http://www.one-p.eu daily 1 2010-08-25T22:15:12Z http://www.one-p.eu/publication/publication/mixed-quan20111223100828 The electron-phonon coupling is critical in determining the intrinsic charge carrier and exciton transport properties in organic materials. In this study, we consider a Su-Schrieffer-Heeger (SSH) model for molecular crystals, and perform numerical benchmark studies for different strategies of simulating the mixed quantum-classical dynamics. These methods, which differ in the selection of initial conditions and the representation used to solve the time evolution of the quantum carriers, are shown to yield similar equilibrium diffusion properties. A hybrid approach combining molecular dynamics simulations of nuclear motion and quantum-chemical calculations of the electronic Hamiltonian at each geometric configuration appears as an attractive strategy to model charge dynamics in large size systems “on the fly,” yet it relies on the assumption that the quantum carriers do not impact the nuclear dynamics. We find that such an approximation systematically results in overestimated charge-carrier mobilities, with the associated error being negligible when the room-temperature mobility exceeds ∼4.8 cm2/Vs (∼0.14 cm2/Vs) in one-dimensional (two-dimensional) crystals. No publisher DeHalleux wp1 2011-12-23T09:08:59Z Publication http://www.one-p.eu/publication/publication/belt-shape20111223100151 Linear π-conjugated oligomers have been widely investi- 18 gated, but the behavior of the corresponding cyclic oligomers is poorly 19 understood, despite the recent synthesis of π-conjugated macrocycles such 20 as [n]cycloparaphenylenes and cyclo[n]thiophenes. Here we present an 21 efficient template-directed synthesis of a π-conjugated butadiyne-linked 22 cyclic porphyrin hexamer directly from the monomer. Small-angle X-ray 23 scattering data show that this nanoring is shape-persistent in solution, even 24 without its template, whereas the linear porphyrin hexamer is relatively 25 flexible. The crystal structure of the nanoring
template complex shows that most of the strain is localized in the acetylenes; the 26 porphyrin units are slightly curved, but the zinc coordination sphere is undistorted. The electrochemistry, absorption, and 27 fluorescence spectra indicate that the HOMO
LUMO gap of the nanoring is less than that of the linear hexamer and less than that 28 of the corresponding polymer. The nanoring exhibits six one-electron reductions and six one-electron oxidations, most of which are 29 well resolved. Ultrafast fluorescence anisotropy measurements show that absorption of light generates an excited state that is 30 delocalized over the whole π-system within a time of less than 0.5 ps. The fluorescence spectrum is amazingly structured and red- 31 shifted. A similar, but less dramatic, red-shift has been reported in the fluorescence spectra of cycloparaphenylenes and was 32 attributed to a high exciton binding energy; however the exciton binding energy of the porphyrin nanoring is similar to those of 33 linear oligomers. Quantum-chemical excited state calculations show that the fluorescence spectrum of the nanoring can be fully 34 explained in terms of vibronic Herzberg
Teller (HT) intensity borrowing. No publisher DeHalleux wp2 2011-12-23T09:02:37Z Publication http://www.one-p.eu/publication/publication/supramolec20111223094928 Molecular dynamics (MD) simulations have been coupled to valence bond/Hartree-Fock (VB/HF) quantum-chemical calculations to evaluate the impact of diagonal and offdiagonal disorder on charge carrier mobilities in self-assembled one-dimensional stacks of a perylene diimide (PDI) derivative. The relative distance and orientation of the PDI cores probed along the MD trajectories translate into fluctuations in site energies and transfer integrals that are calculated at the VB/HF level. The charge carrier mobilities, as obtained from time-of-flight numerical simulations, span several orders of magnitude depending on the relative time scales for charge versus molecular motion. Comparison to experiment suggests that charge transport in the crystal phase is limited by the presence of static defects No publisher DeHalleux wp1 2011-12-23T08:49:59Z Publication http://www.one-p.eu/publication/publication/charge-sep20111129173248 We probe charge photogeneration and subsequent recombination dynamics in neat regioregular poly(3-hexylthiophene) films over six decades in time by means of time-resolved photoluminescence spectroscopy. Exciton dissociation at 10 K occurs extrinsically at interfaces between molecularly ordered and disordered domains. Polaron pairs thus produced recombine by tunneling with distributed rates governed by the distribution of electron-hole radii. Quantum-chemical calculations suggest that hot-exciton dissociation at such interfaces results from a high charge-transfer character. No publisher Wang wp2 2011-12-11T13:54:04Z Publication http://www.one-p.eu/publication/publication/mixed-quan20111117155019 The electron-phonon coupling is critical in determining the intrinsic charge carrier and exciton transport properties in organic