Highest Occupied Molecular Orbital-lowest Unoccupied Molecular Orbital Research Articles

Mechanisms of large red-shift and intensity enhancement of absorption bands by π-bond breaking in a benzoporphyrin sensitizer for photodynamic therapy

A free-base monobenzoporphyrin photosensitizer called verteporfin for photodynamic therapy (PDT) has a benzopyrrole ring with a partial π-bond breaking. The π-bond breaking significantly red-shifts the absorption band (so-called Qy-band) to 688 nm and enhances the absorption intensity. In this paper, we theoretically study the mechanisms of the red-shift and intensity enhancement in verteporfin. Density functional analysis at the B3LYP-6-31G(d,p) level of theory reveals that the π-bond breaking increases the energies of the highest occupied molecular orbital (HOMO) and second lowest unoccupied molecular orbital (LUMO+1) by 0.28 and 0.11 eV, respectively, and decreases the energy of the lowest unoccupied molecular orbital (LUMO) by 0.08 eV, in contrast to the unmoving second lowest occupied molecular orbital (HOMO-1). In the consequence of the higher energy shift of the HOMO and lower energy shift of the LUMO, the HOMO–LUMO gap is significantly narrowed. Therefore, the absorption red-shift in verteporfin is attributed to the HOMO–LUMO gap narrowing. On the other hand, the molecular-orbital shifts split the four electronically excited states, (HOMO, LUMO), (HOMO, LUMO+1), (HOMO-1, LUMO), and (HOMO-1, LUMO+1). The splitting weakens the configuration interactions between the excited states, which enhances the transition probability of the Qy-band.

Electronic and geometric structure of AuxCuy clusters studied by density functional theory

We have determined the stable structures of Au Cu n, Au 2 Cu n, Au 3 Cu n and Au x Cu 8-x clusters. It has been observed that Au Cu n, Au 2 Cu n and Au 3 Cu n systems have two-dimensional (2D) structures up to six atoms and they become three-dimensional (3D) afterwards. Au x Cu 8-x clusters favor 3D structures till the Au 7 Cu 1 cluster. We have found a lowest energy isomer of Au 6 Cu 2 from the literature. Bond lengths, binding energies, density of states (DOS), highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO-LUMO) gaps, ionization potential (IP) and electron affinity (EA) have been calculated for these structures using the first principles density functional theory (DFT) within the generalized gradient approximation (GGA) and the local density approximation (LDA). Generally, we have observed the overlap between s electrons of Cu and p electrons of Au near the Fermi level. Charge transfers are calculated by using the Löwdin analysis. It is observed that one Cu atom does not significantly modify the clusters which have more gold atoms. It is also seen that these clusters generally have nonmagnetic properties and results are consistent with the hybridization between s and d orbitals of Au in Au x Cu 8-x clusters.

Ultrafast Dynamical Study of Pyrene-N,N-dimethylaniline (PyDMA) as an Organic Molecular Diode in Solid State

Femtosecond optical pump-probe spectroscopy has been employed for studying the directly linked electron donor-acceptor system pyrene-N,N-dimethylaniline (PyDMA) in solid state. This DMA-pyrene derivative discussed is being applied as a molecular diode system switching on an ultrafast time scale. Our ultrafast solid-state studies reveal a complex photochemistry of this molecular crystal system. Strong couplings of the optically induced charge-transfer state with the radical ion pair state allow a femtosecond transition of the latter. One could see on the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) description that a pure optical transition switches the system from a conducting to a blocked system because the molecular orbitals (MOs) of DMA moiety lie in a node plane of the LUMO. Within 800 fs the system relaxes back to the ground state and/or forms a radical ion pair, which is the surprising result of our study; when the system was probed further, the system underwent vibrational cooling and enhanced population inversion of the radical ion pair.

