Molecular Orbital Excitation Research Articles

Synthesis, characterization, DFT, and TD-DFT studies of (E)-5-((4,6-dichloro-1,3,5-triazin-2-yl)amino)-4-hydroxy-3-(phenyldiazenyl)naphthalene-2,7-diylbis(hydrogen sulfite)

In this study, (E)-5-((4,6-dichloro-1,3,5-triazin-2-yl)amino)-4-hydroxy-3-(phenyldiazenyl)naphthalene-2,7-diylbis(hydrogen sulfite), a cyanurated H-acid (CHA) azo dye, was synthesized and characterized using FT-IR spectrophotometer and GC-MS spectroscopy. A density functional theory (DFT) based B3LYP and CAM-B3LYP method with 6–311 + G (d,p) basis set analysis was computed for HOMO-LUMO, natural bonding orbitals (NBO), UV-Vis absorptions and excitation interactions, in order to understand its molecular orbital excitation properties. A low Energy gap (Eg) of 2.947 eV was obtained from the molecular orbital analysis, which showed that HOMO to LUMO transition is highly feasible; hence CHA is adequate for diverse electronic and optic applications. Studies of the first five excitations (S0 → S1/S2/S3/S4/S5) of CHA revealed that S0 → S1 and S0 → S3 are π → π* type local excitations distributed around the –N=N– group; S0 → S2, a Rydberg type local excitation; S0 → S4, a highly localized π → π* excitation; while S0 → S5 is an n → π* charge transfer from a benzene ring to –N=N– group. From NBO analysis, we obtained the various donor–acceptor orbital interactions contributing to the stabilization of the studied compound. Most significantly, some strong hyper-conjugations (n → n*) within fragments, and non-bondingand anti-bonding intermolecular (n → n*/π* and π → n*/π*) interactions were observed to contribute appreciable energies. This study is valuable for understanding the molecular properties of the azo dyes compounds and for synthesizing new ones in the future.

DFT and TD-DFT studies on osmacycle dyes with tunable photoelectronic properties for solar cells

B3LYP and CAM-B3LYP functionals have been used to determine structures, electronic and optical properties of osmium-bridged tricyclic aromatic compounds. Calculations show that the optical properties and charge separation features of these osmacycle derivatives can be well modified by incorporating different π-bridge groups. In particular, the newly designed osmacycle dyes 5 and 6 by embedding thiophene and thienothiophene bridge units to osmium polycyclic aromatic system show very strong and broad adsorptions in the whole visible region and excellent charge separation in the first excited state of 1(ππ*) from the hightest occupied molecular orbital to the lowest unoccupied molecular orbital excitation. Furthermore, the predicted relatively high light harvesting efficiency and large driving force for electron injection suggest that they are quite promising for design of high-performance dye-sensitized solar cells.

Core-Level Spectroscopy of Point Defects in Single Layerh-BN

Electron energy-loss spectroscopy (EELS) is used to analyze single-layered hexagonal boron-nitride with or without point defects. EELS profiles using a 0.1 nm probe clearly discriminate the chemical species of single atoms but show different delocalization of the boron and nitrogen K edges. A monovacancy at the boron site is unambiguously identified and the electronic state of its nearest neighboring nitrogen atoms is examined by energy-loss near edge fine structure analysis, which demonstrates a prominent defect state. Theoretical calculations suggest that the observed prepeak originates from the 1s to lowest unoccupied molecular orbital excitation of dangling nitrogen bonds, which is substantially lowered in energy with respect to the three coordinated nitrogen atoms.

Density functional study of structural, electronic, and optical properties of small bimetallic ruthenium‐copper clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Cu(n)Ru(m)](+/0/-) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 nonlocal hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Cu(n Ru(m)](+/0/-) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and assignments of all principal electronic transitions were made and interpreted in terms of contribution from specific molecular orbital excitations.

Structural, electronic, bonding, magnetic, and optical properties of bimetallic [RunAum]0/+ (n + m ≤ 3) clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 non local hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Ru(n)Au(m)](0/+) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and compared to those of pure gold and ruthenium clusters. Assignments of all principal electronic transitions are given and interpreted in terms of contribution from specific molecular orbital excitations.

