Experimental and computational studies on ruthenium(ii) bis-diimine complexes of N,N'-chelate ligands: the origin of changes in absorption spectra upon oxidation and reduction.

This work presents an interpretation of the origin of changes in absorption spectra upon one-electron oxidation and reduction of two ruthenium polypyridyl complexes based on a combination of UV-Vis spectroelectrochemical experiments and theoretical calculations using the Gaussian 09 program. A bis-chelating ligand containing a p-bromobenzoylthiourea unit connected to 1,10-phenanthroline (phen-p-BrBT) has been prepared. Complexation of phen-p-BrBT to ruthenium bis-diimine centres, Ru(N-N)2 [N-N = 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen)], affords octahedral Ru(ii) tris-diimine complexes that are synthesised and structurally characterised. The two complexes exhibit similar MLCT bands and electronic energy levels owing to the similar electronic structures of the bpy and phen ligands. However, [Ru(phen)2(phen-p-BrBT)]2+ exhibits a slightly broader visible region MLCT (metal-to-ligand-charge transfer) band than [Ru(bpy)2(phen-p-BrBT)]2+ as expected from a slightly more delocalised π-electron system in the phen diimine ligands. In addition, the π→π* absorption in the UV is blue-shifted for [Ru(phen)2(phen-p-BrBT)]2+ relative to that for [Ru(bpy)2(phen-p-BrBT)]2+, because of greater stabilisation of the bpy HOMO relative to that of phen. The extra C-C bond in phen produces greater delocalisation of electron density leading to a blue-shift in the π→π* transition. The MLCT band is blue-shifted and diminished in intensity upon oxidation due to stabilisation of the Ru d-orbitals by removal of one electron. A new broad absorption band appears in the UV region upon reduction. The new transition is attributed to a blue-shift of the first MLCT transition for [Ru(bpy)2(phen-p-BrBT)]2+ and a red-shift of the second MLCT transition for [Ru(phen)2(phen-p-BrBT)]2+. The new transitions originate from destabilisation or stabilisation of the ligand LUMO orbitals relative to the Ru d-orbitals. A red-shift of the UV band in the initial complex also contributes to the new band produced upon reduction of [Ru(bpy)2(phen-p-BrBT)]2+. The new band does not involve an n(C[double bond, length as m-dash]S) →π* transition. Although both complexes show subtle differences in behaviour, their spectral changes are distinct, and the origin of changes in their absorption spectra upon oxidation and reduction is successfully interpreted.

Surface Structure of Organic Semiconductor [ n]Phenacene Single Crystals.

Surface structure relaxation of organic semiconductors affects the properties of organic devices, although such relaxation has not been well explored. Only two examples have been experimentally reported; tetracene shows a large surface relaxation, while rubrene exhibits no relaxation. Therefore, a systematic investigation of the surface relaxation is conducted on [ n]phenacenes ( n = 5, 7, and 9). Electron density analyses are performed based on the synchrotron surface X-ray scattering with the aid of first-principles calculations. The results show little surface relaxation in [ n]phenacenes.

Augmented pH-sensitivity absorbance of a ruthenium(ii) bis(bipyridine) complex with elongation of the conjugated ligands: an experimental and theoretical investigation.

