The intramolecular interactions responsible for the planarity observed in poly(3,4-ethylenedioxythiophene) and small 3,4-ethylenedioxythiophene-containing oligomers have been investigated using quantum mechanical methods. Specifically, the relative influence of electron-donating effects, π-conjugation, and geometric restrictions induced by the cyclic substituent and attractive S · · ·O intramolecular noncovalent interactions, which were proposed to be the most relevant factor for such planarity on the self-rigidification observed in these compounds, have been examined considering a wide number of model compounds. Results evidenced that noncovalent S · · ·O interactions, which were postulated on the basis that the nonbonded distances between sulfur and oxygen atoms belonging to neighboring repeating units are significantly shorter than the sum of the van der Waals radii of sulfur and oxygen, are slightly repulsive destabilizing the planar anti conformation. In contrast, the latter arrangement is favored by the π-conjugation produced by both geometric restrictions imposed by the cyclic substituent and the electron-donating effects provided by the oxygen atoms attached to positions three and four of each tiophene ring. Therefore, these factors produce gain in aromaticity and favorable electrostatic interactions when the planarity is reached, compensating the Pauli repulsions between the shared electron pairs of the sulfur and oxygen atoms.

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