Highest Occupied Molecular Orbital States Research Articles

Benzyl Alcohol Photo-oxidation Based on Molecular Electronic Transitions in Metal Halide Perovskites

Vacancy ordered double perovskites Cs2Te(IV)X6 have been found to exhibit molecule-like electronic behavior when X is Cl– or Br– due to the zero-dimensional (0D) nature of their octahedral units. Electronically isolated building blocks, the [TeBr6]2– ionic octahedron, serve as the fundamental electronic unit of the Cs2TeBr6 solid. Herein, a detailed understanding of the Cs2TeBr6 electronic structure and its photoexcitation is presented with consideration of individual molecular orbitals from these isolated octahedral building blocks. Two optical absorption features correspond to two unique electronic transitions, (1) a highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition under 455 nm excitation and (2) mixed transitions including lower HOMO states to LUMO transition and HOMO to higher LUMO states transition under 365 nm excitation. With this in mind, we examined the excitation wavelength-dependent photo-oxidation of benzyl alcohol using Cs2TeBr6 as the photocatalyst. Significant differences in photocatalytic performance were observed, and different forms of activated alcohol radicals were detected under the two excitation wavelengths. As a case study, this work highlights the application of molecule-like halide perovskites in photocatalysis. The highly tunable energy band structures and catalytic centers in perovskites can offer a valuable platform for photocatalytic mechanistic studies and catalyst development in the foreseeable future.

Ab initio investigation of the role of the d-states on the adsorption and activation properties of CO2 on 3d, 4d, and 5d transition-metal clusters.

We report a theoretical investigation of the adsorption and activation properties of CO2 on eight-atom 3d, 4d, and 5d transition-metal (TM) clusters based on density functional theory calculations. From our results and analyses, in the lowest energy configurations, CO2 binds via a chemisorption mechanism on Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt (adsorption energy from -0.49eV on Pt up to -1.40eV on Os), where CO2 breaks its linearity and adopts an angular configuration due to the charge transfer from the clusters toward the C atom in the adsorbed CO2. In contrast, it binds via physisorption on Cu, Ag, and Au and maintains its linearity due to a negligible charge transfer toward CO2 and has a small adsorption energy (from -0.17eV on Cu up to -0.18eV on Ag). There is an energetic preference for twofold bridge TM sites, which favors binding of C with two TM atoms, which enhances the charge transfer ten times than on the top TM sites (onefold). We identified that the strength of the CO2-TM8 interaction increases when the energy values of the highest occupied molecular orbital (HOMO) of the TM8 are closer to the energy values of the lowest unoccupied molecular orbital of CO2, which contributes to maximize the charge transfer toward the molecule. Beyond the energy position of the HOMO states, the delocalization of 5d orbitals plays an important role in the adsorption strength in TM, especially for the iron group, e.g., the adsorption energies are -1.08eV (Fe, 3d), -1.19eV (Ru, 4d), and -1.40eV (Os, 5d).

Doping hepta-alanine with tryptophan: A theoretical study of its effect on the electrical conductance of peptide-based single-molecule junctions.

Motivated by a recent experiment [C. Guo et al., Proc. Natl. Acad. Sci. U. S. A. 113, 10785 (2016)], we carry out a theoretical study of electron transport through peptide-based single-molecule junctions. We analyze the pristine hepta-alanine and its functionalizations with a single tryptophan unit, which is placed in three different locations along the backbone. Contrary to expectations from the experiment on self-assembled monolayers, we find that insertion of tryptophan does not raise the electrical conductance and that the resulting peptides instead remain insulating in the framework of a coherent transport picture. The poor performance of these molecules as conductors can be ascribed to the strongly off-resonant transport and low electrode-molecule coupling of the frontier orbitals. Although the introduction of tryptophan increases the energy of the highest occupied molecular orbital (HOMO) of the peptides in the gas phase, the new HOMO states are localized on the tryptophan unit and therefore essentially do not contribute to coherent charge transport.

