13,408 research outputs found
Polarization and polarizability in extended one-dimensional organic materials
No full textAbstract: The modern theory of polarization in extended insulators is applied to one-dimensional models for conjugated polymers and charge transfer salts. Closed expressions for the dependence of the polarization on the site and bond energy alternations are presented for uncorrelated models, and results from exact real-space diagonalization are obtained for correlated models. Changes in polarization induced by lattice phonons or molecular vibrations are directly related to the intensity of infrared bands in the far and mid-IR, respectively. We model intensities by introducing linear electron-vibration coupling and show that coupling to delocalized electrons generates a combination band consisting of a lattice phonon and a molecular vibration. The displaced dipole operator is defined on a real-space basis allowing for the finite field calculation of linear polarizability in finite size systems with periodic boundary conditions. Size-consistency arguments are used to demonstrate that the resulting polarizability becomes exact in the thermodynamic limit, and numerical calculations demonstrate that this approach leads to reliable results that converge rapidly to the thermodynamic limit. (c) 2005 Elsevier B.V. All rights reserved
Dielectric properties of crystalline organic molecular films in the limit of zero overlap
No full textWe present the calculation of the static dielectric susceptibility tensor and dipole field sums in thin molecular films in the well-defined limit of zero intermolecular overlap. Microelectrostatic and charge redistribution approaches are applied to study the evolution of dielectric properties from one to a few molecular layers in films of different conjugated molecules with organic electronics applications. Because of the conditional convergence of dipolar interactions, dipole fields depend on the shape of the sample and different values are found in the middle layer of a thick film and in the bulk. The shape dependence is eliminated when depolarization is taken into account, and the dielectric tensor of molecular films converges to the bulk limit within a few molecular layers. We quantify the magnitude of surface effects and interpret general trends among different systems in terms of molecular properties, such as shape, polarizability anisotropy, and supramolecular organization. A connection between atomistic models for molecular dielectrics and simpler theories for polarizable atomic lattices is also provided
Modeling the neutral-ionic transition with correlated electrons coupled to soft lattices and molecules
No full textNeutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar p-electron donors (D) and acceptors (A) that form mixed stacks... D+ρA–ρD+ρA–ρD+ρA–ρ... with variable ionicity 0 < r < 1 and electron transfer t along the stack. The microscopic NIT model presented here combines a modified Hubbard model for strongly correlated electrons delocalized along the stack with Coulomb intermolecular interactions treated in mean field. It also accounts for linear coupling of electrons to a harmonic molecular vibration and to the Peierls phonon. This simple framework captures the observed complexity of NITs with continuous and discontinuous r on cooling or under pressure, together with the stack’s instability to dimerization. The interplay of charge, molecular and lattice degrees of freedom at NIT amplifies the nonlinearity of responses, accounts for the dielectric anomaly, and generates strongly anharmonic potential energy surfaces (PES). Dynamics on the ground state PES address vibrational spectra using time correlation functions. When extended to the excited state PES, the NIT model describes the early (<1 ps) dynamics of transient NIT induced by optical CT excitation with a fs pulse. Although phenomenological, the model parameters are broadly consistent with density functional calculations
Towards a unified view of electron-phonon coupling in 1D solids
No full textWe analyze the effect of on-site and on-bond electron-phonon (e-ph) coupling in different classes of quasi one-dimensional (1D) solids in terms of a single model. The model was originally developed to account for e-ph coupling in charge transfer crystals and to interpret their vibrational spectra. The same model is extended to conjugated polymers, showing that important information about the electronic structure can be obtained through a careful analysis of vibrational data. Evidence of e-ph coupling in the spectra of halogen-bridged transition-metal complexes is also presented. The unified model thus applies to different classes of 1D systems and yields transferable e-ph coupling constants
Vibrational spectra of pristine, photoexcited and doped polyacetylene: towards a microscopic model.
No full textElectron-phonon coupling in conjugated polymers: Reference force field and transferable coupling constants for polyacetylene
No full textElectron-Transfer in Molecular Functional Materials
No full textWe discuss electron-transfer processes that govern the physics of several materials or systems of interest for advanced applications. The discussion touches upon several topics, ranging from solvatochromism to solvent-induced symmetry breaking, from excitonic to cooperative effects in molecular crystals, from phase transitions to vibrational contributions to the dielectric constant in organic materials, from spectroscopy to molecular transport. In all these diverse systems electron transfer (ET) plays a major role and is discussed with reference to simple models for delocalized charges
Infrared and Raman modes of polyacetylene and its isotopes: Transferable coupling constants
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