Showing papers on "HOMO/LUMO published in 2005"

Cationic Bis-cyclometalated Iridium(III) Diimine Complexes and Their Use in Efficient Blue, Green, and Red Electroluminescent Devices

Abstract: A series of cationic Ir(III) complexes with the general formula (C∧N)2Ir(N∧N)+PF6- featuring bis-cyclometalated 1-phenylpyrazolyl-N,C2‘ (C∧N) and neutral diimine (N∧N, e.g., 2,2‘-bipyridyl) ligands were synthesized and their electrochemical, photophysical, and electroluminescent properties studied. Density functional theory calculations indicate that the highest occupied molecular orbital of the compounds is comprised of a mixture of Ir d and phenylpyrazolyl-based orbitals, while the lowest unoccupied molecular orbital has predominantly diimine character. The oxidation and reduction potentials of the complexes can be independently varied by systematic modification of either the C∧N or N∧N ligands with donor or acceptor substituents. The electrochemical redox gaps (Eox − Ered) were adjusted to span a range between 2.39 and 3.08 V. All of the compounds have intense absorption bands in the UV region assigned to 1(π−π*) transitions and weaker charge-transfer (CT) transitions that extend to the visible region....

Intermolecular charge transfer between heterocyclic oligomers. Effects of heteroatom and molecular packing on hopping transport in organic semiconductors.

Abstract: For electron or hole transfer between neighboring conducting polymer strands or oligomers, the intrinsic charge-transfer rate is dictated by the charge-resonance integral and by the reorganization energy due to geometric relaxation. To explain conduction anisotropy and other solid-state effects, a multivariate, systematic analysis of bandwidth as a function of intermolecular orientations is undertaken for a series of oligoheterocycles, using first-principles methods. While cofacial oligomers show the greatest bandwidths at a given intermolecular C-C contact distance, for a fixed center-to-center intermolecular distance, tilted pi-stacking increases pi-overlap (particularly for LUMO orbitals) and decreases electrostatic repulsion, yielding optimum tilt angles for packing of approximately 40-60 degrees at small intermolecular separations. The calculations also reveal that bandwidths and intrinsic mobilities of holes and electrons in conjugated oligoheterocycles can be quite comparable.

Efficient Electrogenerated Chemiluminescence from Cyclometalated Iridium(III) Complexes

Abstract: Very efficient electrogenerated chemiluminescence (ECL) phenomena were realized by deliberately tuning electron-transfer reactions from electrochemically generated electron donor to metal complex radical cations. By controlling the relative positions of HOMO and LUMO levels (oxidation potential and reduction potential) of Ir(III) complexes, we could obtain 77 times higher ECL from iridium(III) complexes in the presence of TPA than that of the Ru(bpy)32+/TPA system. This high ECL efficiency of new Ir(III) complexes can be used in many interesting applications such as sensors and luminescent devices.

Study on the prediction of visible absorption maxima of azobenzene compounds

Abstract: The geometries of azobenzene compounds are optimized with B3LYP/6-311G* method, and analyzed with nature bond orbital, then their visible absorption maxima are calculated with TD-DFT method and ZINDO/S method respectively. The results agree well with the observed values. It was found that for the calculation of visible absorption using ZINDO/S method could rapidly yield better results by adjusting OWFπ-π (the relationship between π-π overlap weighting factor) value than by the TD-DFT method. The method of regression showing the linear relationship between OWFπ-π and BLN-N (nitrogen-nitrogen bond lengths) as OWF π-π=−8.1537+6.5638BL N-N, can be explained in terms of quantum theory, and also be used for prediction of visible absorption maxima of other azobenzne dyes in the same series. This study on molecules’ orbital geometry indicates that their visible absorption maxima correspond to the electron transition from HOMO (the highest occupied molecular orbital) to LUMO (the lowest unoccupied molecular orbital).

Controlling high harmonic generation with molecular wave packets.

