radical III are discussed.

The Rayleigh and hyper Rayleigh scattering properties of the binary (H2SO4)(H2O)(n) and ternary (H2SO4)(NH3)(H2O) n clusters are investigated using a quantum mechanical response theory approach. The molecular Rayleigh scattering intensities are expressed using the dipole polarizability alpha and hyperpolarizability beta tensors. Using density functional theory, we elucidate the effect of cluster morphology on the scattering properties using a combinatorial sampling approach. We find that the Rayleigh scattering intensity depends quadratically on the number of water molecules in the cluster and that a single ammonia molecule is able to induce a high anisotropy, which further increases the scattering intensity. The hyper Rayleigh scattering activities are found to be extremely low. This study presents the first attempt to map the scattering of atmospheric molecular clusters using a bottom-up approach.

The Rayleigh light scattering properties of (H2SO4)(a)(NH3)(b) and (H2SO4)(a)((CH3)(2)NH)(b) atmospheric molecular clusters have been investigated using a response theory approach. Using density functional theory the molecular structures and stepwise formation free energies of clusters with a and b up to 4 have been re-investigated. The Rayleigh scattering intensities are calculated from the dipole polarizability tensor a using the CAM-B3LYP functional by applying linear response methods. The intrinsic scattering properties of (H2SO4)(a)(NH3)(b) and (H2SO4)(a)((CH3)(2)NH)(b) indicate that amine containing clusters scatter light significantly more efficiently then their ammonia containing counterparts. Using the Atmospheric Cluster Dynamics Code (ACDC) the steady state cluster concentrations are estimated and the effective scattering is calculated. The effective scattering is shown to be highly dependent on the estimated concentrations and indicates that there exist competitive pathways, such as nucleation and coagulation, which influence the cluster distributions. The frequency dependence of the scattering is found to depend on the cluster composition and show increased responses when clusters contain more bases than acid molecules. Based on structures obtained using semi-empirical molecular dynamics simulations the Rayleigh scattering properties of clusters with up to 20 acid-base pairs are evaluated. This study represents the first step towards gaining a fundamental understanding of the scattering properties of small atmospheric clusters in the ambient atmosphere.

The structure and dynamics of the radical cations produced from benzene, monodeuterated benzene and toluene in various low-temperature matrices were characterized by EPR and ENDOR spectroscopy. It was found that the nature of the matrix had a dramatic effect on the EPR spectra of benzene cation. Rigid structures corresponding to the 2B(1g) and 2B(2g) states are revealed in solid argon and halocarbon (CFCl3) matrices, respectively, whereas only dynamically averaged patterns are observed in other hosts used (krypton, xenon, sulfur hexafluoride). Deuterium monosubstitution has no appreciable effect on the cation structure observed in argon and halocarbon matrices, which implies matrix control of the preferred electronic state. In contrast, the toluene radical cation exhibits only a 2B(2g)-like structure both in argon and in CFCl3 matrices, that is, the internal structural effect strongly predominates over environment effects in this case. The results are discussed in qualitative terms taking into consideration the matrix and substituent effects on the charge distribution in benzene cation.

The intrachain recombination dynamics between oppositely charged polarons is theoretically investigated through the use of a version of the Su-Schrieffer-Heeger (SSH) model modified to include an external electric field, an extended Hubbard model, Coulomb interactions, and temperature effects in the framework of a nonadiabatic evolution method. Our results indicate notable characteristics concerning the polaron recombination: (1) it is found that there exists a critical temperature regime, below which an exciton is formed directly and (2) a pristine lattice is the resulting product of the recombination process, if the temperature is higher than the critical value. Additionally, it is found that the critical electric field regime plays the role of drastically modifying the system dynamics. These facts suggest that thermal effects in the intrachain recombination of polarons are crucial for the understanding of electroluminescence in optoelectronic devices, such as Polymer Light Emitting Diodes.

X-ray absorption spectra of carbon, silicon, germanium, and sulfur compounds have been investigated by means of damped four-component density functional response theory. It is demonstrated that a reliable description of relativistic effects is obtained at both K- and L-edges. Notably, an excellent agreement with experimental results is obtained for L_2,3-spectra—with spin-orbit effects well accounted for—also in cases when the experimental intensity ratio deviate from the statistical one of 1:2. The theoretical results are consistent with calculations using standard response theory as well as recently reported real-time propagation methods in time-dependent density functional theory, and the virtues of different approaches are discussed. As compared to silane and silicon tetrachloride, an anomalous error in the absolute energy is reported for the L_2,3-spectrum of silicon tetrafluoride, amounting to an additional spectral shift of ~ 1 eV. This anomaly is observed also for other exchange-correlation functionals, but it is seen neither at other silicon edges nor at the carbon K-edge of fluorine derivatives of ethene. Considering the series of molecules SiH_4−XF_X with X = 1, 2, 3, 4, a gradual divergence from interpolated experimental ionization potentials is observed at the level of Kohn–Sham density functional theory (DFT), and to a smaller extent with use of Hartree–Fock. This anomalous error is thus attributed partly to difficulties in correctly emulating the electronic structure effects imposed by the very electronegative fluorines, and partly due to inconsistencies in the spurious electron self-repulsion in DFT. Substitution with one, or possibly two, fluorine atoms is estimated to yield small enough errors to allow for reliable interpretations and predictions of L_2,3-spectra of more complex and extended silicon-based systems.

