Frontier Electronic Structure Research Articles

Synthesis and Optoelectronic Features of Rhodamine-Triazole Dyads as Metallochromic Probes for Copper-Selective Chemosensing.

Rhodamine-based chromic materials have attracted significant interest owing to their cation recognition ability with high sensitivity. However, rhodamine chromophores with controllable sensing selectivity towards transition metal species are only at the advent. Herein, three triazole-conjugated rhodamine dyads with different peripheral substituents were synthesized. The key triazole precursors required for the desired chemistry were prepared by adopting our recently developed CTAB catalyzed mechano-click chemistry. Molecular properties derived from photophysics, electrochemistry and surface morphology of the synthesized dyads were analyzed. Furthermore, frontier molecular orbitals, electronic structure and secondary quantum chemical parameters of dyads were also compared. Screening of dyads for their sensing ability towards an array of alkali, alkaline and transition metal ions exhibited a noticeable naked-eye detection of Cu2+ ions and was confirmed by spectrophotometric titration. The specific binding mode of dyads as probable with Cu2+ over other metal ions attributes to the chemoselectivity.

Resonant X-ray photo-oxidation of light-harvesting iron (II/III) N-heterocyclic carbene complexes

Two photoactive iron N-heterocyclic carbene complexes {[hbox {Fe}^{{{rm{II}}}}(hbox {btz})_2(hbox {bpy})]^{2+}} and {[hbox {Fe}^{{rm{III}}}(hbox {btz})_3]^{3+}}, where btz is 3,3’-dimethyl-1,1’-bis(p-tolyl)-4,4’-bis(1,2,3-triazol-5-ylidene) and bpy is 2,2’-bipyridine, have been investigated by Resonant Photoelectron Spectroscopy (RPES). Tuning the incident X-ray photon energy to match core-valence excitations provides a site specific probe of the electronic structure properties and ligand-field interactions, as well as information about the resonantly photo-oxidised final states. Comparing measurements of the Fe centre and the surrounding ligands demonstrate strong mixing of the Fe {hbox {t}_{{rm{2g}}}} levels with occupied ligand pi orbitals but weak mixing with the corresponding unoccupied ligand orbitals. This highlights the importance of pi-accepting and -donating considerations in ligand design strategies for photofunctional iron carbene complexes. Spin-propensity is also observed as a final-state effect in the RPES measurements of the open-shell hbox {Fe}^{{rm{III}}} complex. Vibronic coupling is evident in both complexes, where the energy dispersion hints at a vibrationally hot final state. The results demonstrate the significant impact of the iron oxidation state on the frontier electronic structure and highlights the differences between the emerging class of hbox {Fe}^{{rm{III}}} photosensitizers from those of more traditional hbox {Fe}^{{rm{II}}} complexes.

A soft x-ray probe of a titania photoelectrode sensitized with a triphenylamine dye.

We present a thorough soft x-ray photoelectron spectroscopy (XPS) study of a mesoporous titanium dioxide electrode sensitized with the dye 4-(diphenylamino)phenylcyanoacrylic acid, referred to as "L0." Supported by calculations, the suite of XPS, x-ray absorption spectroscopy, and resonant photoelectron spectroscopy allows us to examine bonding interactions between the dye and the surface and the frontier electronic structure at the molecule-oxide interface. While placing these measurements in the context of existing literature, this paper is intended as a useful reference for further studies of more complex triphenylamine based sensitizers.

Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization.

We revisit the question of kekulene’s aromaticity by focusing on the electronic structure of its frontier orbitals as determined by angle-resolved photoemission spectroscopy. To this end, we have developed a specially designed precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene, which allows us to prepare sufficient quantities of kekulene of high purity directly on a Cu(111) surface, as confirmed by scanning tunneling microscopy. Supported by density functional calculations, we determine the orbital structure of kekulene’s highest occupied molecular orbital by photoemission tomography. In agreement with a recent aromaticity assessment of kekulene based solely on C–C bond lengths, we conclude that the π-conjugation of kekulene is better described by the Clar model rather than a superaromatic model. Thus, by exploiting the capabilities of photoemission tomography, we shed light on the question which consequences aromaticity holds for the frontier electronic structure of a π-conjugated molecule.

Layered Dinitrostilbene-Based Molecular Solids with Tunable Micro/Nanostructures and the Reversible Fluorescent Response to Explosives

The ability to modulate and control the fluorescence properties of molecular solids at the micro/nanoscale is important to develop high-performance optoelectronic materials and sensors. Here we report the tunable one-photon and two-photon fluorescence as well as micro/nanostructures of dinitrostilbene-based (DNS) chromophore by the formation of layered multicomponent crystals with guanidinium cation (GD) through hydrogen-bonding assembly. The as-prepared GD2DNS bulk crystal shows a red-shift emission as well as enhanced photoluminescence quantum yield and fluorescence lifetime compared with those of the Na2DNS sample, which is related to the structural transfer of DNS from staggered arrangement to parallel fashion within the crystal. Periodic density functional theoretical calculations further show that the introduction of different cationic units can modify the frontier orbital distribution and electronic structure of DNS anions within the multicomponent crystals. Moreover, one-dimensional GD2DNS nanobel...