An absorbance-based sensor employing ruthenium bipyridyl with a phenanthroline-fused benzoylthiourea moiety formulated as [Ru(ii)(bpy)2(phen-nBT)](PF6)2 {bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, nBT = n-benzoylthiourea} has been synthesized and characterized by elemental analyses, mass spectrometry, and infrared, ultraviolet-visible, luminescence and nuclear magnetic resonance spectroscopy. The changes in the intensity of absorption and emission of the complex induced by functionalization of the benzoylthiourea ligands with amino and carbonyl in their protonated and deprotonated forms were studied experimentally. The absorption and emission properties of the complex exhibit a strong dependence on the pH (1-11) of the aqueous medium. This work highlights the pH-sensitivity augmentation of the absorption band by elongating the conjugation length in the structure of the ruthenium bipyridine complex. The principle of this work was to design the title compound to be capable of enhancing the differences in the absorption sensitivity responses towards pH between the protonated and deprotonated complexes in the absorption measurement. Along with significant and noticeable changes in the absorption spectra, subsequent theoretical investigations specifically on the electronic and absorbance properties of the title compound were carried out in this study. Protonation of the molecule significantly stabilized the lowest-unoccupied molecular orbital (LUMO), whereas the highest-occupied molecular orbital (HOMO) is greatly destabilized upon deprotonation. A time-dependent density functional theory (TDDFT) calculation in the linear-response (-LR) regime was performed to clarify the origin of the experimentally observed linear dependence of absorption intensity upon pH (1-11). The MLCT band exhibits hyperchromic shift at low pH as indicated by the large transition dipole moment and a wider distribution of the response charge of the molecule, which is induced by the stabilization of the electrostatic potential at the carbonyl moiety by protonation. This study provides the possibility of employing theoretical information to gain insight into the origin of the optical absorption obtained experimentally. The ruthenium complex was designed with an elongated ligand conjugation length and exhibited a tremendously large change in the absorption intensity of the protonated and deprotonated forms, which therefore demonstrates its feasibility as an indicator molecule especially for absorbance measurements.

First-Principles Molecular Dynamics Analysis of Ligand-Free Suzuki-Miyaura Cross-Coupling in Water Solvent: Oxidative Addition Step.

We investigated the oxidative addition of PhX (X = Cl, Br) to a single Pd(0) atom or a PdX- complex in water using first-principles molecular dynamics simulations, with solvent H2O molecules explicitly included in the calculation models, to clarify the origin of the extremely high reactivity of a ligand-free Pd catalyst in an aqueous solution for the Suzuki-Miyaura reaction. The free-energy profiles are estimated using blue moon ensemble sampling to include the entropy effect in chemical reactions in a water solvent. The free-energy barrier of the oxidative addition step is quite low for PhBr, whereas the barrier for PhCl is sizable, indicating that the reaction can proceed at room temperature with a high rate for PhBr but a rather low rate for PhCl. We also investigated the effect of the additional halogen anion on the Pd catalyst as a "supporting ligand". The activation barrier of the oxidative addition step is not affected by the supporting halogen ligand, but the final state is significantly destabilized, which should be important for the following transmetalation step. The solvent effect has also been investigated and discussed.

Suppressing molecular vibrations in organic semiconductors by inducing strain.

Organic molecular semiconductors are solution processable, enabling the growth of large-area single-crystal semiconductors. Improving the performance of organic semiconductor devices by increasing the charge mobility is an ongoing quest, which calls for novel molecular and material design, and improved processing conditions. Here we show a method to increase the charge mobility in organic single-crystal field-effect transistors, by taking advantage of the inherent softness of organic semiconductors. We compress the crystal lattice uniaxially by bending the flexible devices, leading to an improved charge transport. The mobility increases from 9.7 to 16.5 cm(2) V(-1) s(-1) by 70% under 3% strain. In-depth analysis indicates that compressing the crystal structure directly restricts the vibration of the molecules, thus suppresses dynamic disorder, a unique mechanism in organic semiconductors. Since strain can be easily induced during the fabrication process, we expect our method to be exploited to build high-performance organic devices.

Theoretical Verification of Photoelectrochemical Water Oxidation Using Nanocrystalline TiO2 Electrodes.

