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 impact of conformational isomerization on the electronic structures of alkyl-substituted oligothiophenes. STM imaging revealed that on Au(111), oligothiophenes self-assemble into crystal- and chain-like structures, binding to each other via interdigitated alkyl ligands. The molecules adopted varied torsional conformations with alkyl ligands adopting cis- or trans- mutual orientations. For each observed molecular conformation, we used STS mapping to identify a progression of particle-in-a-box-like states corresponding to unoccupied molecular orbitals. Analysis of orbital energies obtained from STS data revealed very similar molecular electronic structures for the different possible molecular conformations. By using density functional theory (DFT) calculations, we showed that the lack of variation in electronic structures among the different oligothiophene conformers implies that the effect of the Au-oligothiophene interaction 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.

Scanning tunneling microscopy of alkyl-substituted oligothiophenes on Au(111): Real-space visualization of molecular electronic structure

253rd National Meeting (2017)

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