Density-functional study of the evolution of the electronic structure of oligomers of thiophene: Towards a model Hamiltonian

Abstract

Density-functional study of the evolution of the electronic structure of oligomers of thiophene Telesca, R.; Bolink, H.; Yunoki, S.; Hadziioannou, G.; Duijnen, P.Th. van; Snijders, J.G.; Jonkman, Harry; Sawatzky, G.A. Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Telesca, R., Bolink, H., Yunoki, S., Hadziioannou, G., Duijnen, P. T. V., Snijders, J. G., ... Sawatzky, G. A. (2001). Density-functional study of the evolution of the electronic structure of oligomers of thiophene: Towards a model Hamiltonian. Physical Review B, 6315(15), [155112]. DOI: 10.1103/PhysRevB.63.155112 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. We present density-functional and time-dependent density-functional studies of the ground, ionic, and excited states of a series of oligomers of thiophene. We show that, for the physical properties, the most relevant highest occupied and lowest unoccupied molecular orbitals develop gradually from monomer molecular orbitals into occupied and unoccupied broad bands in the large length limit. We show that band gap and ionization potentials decrease with size, as found experimentally and from empirical calculations. This gives credence to a simple tight-binding model Hamiltonian approach to these systems. We demonstrate that the length dependence of the experimental excitation spectra for both singlet and triplet excitations can be very well explained with an extended Hubbard-like Hamiltonian, with a monomer on-site Coulomb and exchange interaction and a nearest-neighbor Coulomb interaction. We also study the ground and excited-state electronic structures as functions of the torsion angle between the units in a dimer, and find almost equal stabilities for the transoid and cisoid isomers, with a transition energy barrier for isomerization of only 4.3 kcal/mol. Fluctuations in the torsion angle turn out to be very low in energy, and therefore of great importance in describing even the room-temperature properties. At a torsion angle of 90°the hopping integral is switched off for the highest occupied molecular orbital levels because of symmetry, allowing a first-principles estimate of the on-site interaction minus the next-neighbor Coulomb interaction as it enters in a Hubbard-like model Hamiltonian.

Resolving Band-Structure Evolution and Defect-Induced States of Single Conjugated Oligomers by Scanning Tunneling Microscopy and Tight-Binding Calculations

“…Theoretical descriptions of CP follow one of two routes: a delocalized band theory of solid-state physics or a localized molecular orbital approach of quantum chemistry [2][3][4]. In spite of their different perspectives, both approaches predict that the molecular orbitals of short oligomers develop progressively into a broad band in the long-length limit [5][6][7]. The bandlike features have been detected experimentally for short oligomers only [8,9].…”

“…The majority of attempts follow the so-called oligomer approach, in which the characteristics of short oligomers of increasing chain length are extrapolated to infinite polymers [10,11]. There are still significant discrepancies among different theories, as well as between theories and experimental results, and many fundamental properties of CP including bandwidth, band gap, and the influence of various structural defects, etc., are under debate [7].In particular, precise experimental determination of band structure, i.e., the energy-momentum dispersion, of CP or long oligomers has had limited successes largely due to the difficulty of growing high-purity single crystalline samples [12]. Above all, the existence of defects presents a severe challenge since defects in a 1D system can strongly alter its electronic structure [13].…”

“…The majority of attempts follow the so-called oligomer approach, in which the characteristics of short oligomers of increasing chain length are extrapolated to infinite polymers [10,11]. There are still significant discrepancies among different theories, as well as between theories and experimental results, and many fundamental properties of CP including bandwidth, band gap, and the influence of various structural defects, etc., are under debate [7].…”

Excited state polarizabilities of conjugated molecules calculated using time dependent density functional theory

“…[35][36][37] The TDDFT method as implemented in ADF can be used to calculate accurate excitation energies and has been successfully applied to the excited states of oligothiophenes. 38 The combination of TDDFT with a static uniform electric field makes possible to analyze the electrostatic properties of molecules in their excited state. The dependence of the excitation energy on a static external electric field depends on the changes in dipole moment and polarizability upon excitation.…”

Photophysical Properties of Two New Donor-Acceptor Conjugated Copolymers and Their Model Compounds: Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs)

“…Moreover, they are model compounds for the corresponding polymers [66,67]. In parallel to recent experimental work on oligomers, theoretical efforts have also begun complementing the experimental studies in the characterization of the nature and the properties of their ground-and lowest electronic excited states [68][69][70][71][72][73]. In addition, these approaches have provided significant insight into the electronic and optical properties of conjugated polymers.…”