Mesoscopic anatase nanocrystalline TiO2 (nc-TiO2) electrodes play effective and efficient catalytic roles in photoelectrochemical (PEC) H2O oxidation under short circuit energy gap excitation conditions. Interfacial molecular orbital structures of (H2O)3 &OH(TiO2)9H as a stationary model under neutral conditions and the radical-cation model of [(H2O)3&OH(TiO2)9H]+ as a working nc-TiO2 model are simulated employing a cluster model OH(TiO2)9H (Yamashita/Jono's model) and a H2O cluster model of (H2O)3 to examine excellent H2O oxidation on nc-TiO2 electrodes in PEC cells. The stationary model, (H2O)3&OH(TiO2)9H reveals that the model surface provides catalytic H2O binding sites through hydrogen bonding, van der Waals and Coulombic interactions. The working model, [(H2O)3&OH(TiO2)9H]+ discloses to have a very narrow energy gap (0.3 eV) between HOMO and LUMO potentials, proving that PEC nc-TiO2 electrodes become conductive at photo-irradiated working conditions. DFT-simulation of stepwise oxidation of a hydroxide ion cluster model of OH-(H2O)3, proves that successive two-electron oxidation leads to hydroxyl radical clusters, which should give hydrogen peroxide as a precursor of oxygen molecules. Under working bias conditions of PEC cells, nc-TiO2 electrodes are now verified to become conductive by energy gap photo-excitation and the electrode surface provides powerful oxidizing sites for successive H2O oxidation to oxygen via hydrogen peroxide.

Scanning tunneling microscopy/spectroscopy of picene thin films formed on Ag(111).

Using ultrahigh-vacuum low-temperature scanning tunneling microscopy and spectroscopy combined with first principles density functional theory calculations, we have investigated structural and electronic properties of pristine and potassium (K)-deposited picene thin films formed in situ on a Ag(111) substrate. At low coverages, the molecules are uniformly distributed with the long axis aligned along the [112̄] direction of the substrate. At higher coverages, ordered structures composed of monolayer molecules are observed, one of which is a monolayer with tilted and flat-lying molecules resembling a (11̄0) plane of the bulk crystalline picene. Between the molecules and the substrate, the van der Waals interaction is dominant with negligible hybridization between their electronic states; a conclusion that contrasts with the chemisorption exhibited by pentacene molecules on the same substrate. We also observed a monolayer picene thin film in which all molecules were standing to form an intermolecular π stacking. Two-dimensional delocalized electronic states are found on the K-deposited π stacking structure.

Intermolecular interaction as the origin of red shifts in absorption spectra of zinc-phthalocyanine from first-principles.

We investigate electronic origins of a redshift in absorption spectra of a dimerized zinc phthalocyanine molecule (ZnPc) by means of hybrid density functional theoretical calculations. In terms of the molecular orbital (MO) picture, the dimerization splits energy levels of frontier MOs such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the constituent molecules. Consequently, the absorption wavelength seems to become longer than the monomer as the overlap between the monomers becomes larger. However, for a ZnPc dimer configuration with its cofacially stacked monomer arrangement, the calculated absorption spectra within the time-dependent density functional theory indicates no redshift but blueshift in the Q-band absorption spectrum, i.e., a typical H-aggregate. The origin of the apparently contradictory result is elucidated by the conventional description of the interaction between monomer transition dipoles in molecular dimers [Kasha, M. Radiat. Res. 1963, 20, 55]. The redshift is caused by an interaction between the two head-to-tail transition dipoles of the monomers, while the side-by-side arranged transition dipoles result in a blueshift. By tuning the dipole-dipole interaction based on the electronic natures of the HOMO and the LUMO, we describe a slipped-stacked ZnPc dimer configuration in which the Q-band absorption wavelength increases by as large as 144 nm relative to the monomer Q-band.

First-principles investigation on the segregation of Pd at LaFe1-xPdxO3-y surfaces.

First-principles calculations were performed to investigate the effect of Pd concentration and oxygen vacancies on the stability of Pd at LaFeO3 surfaces. We found a much stronger tendency of Pd to segregate by taking the aggregation of Pd at LaFe1-xPdxO3-y surfaces into consideration, resulting in a pair of Pd-Pd around a vacancy. Moreover, we predicted that one oxygen-vacancy-containing FeO2-terminated surfaces would be stable at high temperatures by comparing the stability of LaFe1-xPdxO3-y surfaces, which further supports our previous conclusion that a Pd-containing perovskite catalyst should be calcined at 1,073 K or higher temperatures in air to enhance the segregation of Pd in the vicinity of surfaces to rapidly transform the Pd catalyst from oxidized to reduced states on the perovskite support.

Local electronic properties at organic-metal interfaces: thiophene derivatives on Pt(111).

