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J Cornil

Publications and source records attributed to J Cornil.

13 recordsLinked to original sources
PubMed2006

Charge hopping in organic semiconductors: influence of molecular parameters on macroscopic mobilities in model one-dimensional stacks.

We present a Monte Carlo approach to estimate how molecular parameters impact hopping rates and charge mobilities in organic pi-conjugated materials. Our goal is to help in establishing structure-properties relationships. As a first step, our approach is illustrated by considering a model system made of a one-dimensional array of pentacene molecules; we describe the variations of the electron-transfer rates and of the resulting charge mobilities as a function of electric field and of the presence of molecular disorder and traps. The results highlight that there is no direct relationship between the degree of spatial overlap among adjacent molecules and charge mobility.

Journal Article↗
PubMed2006

Influence of copolymer interface orientation on the optical emission of polymeric semiconductor heterojunctions.

We have examined the Coulombic interactions at the interface in a blend of two copolymers with intramolecular charge-transfer character and optimized band offsets for photoinduced charge generation. The combination of both time-resolved measurements of photoluminescence, and quantum-chemical modeling of the heterojunction allows us to show that relative orientation across the heterojunction can lead to either a repulsive barrier ( approximately 65 meV) or an attractive interaction which can enhance the charge-transfer processes. We conclude that polymer orientation at the heterojunction can be as important as energy-band offsets in determining the dynamics of charge separation and optical emission.

Journal Article↗
PubMed2006

Pathways for photoinduced charge separation and recombination at donor-acceptor heterojunctions: the case of oligophenylenevinylene-perylene bisimide complexes.

Semiempirical Hartree-Fock techniques have been applied to assess the molecular parameters governing the efficiency of photoinduced charge generation and recombination processes in donor/acceptor complexes involving a three-ring oligophenylenevinylene as donor and perylene bisimide as acceptor. The corresponding rates have been estimated in the framework of the Marcus-Levich-Jortner formalism for different geometries of the complexes. The results indicate that dissociation pathways involving the lowest two charge transfer excited states contribute significantly to the dynamics of the whole process. The rates are found to be strongly sensitive to the relative position of the donor and acceptor units and can be rationalized in terms of symmetry arguments applied to relevant electronic levels.

Journal Article↗
PubMed2005

Impact of the computational method on the geometric and electronic properties of oligo(phenylene vinylene) radical cations.

We report on a quantum-chemical study of the electronic and optical properties of unsubstituted oligo(phenylene vinylene) (OPV) radical cations. Our goal is to distinguish the impact of the choice of molecular geometry from the impact of the choice of quantum-chemical method, on the calculated optical transition energies. The geometry modifications upon ionization of the OPV chains are found to depend critically on the theoretical formalism: Hartree-Fock (HF) geometry optimizations lead to self-localization of the charged defects while pure density functional theory (DFT) results in a complete delocalization of the geometric modifications over the whole conjugated backbone. The electronic structure and vertical transition energy associated with the lowest excited state of the radical cations have been calculated at the post-Hartree-Fock level within a configuration interaction (HF-CI) scheme and using the time-dependent DFT (TD-DFT) formalism for different radical cation geometries. Interestingly, the changes in the calculated optical properties obtained when using different geometric structures are less important within a given method than the differences between methods for a given structure. The optical excitation is localized with HF-CI and delocalized with TD-DFT, almost irrespective of the molecular geometry; as a result, HF-CI excitation energies tend to saturate as the chain length increases, in contrast to the results from TD-DFT.

Journal Article↗
PubMed2004

Characterization of the molecular parameters determining charge transport in anthradithiophene.

The molecular parameters that govern charge transport in anthradithiophene (ADT) are studied by a joint experimental/theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. The hole reorganization energy of ADT has been determined by an analysis of the vibrational structure of the lowest ionization band in the gas-phase photoelectron spectrum as well as by density-functional theory calculations. In addition, various dimers and clusters of ADT molecules have been considered in order to understand the effect of molecular packing on the hole and electron intermolecular transfer integrals. The results indicate that the intrinsic electronic structure, the relevant intramolecular vibrational modes, and the intermolecular interactions in ADT are very similar to those in pentacene.