Synthesis of green-photoluminescent single layer graphene quantum dots: Determination of HOMO and LUMO energy states

Low-cost and high-yield green-photoluminescent single layer graphene quantum dots (SLGQDs) synthesized with only DI water and glucose as precursor using a hydrothermal method. Atomic force microscopy proved that the graphene quantum dots are single layers and transmission electron microscopy images showed that the average width of SLGQDs is about 8nm and optical characterization revealed that SLGQDs are highly photoluminescent. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the SLGQDs against vacuum determined using a cyclic voltammetry technique, and energy band gap was obtained ∼ 2.9 and 3.1eV from cyclic voltammetry and optical measurements, respectively.

Measurements of HOMO-LUMO levels of poly(3-hexylthiophene) thin films by a simple electrochemical method

Though poly(3-hexylthiophene) (P3HT) is one of the most commonly used polymers in organic solar cells, a broad range of values, derived from cyclic voltammetry (CV), has been reported for the lowest unoccupied molecular orbital (LUMO) position (from −3.53 to −2.70 eV); contrastingly, the highest occupied molecular orbital (HOMO) position is reported in a narrow range (from −4.92 to −5.20 eV). As a consequence of this wide distribution for the LUMO position, most researchers choose to use electrochemical techniques for determining only the HOMO position, and estimate the LUMO position by adding the experimental optical band-gap value. Here, three different electrochemical strategies (CV, potentiostatic EIS, and potentiodynamic EIS) for obtaining the HOMO and LUMO position for P3HT films formed under ambient conditions on transparent conductive substrates (indium tin oxide (ITO) glass) are compared. The results are discussed in the frame of limitations of each technique. The cyclic voltammetric response and the data derived from potentiostatic EIS using electric equivalent circuits include the response of all processes involved in the measurements, particularly masking the LUMO response due to the presence of energetic states. In contrast, potentiodynamic EIS measured in a wide frequency range results in a more reliable approach since it allows discerning between middle-frequency-dependent processes associated with energetic states in the gap of the P3HT/ITO films, and low-frequency-dependent processes associated with filling and emptying of LUMO and HOMO states, respectively. It is a powerful and simple method to analyze the electronic structure of semiconductor organic thin films.

Competition of edge effects on the electronic properties and excitonic effects in short graphene nanoribbons

We explore the electronic properties and exciton effects in short graphene nanoribbons (SGNRs), which have two armchair edges and two zigzag edges. Our results show that both of these two types of edges have profound effects on the electronic properties and exciton effects. Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states are alternatively changed between the bulk and the edge states as the lengths of the zigzag edges increase, due to the competition between the states of the two types of edges. The energy gaps, as a function of the lengths of the armchair edges, will then induce two kinds of trends. Furthermore, two kinds of exciton energies and exciton binding energies are found, which can be understood through the two kinds of HOMO and LUMO states in SGNRs. In addition, we find that the three triplet exciton states are not totally energy degenerate in SGNRs due to the spin-polarized states on the zigzag edges.

Photoelectron spectroscopic studies of ultra-thin CuPc layers on a Si(111)-(√3 × √3)R30°-B surface

The adsorption of copper phthalocyanine (CuPc) molecules on Si(111)-(√3 × √3)R30°-B surface is investigated at room temperature under ultra-high vacuum. Crystallographic, chemical and electronic properties of the interface are investigated by low energy electron diffraction (LEED), ultraviolet and X-ray photoemission spectroscopies (UPS, XPS) and X-ray photoemission diffraction (XPD). LEED and XPD results shed light on the growth mechanism of CuPc on this substrate. At one monolayer coverage the growth mode was characterized by the formation of crystalline 3D nanoislands. The molecular packing deduced from this study appears very close to the one of the bulk CuPc α phase. The 3D islands are formed by molecules aligned in a standing manner. XPS core level spectra of the substrate reveal that there is no discernible chemical interaction between molecules and substrate. However there is charge transfer from molecules to the substrate. During the growth, the work function (WF) was found to decrease from 4.50eV for the clean substrate to 3.70eV for the highest coverage (30 monolayers). Within a thickness of two monolayers deposition, an interface dipole of 0.50eV was found. A substrate band bending of 0.25eV was deduced over all the range of exposure. UPS spectra indicate the existence of a band bending of the highest occupied molecular orbital (HOMO) of 0.30eV. The changes in the work function, in the Fermi level position and in the onset of the molecular HOMO state have been used to determine the energy level alignment at the interface.