The valence electronic states of thiophene (TP), 2-thiophenethiol (TT), 2,2'-bithiophene (BTP), and 2,2'-bithiophene-5-thiol (BTT) on Pt(111) were measured by ultraviolet photoemission spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) to elucidate how the local electronic properties at the organic-metal interface are altered by the extent of π-conjugation and substituent effects. First-principles calculations using density functional theory (DFT) were used to assign the observed spectra. TP and BTP chemisorb weakly on Pt(111), whereas TT and BTT are strongly bound to Pt(111) through the S atom with the cleavage of the S-H bond, forming a thiolate. In the MAES spectra, weak emission just below the Fermi level (E(F)) was attributed to a chemisorption-induced gap state (CIGS) produced by orbital mixing between the organic species and Pt(111). The formation of CIGS is responsible for a metallic structure at the organic-metal interface. The relative intensities of CIGSs at E(F) were in the order of TP (flat-lying configuration) > TT > TP (inclined configuration), indicating that the spatial distribution of CIGSs is drastically altered by the strength of the organic-metal bond and the adsorption geometry. In other words, TP (flat-lying geometry) and TT serve as good mediators of the extension of the metal wave function at E(F), which would be closely related to charge transport at organic-metal interfaces.

Adsorption of benzene on noble metal surfaces studied by density functional theory with Van der Waals correction.

We have studied the atomic geometries and the electronic properties of benzene/metal interfaces by using density functional theoretical (DFT) calculations with van der Waals corrections. Adsorption energies of benzene on Cu(111), Ag(111), and Au(111) surfaces calculated by van der Waals density functional proposed by Dion and co-workers agree reasonably well with experimentally reported values, while those calculated by a semi-empirical van der Waals correction proposed by Grimme are overestimated slightly. The work function change induced by benzene adsorption on the three surfaces are quite well reproduced by the semi-empirical correction, suggesting that weak adsorption geometries can be quite well reproduced by DFT with a semi-empirical dispersion correction scheme.

State-selective dissociation of a single water molecule on an ultrathin MgO film.

The interaction of water with oxide surfaces has drawn considerable interest, owing to its application to problems in diverse scientific fields. Atomic-scale insights into water molecules on the oxide surface have long been recognized as essential for a fundamental understanding of the molecular processes occurring there. Here, we report the dissociation of a single water molecule on an ultrathin MgO film using low-temperature scanning tunnelling microscopy. Two types of dissociation pathway--vibrational excitation and electronic excitation--are selectively achieved by means of injecting tunnelling electrons at the single-molecule level, resulting in different dissociated products according to the reaction paths. Our results reveal the advantage of using a MgO film, rather than bulk MgO, as a substrate in chemical reactions.

Density functional theoretical study of pentacene/noble metal interfaces with van der Waals corrections: vacuum level shifts and electronic structures.

In order to clarify factors determining the interface dipole, we have studied the electronic structures of pentacene adsorbed on Cu(111), Ag(111), and Au(111) by using first-principles density functional theoretical calculations. In the structural optimization, a semiempirical van der Waals (vdW) approach [S. Grimme, J. Comput. Chem. 27, 1787 (2006)] is employed to include long-range vdW interactions and is shown to reproduce pentacene-metal distances quite accurately. The pentacene-metal distances for Cu, Ag, and Au are evaluated to be 0.24, 0.29, and 0.32 nm, respectively, and work function changes calculated by using the theoretically optimized adsorption geometries are in good agreement with the experimental values, indicating the validity of the present approach in the prediction of the interface dipole at metal/organic interfaces. We examined systematically how the geometric factors, especially the pentacene-substrate distance (Z(C)), and the electronic properties of the metal substrates contribute to the interface dipole. We found that at Z(C) > or = 0.35 nm, the work function changes (Delta phi's) do not depend on the substrate work function (phi(m)), indicating that the interface level alignment is nearly in the Schottky limit, whereas at Z(C) < or = 0.25 nm, Delta phi's vary nearly linearly with phi(m), and the interface level alignment is in the Bardeen limit. Our results indicate the importance of both the geometric and the electronic factors in predicting the interface dipoles. The calculated electronic structure shows that on Au, the long-range vdW interaction dominates the pentacene-substrate interaction, whereas on Cu and Ag, the chemical hybridization contributes to the interaction.