Molecular and spectroscopic properties of hydridecarbonyl ruthenium complexes with pyrazine carboxylic acid ligands

The hydride carbonyl ruthenium(II) [RuH(CO)(pyzCOO)(PPh 3) 2] ( 1), [RuH(CO)(pyz-2,3-COO[CH 3])(PPh 3) 2]·H 2O ( 2) and dinuclear Ru(II)/Ru(III) [RuH(CO)(PPh 3)(pyz-2,3-COO)Ru(CO)Cl 2(PPh 3) 2] ( 3) complexes were synthesized and characterized by IR, 1H, 31P NMR, UV–Vis spectroscopy and X-ray crystallography. The experimental studies were complemented by quantum chemical calculations, which were used to identify the nature of the interactions between the ligands and the central ion, and the orbital composition in the frontier electronic structure. Based on a molecular orbital scheme, the calculated results allowed the interpretation of the UV–Vis spectra obtained at an experimental level. The luminescence property of the complex 2 was determined. The ac magnetic susceptibility measurements showed a residual magnetism evidenced by the small values of the molar susceptibility, not exceeding 0.5 emu/mol at 2 K, a lack of a Curie-Weiss region and weak magnetic interactions below 20 K.

Theoretical Characterization of a Typical Hole/Exciton-Blocking Material Bathocuproine and Its Analogues

The structural, electronic, and carrier transport properties of bathocuproine (BCP), which is a typical hole/exciton-blocking material applied in organic light-emitting diodes (OLEDs), have been investigated based on density functional theory (DFT) and ab initio HF method. The detail characterizations of frontier electronic structure and lowest-energy optical transitions have been studied by means of time-dependent density functional theory (TD-DFT). Five BCP analogues, o-phenanthroline (1), 2,9-dimethyl-1,10-phenanthroline (2), 2,9-diphenyl-1,10-phenanthroline (3), 4,7-diphenyl-1,10-phenanthroline (4), and 2,9-bis(trifluoromethyl)-1,10-phenanthroline (5) have also been studied in order to select more suitable candidates of efficient hole-blocking materials. The calculated results showed that rigid planar structures, conjugate degrees, and substitute groups play crucial roles in the hole/exciton-blocking and electron-transport properties of these materials. The calculated geometries, ionization energies (IP), and energy gap between the singlet ground state and triplet excited state (E(T1)) were well in agreement with the experimental results. On the basis of the incoherent transport model, the calculated electron mobility of BCP is 1.79 x 10(-2) cm(2)/(V s), which is comparable to experimental results of 1.1 x 10(-3) cm(2)/(V s). The electron mobilities for compounds 1, 4, and 5 are 3.45 x 10(-2), 2.90 x 10(-2), and 1.40 x 10(-2) cm(2)/(V s), respectively. The calculated results indicated that compounds 1, 4, and 5 may be more effective hole/exciton-blocking materials than BCP.

Near-edge x-ray absorption studies of Na-doped tetracyanoethylene films: A model system for theV(TCNE)xroom-temperature molecular magnet

$\mathrm{V}{(\mathrm{TCNE})}_{x}$, with $\mathrm{TCNE}=\text{tetracyanoethylene}$ and $x\ensuremath{\sim}2$, is an organic-based molecular magnet with potential to be used in spintronic devices. With the aim of shedding light on the unoccupied frontier electronic structure of $\mathrm{V}{(\mathrm{TCNE})}_{x}$ we have studied pristine TCNE and sodium-intercalated TCNE by near edge x-ray absorption fine structure (NEXAFS) spectroscopy as well as with theoretical calculations. Sodium-intercalated TCNE was used as a model system of the more complex $\mathrm{V}{(\mathrm{TCNE})}_{x}$ and both experimental and theoretical results of the model compound have been used to interpret the NEXAFS spectra of $\mathrm{V}{(\mathrm{TCNE})}_{x}$. By comparing the experimental and theoretical C $K$-edge of pristine TCNE, the contributions from the various carbon species (cyano and vinyl) could be disentangled. Upon fully sodium intercalation, TCNE is $n$ doped with one electron per molecule and the features in the C and N $K$-edge spectra of pristine TCNE undergo strong modification caused by partially filling the TCNE lowest unoccupied molecular orbital (LUMO). When comparing the C and N $K$-edge NEXAFS spectra of fully sodium-doped TCNE with $\mathrm{V}{(\mathrm{TCNE})}_{x}$, the spectra are similar except for broadening of the features which originates from structural disorder of the $\mathrm{V}{(\mathrm{TCNE})}_{x}$ films. The combined results from the model system and $\mathrm{V}{(\mathrm{TCNE})}_{x}$ suggest that the lowest unoccupied molecular orbital with density on the nitrogen atoms in $\mathrm{V}{(\mathrm{TCNE})}_{x}$ has no significant hybridization with vanadium and is similar to the so-called singly occupied molecular orbital of the TCNE anion. This suggests that the LUMO of $\mathrm{V}{(\mathrm{TCNE})}_{x}$ is ${\mathrm{TCNE}}^{\ensuremath{-}}$ or vanadiumlike, in contrast to the frontier occupied electronic structure where the highest occupied molecular orbital is a hybridization between $\mathrm{V}(3d)$ and cyano carbons. The completely different nature of the unoccupied and occupied frontier electronic structure of the material will most likely affect both charge injection and transport properties of a spintronic device featuring $\mathrm{V}{(\mathrm{TCNE})}_{x}$.