Adsorption study of copper phthalocyanine on Si(111)(√3 × √3)R30°Ag surface

The adsorption of copper phthalocyanine (CuPc) molecules on Si(111)(√3×√3)R30°Ag surface is studied at room temperature under ultra high vacuum. Crystallographic, chemical and electronic properties of the interface are investigated by low energy electron diffraction (LEED), ultraviolet and X-ray photoemission spectroscopies (UPS, XPS) and X-ray photoemission diffraction (XPD). LEED and XPD results indicate that after one monolayer deposition the molecular layer is highly ordered with a flat lying adsorption configuration. The corresponding pattern reveals the coexistence of three symmetrically equivalent orientations of molecules with respect to the substrate. XPS core level spectra of the substrate reveal that there is no discernible chemical interaction between molecules and substrate; however there is evidence of Fermi level movement. During the growth, the work function was found to decrease from 4.90eV for the clean substrate to 4.35eV for the highest coverage (60 monolayers). Within a thickness of two monolayer deposition an interface dipole of 0.35eV and a band bending of 0.2eV have been found. UPS spectra indicate the existence of a band bending of the highest occupied molecular orbital (HOMO) of 0.55eV. The changes in the work function, in the Fermi level position and in the HOMO state have been used to determine the energy level alignment at the interface.

Electronic structure, stacking energy, partial charge, and hydrogen bonding in four periodic B-DNA models.

We present a theoretical study of the electronic structure of four periodic B-DNA models labeled (AT)(10), (GC)(10), (AT)(5)(GC)(5), and (AT-GC)(5) where A denotes adenine, T denotes thymine, G denotes guanine, and C denotes cytosine. Each model has ten base pairs with Na counterions to neutralize the negative phosphate group in the backbone. The (AT)(5)(GC)(5) and (AT-GC)(5) models contain two and five AT-GC bilayers, respectively. When compared against the average of the two pure models, we estimate the AT-GC bilayer interaction energy to be 19.015 Kcal/mol, which is comparable to the hydrogen bonding energy between base pairs obtained from the literature. Our investigation shows that the stacking of base pairs plays a vital role in the electronic structure, relative stability, bonding, and distribution of partial charges in the DNA models. All four models show a highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gap ranging from 2.14 to 3.12 eV with HOMO states residing on the PO(4) + Na functional group and LUMO states originating from the bases. Our calculation implies that the electrical conductance of a DNA molecule should increase with increased base-pair mixing. Interatomic bonding effects in these models are investigated in detail by analyzing the distributions of the calculated bond order values for every pair of atoms in the four models including hydrogen bonding. The counterions significantly affect the gap width, the conductivity, and the distribution of partial charge on the DNA backbone. We also evaluate quantitatively the surface partial charge density on each functional group of the DNA models.

Molecular electronic states in charge transfer complex studied by x-ray absorption spectroscopy

The electronic states of tetrathiafulvalene (TTF: TTF = C6H4S4) molecule in organic ferroelectric TTF-p-bromanil (TTF-BA: BA = C6Br4O2) and TTF crystals have been investigated by x-ray absorption spectroscopy (XAS) measurement at S K-edge. We elucidated that the peak structure at 2470.5 eV directly reflects the existence of hole in the highest occupied molecular orbital (HOMO) state of the TTF molecule in TTF-BA; that is consistent with the ionic TTF molecule (TTF+). The XAS of TTF-BA was evaluated on the basis of first-principles calculations, and the calculated spectra reproduce well the shape of experimental spectrum and the peak energy of the HOMO state.