Theoretical investigation of the electronic structure of the Alq(3)/Mg interface.

We have studied the atomic geometries and the electronic properties of the tris-(8-hydroxyquinoline) aluminum (Alq(3))/Mg interface by using density functional theoretical calculations. We have found that the chemical bond is formed between the O atoms of Alq(3) and the substrate Mg atoms, and the stability of the interface structures depends on the number of O-Mg bonds. In the up configurations, where two or three O-Mg chemical bonds are formed and the Alq(3) molecular dipoles are oriented up to the vacuum side, the work function is decreased by as much as 1.1 eV or more. The interface dipole is dominated by the orientation of the molecular dipoles of Alq(3). The interface gap state reported from experiments is ascribed to the highest occupied molecular orbital (HOMO) levels of the down configurations, which may coexist with the dominant up configurations.

First-principles theoretical study of Alq3Al interfaces: origin of the interfacial dipole.

We have studied the atomic geometries and the electronic properties of the tris-(8-hydroxyquinoline) aluminum (Alq(3))Al interfaces by using density functional theoretical calculations, and clarified the origin of the interfacial dipole moment. We have examined various possible adsorption geometries of Alq(3) on Al surfaces and calculated the work function change induced by adsorption of Alq(3) on Al surfaces. We found that the stability depends crucially on the number of O-Al bonds formed at the interface, and Alq(3) tends to expose its O atoms to the Al substrate side and its N atoms to the vacuum side. Although the binding energies are influenced by the poor description of the van der Waals interaction by the density functionals used, the resulting bonding configurations are found to give correct binding energies when the van der Waals interaction is taken into account based on the recently proposed van der Waals density functional [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)]. This bonding configuration arranges molecular permanent dipoles of Alq(3) directed towards the vacuum, leading to the decrease of the surface work function. The calculated interface dipoles agree reasonably well with the experimental results and the origin of the interface dipole formation mainly comes from the alignment of the permanent dipoles of Alq(3). The HOMO levels of the Alq(3) molecules significantly depend on the orientation of the molecular permanent dipoles and the interfacial gap state observed by experiments is ascribed to the coexistence of the two orientations of the molecular dipole moments.

Role of molecular orbitals near the fermi level in the excitation of vibrational modes of a single molecule at a scanning tunneling microscope junction.

Inelastically tunneled electrons from the tip of a scanning tunneling microscope were used to induce S-S bond dissociation of a (CH(3)S)(2) and lateral hopping of a CH(3)S on Cu(111) at 4.7 K. Both experimental results and theoretical calculations confirm that the excitation mechanism of the vibrationally induced chemistry reflects the projected density of states of molecular orbitals that appear near the Fermi level as a result of the rehybridization of the orbitals between the adsorbed molecules and the substrate metal atoms.

A long-range-corrected time-dependent density functional theory.

We apply the long-range correction (LC) scheme for exchange functionals of density functional theory to time-dependent density functional theory (TDDFT) and examine its efficiency in dealing with the serious problems of TDDFT, i.e., the underestimations of Rydberg excitation energies, oscillator strengths, and charge-transfer excitation energies. By calculating vertical excitation energies of typical molecules, it was found that LC-TDDFT gives accurate excitation energies, within an error of 0.5 eV, and reasonable oscillator strengths, while TDDFT employing a pure functional provides 1.5 eV lower excitation energies and two orders of magnitude lower oscillator strengths for the Rydberg excitations. It was also found that LC-TDDFT clearly reproduces the correct asymptotic behavior of the charge-transfer excitation energy of ethylene-tetrafluoroethylene dimer for the long intramolecular distance, unlike a conventional far-nucleus asymptotic correction scheme. It is, therefore, presumed that poor TDDFT results for pure functionals may be due to their lack of a long-range orbital-orbital interaction.