Valence Effective Hamiltonian Research Articles

ChemInform Abstract: Electronic Structure of Tetraphenyldithiapyranylidene: A Valence- Effective Hamiltonian Theoretical Investigation.

We present a theoretical investigation of the electronic structure of tetraphenyldithiapyranylidene (DIPSΦ4) using the nonempirical valence effective Hamiltonian (VEH) method. Molecular geometries are optimized at the semiempirical PM3 level which predicts an alternating nonaromatic structure for the dithiapyranylidene (DIPS) framework. The VEH one‐electron energy level distribution calculated for DIPSΦ4 is presented as a theoretical XPS simulation and is analyzed by comparison to the electronic structure of its molecular components DIPS and benzene. The theoretical VEH spectrum is found to be fully consistent with the experimental solid‐state x‐ray photoelectron spectroscopy (XPS) spectrum and an excellent quantitative agreement between theory and experiment is achieved when comparing the energies of the main peaks. A detailed interpretation of all the experimental photoemission bands is reported in the light of the VEH results.

Theoretical investigation of the electronic and optical properties of oligothiophenes upon methyl, thiol, and thiomethyl substitutions

The geometric, electronic, and optical properties of oligothiophenes of 2, 4, 6 and 8 units of head-to-head–tail-to-tail (HH–TT) regioselectivity and substituted by methyl, thiol, and thiomethyl groups have been characterized in their neutral and p-doped states with quantum-chemical calculations derived from semiempirical Hartree–Fock approaches and with the nonempirical valence effective Hamiltonian (VEH) method. Such calculations provide a good insight into the electronic properties measured for electropolymerized chains.

Electronic structure of pristine and sodium doped poly(p-pyridine)

The electronic structure of pristine and sodium-doped poly(p-pyridine) has been studied using both ultraviolet and x-ray photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum-chemical calculations. Electronic band-structure calculations are performed for isolated chains with different connectivity patterns (head-to-tail and head-to-head), using the valence effective Hamiltonian (VEH) method, with geometries derived from optimizations using the Austin Model 1 Hamiltonian. The density-of-valence-states are derived directly from the VEH band structure. Excellent agreement is obtained between the theoretical simulations and the experimental data, which allows for a detailed assignment of the different peaks in the spectra. The C(1s) and N(1s) shake-up spectra of poly(p-pyridine) are analyzed on the basis of corresponding data for pyridine in the gas phase. Upon sodium doping of poly(p-pyridine), new states are observed within the otherwise forbidden energy gap. These new states can be assigned to the formation of bipolarons.

Monodisperse poly(triacetylene) oligomers extending from monomer to hexadecamer: joint experimental and theoretical investigation of physical properties

A series of monodisperse Et3-Si-end-capped poly(triacetylene) (PTA) oligomers ranging from monomer to hexadecamer was prepared by a fast and efficient statistical deprotection-oxidative Hay oligomerization protocol. The PTA oligomers exhibit an increasingly deep-yellow color with lengthening of the pi-conjugated backbone, feature excellent solubility in aprotic solvents, and exhibit melting points up to > 22 degrees C for the hexadecameric rod. This new dramatically extended oligo(enediyne) series now enables to investigate the evolution of the physico-chemical effects in PTAs beyond the linear 1/n versus property regime into the higher oligomer region where saturation becomes apparent. We report the results of joint experimental and theoretical studies, including analysis of the 13C NMR spectra, evaluation of the linear (UV/ Vis) and nonlinear [third-harmonic generation (THG) and degenerate four-wave mixing (DFWM)] optical properties, and characterization of the redox properties with cyclic and steady-state voltammetry. Up to the hexadecameric rod, an increasingly facile one-electron reduction step is observed, showing at the stage of the dodecamer, a leveling off tendency from the linear correlation between the inverse number of monomer units and the first reduction potential. The effective conjugation length (ECL) determined by means of UV/Vis spectroscopy revealed a pi-electron-delocalization length of about n = 10 monomeric units, which corroborates well with the oligomeric length for which in the 13C NMR spectrum C(sp2) and C(sp) resonances start to overlap. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) measurements revealed for the second-order hyperpolarizability gamma a power law increase gammma-alpha-n(a) for oligomers up to the octamer with exponential factors a= 2.46+/-0.10 and a=2.64+/-0.20, respectively, followed by a smooth saturation around n = 10 repeating units. The power law coefficient a calculated with the help of the valence effective Hamiltonian (VEH) method combined to a sum-over-states (SOS) formalism corroborates well with the values found by both THG and DFWM experiments. Up to the PTA heptamer, INDO (intermediate neglect of differential overlap)-calculated gas-phase ionization potentials and electron affinities obey a linear relationship as a function of the inverse number of monomer units displaying a strong electron-hole symmetry. The onset of saturation for the electron affinity is calculated to occur around the octamer, in accordance with experimentally obtained results from electrochemical measurements.