First-principles study of Carbz-PAHTDDT dye sensitizer and two Carbz-derived dyes for dye sensitized solar cells

Two new carbazole-based organic dye sensitizers are designed and investigated in silico. These dyes are designed through chemical modifications of the π-conjugated bridge of a reference organic sensitizer known as Carbz-PAHTDDT (S9) dye. The aim of designing these dyes was to reduce the energy gap between their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and to red-shift their absorption response compared to those of the reference S9 dye sensitizer. This reference dye has a reported promising efficiency when coupled with ferrocene-based electrolyte composition. To investigate geometric and electronic structure, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were conducted on the new dyes as well as the reference dye. The present study indicated that the long-range correction to the theoretical model in the TD-DFT simulation is important to produce accurate absorption wavelengths. The theoretical studies have shown a reduced HOMO-LUMO gap and red-shifted absorption spectra for both of the new candidate dyes. In particular, the new S9-D1 dye is found to have significant reduced HOMO-LUMO energy gap, greater push-pull character and higher wavelengths of absorption when compared to the reference dye. Such findings suggest that the new dyes are promising and suitable for optoelectronic applications.

An ab initio study of the interaction of a single Li atom with single-walled SiGe (6,6) nanotubes and consequences of Jahn–Teller effect

A systematic study of a Li atom and four types of SiGe (6,6) nanotubes (NTs) of length ~20 A has been performed to explore SiGeNTs’ viability as anode material in Li-ion batteries. For all structures, full geometry and spin optimizations were performed without any symmetry constraints. We used the hybrid functional B3LYP and an all electron 6-311G**//3-21G* basis set implemented in GAUSSIAN 09 suite of software. All possible internal and external adsorption sites were considered, and it was found that some tubes deformed as a result of the Jahn–Teller effect. Out of all four types, type III NTs were found to retain the tubular shape for most cases of adsorption. Our calculations predict that SiGeNTs attract Li atom on top of Si, on top of Ge, and on top of hollow sites with adsorption energies ranging from 0.888 to 1.657 eV. Most probable sites for external and internal adsorptions were quasi on top of Ge site and quasi hollow site, respectively. After adsorption of the Li atom, Jahn–Teller active orbitals of the systems vanished from the frontier orbital region, indicating that more Li atoms can be adsorbed while keeping the desired tubular shape. In general, the difference between the highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level decreased with respect to the bare SiGeNT values. The decreased HOMO–LUMO gaps vary between 0.685 and 0.930 eV. Charge population analysis indicated that charge generally accumulated around the nearest neighbor atoms to the Li atom. We conclude that with the careful placement of Li atoms, SiGeNTs can be very successful as anode material in Li-ion battery technology.

Density Functional Theory Study on the Oxidation of Hydrosilylated Silicon Nanocrystals

As a leading surface modification approach, hydrosilylation enables freestanding silicon nanocrystals (Si NCs) to be well dispersed in a desired medium. Although hydrosilylation-induced organic layers at the NC surface may somehow retard the oxidation of Si NCs, oxidation eventually occurs to Si NCs after relatively long time exposure to air. We now investigated the oxidation of hydrosilylated Si NCs in the frame work of density functional theory (DFT). Three oxygen configurations that may be introduced by the oxidation of a Si NC are considered. It is found that a hydrosilylated Si NC is less prone to oxidation than a fully H-passivated Si NC in the point of view of thermodynamics. At the ground state, backbond oxygen (BBO) and hydroxyl (OH) hardly change the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a hydrosilylated Si NC. At the excited state, the decrease in the HOMO–LUMO gap induced by the introduction of doubly bonded oxygen (DBO) is more significant than that induced by the introduction of BBO or OH. We have correlated the changes in the optical absorption (emission) of a hydrosilylated Si NC after oxidation to those of the HOMO–LUMO gap at the ground state (excited state).