Electron Spectroscopic Studies of Bis-(2,2‘-bipyridine)-(4,4‘-dicarboxy-2,2‘-bipyridine)-ruthenium(II) and Bis-(2,2‘-bipyridine)-(4,4‘-dicarboxy-2,2‘-bipyridine)-osmium(II) Adsorbed on Nanostructured TiO2and ZnO Surfaces

The adsorption of the dyes bis-(2,2‘-bipyridine)-(4,4‘-dicarboxy-2,2‘-bipyridine)-ruthenium(II) and bis-(2,2‘-bipyridine)-(4,4‘-dicarboxy-2,2‘-bipyridine)-osmium(II) on nanostructured TiO2 and ZnO was investigated by photoelectron spectroscopy (PES) using synchrotron radiation. Comparison of the results gives insight into the electronic structure of the molecules, as well as to their interaction with the substrates. The identity of the central metal ion of the dye complex was found not to influence the bonding mode of the molecule on the surfaces. However, when comparing the adsorption on the two metal oxides, the chemical state of the carboxyl group was found to be different. Binding to TiO2 results in four chemically equivalent carboxyl oxygens, while the oxygens are found chemically nonequivalent for adsorption onto ZnO. In addition, information concerning the frontier electronic structure was obtained by measuring the valence band region by PES using different photon energies and the N1s near edge X-r...

QSAR treatment of electronic substituent effects using frontier orbital theory and topological parameters

A methodology for the estimation of Hammett substituent constants from computational-based descriptors utilizing quantitative structure activity/property relationships (QSAR/QSPR) formalism is presented. Electronic descriptors derived from quantum chemical calculations and molecular topology were used to generate computational-based analogues of empirical Hammett substituent constants from statistical analysis. Global quantum chemical reaction indices were drawn from frontier orbital theory and density functional theory and formulated from AM1-based calculations. A localized index based on the electrotopological state index was used to encode information on individual group properties. From a training set consisting of 150 meta and para-substituted benzoic acids, statistical analysis of computational-based descriptors as a function of empirical substituent constants yielded a five-parameter QSAR/QSPR model which generates computational-based constants exhibiting a strong correlation with empirical values (r2 = 0.958). Both internal (PRESS) and external (independent testing set of benzoic acids) validation procedures suggest that the electronic effects QSAR/QSPR model derived in this work from computational-based parameters is a statistically viable paradigm. Both predicted and empirical constants were used in Hammett-type validation analyses as functions of chemical, biological, and spectroscopic data for thirty structurally diverse meta and parasubstituted aromatic testing sets. Statistical measures of ensuing correlations were examined and compared, and the empirical and predicted results were of similar quality. Validation results reveal that a large number of computational-based substituent constants can be accurately estimated from semiempirical AM1 frontier orbital energies and electronic structure information obtained directly from substituted benzoic acids without the aid of empirical parametrization.

Electronic structure and optical properties of electroluminescent spiro-type molecules

The electronic structure and optical properties of the propeller-shaped spiro molecules, 2,2′-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-9,9′-spirobifluorene, and 2,2′,7,7′-tetrakis (biphenyl-4-yl)-9,9-spirobifluorene, have been studied by photoelectron spectroscopy, Raman spectroscopy, and spectroscopic ellipsometry. The experimental spectra are analyzed with the help of results from quantum-chemical calculations. The excellent agreement between experiment and theory allows for a detailed characterization of the frontier electronic structure and lowest-energy optical transitions in these novel compounds. It follows that most of the electronic and optical properties of the these spiro-molecules can be deduced by consideration of a single branch; in this case a sexi-phenyl and PBD branch. In particular, although the branches in the spiro molecules are perpendicular to each other, there is considerable interaction at the spiro-center between the π-electron systems of the two branches, indicating, among other things, that chemistry involving either branch is dependent on the other branch.

Frontier electronic structure in gas-phase epoxy and hydroxy cyclohexenes by means of photoelectron and electron transmission spectroscopies

Ionisation energy (Ei) and electron affinity (Ea) values in benzene oxide, benzene hydrate, the corresponding monoenes and other related molecules have been measured by means of UV photoelectron spectroscopy and electron transmission spectroscopy. The valence filled levels do not show evidence for strong σco–π or oxygen lone pair–π mixing in the alcohols nor in the epoxides. Although the energy perturbations, caused by the hydroxy and epoxy substituents, to adjacent filled π orbitals are small, the electron transmission spectra show that the empty π* orbitals are affected to very different extents, the epoxy substituent producing a sizeable electron affinity increase. This finding indicates the occurrence of strong interaction between the empty π* orbitals and low-energy empty σ* orbitals localised at the three-membered cycle. The experimental data have been compared with the orbital energies predicted by ab initio 3-21G calculations.