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ABSTRACT: We investigate theoretically the geometric structure and vibrational properties of complexes of polyenes with aluminum atoms that constitute model systems for the species formed at the interface between aluminum and polyacetylene. The calculations are performed with two quantum-mechanical techniques: ab initio Hartree−Fock and density functional theory in the local spin density approximation. These methods are first applied to a polyene molecule, all-trans octatetraene, and the calculated vibrational spectra are compared to existing experimental and theoretical data. The molecule is then made to interact with two aluminum atoms in various configurations. Since the metal atoms form covalent bonds with carbon atoms in the central part of octatetraene, strong geometric modifications occur along the conjugated system, which in turn deeply affect the vibrational spectra. These results allow us to derive the expected infrared signature of the chemical species present at the interface.
05/1997;
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ABSTRACT: We have employed quantum chemical methods, at the local spin density approximation level, to study the interaction between an organic semiconductor, polythiophene, and potential metals for hole injecting contacts in devices: vanadium, chromium, copper, and gold. The results show that there is a strong interaction between vanadium and the thiophene ring, mainly due to covalent bond formation between the metal and the S and Cα atoms of the thiophene. Vanadium is therefore predicted to provide good conditions for chemisorption and mechanical stability at the polymer/contact interface. A similar, but considerably weaker, covalent interaction is found between chromium and all the conjugated atoms of the thiophene molecule. For both these metals, the interactions cause the thiophene ring to lose its aromaticity and planarity which, as a consequence, would interrupt the π‐electron system in a polymer and impair charge transport along the chains. In the case of copper, the metal is found to react only with the sulfur atom of the thiophene and to a very small extent. For gold, the results indicate that there are no significant chemical interactions. Our results are in good qualitative agreement with XPS measurements performed during metallization of thin films of poly(3‐hexylthiophene). The calculations confirm the general trend of reactivities for this series of metals: V≳Cr≳Cu≳Au=0. © 1995 American Institute of Physics.
The Journal of Chemical Physics 04/1995; 102(15):6153-6158. · 3.33 Impact Factor
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ABSTRACT: The interactions between aluminum atoms and model molecules representing trans‐polyacetylene are studied quantum chemically by a local density functional method. We focus on the chemical and electronic structure of the organoaluminum complexes. Special emphasis is put on a comparison between results at the local spin density approximation and ab initio Hartree–Fock levels. In unmetallized polyenes, the density functional method provides a very good description of the carbon–carbon bond lengths of conjugated systems; in the case of hexatriene, it reproduces the bond dimerization in very good agreement with experimental measurements. Upon metallization, a strong covalent interaction between aluminum and carbon is found. The Al–C bond formation induces an interruption of the bond alternation pattern and reduces the π‐conjugation in the oligomer, in qualitative agreement with photoelectron spectroscopy data and previous theoretical results at the Hartree–Fock level. Notably, the π‐electron levels in the organoaluminum complexes maintain delocalization. In contrast to Hartree–Fock results where an aluminum atom binds to a single carbon, the interactions calculated with the local spin density approximation lead to (i) formation of multicenter aluminum–carbon bonding; (ii) near planarity of the polyene molecule; and (iii) a lower degree of charge transfer from the metal atom to the polymer.
The Journal of Chemical Physics 06/1994; 100(12):9258-9264. · 3.33 Impact Factor
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ABSTRACT: The interactions between different low work function metals aluminium, calcium and sodium, and α,ω‐diphenyltetradecaheptaene, a model molecule for certain conjugated polymers, have been investigated using both x‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum chemical calculations performed within the local spin density (LSD) approximation methodology. The metals are found to interact with the conjugated system in very different ways. Aluminium forms a covalent bond, which strongly modifies the π‐electronic structure of the conjugated molecule, while both the sodium and the calcium atoms act as doping agents, inducing new states in the otherwise forbidden bandgap. These new gap states can be viewed as a soliton–antisoliton pair for the Na/DP7 and a bipolaronic‐like defect for Ca/DP7.
The Journal of Chemical Physics 04/1994; 100(9):6765-6771. · 3.33 Impact Factor
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ABSTRACT: We have investigated the chemical nature and the electronic structure of the interface between a low work function metal, aluminum, and a conjugated polymer semiconductor, polythiophene. We have studied the initial stages of the interface formation by depositing the metal onto the surface of a polymer film. Charge transfer processes between the metal and the polymer are analyzed using core‐level x‐ray photoelectron spectroscopy (XPS); the evolution upon metallization of the valence electronic levels directly related to the polymer electronic structure is followed with ultraviolet photoelectron spectroscopy (UPS). With these techniques, we investigate the deposition of aluminum on two polythiophene systems (i) the alkyl‐substituted poly‐3‐octylthiophene and (ii) the α‐sexithiophene oligomer. The experimental data are compared to the results of a recent quantum chemical study on model systems consisting of thiophene oligomers (up to sexithiophene) interacting with a few Al atoms. The interaction of polythiophene with Al atoms is found to modify dramatically the structure of the conjugated backbone, as strong carbon–aluminum bonds are formed in the α positions of the thiophene rings. A large charge transfer takes place from the Al atoms to the polymer chain, and the upper π levels of the polymer are strongly affected. The metallization is contrasted to the doping of conjugated polymers with alkali metals.
The Journal of Chemical Physics. 06/1993; 99(1):664-672.