Chemical and electronic structure of a novel conjugated polymer: poly(4,4′-diphenylene diphenylvinylene)

The chemical and electronic structure of a highly luminescent conjugated polymer poly(4,4′-diphenylene diphenyl-vinylene) has been investigated using a combined experimental and theoretical approach based on photoelectron spectroscopy, optical absorption and semiempirical AM1 and INDO/SCI, as well as Valence Effective Hamiltonian (VEH), calculations. Details are presented on the ground state geometry, calculated electronic band structure, comparisons between the calculated and experimental, as obtained by UPS, density-of-valence-states, and nature of the optical transitions.

Electornic structure of emeraldine and pernigraniline base: A joint theoretical and experimental study

The electronic structure of the emeraldine and pernigraniline base forms of polyaniline is revisited with a combined experimental and theoretical approach. On the one hand, high-resolution valence band photoelectron spectra are presented. High-quality electronic structural information is obtained due to the use of chemically pure and well-defined materials, which could be studied for the first time with a high-resolution photoelectron spectroscopy facility. On the other hand, the electronic band structures and the density of valance states spectra associated with these polymers are calculated by means of the pseudopotential valence effective Hamiltonian (VEH) technique. The excellent agreement observed for the first time between the experimental and theoretical results obtained on these polymers clearly validates the description of the emeraldine and pernigraniline valence band electronic structure within the VEH formalism and allows us to evaluate the ionization potential of both polymers.

Influence of silicon atoms on the ?-conjugation in electroluminescent polymers

By means of the valence effective Hamiltonian (VEH) method, we calculated the electronic structure for a polymer containing poly(para-phenylene vinylene) (PPV) and dimethoxy-poly(para-phenylene) (PPP) units as well as silicon atoms used as spacers. The equilibrium geometry was obtained by an AM1 calculation. We obtained a band gap of 3.2 eV, which corresponds to emittance in the blue part of the spectrum. This polymer is used as the active layer in light-emitting diodes. © 1997 John Wiley & Sons, Inc.

Photon energy dependence of valence band photoemission and resonant photoemission of polystyrene

The photon energy dependence of valence band photoemission from polystyrene thin films has been investigated using synchrotron radiation in the range 150–450 eV, covering the carbon K edge. Six valence peaks were found which show an energy dependence that can be qualitatively explained by the relative contributions of 2s and 2p atomic orbitals to the molecular orbital photoemission cross-section. The experimental results are generally in agreement with valence effective Hamiltonian (VEH) calculations in the literature. In the vicinity of the carbon 1s absorption edge, there is a strong enhancement of the valence band emission because of resonant photoemission, reported here in organic polymers for the first time. The relative contribution of spectator and participator decay channels varies among the valence orbitals, in a way that can be associated with the spatial distribution of the benzene-derived molecular orbitals involved in the resonance transition.

Theoretical determination of the molecular and solid-state electronic structures of phthalocyanine and largely extended phthalocyanine macrocycles

The molecular and solid-state electronic structures of metal-free phthalocyanine and a series of linearly benzoannulated phthalocyanines have been investigated using the valence effective Hamiltonian (VEH) quantum-chemical method. Geometry optimizations show that, from the molecular structure standpoint, phthalocyanine-based macrocycles are the result of joining four polyacenic units to the C8N8 central ring. The electronic structure calculated for the parent phthalocyanine is compared with that of porphyrin, and the consequences of benzoannulation and meso-tetraaza substitution on the optical properties of phthalocyanines are discussed. The VEH results obtained for extended phthalocyanines are in agreement with available photoemission, cyclic voltammetry and optical absorption data and help to rationalize the evolution of the electronic properties. The first ionization energy is predicted to decrease with linear benzoannulation and asymptotically converges to an extrapolated value of –5.7 eV. Strikingly, a non-convergent behaviour is obtained for the HOMO–LUMO energy gap and very low excitation energies are predicted for extended phthalocyanines. Band-structure calculations have been performed for one-dimensional stacks of the molecules investigated. The variation of the bandwidth with the staggering angle and the intermolecular separation provides a coherent picture of the electrical conductivities observed experimentally in crystals and polymers. Very small bandgaps lower than 0.5 eV are predicted for extended phthalocyanines.