Interaction of doxorubicin with the subcellular structures of the sensitive and Bcl-xL-overexpressing MCF-7 cell line: Confocal and low-energy-loss transmission electron microscopy

The present investigation was directed to examine the interaction of the anti-cancer agent doxorubicin (DOX) with the subcellular compartments of the drug sensitive and Bcl-xL-overexpressing (Bcl-xL) MCF-7 cells using confocal and low-energy-loss transmission electron microscopy (LELTEM). Intracellular detection of DOX with LELTEM was carried out without specific antibodies or heavy metal stains but via the electron-induced molecular orbital excitation of the drug. Cells were incubated with 10 μM DOX for 1 min, 1, 24, and 48 h and then examined live by confocal microscope and as very thin sections in an electron microscope equipped with an energy filter having an energy resolution of 1 eV. Ultrastructural localization of DOX, obtained from pairs of images taken at energy losses of 3 ± 1 and 10 ± 1 eV, were analyzed and correlated with the confocal observations. When the sensitive and Bcl-xL cells were examined under the confocal microscope after 1 min, DOX uptake could not be detected in the nuclei nor in the cytoplasm whereas LELTEM observation revealed that at this stage of incubation the drug has already been incorporated by both cell types and that the nuclear membrane, nucleolus, and mitochondria of the Bcl-xL cells were temporally less DOX-responsive as compared to the sensitive cells. As the incubation time increased, nuclear membranes and nucleoli of both cell types appeared equally sensitive to DOX, nonetheless, mitochondria of the Bcl-xL cells remained invulnerable to DOX for 24 h. The results point to LELTEM feasibility to better characterize yet unresolved cellular events caused by DOX and suggest a transitory role for Bcl-xL overexpression in protecting the cellular compartments from DOX invasion.

Low-energy-loss electron microscopy of doxorubicin in human breast cancer MCF-7 cells: Localization by color

The distribution of the anti-cancer drug doxorubicin (DOX) in human breast cancer MCF-7 cells was imaged directly by low-energy-loss electron microscopy (EM) without specific antibodies or heavy metal stains, using only the electron-induced molecular orbital excitation of the drug. Cells treated with DOX were examined live by confocal fluorescence microscopy and as very thin sections in an electron microscope equipped with an electron energy filter having an energy resolution of 1 eV. The distribution of DOX obtained by EM from pairs of images at energy losses of 3 ± 1 eV and 10 ± 1 eV agreed with fluorescence microscope observations, but provided much more detail, easily distinguishing localization between nuclear membrane and perimembrane compartments and between vacuolated nucleoli and perinucleolar chromatin. Treatment times up to 1 h and DOX concentrations up to 30 μM indicated a progression of DOX ingress from higher concentrations in the nuclear membrane to labeling of the nucleolus. Subsequently DOX moved into perinucleolar chromatin and concentrated in perimembrane chromatin aggregations. Quantification of the DOX signal indicated a decay half-life of 320 e/Å 2 under electron irradiation, whereas each image at 3000× required 10 e/Å 2. The results point to a new field of high resolution microanalysis: color electron microscopy.

Resonant inelastic x-ray scattering study of overdopedLa2−xSrxCuO4

Resonant inelastic x-ray scattering (RIXS) at the copper K absorption edge has been performed for heavily overdoped samples of La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x= 0.25$ and 0.30. We have observed the charge transfer and molecular-orbital excitations which exhibit resonances at incident energies of $E_i= 8.992$ and 8.998 keV, respectively. From a comparison with previous results on undoped and optimally-doped samples, we determine that the charge-transfer excitation energy increases monotonically as doping increases. In addition, the $E_i$-dependences of the RIXS spectral weight and absorption spectrum exhibit no clear peak at $E_i = 8.998$ keV in contrast to results in the underdoped samples. The low-energy ($\leq 3$ eV) continuum excitation intensity has been studied utilizing the high energy resolution of 0.13 eV (FWHM). A comparison of the RIXS profiles at $(\pi ~0)$ and $(\pi ~\pi)$ indicates that the continuum intensity exists even at $(\pi ~\pi)$ in the overdoped samples, whereas it has been reported only at $(0 ~0)$ and $(\pi ~0)$ for the $x=0.17$ sample. Furthermore, we also found an additional excitation on top of the continuum intensity at the $(\pi ~\pi)$ and $(\pi ~0)$ positions.