Dimerization↗
PubMed2002

Photoinduced electron-transfer processes along molecular wires based on phenylenevinylene oligomers: a quantum-chemical insight.

Quantum-chemical techniques are applied to model the mechanisms of photoinduced charge transfer from a pi-electron donating group (tetracene, D) to a pi-electron-acceptor moiety (pyromellitimide, A) separated by a bridge of increasing size (p-phenylenevinylene oligomers, B). Correlated Hartree-Fock semiempirical approaches are exploited to calculate the four main parameters controlling the transfer rate (k(RP)) in the framework of Marcus-Jortner-Levich's formalism: (i) the electronic coupling between the initial and final states; (ii) and (iii) the internal and external reorganization energy terms; and (iv) the variation of the free Gibbs energy. The charge transfer is shown to proceed in these compounds through two competing mechanisms, coherent (superexchange) versus incoherent (bridge-mediated) pathways. While superexchange is the dominant mechanism for short bridges, incoherent transfer through hopping along the phenylene vinylene segment takes over in longer chains (for ca. three phenylenevinylene repeat units). The influence of the chemical structure of the pi-conjugated phenylenevinylene bridge on the electronic properties and the rate of charge transfer is also investigated.

Journal Article↗
PubMed2002

Organic semiconductors: a theoretical characterization of the basic parameters governing charge transport.

Organic semiconductors based on pi-conjugated oligomers and polymers constitute the active elements in new generations of plastic (opto)electronic devices. The performance of these devices depends largely on the efficiency of the charge-transport processes; at the microscopic level, one of the major parameters governing the transport properties is the amplitude of the electronic transfer integrals between adjacent oligomer or polymer chains. Here, quantum-chemical calculations are performed on model systems to address the way transfer integrals between adjacent chains are affected by the nature and relative positions of the interacting units. Compounds under investigation include oligothienylenes, hexabenzocoronene, oligoacenes, and perylene. It is shown that the amplitude of the transfer integrals is extremely sensitive to the molecular packing. Interestingly, in contrast to conventional wisdom, specific arrangements can lead to electron mobilities that are larger than hole mobilities, which is, for instance, the case of perylene.

Dimerization↗
PubMed2002

Negative differential resistance in phenylene ethynylene oligomers.

The origin of the sharp peak profile (i.e., negative differential resistance, NDR) observed in the I/V curves of three-ring phenylene ethynylene oligomers is a topic of major current interest. Here, quantum-chemical calculations are performed to analyze the evolution of the one-electron structure of an unsubstituted three-ring oligomer under the influence of a static electric field (which models the driving voltage applied in the experiments). The results indicate that the rotation of the central ring of the oligomer induces resonant tunneling processes over a limited voltage range. This can thus be responsible for the NDR signature observed experimentally.

Journal Article↗
PubMed2001

Negative differential resistance behavior in conjugated molecular wires incorporating spacers: a quantum-chemical description.

Recent experimental studies have demonstrated that single molecules or a small number of self-assembled molecules can perform the basic functions of traditional electronic components, such as wires and diodes. In particular, molecular wires inserted into nanopores can be used as active elements for the fabrication of resonant tunneling diodes (RTDs), whose I/V characteristics reveal a Negative Differential Resistance (NDR) behavior (i.e., a negative slope in the I/V curve). Here, quantum-chemical calculations are used to describe on a qualitative basis the mechanism leading to NDR in polyphenylene-based molecular wires incorporating saturated spacers. This description is based on the characterization of the evolution of the wire electronic structure as a function of a static electric field applied along the molecular axis, which simulates the driving voltage between the two electrodes in the RTD devices. We illustrate that the main parameters controlling the NDR behavior can be modulated through molecular engineering of the wires.

Journal Article↗
PubMed2000

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.

Journal Article↗