title: Oligothiophene wires: impact of torsional conformation on the electronic structure.
abstract: Charge transport in polymer- and oligomer-based semiconductor materials depends strongly on the structural ordering of the constituent molecules. Variations in molecular conformations influence the electronic structures of polymers and oligomers, and thus impact their charge-transport properties. In this study, we used Scanning Tunneling Microscopy and Spectroscopy (STM/STS) to investigate the electronic structures of different alkyl-substituted oligothiophenes displaying varied torsional conformations on the Au(111) surface. STM imaging showed that on Au(111), oligothiophenes self-assemble into chain-like structures, binding to each other via interdigitated alkyl ligands. The molecules adopted distinct planar conformations with alkyl ligands forming cis- or trans- mutual orientations. For each molecule, by using STS mapping, we identify a progression of particle-in-a-box-like states corresponding to the LUMO, LUMO+1 and LUMO+2 orbitals. Analysis of STS data revealed very similar unoccupied molecular orbital energies for different possible molecular conformations. By using density functional theory calculations, we show that the lack of variation in molecular orbital energies among the different oligothiophene conformers implies that the effect of the Au-oligothiophene interaction on molecular orbital energies is nearly identical for all studied torsional conformations. Our results suggest that cis-trans torsional disorder may not be a significant source of electronic disorder and charge carrier trapping in organic semiconductor devices based on oligothiophenes.
authors.affiliation: Department of Chemistry and Biochemistry, Materials Science Institute, Oregon Center for Optical, Molecular and Quantum Science, University of Oregon, 1253 University of Oregon Eugene, OR 97403, USA. gnazin@uoregon.edu.
authors.forename: D A
authors.initials: DA
authors.lastname: Kislitsyn
authors.affiliation: Department of Chemistry and Biochemistry, Materials Science Institute, Oregon Center for Optical, Molecular and Quantum Science, University of Oregon, 1253 University of Oregon Eugene, OR 97403, USA. gnazin@uoregon.edu.
authors.forename: B N
authors.initials: BN
authors.lastname: Taber
authors.affiliation: Department of Chemistry and Biochemistry, Materials Science Institute, Oregon Center for Optical, Molecular and Quantum Science, University of Oregon, 1253 University of Oregon Eugene, OR 97403, USA. gnazin@uoregon.edu.
authors.forename: C F
authors.initials: CF
authors.lastname: Gervasi
authors.affiliation: Department of Polymer Science and Engineering, University of Massachusetts-Amherst, Silvio O. Conte National Center for Polymer Research, 120 Governors Drive Amherst, MA 01003, USA.
authors.forename: L
authors.initials: L
authors.lastname: Zhang
authors.affiliation: Center for Advancing Electronics Dresden, Dresden University of Technology, 01062 Dresden, Germany.
authors.forename: S C B
authors.initials: SC
authors.lastname: Mannsfeld
authors.affiliation: Department of Chemistry and Biochemistry, Materials Science Institute, Oregon Center for Optical, Molecular and Quantum Science, University of Oregon, 1253 University of Oregon Eugene, OR 97403, USA. gnazin@uoregon.edu.
authors.forename: J S
authors.initials: JS
authors.lastname: Prell
authors.affiliation: Department of Polymer Science and Engineering, University of Massachusetts-Amherst, Silvio O. Conte National Center for Polymer Research, 120 Governors Drive Amherst, MA 01003, USA.
authors.forename: A L
authors.initials: AL
authors.lastname: Briseno
authors.affiliation: Department of Chemistry and Biochemistry, Materials Science Institute, Oregon Center for Optical, Molecular and Quantum Science, University of Oregon, 1253 University of Oregon Eugene, OR 97403, USA. gnazin@uoregon.edu.
authors.forename: G V
authors.initials: GV
authors.lastname: Nazin