A computational benchmark study on X-ray absorption spectra of water has been performed by means of transition-potential density functional theory (TP-DFT), damped time-dependent density functional theory (TDDFT), and damped coupled cluster (CC) linear response theory. For liquid water, using TDDFT with a tailored CAM-B3LYP functional and a polarizable embedding, we find that an embedding with over 2000 water molecules is required to fully converge spectral features for individual molecules, but a substantially smaller embedding can be used within averaging schemes. TP-DFT and TDDFT calculations on 100 MD structures demonstrate that TDDFT produces a spectrum with spectral features in good agreement with experiment, while it is more difficult to fully resolve the spectral features in the TP-DFT spectrum. Similar trends were also observed for calculations of bulk ice. In order to further establish the performance of these methods, small water clusters have been considered also at the CC2 and CCSD levels of theory. Issues regarding the basis set requirements for spectrum simulations of liquid water and the determination of gas-phase ionization potentials are also discussed.

Charge generation in organic solar cells is a fundamental yet heavily debated issue. This article gives a balanced review of different mechanisms proposed to explain efficient charge generation in polymer-fullerene bulk-heterojunction solar cells. We discuss the effect of charge-transfer states, excess energy, external electric field, temperature, disorder of the materials, and delocalisation of the charge carriers on charge generation. Although a general consensus has not been reached yet, recent findings, based on both steady-state and transient measurements, have significantly advanced our understanding of this process.

We have investigated the ferroelectric polarization switching properties of trialkylbenzene-1,3,5-tricarboxamide (BTA), which is a model system for a large class of novel organic ferroelectric materials. In the solid state BTAs form a liquid crystalline columnar hexagonal phase that provides long range order that was previously shown to give rise to hysteretic dipolar switching. In this work the nature of the polar switching process is investigated by a combination of dielectric relaxation spectroscopy, depth-resolved pyroelectric response measurements, and classical frequency- and time-dependent electrical switching. We show that BTAs, when brought in a homeotropically aligned hexagonal liquid crystalline phase, are truly ferroelectric. Analysis of the transient switching behavior suggests that the ferroelectric switching is limited by a highly dispersive nucleation process, giving rise to a wide distribution of switching times.

We investigate the polarization loss in the archetypical molecular organic ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA). We prove that the polarization loss is due to thermally activated R-relaxation,which is a collective reversal of the amide dipole moments in ferroelectric domains. By applying a weakelectrostatic field both the polarization loss and the R-relaxation are suppressed, leading to anenhancement of the retention time by at least several orders of magnitude. Alternative loss mechanismsare discussed and ruled out. By operating the thin-film devices slightly above the crystalline to liquidcrystalline phase transition temperature the retention time of one compound becomes more than12 hours even in absence of supportive bias, which is among the longest reported so far for organicferroelectric materials.

The frontier electronic structures of a series of organic dye molecules containing a triphenylaminemoiety, a thiophene moiety and a cyanoacrylic acid moiety have been investigated byphotoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS), X-ray emissionspectroscopy (XES) and resonant photoelectron spectroscopy (RPES). The experimental resultswere compared to electronic structure calculations on the molecules, which are used to confirmand enrich the assignment of the spectra. The approach allows us to experimentally measure andinterpret the basic valence energy level structure in the dye, including the highest occupied energylevel and how it depends on the interaction between the different units. Based on N 1s X-rayabsorption and emission spectra we also obtain insight into the structure of the excited states,the molecular orbital composition and dynamics. Together the results provide an experimentallydetermined energy level map useful in the design of these types of materials. Included are alsoresults indicating femtosecond charge redistribution at the dye/TiO2 interface.

To realize the optical transfer of electron spin information, developing a semiconductor layer for efficient transport of spin-polarized electrons to the active layers is necessary. In this study, electron spin transport from a GaAs/Al0.3Ga0.7As superlattice (SL) barrier to In0.5Ga0.5As quantum dots (QDs) is investigated at room temperature through a combination of time-resolved photoluminescence and rate equation analysis, separating the two transport processes from the GaAs layer around the QDs and SL barrier. The electron transport time in the SL increases for a thicker quantum well (QW) of SL due to the weaker wavefunction overlap between adjacent QWs. Additionally, the degree of conservation of spin polarization during transport varies with QW thickness. Rate equation analysis demonstrates an electron transport from SL to QDs while maintaining a high spin polarization for thick QWs. The achieved spin-conserved electron transport can be attributed to the combination of electron transport being sufficiently faster than the spin relaxation in SL and the suppressed spin relaxation in the p-doped GaAs layer capping the QDs. The findings indicate that SL is a promising candidate as an electron spin transport layer for optical spin devices.