Abstract: We show that, by controlling the alignment of molecules, we can influence the high harmonic generation process. We observed strong intensity modulation and spectral shaping of high harmonics produced with a rotational wave packet in a low-density gas of N2 or O2. In N2, where the highest occupied molecular orbital (HOMO) has sigma(g) symmetry, the maximum signal occurs when the molecules are aligned along the laser polarization while the minimum occurs when it is perpendicular. In O2, where the HOMO has pi(g) symmetry, the harmonics are enhanced when the molecules are aligned around 45 degrees to the laser polarization. The symmetry of the molecular orbital can be read by harmonics. Molecular wave packets offer a means of shaping attosecond pulses.

Platinum chromophore-based systems for photoinduced charge separation: a molecular design approach for artificial photosynthesis.

Abstract: Photoinduced charge separation is a fundamental step in photochemical energy conversion. In the design of molecularly based systems for light-to-chemical energy conversion, this step is studied through the construction of two- and three-component systems (dyads and triads) having suitable electron donor and acceptor moieties placed at specific positions on a charge-transfer chromophore. The most extensively studied chromophores in this regard are ruthenium(II) tris(diimine) systems with a common 3MLCT excited state, as well as related ruthenium(II) bis(terpyridyl) systems. This Forum contribution focuses on dyads and triads of an alternative chromophore, namely, platinum(II) di- and triimine systems having acetylide ligands. These d8 chromophores all possess a 3MLCT excited state in which the lowest unoccupied molecular orbital is a π* orbital on the heterocyclic aromatic ligand. The excited-state energies of these Pt(II) chromophores are generally higher than those found for the ruthenium(II) tris(diimin...

From Blue to Red: Syntheses, Structures, Electronic and Electroluminescent Properties of Tunable Luminescent N,N Chelate Boron Complexes

Abstract: A comprehensive study of a series four-coordinate boron compounds with the general formula of BPh 2 (N,N), where N,N are bidentate chelate ligands containing both neutral and negatively charged nitrogen donor atoms has been conducted. The structures of the boron complexes were examined via single-crystal X-ray diffraction. The series of molecules display bright luminescence with emission maxima λ m a x ranging from blue to red, depending on the nature of the N,N chelate ligand. The electronic effects and their consequences on the luminescent properties of the complexes due to the CH replacement of the chelate ligand by a nitrogen atom, the increase of conjugation, or the change of substituents on the chelate ligand have been examined using electrochemical analysis, UV-visible, and fluorescence spectroscopic methods, and by molecular orbital calculations (Gaussian 98). Experimental data and MO calculation results established that the emission of this class of compounds is caused by π-π* transitions centered on the chelate ligand. Furthermore, the experimental and theoretical results consistently and conclusively established that electron withdrawing groups on the negatively charged N-donor portion of the chelate ligand causes a decrease in the highest occupied molecular orbital (HOMO) energy level, thus increasing the energy gap. The CH replacement by a nitrogen atom on the negatively charged portion of the chelate ligand causes a dramatic decrease of the HOMO energy level, and the increase of conjugation in the chelate ligand significantly decreases the energy gap. Blue and red electroluminescent (EL) devices were fabricated successfully using two representative boron compounds from the series. The new boron compounds have been found to be able to function as both emitters and electron transport materials in EL devices.

Size and ligand effects on the electrochemical and spectroelectrochemical responses of CdSe nanocrystals

