Density-functional study of the evolution of the electronic structure of oligomers of thiophene: Towards a model Hamiltonian - art. no. 155112

We present density-functional and time-dependent density-functional studies of the ground, ionic, and excited states of a series of oligomers of thiop hene. We show that, for the physical properties, the most relevant highest occupied and lowest unoccupied molecular orbitals develop gradually from mo nomer molecular orbitals into occupied and unoccupied broad bands in the la rge length limit. We show that band gap and ionization potentials decrease with size, as found experimentally and from empirical calculations. This gi ves credence to a simple tight-binding model Hamiltonian approach to these systems. We demonstrate that the length dependence of the experimental exci tation spectra for both singlet and triplet excitations can be very well ex plained with an extended Hubbard-like Hamiltonian, with a monomer on-site C oulomb and exchange interaction and a nearest-neighbor Coulomb interaction. We also study the ground and excited-state electronic structures as functi ons of the torsion angle between the units in a dimer, and find almost equa l stabilities for the transoid and cisoid isomers, with a transition energy barrier for isomerization of only 4.3 kcal/mol. Fluctuations in the torsio n angle turn out to be very low in energy, and therefore of great importanc e in describing even the room-temperature properties. At a torsion angle of 90 degrees the hopping integral is switched off for the highest occupied m olecular orbital levels because of symmetry, allowing a first-principles es timate of the on-site interaction minus the next-neighbor Coulomb interacti on as it enters in a Hubbard-like model Hamiltonian.