New pyrido[3,4-b]pyrazine-based sensitizers for efficient and stable dye-sensitized solar cells

A series of new pyrido[3,4-b]pyrazine-based organic sensitizers (PP-I and APP-I–IV) containing different donors and π-spacers have been synthesized and employed in dye-sensitized solar cells (DSSCs). The absorption spectra properties of dyes are analysed by density functional theory (DFT). The calculated results in combination with the experiments suggest that the absorption characteristics and excited state features will mainly be dominated by charge transfer transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and to higher LUMO orbitals. Furthermore, attaching the octyloxy groups significantly extends the π-conjugation of the donor in APP-IV, which raises the HOMO energy and facilitates its oxidation. As a consequence, APP-IV exhibits the lowest HOMO–LUMO energy gap among all dyes, which, in turn, corresponds well with the red shift of the absorption spectra. Transient photovoltage and photocurrent decay experiments as well as electrochemical impedance spectroscopy indicate that the electron lifetime and charge recombination resistance are increased due to the introduction of octyloxy chains on the donor unit, resulting in the high photovoltage based on APP-IV. It was found that APP-IV based DSSCs with liquid electrolyte display the highest power conversion efficiency (PCE) of 7.12%. Importantly, a PCE of 6.20% has been achieved for APP-IV based DSSCs with ionic-liquid electrolytes and retained 97% of the initial value after continuous light soaking for 1000 h at 60 °C. This renders these pyrido[3,4-b]pyrazine-based sensitizers quite promising candidates for highly efficient and stable DSSCs.

Quasiparticle spectra and excitons of organic molecules deposited on substrates:G0W0-BSE approach applied to benzene on graphene and metallic substrates

We present an alternative methodology for calculating the quasiparticle energy, energy loss, and optical spectra of a molecule deposited on graphene or a metallic substrate. To test the accuracy of the method it is first applied to the isolated benzene (${\text{C}}_{6}{\text{H}}_{6}$) molecule. The quasiparticle energy levels and especially the energies of the benzene excitons (triplet, singlet, optically active and inactive) are in very good agreement with available experimental results. It is shown that the vicinity of the various substrates [pristine/doped graphene or (jellium) metal surface] reduces the quasiparticle highest occupied molecular orbital--lowest unoccupied molecular orbital (HOMO-LUMO) gap by an amount that slightly depends on the substrate type. This is consistent with the simple image theory predictions. It is even shown that the substrate does not change the energy of the excitons in the isolated molecule. We prove (in terms of simple image theory) that energies of the excitons are indeed influenced by two mechanisms which cancel each other. We demonstrate that the benzene singlet optically active (${E}_{1u}$) exciton couples to real electronic excitations in the substrate. This causes it substantial decay, such as $\ensuremath{\Gamma}\ensuremath{\approx}174$ meV for pristine graphene and $\ensuremath{\Gamma}\ensuremath{\approx}362$ meV for metal surfaces as the substrate. However, we find that doping graphene does not influence the ${E}_{1u}$ exciton decay rate.

Investigation of Carbon Monoxide Adsorption on Cationic Gold- Palladium Clusters

Density functional calculations have been performed for the carbon monoxide molecule adsorption on Au n Pd+ m (n+m ≤ 6) clusters. In the process of CO adsorption, small Au clusters and Pd clusters tend to be an Au atom and three Pd atoms adsorption, respectively. For the mixed Au-Pd clusters, an Au atom, a Pd atom, two atoms consisted of an Au atom and a Pd atom, two Pd atoms, and three Pd atoms adsorption structures are displayed. The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps and natural bond orbital charge population are calculated. Moreover, CO adsorption energy, CO stretching frequency, and CO bond length (upon adsorption) are also analysed in detail. The results predict that the adsorption strength of Au clusters with CO and the C-O vibration strength is enhanced and reduced after doping of Pd in the Au n Pd m CO+ complexes, respectively

Bond overlap population analysis on through-space electron transfer in [3n]cyclophanes (n = 2 – 5)

Through-space electron transfer between the benzene rings of [3n]cyclophanes, n=2 – 5, was investigated using density functional theory calculations, CAM-B3LYP/6-311++G**. An external point charge, q, was placed on the axis perpendicular to a benzene ring of the cyclophane molecule. The dependence of the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap and bond overlap population (BOP) of cyclophanes on the values of q and n are discussed. The HOMO–LUMO gap generally decreases as q decreases and n increases. It is found from the BOP analysis that electron transfer occurs mainly through LUMO+2 for n=2, LUMO for n=3 and n=4, and LUMO+4 for n=5. The result that the values of the BOP for n=2 and 5 increase as q decreases means that the through-space electron transfer is enhanced by the negative charge on the neighboring molecule in the solid.