Abstract: The electrochemical properties of CdSe quantum dots with electrochemically inactive surface ligands (TOPO) have been investigated in comparison with the analogous nanocrystals containing electrochemically active oligoaniline ligands. The TOPO-capped nanocrystals have been studied in a wide size range (from 3 to 6.5 nm) with the goal to amplify the influence of the quantum confinement effect on the electrochemical response. The determined HOMO and LUMO levels have been found in good agreement with the ones obtained from photoluminescence studies and those predicted theoretically. Ligand exchange with aniline tetramer significantly influences the voltammetric peaks associated with the HOMO oxidation and the LUMO reduction of the quantum dots, which are shifted to higher and lower potentials, respectively. These shifts are interpreted in terms of the positive ligand charging which precedes the oxidation of the nanocrystals and the insulating nature of the ligand in the case of the nanocrystal reduction. The ligand-nanocrystal interactions have also been studied by UV-Vis-NIR and Raman spectroelectrochemistry in comparison with a specially prepared model compound which, apart from the anchoring function is identical to the grafted oligoaniline ligand. Both spectroelectrochemical techniques clearly indicate the same nature of the oxidation/reduction pathway for both the model compound and the grafted ligand. The influence of the grafting is manifested by a shift in the onset of the ligand oxidation as compared to the case of the "free" model compound. Since both components (ligands and nanocrystals) mutually influence their electrochemical and spectroelectrochemical properties, the newly developed system can be considered as a true molecular hybrid. Such hybrids are of interest because the potential zone of the ligand electroactivity is well separated from that of the nanocrystals and, as a result, the organic part can be electrochemically switched between the semiconducting and the conducting states with no change in the oxidation state of the nanocrystal. The newly developed system offers therefore the possibility of an electrical addressing of individual nanocrystals via the conducting ligands.

Substituent Effects on Redox Potentials and Optical Gap Energies of Molecule-like Au38(SPhX)24 Nanoparticles

Abstract: A molecule-like substituent effect on redox formal potentials in the nanoparticle series Au38(SPhX)24 has been discovered. Electron-withdrawing “X” substituents energetically favor reduction and disfavor oxidation, and give formal potentials that correlate with Hammett substituent constants. The ligand monolayer of the nanoparticles is shown, thereby, to play a strong role in determining electronic energies of the nanoparticle core and is more than simply a protecting or capping layer. The substituent effect does not, however, detectably change the HOMO−LUMO gap energy, being identical for the HOMO and LUMO levels and presumably inductive in nature.

Evidence for d orbital aromaticity in square planar coinage metal clusters.

Abstract: Quantitative evidence for the existence of aromaticity involving the d orbitals of transition metals is provided for the first time. The doubly bridged square planar (D4h) coinage metal clusters (M4Li2, M = Cu (1), Ag (2), and Au (3)) are characterized as aromatic by their substantial nucleus independent chemical shifts (NICS) values in the centers (−14.5, −14.1, and −18.6, respectively). Nevertheless, the participation of p orbitals in the bonding (and cyclic electron delocalization) of 1−3 is negligible. Instead, these clusters benefit strongly from the delocalization of d and to some extent s orbitals. The same conclusion applies to Tsipis and Tsipis' H-bridged D4h Cu4H4 ring (4). Canonical MO−NICS analysis of structures 1−3 shows the total diatropic d orbital contributions to the total NICS to be substantial, although the individual contributions of the five sets of filled d orbitals vary. The d orbital aromaticity of Cu4Li2 also is indicated by its atomization energy, 243.2 kcal/mol, which is larger ...

Gold-caged metal clusters with large HOMO-LUMO gap and high electron affinity.

Abstract: We report a series of isoelectronic gold-caged metal clusters, M@Au14 (M = Zr, Hf), and anion clusters, M@Au14- (M = Sc, Y), all having a calculated HOMO−LUMO gap larger than the well-known tetrahedral cluster Au20the 3D metal cluster with a very large measured HOMO−LUMO gap (1.77 eV). The clusters M@Au14 (M = Sc, Y) also exhibit a calculated electron affinity (EA) and vertical detachment energy (VDE) not only higher than the “superhalogen” icosahedral Al13 cluster but also possibly even higher than a Cl atom which has the highest (measured) elemental EA or VDE (3.61 eV).