materials. In this study, we consider a Su-Schrieffer-Heeger (SSH) model for molecular crystals, and perform numerical benchmark studies for different strategies of simulating the mixed quantum-classical dynamics. These methods, which differ in the selection of initial conditions and the representation used to solve the time evolution of the quantum carriers, are shown to yield similar equilibrium diffusion properties. A hybrid approach combining molecular dynamics simulations of nuclear motion and quantum-chemical calculations of the electronic Hamiltonian at each geometric configuration appears as an attractive strategy to model charge dynamics in large size systems “on the fly,” yet it relies on the assumption that the quantum carriers do not impact the nuclear dynamics. We find that such an approximation systematically results in overestimated charge-carrier mobilities, with the associated error being negligible when the room-temperature mobility exceeds ∼4.8 cm2/Vs (∼0.14 cm2/Vs) in one-dimensional (two-dimensional) crystals. No publisher Wang wp2 2012-01-19T14:43:44Z Publication http://www.one-p.eu/publication/publication/does-supra20110225085914 We have developed a theoretical platform for modelling temperature-dependent exciton transport in organic materials, using indenofluorene trimers as a case study. Our atomistic molecular dynamics simulations confirm the experimentally observed occurrence of a liquid crystalline smectic phase at room temperature and predict a phase transition to the isotropic phase between 375 and 400 K. Strikingly, the increased orientational disorder at elevated temperatures barely affects the ability of excitons to be transported over large distances, though disorder influences the directionality of the energy diffusion process. Detailed quantum-chemical calculations show that this result arises from a trade-off between reduced excitonic couplings and increased spectral overlap at high temperatures. Our results suggest that liquid crystalline oligomeric materials could be promising candidates for engineering optoelectronic devices that require stable and controlled electronic properties over a wide range of temperatures and supramolecular arrangements. No publisher Muccioli wp2 2011-08-31T11:58:52Z Publication http://www.one-p.eu/publication/publication/charge-tra20101220224821 No publisher DeHalleux wp1 2011-08-31T10:20:44Z Publication http://www.one-p.eu/publication/publication/influence-20101118142205 Normal mode sampling and molecular dynamics simulations are coupled to a valence-bond/Hartree-Fock approach to evaluate the impact of the lattice and molecular vibrations on site energies in anthracene single crystals. The calculations are conducted in the temperature range 0-400 K and show substantial contributions from high-frequency modes, which calls for a quantum-mechanical model even at room temperature. External reorganization energies are also obtained from these modeling studies and found to be much smaller than their internal counterparts. Implications for charge transport in organic single crystals are discussed. No publisher DeHalleux wp1 2011-03-29T13:19:01Z Publication http://www.one-p.eu/publication/publication/electronic20100816130045 We report on the recent progress achieved in modeling the electronic processes that take place at interfaces between π-conjugated materials in organic opto-electronic devices. First, we provide a critical overview of the current computational techniques used to assess the morphology of organic:organic heterojunctions; we highlight the compromises that are necessary to handle large systems and multiple time scales while preserving the atomistic details required for subsequent computations of the electronic and optical properties. We then review some recent theoretical advances in describing the ground-state electronic structure at heterojunctions between donor and acceptor materials and highlight the role played by charge-transfer and long-range polarization effects. Finally, we discuss the modeling of the excited-state electronic structure at organic:organic interfaces, which is a key aspect in the understanding of the dynamics of photoinduced electron-transfer processes. No publisher Muccioli wp1 2011-03-29T13:24:34Z Publication http://www.one-p.eu/publication/publication/fluorescen20100726144148 We report upon a theoretical study of singlet exciton migration and relaxation within a model conjugated polymer chain. Starting from poly(2-methoxy-5-((2-ethylhexyl)oxy)-1,4- phenylenevinylene) polymer chains, we assume that the pi-conjugation is disrupted by conformational disorder of the chain itself, giving rise to a localized Frenkel exciton basis. Electronic coupling between segments as determined by the coupling between the transition densities of the localized excitons gives rise to delocalized exciton states. Using a kinetic Monte Carlo approach to compute the exciton transfer kinetics within the manifold of either the dressed chromophore site basis or dressed eigenstate basis, we find that the decay of the polarization anisotropy of the exciton is profoundly affected by the delocalization of