Vibrational spectra, optical properties, NBO and HOMO–LUMO analysis of L-Phenylalanine L-Phenylalaninium Perchlorate: DFT calculations

In this work, we report a combined experimental and theoretical study of a nonlinear optical material, L-Phenylalanine L-Phenylalaninium Perchlorate. Single crystals of the title compound have been grown by slow evaporation of an aqueous solution at room temperature. Theoretical calculations were preceded by redetermination of the crystal X-ray structure. The compound crystallizes in the non-centro symmetric space group P212121 of the orthorhombic system. The FT-IR and Raman spectra of the crystal were recorded and analyzed. The density functional theory (DFT) computations have been performed at B3LYP/6-31G(d) level to derive equilibrium geometry, vibrational wavenumbers, intensity and NLO properties. All observed vibrational bands have been discussed and assigned to normal mode or to combinations on the basis of our DFT calculations as a primary source of attribution and also by comparison with the previous results for similar compounds. Natural bond orbital analysis was carried out to demonstrate the various inter-and intramolecular interaction that are responsible of the stabilization of the compound. The lowering of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy gap appears to be the cause of its enhanced charge transfer interaction leading to high NLO activity.

Modeling of efficient charge transfer materials of 4,6-di(thiophen-2-yl)pyrimidine derivatives: Quantum chemical investigations

The 4,6-di(thiophen-2-yl)pyrimidine has alternate pi-rich and pi-poor units. To reduce the HOMO–LUMO energy gap and improve the intra-molecular charge transfer pi-backbone has been elongated along with push–pull strategy. The ground state geometries have been optimized by using density functional theory. The frontier molecular orbitals, i.e., highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) have been conferred. The absorption spectra have been computed by using time dependent density functional theory. On the basis of ionization potentials, electron affinities and reorganization energies charge transfer properties have been discussed. We tuned the electronic, photophysical and charge transfer properties of 4,6-di(thiophen-2-yl)pyrimidine derivatives. It is expected that new designed derivatives might be better/comparable to commonly used hole transfer material (pentacene). The smaller reorganization energies revealing that the electron transfer properties of new designed derivatives might be better/comparable to commonly used electron transfer materials (tris(8-hydroxyquinolinato)aluminum). The structure–property relationship has been discussed.

Photo‐Stable Organic Thin‐Film Transistor Utilizing a New Indolocarbazole Derivative for Image Pixel and Logic Applications

Small molecule pentacene layer has been a representative among many organic thin‐film transistor (OTFT) channels with decent p‐type mobilities, but it is certainly light‐sensitive due to its relatively small highest occupied molecular orbital‐lowest unoccupied molecular orbital (HOMO‐LUMO) gap (1.85 eV). Although a few other small molecule‐based layers have been reported later, their photo‐stabilities or related device applications have hardly been addressed. Here, a new photostable organic layer is reported, heptazole (C26H16N2), which has almost the same HOMO level as that of pentacene but with a higher HOMO‐LUMO gap (≈2.95 eV). This heptazole OTFT displays a decent mobility comparable to that of conventional amorphous Si TFTs, showing good photostability unlike pentacene OTFTs. An image pixel driving the photostable heptazole OTFT connected to a pentacene/Al Schottky photodiode is demonstrated. This heptazole OTFT also conveniently forms a logic inverter coupled with a pentacene OTFT, sharing Au for source/drain.

