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LUMO Gap (lumo + gap)

Distribution by Scientific Domains

Chemistry	90%

Selected Abstracts

Selective Tuning of the HOMO,LUMO Gap of Carbazole-Based Donor,Acceptor,Donor Compounds toward Different Emission Colors

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 9 2010
Huaqiang Zhang
Abstract Carbazole-based donor,acceptor compounds with tunable HOMO,LUMO gaps were synthesized by Suzuki and Sonogashira cross-coupling reactions. Their optical and electrochemical properties were fully characterized. The results show that materials with different emission colors ranging from blue to green to orange could be obtained. The experimental results were also supported by theoretical calculations. [source]

Density-Functional Calculation of the 183W and 17O NMR Chemical Shifts for Large Polyoxotungstates

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 6 2006
Jose Gracia
Abstract A phosphane sulfate relativistic DFT method (ZORA)has been used to calculate the 183W and 17O NMR chemicalshifts for large polyoxotungstates, including W6O192,,CH3OTiW5O183,, W5O18WIINO3,, W10O324,, ,-,-,-XW12O40n,, ,-PW9O28Br63,, P2W18O626,, PW2O143,, and W7O246,, based on their optimized molecular structures. Despite sizeable deviations between the calculated and experimental values, the calculations correctly reproduce the trends in the change of the chemical shift for both nuclei. As expected, the diamagnetic term is almost constant throughout the whole series. The change in the chemical shifts is shown to be determined by the paramagnetic term, which depends on the electronic structure of the whole anion under study and, in particular, on the local geometry around a given tungsten atom. On the other hand, there is no correlation between the chemical shift and HOMO,LUMO gap, showing that deeper occupied levels and several unoccupied orbitals play an important role in the paramagnetic term. However, analysis of the components of the paramagnetic shielding has shown that the most significant transitions determining the change of both 183W and 17O NMR chemical shifts for anions consisting of tungsten and oxygen atoms are the occupied,occupied and not the occupied,virtual ones.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

Energy-Modulated Heterostructures Made with Conjugated Polymers for Directional Energy Transfer and Carrier Confinement,

ADVANCED FUNCTIONAL MATERIALS, Issue 15 2007
R. Favarim
Abstract In this paper we demonstrate that multilayer structures with modulated bandgaps can be used for efficient energy transfer and carrier confinement inside a nanostructured film of a light-emitting polymer. The films were produced with the layer-by-layer technique (LbL) with a poly(p -phenylene vinylene) (PPV) precursor and a long chain dodecylbenzenesulfonate ion (DBS). DBS is incorporated selectively into the precursor chain, and with a rapid, low temperature conversion process (100,°C) superstructures with variable HOMO,LUMO gap could be formed along the deposition direction by changing the DBS concentration. Structures with different stair-type energy modulations were produced, which are thermally stable and reproducible, as demonstrated by UV-VIS. absorption measurements. Energy differences of up to 0.5,eV between the lowest and highest conjugated layers inside the stair structure could be achieved, which was sufficient to guide the excitation over long distances to the lower bandgap layer. [source]

Domino Diels,Alder Cycloadditions of Arynes: New Approach to Elusive Perylene Derivatives

CHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2010
Alejandro Criado
A game of dominos: Domino [4+2]/[4+2] cycloadditions between 1,8-difurylnaphthalene and arynes provide the corresponding adducts in a highly diastereoselective manner. This new synthetic methodology enables the preparation of elusive perylene derivatives with a remarkable reduced HOMO,LUMO gap, a crucial feature for organic semiconductors. [source]

Synthesis and Characterization of the D5h Isomer of the Endohedral Dimetallofullerene Ce2@C80: Two-Dimensional Circulation of Encapsulated Metal Atoms Inside a Fullerene Cage

CHEMISTRY - A EUROPEAN JOURNAL, Issue 37 2009
Michio Yamada Dr.
Abstract Herein we show the synthesis and characterization of the second known Ce2@C80 isomer. A 13C,NMR spectroscopic study revealed that the structure of the second isomer has D5h symmetry. Paramagnetic NMR spectral analysis and theoretical calculation display that the encapsulated Ce atoms circulate two-dimensionally along a band of ten contiguous hexagons inside a D5h -C80 cage, which is in sharp contrast to the three-dimensional circulation of two Ce atoms in an Ih -C80 cage. The electronic properties were revealed by means of electrochemical measurements. The D5h isomer of Ce2@C80 has a much smaller HOMO,LUMO gap than cluster fullerenes (M3N@C80, M=Sc, Tm, and Lu) with the same D5h -C80 cages. The chemical reactivity was investigated by using disilirane as a chemical probe. The high thermal reactivity toward 1,1,2,2-tetramesityl-1,2-disilirane is consistent with the trends of the redox potentials and the lower LUMO level of the D5h isomer of Ce2@C80 compared with that of C60. [source]