the exciton over multiple basis segments. Two time scales emerge from the exciton migration simulations: a short, roughly 10 ps, time scale corresponding to rapid hopping about the initial excitation site followed by a slower, 180 ps, component corresponding to long range hopping. We also find that excitations can become trapped at long times when the hopping rate to lower-energy states is longer than the radiative lifetime of the exciton. No publisher DeHalleux wp2 wp3 2011-03-29T13:38:23Z Publication http://www.one-p.eu/publication/publication/2d-exciton20100726120211 A number of organic crystals show anisotropic excitonic couplings, with weak interlayer interactions between molecules that are more strongly coupled within the layers. The resulting energy carriers are intralayer 2D excitons that diffuse along the interlayer direction. We model this analytically for infinite layers and using quantum-chemical calculations of the electronic couplings for anthracene clusters. We show that the exciton hopping rates and diffusion lengths depend in a subtle manner on the size and shape of the interacting aggregates, temperature, and the presence of energetic disorder. No publisher DeHalleux wp2 2011-03-29T13:40:54Z Publication http://www.one-p.eu/publication/publication/on-the-int20100107160606 The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organics-based photovoltaic cells. Here, we perform quantum-chemical and micro-electrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules, rather than in a charge transfer from donor to acceptor. The local dipole is found to fluctuate in sign and magnitude over the interface and appears as a sensitive probe of the relative arrangements of the pentacene and C60 molecules (and of the resulting local electrical fields sensed by the molecular units). No publisher Olivier wp2 2011-03-29T14:14:59Z Publication http://www.one-p.eu/publication/publication/organic20120090928103043 Organic semiconductors are characterized by localized states whose energies are predominantly determined by electrostatic interactions with their immediate molecular environment. As a result, the details of the energy landscape at heterojunctions between different organic semiconductors cannot simply be deduced from those of the individual semiconductors, and they have so far remained largely unexplored. Here, microelectrostatic computations are performed to clarify the nature of the electronic structure and geminate pair energetics at the pentacene/C60 interface, as archetype for an interface between a donor molecule and a fullerene electron acceptor. The size and orientation of the molecular quadrupole moments, determined by material choice, crystal orientation, and thermodynamic growth parameters of the semiconductors, dominate the interface energetics. Not only do quadrupoles produce direct electrostatic interactions with charge carriers, but, in addition, the discontinuity of the quadrupole field at the interface induces permanent interface dipoles. That discontinuity is particularly striking for an interface with C60 molecules, which by virtue of their symmetry possess no quadrupole. Consequently, at a pentacene/C60 interface, both the vacuum-level shift and geminate pair dissociation critically depend on the orientation of the pentacene p-system relative to the adjacent C60. No publisher Cheyns wp2 2011-03-29T14:40:47Z Publication http://www.one-p.eu/publication/publication/theoretica20090904103322 We present a joint molecular dynamics (MD)/kinetic Monte Carlo (KMC) study aimed at the atomistic description of charge transport in stacks of liquid-crystalline tetraalkoxy-substituted, metal-free phthalocyanines. The molecular dynamics simulations reproduce the major structural features of the mesophases, in particular, a phase transition around 340 K between the rectangular and hexagonal phases. Charge transport simulations based on a Monte Carlo algorithm show an increase by 2 orders of magnitude in the hole mobility when accounting for the rotational and translational dynamics. The results point to the formation of dynamical structural defects along the columns. No publisher Olivier wp1 wp2 2011-03-29T14:46:40Z Publication http://www.one-p.eu/publication/publication/influence-20090902134807 We have performed classical molecular dynamics simulations and quantum-chemical calculations on molecular crystals of anthracene and perfluoropentacene. Our goal is to characterize the amplitudes of the room-temperature molecular displacements and the corresponding thermal fluctuations in electronic transfer integrals, which constitute a key parameter for charge transport in organic semiconductors. Our calculations show that the thermal fluctuations lead to Gaussian-like distributions of the transfer integrals centered around the values obtained for the equilibrium crystal geometry. The calculated distributions have been plugged into Monte-Carlo simulations of hopping transport, which show that lattice vibrations impact charge transport properties to various degrees depending on the actual crystal structure. No publisher Olivier wp1 2011-03-29T14:48:47Z Publication