Theoretical Study of Bis(phenylethynyl)thienoacenes as Precursors of Molecular Wires for Molecular Electronics

The geometries and electronic properties of a series of bis(phenylethynyl)thienoacenes as precursors of n-type materials with desirable (opto)electronic properties have been studied from a theoretical perspective: 2,5-bis-phenylethynyl-thiophene (PhEtT), 2,5-bis-phenylethynyl-thieno[3,2-b]thiophene (PhEtT2), 2,6-bis-phenylethynyl-dithieno[3,2-b;2',3'-d]thiophene (PhEtT3), and 2,6-bis-phenylethyny1-3,4,7,8-tetrathia-dicyclopenta[a,e]pentalene (PhEtT4). This has allowed performance of a comparative study of the influence of the number of fused thiophene rings on the studied properties. The (opto)electronic properties and conducting capability were investigated through the HOMO -> LUMO excitation energy, bond length alternation (BLA), LUMO energy, electron affinity, and intramolecular reorganization energy. Also, the influence of planarity on electrical conductivity has been analyzed by means of the natural bond orbital (NBO) approach together with the theory of atoms in molecules (AIM). From this study it follows that the increase in the number of fused thiophene rings does not significantly affect those properties related to aromaticity such as BLA, rotational barriers, or charge transference between the different aromatic rings, while that increase leads to an easier charge injection and brings on an enhancement of the n-type character.