4,4′-Dithiodipyridine on Au(111): A Combined STM, STS, and DFT Study

We studied the adsorption of 4,4′-dithiodipyridine (PySSPy) on Au(111) under ultrahigh vacuum conditions and at low-temperature both experimentally by means of scanning tunneling microscopy and spectroscopy (STM and STS) and theoretically by density functional theory (DFT). We find PySSPy molecules, characterized by their elongated appearance in STM, assembled in islands that reside exclusively in fcc regions of the herringbone reconstructed Au (HB) terraces. A triangular structure motif dominates the local arrangement of the PySSPy. DFT calculations reveal a virtually planar adsorption geometry of the PySSPy with the S–S bond of the molecule almost parallel to the Au–Au bonds of the substrate underneath. Though van der Waals forces dominate the stability of the adsorbed PySSPy, there is also a covalent contribution to the PySSPy/Au interaction. As a consequence, the PySSPy structures are relatively unstable. As found by STS, the highest occupied molecular orbital (HOMO) is located at around −0.7 eV below the Fermi energy (Ef), in good agreement with the HOMO provided by DFT. The position of the lowest unoccupied molecular orbital (LUMO) is about 3.2 eV above Ef, leaving the HOMO–LUMO gap practically unchanged compared to a free molecule. The molecules exhibit a strong propensity to dissociate into monomers forming pyridine thiolates (PyS) at step edges or elbow sites of the HB. At room temperature, all PySSPy on the substrate dissociate resulting in linear chains of PyS characterized by a drop of the LUMO by about −0.9 eV.

Quantum chemical calculations and experimental studies of third-order nonlinear optical properties of conjugated TTF–quinones

To investigate microscopic third-order nonlinear optical (NLO) behaviour of two tetrathiafulvalene (TTF) derivatives, TTF–diquinone triad (1) and TTF–monoquinone dyad (2), we have computed both dispersion-free and also dispersion of dipole polarizabilities (α) and third-order hyperpolarizabilities (γ) at 532nm wavelength using time-dependent Hartree–Fock (TDHF) method. The one-photon absorption (OPA) characterizations of the title molecules have been theoretically obtained by means of configuration interaction (CI) method with all doubly occupied molecular orbitals. We have also calculated the dynamic third-order susceptibilities (χ(3)) using the TDHF method. Our theoretical results on the maximum OPA wavelengths, third-order susceptibilities and corresponding microscopic NLO responses are in good agreement with the previous experimental observations of the examined TTF-based molecules. The highest occupied molecular orbitals (HOMO), the lowest unoccupied molecular orbitals (LUMO) and the HOMO–LUMO band gaps for 1 and 2 have been evaluated by density functional theory (DFT) quantum mechanical calculations at B3LYP/6–31G(d,p) level.

Electronic Properties of [Core+exo]-type Gold Clusters: Factors Affecting the Unique Optical Transitions

Unusual visible absorption properties of [core+exo]-type Au6 (1), Au8 (2), and Au11 (3) clusters were studied from experimental and theoretical aspects, based on previously determined crystal structures. Unlike conventional core-only clusters having no exo gold atoms, these nonspherical clusters all showed an isolated visible absorption band in solution. Density functional theory (DFT) studies on corresponding nonphenyl models (1'-3') revealed that they had similar electronic structures with discrete highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) bands. The theoretical spectra generated by time-dependent DFT (TD-DFT) calculations agreed well with the experimentally measured properties of 1-3, allowing assignment of the characteristic visible bands to HOMO-LUMO transitions. The calculated HOMO-LUMO transition energies increased in the order Au11 < Au6 < Au8, as was found experimentally. Frontier orbital analyses indicated that the HOMO and LUMO were both found in proximity to the terminal Au3 triangles containing the exo gold atom, with the HOMO → LUMO transition occurring in the core → exo direction. The HOMO/LUMO distribution patterns of 1' and 3' were similar to each other but were markedly different from that of 2', which has longer core-to-exo distances. These findings showed that not only nuclearity (size) but also geometric structures have profound effects on electronic properties and optical transitions of the [core+exo]-type clusters.