Highly Selective Colorimetric and Electrochemical Pb2+ Detection Based on TTF-π-Pyridine Derivatives

Abstract: The pyridineethenyl-substituted tetrathiafulvalene (TTF) compounds, 4-(4-pyridineethenyl)tetrathiafulvalene (1a) and 4,4‘(5‘)-[bis-(4-pyridineethenyl)]tetrathiafulvalene (2a) together with the styryl-substituted TTF compounds, 4-styryltetrathiafulvalene (1b) and 4,4‘(5‘)-bis-styryltetrathiafulvalene (2b), have been designed and synthesized. All these compounds exhibit strong absorption bands in the range of 370 to 550 nm, which are assigned to the intramolecular charge-transfer transition from the HOMO in TTF to the LUMO in the pyridyl or phenyl group. Compared to compounds 1b and 2b, the pyridineethenyl-substituted TTF compounds 1a and 2a show remarkable sensing and coordinating properties to Pb2+. With the addition of micromolar concentrations of Pb2+ to the solution, 1a or 2a displays dramatic changes in the UV−vis absorption spectrum, 1H NMR spectrum, and redox property.

Full Emission Color Tuning in Bis-Dipolar Diphenylamino-Endcapped Oligoarylfluorenes

Abstract: A novel series of monodisperse bis-dipolar emissive oligoarylfluorenes, OF(2)Ar-NPh, bearing an electron affinitive core, 9,9-dibutylfluorene as conjugated bridges, and diphenylamino as end-caps was successfully synthesized by a convergent approach using palladium catalyzed Suzuki cross-coupling. The results of optical and electrochemical investigations showed that the HOMO, LUMO, and energy gap of these diphenylamino endcapped oligoarylfluorenes can easily be modified or tuned by the use of various electron affinitive central aryl cores that included dibenzothiophene, phenylene, oligothiophenes, 2,1,3-benzothiadiazole, 4,7-dithien-2-yl-2,1,3-benzothiadiazole, thiophene S,S‘-dioxide, and dibenzothiophene S,S‘-dioxide as well as the extent of the π-conjugated core. As a result, their emission bands measured in chloroform can cover the full UV−vis spectrum (from 412 to 656 nm). In contrast to the common dipolar chromophores, most of OF(2)Ar-NPhs can form morphologically stable amorphous thin films (Tg = 88−...

Phenothiazine-phenylquinoline donor-acceptor molecules : Effects of structural isomerism on charge transfer photophysics and electroluminescence

Abstract: Large differences in the intramolecular charge-transfer fluorescence quantum yields and electroluminescence efficiencies were observed among the isomeric donor−acceptor molecules 2-(4-phenyl-2-quinolyl)-10-methylphenothiazine (2PQMPT) and 3-(4-phenyl-2-quinolyl)-10-methylphenothiazine (3PQMPT). In solution, the 2PQMPT isomer had a larger positive solvatochromism and thus a greater degree of charge transfer, whereas 3PQMPT had a larger fluorescence quantum yield (71%) compared to 2PQMPT (46%). High brightness (23750 cd/m2) and high efficiency (8.18 cd/A, 4.45 lm/W, 2.42% external quantum efficiency at 1015 cd/m2) green electroluminescence was achieved from 3PQMPT diodes. In contrast, green light-emitting diodes with lower brightness (8900 cd/m2) and efficiencies (4.79 cd/A, 2.36 lm/W, 1.41% external quantum efficiency at 690 cd/m2) were obtained from 2PQMPT. The two isomeric donor−acceptor molecules had identical HOMO (5.1 eV) and LUMO (2.4 eV) energy levels derived from electrochemistry. Density functiona...

Chemical Reactivity of Sc3N@C80 and La2@C80

Abstract: Sc3N@C80 has a lower thermal reactivity than La2@C80, although Sc3N@C80 has the same carbon cage (Ih) and oxidation state (C806-) as La2@C80. This result is attributed to the difference in the energy level and distribution of LUMO between Sc3N@C80 and La2@C80.