Electronic structure of poly( p-(disilanylene)phenylene)

We present the geometrical and electronic structures of several isomers of poly( p-(disilanylene)phenylene), The structural analysis, performed at the 3-21G * level, shows that the isomers with the phenylene group perpendicular to the silicon backbone are the more stable conformations, displaying almost the same energy. The electronic properties, as obtained from the valence-effective Hamiltonian (VEH) band structure calculations, strongly depend on the disposition of the phenylene group into the polymeric backbone. The VEH predicts a wide and asymmetric absorption band in excellent agreement with UV experimental data.

Electronic structure of statistical and block poly( p-phenylene)-polyacetylene copolymers

We investigate theoretically the electronic structure of statistical and block copolymers of poly( p-phenylene) (PPP) and polyacetylene (PA); the calculations are performed using the negative factor counting (NFC) technique coupled to the valence effective Hamiltonian (VEH) method. As a consequence of the energy gap difference in the parent polymers, the bandgaps of PPP-PA copolymers can be tuned over the whole visible range depending on the relative concentrations and distributions of PA and PPP segments. An interesting result is the formation of spatially localized frontier orbitals within the PA moieties when these segments cluster (block copolymers).

Theoretical investigation of nitrogen-containing polyisothianaphthene derivatives

We use a quantum-chemical approach to investigate the geometric and electronic structure of aza and diaza derivatives of polyisothianaphthene (PITN). Semiempirical Austin Model 1 (AM1) calculations are performed on a series of oligomers of increasing chain length; the oligomers are capped by various end groups that lead to either aromatic or quinoid geometric structures. The relative stabilities of the aromatic and quinoid polymers are estimated on the basis of the energy per repeat unit (Epru) and the electronic properties are evaluated with the Valence Effective Hamiltonian (VEH) method.

Experimental and Theoretical Studies of the Electronic Structure of Poly(p-phenylenevinylene) and Some Ring-Substituted Derivatives

The electronic structure of a conjugated polymer of current interest in organic LED's, poly(p-phenylenevinylene), or PPV, has been studied by ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. The focus of this work is on the nature of the pi-electronic band structure nearest the Fermi level and the physical influence of finite torsion angles, the geometry of the polymer backbone, on the electronic properties of the system. Details of the ct-electronic bands, as reflected in the associated density-of-states, are observed clearly in the spectra, from which some underlying geometrical details of the polymer system can be deduced. The experimental spectra have been analyzed theoretically using band structure calculations based upon the valence effective Hamiltonian (VEH) model. In addition, in order to control the band structure, three ring-substituted derivatives of PPV, each of which induces a different bonding geometry in the backbone, have been studied. The changes in the experimental results can be explained on the basis of both physical and chemical interactions of the substituents with the backbone, which lead to geometrical changes along the backbone, which influence the ct-bandwidths and contribute to differences in both the optical absorption threshold and the binding energy of the valence band edge.

Modulation of the electronic properties of conjugated polymers through design of polymer backbone

We present the results of valence effective hamiltonian (VEH) band structure calculations on stereoregular block copolymers of poly(thienylene vinylene) and poly(pyrrylene vinylene). We have examined the evolution of the electronic properties as a function of the extension and distribution of the block copolymer sequences. In both cases, the electronic-properties deviate from a linear evolution as a consequence of the localization of both HOCO and LUCO.

Some chemical and electronic structures of the non-conjugated polymer poly(3,3′-phthalidylidene-4,4′-biphenylene)

The valence band structure of poly(3,3′-phthalidylidene-4,4′-biphenylene) has been studied using ultraviolet photoelectron spectroscopy (UPS). This polymer exhibits a dielectric-to-metal phase transition under the influence of external forces and is therefore a good candidate for studying charge instabilities in conjugated polymers. Interpretation of the UPS spectra is facilitated using quantum chemical calculations based on the semi-empirical Austin Model 1 (AM1) method, for geometry optimization, and the valence effective Hamiltonian (VEH) method, for calculating the energy band structure diagram, from which the density of valence states (DOVS) in the solid material is derived. The calculated DOVs is in good agreement with the experimental UPS spectra, enabling an identification of features in the experimental data in terms of specific features in the energy band structure.