Low energy (e,2e) studies from CH4: Results from symmetric coplanar experiments and molecular three-body distorted wave theory

Low energy experimental and theoretical triply differential cross sections are presented for electron impact ionization of methane (CH(4)) for both the highest occupied molecular orbital (HOMO) and next highest occupied molecular orbital (NHOMO). The HOMO is a predominantly p-type orbital which is labeled 1t(2) and the NHOMO is predominantly s-type labeled 2a(1). Coplanar symmetric (symmetric both in final state electron energies and observation angles) are presented for final state electron energies ranging from 2.5 to 20 eV. The theoretical M3DW (molecular three-body distorted wave) results are in surprisingly good agreement with experiment for the HOMO state and less satisfactory agreement for the NHOMO state. The molecular NHOMO results are also compared with the ionization of the 2s shell of neon which is the isoelectronic atom.

Conventional Optics from Unconventional Electronics in ZnO Quantum Dots

We study the electronic and optical properties of ZnO quantum dots within the atomistic empirical pseudopotential framework. The highest occupied molecular orbital (HOMO) is found to be of orbital P character for structures larger than 2.6 nm in diameter. We identify the origin of this unconventional situation in the electronic character of the HOMO state, originating from an even mixture of the A- and B-bands of the Wurtzite band structure. This situation, however, does not lead to an orbitally dark exciton ground state, as one might expect. Coulomb interactions lower the bright (electron-S−hole-S) exciton below the orbitally forbidden (electron-S−hole-P) exciton to recover the conventional situation of an orbitally allowed, but spin-forbidden, exciton ground state and a Stoke’s shift originating from electron−hole exchange interactions.

Negative differential resistance in molecular junctions: Application to graphene ribbon junctions

Using self-consistent calculations based on nonequilibrium Green's function formalism, the origin of negative differential resistance (NDR) in molecular junctions and quantum wires is investigated. Coupling of the molecule to electrodes becomes asymmetric at high bias due to asymmetry between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. This causes appearance of an asymmetric potential profile due to a depletion of charge and reduction of screening near the source electrode. With increasing bias, this sharp potential drop leads to an enhanced localization of the HOMO and LUMO states in different parts of the system. The reduction in overlap, caused by localization, results in a significant reduction in the transmission coefficient and current with increasing bias. An atomic chain connected to two graphene ribbons was investigated to illustrate these effects. For a chain substituting a molecule, an even-odd effect is also observed in the NDR characteristics.

Energy-Level Alignment, Ionization, and Stability of Bio-Amino Acids at Amino Acid/Si Junctions

The electronic structures of 20 bio-amino acids and amino acid/Si junctions are studied using ab initio calculations. It is shown that the amino acids can be classified into two groups depending on where the highest occupied molecular orbital (HOMO) state is localized. This classification is possible owing to the molecular structural geometry and the constituent atoms in the residue part of amino acids. Moreover, we found that, owing to the hybridization of electronic states between amino acids and Si substrate, the optical transition from the HOMO state of the amino acid to the conduction band states of Si becomes possible at the amino acid/Si interface. This result indicates the possibility of the optical ionization of amino acid by producing amino acid/semiconductor junctions. The present results provide basic data not only for the microscopic understanding of protein electronic structures but also for the electronic design of new protein functions.

Electron Transport in a π-Stacking Molecular Chain

We have investigated the electronic structure and transport properties of a pi-stacking molecular chain which is covalently bonded to a H/Si(100) surface, using the first-principles density functional theory approach combined with Green's function method. The highest occupied molecular orbital (HOMO) dispersion is remarkably reduced, but remains noticeable ( approximately 0.1 eV), when a short pi-stacking styrene wire is cut from an infinitely long wire and sandwiched between metal electrodes. We find that the styrene chain's HOMO and lowest unoccupied molecular orbital (LUMO) states are not separated from Si, indicating that it does not work as a wire. By substituting -NO2 or -NH2 for the top -H of styrene, we are able to shift the position of the HOMO and LUMO with respect to the Fermi level. More importantly, we find that the HOMO of styrene-NH2 falls into the band gap of the substrate and is localized in the pi-stacking chain, which is what we need for a wire to be electrically separated from the substrate. The conductance of such an assembly is comparable to that of Au/benzene dithiolate/Au wire based on chemical bonding, and its tunability makes it a promising system for a molecular device.