Understanding the electronic structure, optical, and vibrational properties of theFe8Br8single-molecule magnet

We present the results of our all-electron density functional calculation on the electronic structure and optical properties of ${[{\mathrm{Fe}}_{8}{\mathrm{O}}_{2}{(\mathrm{O}\mathrm{H})}_{12}{({\mathrm{C}}_{6}{\mathrm{H}}_{15}{\mathrm{N}}_{3})}_{6}{\mathrm{Br}}_{6}]}^{2+}$, the functional building block of the $[{({\mathrm{C}}_{6}{\mathrm{H}}_{15}{\mathrm{N}}_{3})}_{6}{\mathrm{Fe}}_{8}{({\ensuremath{\mu}}_{3}\ensuremath{-}\mathrm{O})}_{2}{({\ensuremath{\mu}}_{2}\ensuremath{-}\mathrm{O}\mathrm{H})}_{12}]{\mathrm{Br}}_{7}({\mathrm{H}}_{2}\mathrm{O})\mathrm{Br}.8{\mathrm{H}}_{2}\mathrm{O}$ single-molecule magnet. The calculated results are compared with the experimentally measured optical spectrum, and the primary features of the excitations are assigned. Excellent agreement is obtained. The calculated spin minority gap is $0.23\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, the spin majority gap is $0.54\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and the spin majority to minority gap is $0.15\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Notably, the low-energy spin minority gap is not a dipole-allowed excitation. Experimentally, we find that $[{({\mathrm{C}}_{6}{\mathrm{H}}_{15}{\mathrm{N}}_{3})}_{6}{\mathrm{Fe}}_{8}{({\ensuremath{\mu}}_{3}\ensuremath{-}\mathrm{O})}_{2}{({\ensuremath{\mu}}_{2}\ensuremath{-}\mathrm{O}\mathrm{H})}_{12}]{\mathrm{Br}}_{7}({\mathrm{H}}_{2}\mathrm{O})\mathrm{Br}.8{\mathrm{H}}_{2}\mathrm{O}$ is a semiconductor, with a dipole-allowed optical gap of approximately $0.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (peak position at $\ensuremath{\sim}0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) and an additional optically observable electronic excitation centered at $0.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The latter feature is weak and broad, with superimposed ($\mathrm{C}\mathrm{H}$, $\mathrm{N}\mathrm{H}$, and $\mathrm{O}\mathrm{H}$) vibrational structure. We suggest that the $0.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ peak in the optical conductivity may be attributable to the spin majority highest occupied molecular orbital to spin minority lowest unoccupied molecular orbital excitation, activated by inclusion of spin-orbit coupling, which mixes states of different symmetry and spin. Overall, we find that the low-energy excitations are dominated by $\mathrm{Fe}\phantom{\rule{0.3em}{0ex}}d\ensuremath{\rightarrow}\mathrm{Fe}\phantom{\rule{0.3em}{0ex}}d$ interionic excitations and that the dipole-allowed excitations involve antiferromagnetically coupled pairs of Fe ions. At higher energy, the features are primarily due to $\mathrm{O}\phantom{\rule{0.3em}{0ex}}p\ensuremath{\rightarrow}\mathrm{Fe}\phantom{\rule{0.3em}{0ex}}d$ charge transfer transitions. A discussion of the vibrational properties is also included along with an analysis of how the electronic structure results support reduced moments on the minority spin Fe ions.

Molecular orbital excitations in cuprates: Resonant inelastic x-ray scattering studies

We report resonant inelastic x-ray scattering studies of electronic excitations in a wide variety of cuprate compounds. Specifically, we focus on the charge-transfer type excitation of an electron from a bonding molecular orbital to an antibonding molecular orbital in a copper oxygen plaquette. Both the excitation energy and the amount of dispersion are found to increase significantly as the copper oxygen bond-length is reduced. We also find that the estimated bond-length dependence of the hopping integral t_pd is much stronger than that expected from tight-binding theory.

Localization of chromophore absorption signals in TEM with an improved prism-mirror-prism filter.