Interbase electronic coupling for transport through DNA

Abstract: We develop an approach to derive single-state tight-binding sSSTBd model for electron transport in the vicinity of valence-conduction bands of polysGd-polysCd and polysAd-polysTd DNA. The SSTB parameters are derived from first principlesand are used to model charge transport through finite length DNA. We investigate the rigor of reducing the full DNA Hamiltonian to SSTB model. While the transmission coefficient spectrum is preserved, its position shifts in energy. Thymine is poorly represented and its peak height is substantially reduced. This is attributed to the abstraction of the HOMO-LUMO sHOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbitald coupling to other eigenstates in the nearest-neighbor DNA bases, and can be corrected within second-order time-independent perturbation theory. Interstrand charge transport has also been analyzed and it is found that hopping to the nearest neighbor in the complementary strand is the most important process except in the valence band of polysGd-polysCd, where hopping to the second nearest neighbor between 38-ends is the most dominant process. As a result, transport between 38-ends and 58-ends in the vicinity of valence band of polysGd-polysCd is asymmetric.

Unique CO chemisorption properties of gold hexamer: Au6(CO)n- (n = 0-3)

Abstract: Elucidating the chemisorption properties of CO on gold clusters is essential to understanding the catalytic mechanisms of gold nanoparticles. Gold hexamer Au(6) is a highly stable cluster, known to possess a D(3)(h) triangular ground state structure with an extremely large HOMO-LUMO gap. Here we report a photoelectron spectroscopy (PES) and quasi-relativistic density functional theory (DFT) study of Au(6)-CO complexes, Au(6)(CO)(n)(-) and Au(6)(CO)(n) (n = 0-3). CO chemisorption on Au(6) is observed to be highly unusual. While the electron donor capability of CO is known to decrease the electron binding energies of Au(m)(CO)(n)(-) complexes, CO chemisorption on Au(6) is observed to have very little effect on the electron binding energies of the first PES band of Au(6)(CO)(n)(-) (n = 1-3). Extensive DFT calculations show that the first three CO successively chemisorb to the three apex sites of the D(3)(h) Au(6). It is shown that the LUMO of the Au(6)-CO complexes is located in the inner triangle. Thus CO chemisorption on the apex sites (outer triangle) has little effect on this orbital, resulting in the roughly constant electron binding energies for the first PES band in Au(6)(CO)(n)(-) (n = 0-3). Detailed molecular orbital analyses lead to decisive information about chemisorption interactions between CO and a model Au cluster.

Excited-State Behavior of trans and cis Isomers of Stilbene and Stiff Stilbene: A TD-DFT Study

Abstract: The first part of the isomerization path on the two lowest excited states of trans and cis isomers of stilbene and stiff stilbene is investigated by means of TD-PBE0 calculations in the gas phase and in heptane solution. Solvent effects are taken into account by the PCM model. The excited-state optimized structures and the computed absorption and emission frequencies are in good agreement with the available experimental results. In all of the examined compounds, the isomerization process before barrier crossing occurs on the HOMO → LUMO bright state, whereas the role played by other single-excitation states appears negligible. The relative energy barriers on the isomerization paths are consistent with the experimental excited-state lifetimes, suggesting a unifying picture of the isomerization process in stilbene-like molecules.

Basis set effects on relative energies and HOMO–LUMO energy gaps of fullerene C36

Abstract: Fifteen C36 isomers were examined to determine the influence that the quality of basis sets has on the geometry parameters, the relative stability and HOMO–LUMO energy gaps of fullerene isomers calculated with density functional theory. It is worthwhile to note that the geometry parameters of all C36 isomers are insensitive to basis sets. On the other hand, one set of d-type polarization functions plays an important role in evaluating relative stability and HOMO–LUMO energy gaps, while diffuse functions are not effective. To obtain reliable energies, at least a double-zeta plus polarization basis set is required, and a triple-zeta plus polarization basis set is suggested to lead to accurate energies at a reasonable computational cost.