The adsorption of phenol on flat and stepped Pt and Rh surfaces and the dissociation of hydrogen from the hydroxyl group of phenol on Pt(111) and Rh(111) were studied by density functional calculations. On both Pt(111) and Rh(111), phenol adsorbs with the aromatic ring parallel to the surface and the hydroxyl group tilted away from the surface. Furthermore, adsorption on stepped surfaces was concluded to be unfavourable compared to the (111) surfaces due to the repulsion of the hydroxyl group from the step edges. Transition state calculations revealed that the reaction barriers, associated with the dissociation of phenol into phenoxy, are almost identical on Pt and Rh. Furthermore, the oxygen in the dissociated phenol is strongly attracted by Rh(111), while it is repelled by Pt(111).

When single crystals of the 5-membered heterocyclic ring structure 2-oxazolidinone (C3O2NH5) are irradiated at room temperature, the major radical formed (R1) decays during a period of a few hours, leaving a broad, unstructured EPR spectrum not amenable to analysis. Cooling the crystals to about 140 K immediately after irradiation at room temperature allows analysis of the R1 radical by EPR and ENDOR. The radical is formed by a net H-abstraction from one of the two methylene groups of the molecule. A full EPR and ENDOR analysis of four proton interactions (one a-coupling, and three ß-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carbon-centered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C1-H bond with respect to the N-C1-C2 plane is estimated to be 13-15° (bending angle 0˜ 7°). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogen-bonded proton hyperfine interactions. © The Owner Societies 2002.

The structure of dimeric cations of benzene and toluene formed in X- irradiated halocarbon matrices containing relatively high concentration of the solutes was investigated. EPR and ENDOR (electron nuclear double resonance) spectra of these dimer cations were observed and accurate values of the hf coupling constants were obtained. The ENDOR spectrum of the dimeric radical cation of benzene, (C6H6)2+, exhibited hf couplings due to twelve equivalent protons and the isotropic coupling (a(iso)) was almost one-half of that in the monomer cation. ENDOR transitions with a rhombic symmetry were observed in a CFCl3 matrix, whereas clear axially symmetric transitions with A(is parallel with) < A(is perpendicular to) were obtained in CF3CCl3 even at 50 K. The rhombic dipolar coupling tensor was used as a parameter to evaluate the distance between the two partner rings. As regards the toluene dimer cation, (CH3C6H5)2+, ENDOR transitions of the CH3 and H(4,4') protons were observed. The isotropic hf coupling of the CH3 protons deviated even more strongly from the half value, being rather close to one- third of the value of the monomer. The hf coupling of the H(4,4') protons was almost half the coupling of the monomer. It was suggested that the anomalous hf couplings of the CH3 protons were due to the interaction between the two rings through the s-bond. Density functional theory (DFT) calculations were employed to obtain the optimized geometries and hf coupling tensors and suggested sandwich structures in both dimers. Furthermore, the distances between the two rings and the anomalous CH3 protons hf coupling in the (CH3C6H5)2+ were successfully evaluated.

X-irradiated Li(N2H5)SO4 single crystals were investigated using ESR and ENDOR spectroscopy at several temperatures. The hydrazine radical cation N2H4.+ was selectively produced by irradiation at room temperature. From the analysis of the orientation dependent ENDOR spectra, the 1H-hfc tensors of the cation radical were precisely obtained and the radical structure was supported by theoretical calculations. It is suggested that the cation radical has a planar p* structure D(2h) (2B(2g)) in the crystal down to 230 K. By using the evaluated 1H-hf tensor the powder ESR line shape was successfully simulated. Concomitant with the radical formation, the N-N bond of N2H4.+ is suggested to reorient so as to optimize hydrogen bond interactions. 1H-ENDOR line splitting for the N2H4.+ radical was observed at temperatures below 230 K. Apparently this splitting is due to a reversible structural change where one of the NH2 moleties in N2H4.+ becomes slightly bent out of the molecular plane, whereas the other one remains planar. This deformation evidently arises from interactions between the cation radical, adjacent H2N-NH3+ molecules and the SO4-LiO4 framework. Interacting N2H5+···N2H4.+···N2H5+ molecules along the c-axis are proposed to explain the deformation mechanism.

The nature of quartet state nitrogen atoms formed in gamma -irradiated NaN3 powder was investigated by EPR, ENDOR and ENDOR-induced EPR (EIE). The ENDOR spectrum observed at 110 K clearly showed two sets of N-14 signal pairs centered at a(N)/2 (half the N-14-hyperfine splitting) and at 3a(N)/2, strongly suggesting the existence of quartet state N-14 atoms (S-4(3/2)). At 160 K, the EPR spectrum exhibited a clear fine structure. accompanied by a hyperfine coupling due to N-14. The zero-field splitting, D, gradually increased with decreasing temperature from 170 to 40 K, and then became nearly constant. The temperature dependence of D was explained in terms of a local lattice vibration coupled with spin states of N-14 atoms. At below 95 K, a new fine structure with larger D was observed. The observed two D-values were attributed to 14N atoms located at two different interstitial sites in the monoclinic NaN3 structure.