First-principles studies of peptide fibres

The electronic properties of a single Gly, Cys and Trp peptide fibres (PFs), in separated and closely spaced forms, are studied with ab initio calculations. By decreasing the distance between separated Gly and Cys PFs, respectively, the energy gap between highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of separated fibres tends to narrow down and we investigate the smallest HOMO–LUMO gap for the separated Trp PFs of 3.36 eV, without any electronic states between the gap. The band gap of closely spaced Trp and Gly PFs is predicted to be 0.26 and 3.31 eV, indicating an organic semiconductor character. Series of deep electronic bands (DEBs) in the band gap are found for the closely spaced Cys PFs, helping recombination in the system with the largest DEB gap of 2.26 eV. The electronic properties of all three fibres are mainly decided by the peptide bonds and the functional side chains. The results show that design of the side chains on the PF backbone is a promising method for the application of organic semiconductor and biological compatible devices.

Theoretic design of 1,2,3,4-tetrazine-1,3-dioxide-based high-energy density compounds with oxygen balance close to zero

Density functional theory method was used to study the heats of formation (HOFs), electronic structure, energetic properties, and thermal stability for a series of 1,2,3,4-tetrazine-1,3-dioxide derivatives with different substituents and bridge groups. It is found that the groups –NO2, –C(NO2)3, and –N=N– play a very important role in increasing the HOFs of the derivatives. The effects of the substituents on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels and HOMO–LUMO gaps are coupled to those of different substituents and bridges. The calculated detonation velocities and pressures indicate that the group –NO2, –NF2, –ONO2, –C(NO2)3, or –NH– is an effective structural unit for enhancing the detonation performance for the derivatives. An analysis of the bond dissociation energies for several relatively weak bonds indicates that incorporating the groups –NO2, –NF2, –ONO2, –C(NO2)3, and –N=N– into parent ring decreases their thermal stability. Considering the detonation performance and thermal stability, 18 compounds may be considered as the target compounds holding the greatest potential for synthesis and use as high-energy density compounds. Among them, the oxygen balances of four compounds are equal to zero. These results provide basic information for the molecular design of the novel high-energy compounds.

Atomic structure and edge magnetism in MoS2+x parallelogram shaped platelets

The structural, electronic, and magnetic properties of MoS(2+x) parallelogram shaped platelets having m and n Mo atoms on the adjoining edges have been studied using first principles calculations and by varying m and n from 1 to 6. These platelets have 100% S coverage on two adjoining edges while 50% S coverage on the other two edges. The structural stability of the platelets increases with size but the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap in general decreases. There is a triangular metallic corner at the intersection of 100% S covered edges of the platelets when m = n. On the other hand magnetism is observed on the 50% S covered edges of the platelets for the sizes greater than that of the (3,4) platelet. The magnetic moments mainly arise from the undercoordinated S(2c) atoms at the 50% S covered edges rather than from Mo atoms. The criteria for the existence of the magnetic moments on S(2c) atoms are suggested and the electronic structure of the platelets on the edges as well as inside is discussed.

A CBP derivative as bipolar host for performance enhancement in phosphorescent organic light-emitting diodes

N,N′-Dicarbazolyl-4,4′-biphenyl (CBP) is one of the most successful uni-polar host materials for phosphorescent organic light-emitting diodes (PhOLEDs). We report the synthesis and properties of one novel CBP derivative, CBP-CN, with two cyano groups (CN) at the 3-site of carbazole rings. The strong electron-withdrawing CN group was introduced with the expectation to promote electron-injecting/-transporting abilities and to achieve bipolar features for CBP-CN. In comparison with the parent CBP, CBP-CN possesses lowered HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels and dramatically increased Tg (glass transition temperature, 162 °C), but unaltered HOMO–LUMO band gap and triplet energy (2.69 eV). Green and red PhOLEDs were fabricated with CBP-CN as hosts for traditional iridium phosphors. The maximum luminance efficiency (ηL) of 80.61 cd A−1 (23.13%) was achieved for the green PhOLED, and 10.67 cd A−1 (15.54%) for the red one, which represent efficiency increases of 25–33% compared with those of the best devices with CBP host and are even among the best data for phosphorescent OLEDs reported so far. The theoretical calculation and the carrier-only devices investigation confirmed that the electron-injecting/-transporting character and the bipolar nature of CBP-CN should be responsible for the performance enhancements.