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Molecular Crystals and Liquid Crystals 05/1993; 228(1):43-48. · 0.58 Impact Factor
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ABSTRACT: Molecular model systems are used to quantum chemically investigate the interface between aluminum and trans‐polyacetylene. Modifications to the chemical and electronic structure of trans‐polyacetylene oligomers upon interaction with a submonolayer of aluminum are studied at the semiempirical and ab initio Hartree–Fock levels. An aluminum atom is found to react strongly with a carbon atom of the trans‐polyacetylene chain to form a heteropolar covalent bond. In this process, the binding carbon evolves from an sp2‐ to an sp3‐hybridized electronic structure. Significant contributions from Al 3s and 3p atomic orbitals are found in the frontier molecular orbitals in aluminum/polyene complexes. This results in the fact that despite the presence of sp3 sites due to Al–C bonds, which reduces π conjugation along the chain, a large degree of delocalization in π levels is maintained. Our calculations are discussed in relation to experimental ultraviolet photoelectron spectra (UPS) taken during initial stages of aluminum deposition on oxygen‐free films of trans‐polyacetylene oligomers.
The Journal of Chemical Physics 02/1993; 98(5):4253-4262. · 3.33 Impact Factor
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ABSTRACT: The advantages of using model systems for spectroscopic studies of conjugated polymers and interface formation, as well as for charge-induced electronic and geometric structural changes, are discussed. The electronic structure of a diphenylpolyene, α,ω-diphenyltetradecaheptaene, or DP7, is an example of a model molecular system studied using X-ray and Ultraviolet Photoelectron Spectroscopy, XPS, and UPS. The spectra are interpreted with the help of the results from MNDO, VEH and INDO/S-CI quantum chemical calculations. The frontier orbitals of DP7 are localized mostly on the polyene chain portion of the molecule, resulting in a high degree of separation of the phenyl and polyene parts of the π-system. The INDO calculations show two regions of shake-up features corresponding to a benzene-like part and a polyene-like part. The most important individual shake-up transitions, which contribute to the two observed shake-up spectral features, involve one-electron redistributions separable into contributions from the polyene chain and from the phenyl groups. The analysis indicates the extent to which many chemical and electronic properties of DP7 are expected to be similar to those of (at least short chain) trans-polyacetylene.
Synthetic Metals 51:187-195. · 1.83 Impact Factor
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ABSTRACT: Recently, it has been shown that ordered overlayers of many organic and polymeric materials can be made on specially-prepared ultra-thin films of polytetrafluoroethylene, or PTFE [1]. We have prepared PTFE films, in the range of 15 to 35 Å in thickness, by a slightly modified version of the hot-dragging method of Pooley and Tabor [2], and studied the electronic structure using ultraviolet and X-ray photoelectron spectroscopy, UPS and XPS. The electronic structure results are compared with new ab initio quantum chemical electronic structure calculations.
Synthetic Metals.
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ABSTRACT: A molecular quantum chemical approach is used to study the aluminum on PPV (Poly(p-Phenylene Vinylene)) interface. We focus on modifications to the chemical and electronic structure of the polymer upon interaction with a submonolayer of aluminum. A model system, trans-stilbene, is taken to investigate the nature of the Al-PPV bonding. Energetically favorable conformations are then used as prototypes to study the evolution of the electronic structure as modified by the reaction with aluminum. Results at the ab initio Hartree-Fock level indicates that Al atoms react with the vinylene linkage rather than the phenyl groups at early stages of interface formation.
Synthetic Metals.
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P. Dannetun,
M. Lögdlund,
M. Fahlman,
M. Boman,
S. Stafström,
W.R. Salaneck,
R. Lazzaroni, C. Fredriksson,
J.L. Brédas,
S. Graham,
R.H. Friend,
A.B. Holmes,
R. Zamboni,
C. Taliani
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ABSTRACT: The interaction between aluminum and α-ω-diphenyltetradecaheptaenee (DP7), α-sexithienyl (6T), and poly(p-phenylenevinylene) (PPV), respectively have been studied using both X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS). The UPS valence band spectra, are interpreted with the help of quantum chemical calculations based upon Modified Neglect of Diatomic Overlap (MNDO), Valence Effective Hamitonian (VEH) and ab initio Hartree-Fock methods. DP7 is a model molecule for polyacetylene, while 6T is a model molecule (an oligomer) of polythiophene. The results indicate that aluminum reacts strongly with the surfaces of all of the materials studied. The π-electronic structure of each material was strongly modified. Furthermore, aluminum reacts preferentially with the polyene partof DP7, with the vinylene part of PPV, and with the α-carbons of the thiophene nits of 6T.
Synthetic Metals.
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ABSTRACT: The chemical and electronic structure of the interface between aluminum and several proto-typical conjugated systems is investigated with a combined experimental and theoretical approach. The experiments consists of following the evolution of the polymer surface during the early stages of aluminum deposition, with X-ray and Ultraviolet Photoelectron Spectroscopies (XPS, UPS). In parallel, quantum chemical calculations are performed on model oligomer systems interacting with isolated Al atoms. Aluminum is found to interact strongly with the polymer chain. New covalent Al-carbon bonds are formed along the polymer backbone; the chain geometry is deeply modified and the π electron conjugation can be dramatically reduced.
Electrochimica Acta.
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ABSTRACT: The chemical structure and vibrational properties of the interface between aluminum and polyacetylene are studied theoretically with a quantum-chemical approach. A density-functional-based technique is used to perform calculations on model systems for the interface, consisting of a polyene oligomer interacting with two aluminum atoms. The bonding configuration of the aluminum atoms on the molecule is investigated and the vibrational frequencies of the aluminum/polyene complex are determined. The analysis of the data provides the vibrational signature which is expected for the chemical species generated during the initial stages of the interface formation.
Synthetic Metals 76:225-228. · 1.83 Impact Factor