Spectroscopic evidence of strongπ−πinterorbital interaction in a lead-phthalocyanine bilayer film attributed to the dimer nanostructure

We demonstrate spectroscopic evidence of $2t$ level splitting of the highest occupied molecular orbital (HOMO) state, which is characterized by twice the intermolecular transfer integral $t$, with ultraviolet photoelectron spectroscopy in a well-ordered bilayer of lead-phthalocyanine deposited on graphite. The HOMO state splits clearly by $2t=0.35\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ upon bilayer formation at $295\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and the splitting increases to $0.41\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at $47\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The splitting is ascribed to dimer nanostructures with antiparallel molecular electric dipoles.

Generalization of Einstein relation for doped organic semiconductors

Under the assumption of Gaussian energy distributions of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), analytical expressions of generalized Einstein relation for electron and hole transport in doped organic semiconductor thin films are developed. Numerical calculations show that, although traditional Einstein relation still holds for low carrier concentrations, that is, the diffusion-coefficient-to-mobility ratio in units of kBT/q, with kB the Boltzmann’s constant, T the temperature and q the elementary charge, equals 1. But when the electron (hole) concentration is high, the diffusion-coefficient-to-mobility ratio for electrons (holes) changes strongly with the electron (hole) concentration, the doping level, the mean energy of LUMOs (HOMOs) of the dopant ELd (EHd) and the host EL (EH), as well as their variances. Dopants with ELd EH) affect the diffusion-coefficient-to-mobility ratio mainly in the range of low and middle carrier concentrations, while those with ELd>EL (EHd<EH) have significant effect only in the range of high carrier concentrations. It was found that there can be a maximum in the dependence of the diffusion-coefficient-to-mobility ratio on the quasi-Fermi energy or carrier concentration exist, for appropriate values of the doping level, the mean energy and variance of LUMO or HOMO states of the dopant.

Electrophosphorescent Light Emitting Devices Using Mixed Ligand Ir(III) Complexes

ABSTRACTWe have fabricated phosphorescent light emitting devices using mixed ligand bisorthometalated Ir(III) complexes, chlorobis-(2-phenylpyridinato-N,C2')pyridineiridium(III) [Ir(Cl)(ppy)2(py)] and chlorobis-(2-phenylpyridinato-N,C2')triphenylphosphine iridium(III) [Ir(Cl)(ppy)2P(Ph)3] chlorobis-(2-phenylpyridinato-N,C2')tri-n-butylphosphine iridium(III) [Ir(Cl)(ppy)2P(n-Bu)3], where ppy is the orthometalating ligand. These complexes vary in their HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital) and emissive states in accordance with the φ-accepting abilities of the non-orthometalating ligands. The lifetimes and EL spectrum of the devices were studied and compared.

Chemical selectivity in the dissociative ionization of organic molecules by low-energy positrons.

The ionization and dissociation, induced by positrons having kinetic energies in the range 0.3--3 eV, of a large number of organic molecules have been studied. The required energies for ionization and dissociation are supplied through the annihilation of electrons by the positrons. Cross sections for these interactions are strong functions of molecular size, molecular structure, and bond type. Fragmentation usually occurs between atoms linked by \ensuremath{\sigma} bonds. Multiple bonds tend to stabilize the molecules against fragmentation. Fragmentation induced under the low-energy process occurs preferably in the linear groups of molecules rather than in ring structures. Ionization and dissociation cross sections of large alkane molecules are higher than those of smaller molecules. A possible explanation of the fragmentation phenomena is that positrons annihilate electrons in energy levels below the highest occupied molecular orbital (HOMO), leaving the molecules in excited states. Multiple bonds in molecules tend to promote higher populations near the HOMO state; annihilation of electrons near the HOMO state does not result in fragmentation.