DFT study on the antioxidant activity of rosmarinic acid

Abstract: A theoretical calculation based on the density functional theory (DFT) has been performed to understand the antioxidant activity of rosmarinic acid in view of a molecular structure According to the geometry of the ground state molecule and its free radical, the HOMO and LUMO, the O–H bond dissociation energy (BDE), and the single electron density distribution of the radicals, we interpreted the capacity of scavenging the radicals for rosmarinic acid by its structure Compared with flavonoids, the higher antioxidant activity of rosmarinic acid may be from the abstraction of hydrogen atoms of the ortho -position hydroxyls on the rings A and B This abstraction can occur continuously to form a semiquinone structure, or even to form a quinone structure The activity of the ring A is similar with the ring B However, the ring B is a stronger electron donor than the ring A, and, the radical formed from the H-abstraction of the ring A is more stable than that of the ring B We also found that there is a good relationship between the BDE, which is used to show the stability of the parent molecule and the unpaired electron delocalization correlated to the stability of the free radical These theoretical researches will be helpful to the development for the antioxidant compounds

Synthesis and Characterization of n-Type Materials for Non-Doped Organic Red-Light-Emitting Diodes

Abstract: Two compounds, 2,3-dicyano-5,6-di(4′-diphenylamino-biphenyl-4-yl)pyrazine (CAPP) and 6,7-dicyano-2,3-di(4′-diphenylamino-biphenyl-4-yl)quinoxaline (CAPQ), capable of intramolecular charge transfer, have been designed and synthesized in high yield by a convenient procedure. The compounds have been fully characterized spectroscopically. They have a high thermal stability and show bright light emission both in non-polar solvents and in the solid state. Moreover, they exhibit excellent reversible oxidation and reduction waves. The higher energy level of the highest occupied molecular orbital (–5.3 eV) and the triphenylamine group are advantageous for hole-injection/transport. In addition, the high electron affinities of 3.4 eV and the observed reversible reductive process suggest that these compounds enhance electron injection and have potential for use in electron transport. Three types of non-doped red-light-emitting diodes have been studied using CAPP and CAPQ as the electron-transporting and host-light-emitting layers, respectively. The devices exhibit red electroluminescence (EL), and constant Commission Internationale de l'Eclairage coordinates have been observed on increasing the current density. Pure red EL of CAPP, with a maximum brightness of 536 cd m–2 and an external quantum efficiency of 0.7 % in ambient air, was achieved.

Fluorene‐based alternating polymers containing electron‐withdrawing bithiazole units: Preparation and device applications

Abstract: We report here the synthesis via Suzuki polymerization of two novel alternating polymers containing 9,9-dioctylfluorene and electron-withdrawing 4,4′-dihexyl-2,2′-bithiazole moieties, poly[(4,4′-dihexyl-2,2′-bithiazole-5,5′-diyl)-alt-(9,9-dioctylfluorene-2,7-diyl)] (PHBTzF) and poly[(5,5′-bis(2″-thienyl)-4,4′-dihexyl-2,2′-bithiazole-5″,5″-diyl)-alt-(9,9-dioctylfluorene-2,7-diyl)] (PTHBTzTF), and their application to electronic devices. The ultraviolet–visible absorption maxima of films of PHBTzF and PTHBTzTF were 413 and 471 nm, respectively, and the photoluminescence maxima were 513 and 590 nm, respectively. Cyclic voltammetry experiment showed an improvement in the n-doping stability of the polymers and a reduction of their lowest unoccupied molecular orbital energy levels as a result of bithiazole in the polymers' main chain. The highest occupied molecular orbital energy levels of the polymers were −5.85 eV for PHBTzF and −5.53 eV for PTHBTzTF. Conventional polymeric light-emitting-diode devices were fabricated in the ITO/PEDOT:PSS/polymer/Ca/Al configuration [where ITO is indium tin oxide and PEDOT:PSS is poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid)] with the two polymers as emitting layers. The PHBTzF device exhibited a maximum luminance of 210 cd/m2 and a turn-on voltage of 9.4 V, whereas the PTHBTzTF device exhibited a maximum luminance of 1840 cd/m2 and a turn-on voltage of 5.4 V. In addition, a preliminary organic solar-cell device with the ITO/PEDOT:PSS/(PTHBTzTF + C60)/Ca/Al configuration (where C60 is fullerene) was also fabricated. Under 100 mW/cm2 of air mass 1.5 white-light illumination, the device produced an open-circuit voltage of 0.76 V and a short-circuit current of 1.70 mA/cm2. The fill factor of the device was 0.40, and the power conversion efficiency was 0.52%. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1845–1857, 2005