Oligo- and Polyselenophenes: A Theoretical Study

CHEMISTRY - A EUROPEAN JOURNAL, Issue 34 2009
Sanjio
Abstract Recently, a family of conducting polyselenophenes was synthesized, and they were shown to have a number of interesting properties. Here we have studied oligoselenophenes, their cation radicals and dications up to the 50-mer (50,Se), as well as polyselenophene at the B3LYP/6-31G(d) level of theory, and compared them with the corresponding oligothiophenes. Although the calculations reveal many similarities between oligo- and polyselenophenes and their thiophene-based counterparts, they also show the important differences between those two types of conjugated systems. Oligo- and polyselenophenes have a more quinoid character, lower band gap, and importantly, they are more difficult to twist. The theoretical results suggest that the HOMO,LUMO gap (band gap), bond-length alternation (BLA), and charge distribution in oligo- and polyselenophenes are strongly dependent on inter-ring twisting, yet twisting costs little energy. The inter-ring distances in oligo- and polyselenophenes are shorter than the related distances in oligothiophenes, whereas the bond lengths within the selenophene rings are comparable to those of the corresponding oligothiophenes. [source]

Novel Zinc Porphyrin Sensitizers for Dye-Sensitized Solar Cells: Synthesis and Spectral, Electrochemical, and Photovoltaic Properties

CHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2009
Cheng-Wei Lee
Abstract Novel meso - or ,-derivatized porphyrins with a carboxyl group have been designed and synthesized for use as sensitizers in dye-sensitized solar cells (DSSCs). The position and nature of a bridge connecting the porphyrin ring and carboxylic acid group show significant influences on the spectral, electrochemical, and photovoltaic properties of these sensitizers. Absorption spectra of porphyrins with a phenylethynyl bridge show that both Soret and Q,bands are red-shifted with respect to those of porphyrin 6. This phenomenon is more pronounced for porphyrins 3 and 4, which have a ,-conjugated electron-donating group at the meso position opposite the anchoring group. Upon introduction of an ethynylene group at the meso position, the potential at the first oxidation alters only slightly whereas that for the first reduction is significantly shifted to the positive, thus indicating a decreased HOMO,LUMO gap. Quantum-chemical (DFT) results support the spectroelectrochemical data for a delocalization of charge between the porphyrin ring and the amino group in the first oxidative state of diarylamino-substituted porphyrin 5, which exhibits the best photovoltaic performance among all the porphyrins under investigation. From a comparison of the cell performance based on the same TiO2 films, the devices made of porphyrin 5 coadsorbed with chenodeoxycholic acid (CDCA) on TiO2 in ratios [5]/[CDCA]=1:1 and 1:2 have efficiencies of power conversion similar to that of an N3 -based DSSC, which makes this green dye a promising candidate for colorful DSSC applications. [source]

Current,Voltage Characteristics of a Homologous Series of Polycyclic Aromatic Hydrocarbons

CHEMISTRY - A EUROPEAN JOURNAL, Issue 26 2007
Thilo Böhme Dr.
Abstract A novel alkyl-substituted polycyclic aromatic hydrocarbon (PAH) with D2h symmetry and 78 carbon atoms in the aromatic core (C78) was synthesized, thereby completing a homologous series of soluble PAH compounds with increasing size of the aromatic , system (42, 60, and 78 carbon atoms). The optical band gaps were determined by UV/Vis and fluorescence spectroscopy in solution. Scanning tunneling microscopy (STM) and spectroscopy (STS) revealed diode-like current versus voltage (I,V) characteristics through individual aromatic cores in monolayers at the interface between the solution and the basal plane of graphite. The asymmetry of the current,voltage (I,V) characteristics increases with the increasing size of the aromatic core, and the concomitantly decreasing HOMO,LUMO gap. This is attributed to resonant tunneling through the HOMO of the adsorbed molecule, and an asymmetric position of the molecular species in the tunnel junction. Consistently, submolecularly resolved STM images at negative substrate bias are in good agreement with the calculated pattern for the electron densities of the HOMOs. The analysis provides the basis for tailoring rectification with a single molecule in an STM junction. [source]

Electronic Properties and Reactivity of Short-Chain Oligomers of 3,4-Phenylenedioxythiophene (PheDOT)

CHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2006
Igor F. Perepichka Dr.
Abstract The dimer and trimer of 3,4-phenylenedioxythiophene (PheDOT) have been synthesized. Unlike the parent systems based on 3,4-ethylenedioxythiophene (EDOT), these compounds are quite stable under atmospheric conditions. The electronic absorption spectra of di- and tri-PheDOT exhibit a well-resolved vibronic fine structure indicative of self-rigidification of the conjugated structure by noncovalent intramolecular sulfur,oxygen interactions. Comparison of UV-visible data for the PheDOT oligomers with those of the corresponding EDOT oligomers reveals a faster decrease of the HOMO,LUMO gap with chain length for the former. Cyclic voltammetric data show that whereas PheDOT oxidizes at a lower potential than EDOT, the PheDOT dimer and trimer exhibit much higher oxidation potentials than their EDOT-based analogues. A comparative analysis of the electropolymerization of the three PheDOT-based systems shows that although PheDOT is very difficult to polymerize, its dimer and trimer can be readily electropolymerized. This unexpected increase of reactivity with chain extension is discussed with the aid of theoretical calculations. [source]