Theoretical study of the interaction between sodium and oligomers of poly( p-phenylenevinylene) and poly( p-phenylene)

The semi-empirical Austin Model 1 and the non-empirical pseudo-potential valence effective Hamiltonian (VEH) methods as well as the local spin density (LSD) approximation technique have been applied to the investigation of the doping-induced electronic and geometrical changes in some conjugated molecules related to poly( p-phenylene) and poly( p-phenylenevinylene) (PPV): biphenyl, stilbene and a phenyl-capped dimer of PPV. The theoretical results are compared with experimental valence band spectra, as recorded by ultraviolet photoelectron spectroscopy (UPS). The experimental UPS studies show that two ingap states are detected upon doping with alkali metals. The energy splitting between the two in-gap states increases as the molecule size decreases. The results of the LSD calculations agree very well with the experimental results, while the VEH method overestimates the energy splitting for the small molecules. The LSD modelling also indicates a destabilization of several high binding energy valence levels, due to the presence of counter-ions, in agreement with experiment.

The evolution of charge-induced gap states in degenerate and non-degenerate conjugated molecules and polymers as studied by photoelectron spectroscopy

We report the results of ultraviolet photoelectron spectroscopy (UPS) studies of the interaction between sodium and conjugated systems for a series of diphenylpolyees and diffrent oligomers of poly( p-phenylenevinylene) (PPV). The diphenylpolyenes include molecules containing two (i.e., stilbene) to 14 carbon atoms in the polyene part; stilbene itself can also be considered as a phenyl-capped monomer of PPV. Furthermore, a PPV oligomer with three phenylene units, as well as PPV itself, has been studied. The experimental results are interpreted with the help of quantum-chemical calculations using the Hartree-Fock semi-empirical Austin Model 1 (AM1) and valence-effective Hamiltonian (VEH) methods. An important result is that all the systems react strongly with sodium; at high doping levels two new doping-induced states are detected above the valence band edge of the pristine material. In the case of saturation-doped diphenylpolyenes (i.e., two sodiums per molecule), the new states can be discussed in terms of soliton-antisoliton pairs confined within the polyene part of the molecules; in contrast, the self-localized states induced in PPV and its oligomers have to be referred to as bipolarons.

A theoretical study of the chemical structure of the non-conjugated polymer poly(3,3′-phthalidylidene-4,4′-biphenylene)

The equilibrium geometry of an isolated chain of poly(3,3′-phthalidylidene-4,4′-biphenylene), or PPB, has been studied using quantum chemical calculations based upon the Austin Model 1 (AM1) method. The band structure of an isolated PPB chain has been calculated, using the AM1 geometry as input, with the help of the valence-effective Hamiltonian (VEH) model. From the electronic structure results, the density of valence states has been derived for comparison with experimental data. Both the neutral and the n-doped polymer chains have been simulated. The results are compared with features of the polymer deduced through other measurements reported in the literature.

Theoretical Investigation of the Magnetic Dipole and Electric Quadrupole Contributions to the Second-Harmonic Generation in C60 and C70

Abstract We develop the Valence Effective Hamiltonian method to calculate the magnetic dipole (MD) and electric quadrupole (EQ) contributions to the hyperpolarizability β of molecular systems. For both the C60 and C70 molecules, we find that the MD contribution leads to a β value which is several times larger than that of the EQ contribution. Taking into account the local field correction factor for C60 in solid state, we calculate x (2) susceptibility values that are in excellent agreement with recent experimental data.

Theoretical investigation of the base and protonated forms of polypyrrylenemethine

Results of ab initio SCF crystal orbital band structure calculations of the base and protonated forms of polypyrrylenemethine (PPM) are presented. The important electronic properties, such as energy gap, ionization potential and electron affinity energy are reported. The results are compared with the data of polyacetylene (PA) and the results of the valence effective hamiltonian (VEH) method. The bandgap value of the base form of PPM is almost the same as that of PA, while that of the protonated form of PPM is much reduced. The charge distributions of the systems on the basis of Mulliken polpulation analysis are discussed and the electronic density of states of valence bands and conduction bands of both the base and protonated form of PPM are determined.