Deformed Phthalocyanines: Synthesis and Characterization of Zinc Phthalocyanines Bearing Phenyl Substituents at the 1‐, 4‐, 8‐, 11‐, 15‐, 18‐, 22‐, and/or 25‐Positions

Abstract: The synthesis of a series of zinc phthalocyanines partially phenyl-substituted at the 1-, 4-, 8-, 11-, 15-, 18-, 22-, and/or 25-positions (the so-called alpha-positions) is reported. Macrocycle formation based on 3,6-diphenylphthalonitrile, o-phthalonitrile, and zinc acetate predominantly yielded the near-planar disubstituted complex and opposite tetrasubstituted isomer, while the lithium method yielded the sterically hindered hexasubstituted complex and adjacent tetrasubstituted isomer. All compounds have been characterized by 1H NMR, MALDI-TOF-MS, and elemental analysis methods. In addition, crystal structures have been solved for the di-, hexa-, and octasubstituted complexes and the adjacent tetrasubstituted isomer. DFT geometry optimization calculations predict more highly deformed structures than those observed in the crystals. The packing force of the crystals cannot therefore be ignored, particularly for the less phenyl-substituted derivatives. The crystal structures have revealed that overlap of the phenyl groups causes substantial deformation of the phthalocyanine (Pc) ligands within the crystals, while strong pi-pi stacking in the remainder of the Pc moiety lacking phenyl substituents can suppress the impact of the deformation. Absorption spectra show sizable red shifts of the Q-band with increasing number of phenyl groups. Analysis of the results of absorption spectra and electrochemical measurements reveals that a substantial portion of the red shift is attributable to the ring deformations. Molecular orbital calculations lend further support to this conclusion. A moderately intense absorption band emerging at around 430 nm for highly deformed octaphenyl-substituted zinc Pc can be assigned to the HOMO-->LUMO+3 transition, which is parity-forbidden for planar Pcs, but becomes allowed since the ring deformations remove the center of symmetry.

Synthesis of 3,4-diphenyl-substituted poly(thienylene vinylene) low-band-gap polymers via the dithiocarbamate route

Abstract: It has been demonstrated that the dithiocarbamate precursor route is a suitable pathway towards poly(thienylene vinylene) (PTV)-type low-band-gap polymers. Two particular dithiocarbamate precursor polymers and the corresponding conjugated polymers, poly(3,4-diphenyl-2,5-thienylene vinylene) and poly(3,4-bis(4-butylphenyl)-2,5-thienylene vinylene), have been studied. The introduction of butyl side chains in the latter leads to excellent solubility in common organic solvents. Both polymers have been prepared in a straightforward manner and in good yield. The thermal conversion of the precursor polymers into the conjugated structure was studied with in situ FT-IR and UV−vis spectroscopy. Also, with the latter technique, the band gap was determined and the thermochromic effect was studied and compared with the unsubstituted PTV. The HOMO and LUMO levels of the polymers were determined from UV−vis and electrochemical measurements.

Ab initio analysis of electron currents in thioalkanes

Abstract: A combined density functional theory and Green's function procedure is used to calculate the electrical characteristics of a group of alkanethiols representing possible experimental settings. It is found that the current running through the molecule is the sum of the contributions from all molecular orbitals each presenting a barrier to electron transport equal to their energy difference from the Fermi level of the contacts. For the alkanethiols the location of these intrinsic barriers are at 0.78 eV (highest occupied molecular orbital (HOMO) and HOMO-1), 1.99 eV (HOMO-2 and HOMO-3), 2.07 eV (LUMO and LUMO1), and 2.67 eV (HOMO-4). However, barriers obtained through fittings to known models do not bear any physical meaning at the molecular level, as they are sort of exponential average of the intrinsic barriers. We have also found that the exponential dependence of the current with the length of the alkane is practically independent of the contact nature, perhaps due to the large resistance of the alkanes.