Noncovalent Close Contacts in Fluorinated Thiophene-Phenylene-Thiophene Conjugated Units: Understanding the Nature and Dominance of O•••H versus S•••F and O•••F Interactions with Respect to the Control of Polymer Conformation

Using a simple pi-conjugated trimer, EDOT-phenylene-EDOT (EDOT = 3,4-ethylenedioxythiophene), we evaluate the effect that fluorine substituents have upon changes in conformation, conjugation, and oxidation potentials in pi-conjugated structures. These variations are assessed as a function of the fluorine atom's propensity to feature in hydrogen and/or halogen bonding with other heteroatoms. The molecular motif was chosen because the EDOT unit presents the possibility of competing O center dot center dot center dot X or S center dot center dot center dot X noncovalent contacts (where X = H or F). Such nonbonding interactions are acknowledged to be highly influential in dictating molecular and polymer morphology and inducing changes in certain physical properties. We studied four compounds, beginning with an unsubstituted bridging phenylene ring and then adding one, two, or four fluorine units to the parent molecule. Our studies involve single-crystal X-ray diffraction studies, cyclic voltammetry, absorption spectroscopy, and density functional theory calculations to identify the dominant noncovalent interactions and elucidate their effects on the molecules described. Experimental studies have also been carried out on the corresponding electrochemically synthesized polymers to confirm that these noncovalent interactions and their effects persist in polymers. Our findings show that hydrogen bonding and halogen bonding feature in these molecules and their corresponding polymers.