A corrected prism-mirror-prism electron energy filter with curved entrance and exit faces was designed and incorporated into a Zeiss EM902 transmission electron microscope. The installation of the new filter required little modification to the existing microscope geometry and electronics. The filter had an energy resolution of 1.1 eV over the full image plane (acceptance half angle 10 mradian). The improved energy resolution was applied in studies of the low electron energy loss region that includes molecular orbital excitations or chromophore energy absorptions. Chromophore signal behaviour under electron irradiation was characterized using embedded crystals of hematin and of the dye mercury orange. Images of these crystals confirmed the expected decrease in signal intensity on shifting the selected energy loss from the region of molecular orbital excitations (less than approximately 5 eV) to higher energy losses. Electron irradiation-induced fading of the chromophore signal from hematin and mercury orange yielded similar 1/e dose values of 1.1 x 10(5) e(-) nm(-2) and 1.4 x 10(5) e(-) nm(-2) respectively. In a cellular context, chromophore signals were obtained from energy-filtered images of RIF-1 cell sections containing the photosensitizer chlorin e6 and from sections of BS-C-1 cells with cytoskeletal labelling via FITC-conjugated antibodies. The respective signals were extracted using a dose-dependent method or a shift in selected energy. Chromophore signal distributions were in agreement with fluorescence light microscopic images, but provided detail at higher spatial resolution.

Low energy loss electron microscopy of chromophores

A novel prism-mirror-prism imaging electron spectrometer with 1 eV energy resolution for a transmission electron microscope permits imaging with spectral energies corresponding to light-optical colour absorptions. The instrument selects the molecular orbital excitations of natural chromophores or of specific dyes normally used in biological light microscopy for delineation and chemical identification, but images them with electron microscopic detail. Heavy atom contrast agents customarily used in electron microscopy are not required. The first results exploit the intrinsic red colour of hematin molecules to demonstrate the potential of the technique and address its spatial resolution. Glycosaminoglycans in cartilage stained with Alcian blue are selectively depicted in situ by means of the electron-induced molecular absorption of this chromophore. Thus, with the use of specific colours the direct or indirect analysis of local chemistry by electron microscopy is possible, and can be carried out with a depiction of spatial detail as small as 16A˚, or at least 100-fold finer than observed by light microscopy.

Preresonance Raman spectra for imidazole and imidazolium ion: interpretation of the intensity enhancement from a precise assignment of normal modes

The change of the intensity of Raman bands at resonance relative to that observed far from resonance for Im and ImH + interpreted from the change in their geometry in going from the ground to the excited state. Thus, the vibrational composition for each normal mode in the ground state is obtained from a normal coordinate analysis carried out on the basis of ab initio 3-21G force fields. Then, the change in bond length in the excited state is deduced from the change in bond orders calculated for the lowest lying π-π* molecular orbital excitations. This has allowed the calculation of relative resonance Raman intensities which are discussed by comparison with the measured relative intensities for Raman bands at excitation varying from 514.5 to 200 nm. The satisfactory agreement obtained between observed and calculated data gives another criterion for the reliability of the force fields used.

Core excitons and the dielectric response of polystyrene and poly(2-vinylpyridine) from 1 to 400 eV

The complete spectra of valence and core electronic excitations of atactic polystyrene, PS, and poly(2-vinylpyridine), PVP, from 1 to 400 eV were measured by electron energy loss spectroscopy. Energy losses of 80 keV electrons transmitted through 1000 Å thick films were measured with a resolution of 0.1 eV. The spectrum of PS contains several previously unreported broad peaks between 9 and 20 eV which coincide with structure observed in gas phase benzene spectra and which have been attributed to Rydberg states, but which are described here as valence molecular orbital excitations. The dielectric response function of PS and PVP from 1 to 100 eV is computed from energy loss data. The calculated reflectivity and refractive index are in excellent agreement with recent optical measurements. The spectrum of carbon core electron excitations above 285 eV consists of a series of sharp peaks due to transitions to empty molecular states (core excitons) followed by transitions to a free-electron-like continuum of states. The core exciton spectrum is quantitatively described in a model based on a molecular orbital ground state calculation (CNDO/S). The results show that such spectra provide a measurement of the distribution of empty molecular orbitals. In addition, shifted exciton peaks are observed which correspond to inequivalent core electron binding energies due to differences in net atomic charge.

Alternant molecular-orbital excitations by the generator-coordinate method in the electron gas

A combination of the generator-coordinate method and the alternant molecular-orbital method is applied to the electron-gas problem. The resulting ground and collective excited states are analyzed and discussed with reference to their density and spin density. The restricted Hartree-Fock alternant molecular-orbital instability found earlier now shows up as a vanishing excitation frequency.