Synthesis, Thermal Stability, and Ionic Conductivity of Phenyl-Extended Viologen Ditriflate Salts: Tailoring Properties via Oxyethylene Chain Length

Abstract

Six new phenyl-extended viologen salts with oxyethylene groups paired with triflate anions (OTF) have been synthesized using anionic ring-opening and ring-closing reactions (ANRORC) and metathesis reactions with LiOTf salts. The salts, which contain terminal oxyethylene chains of varying lengths ((CH2CH2O) n , n = 1, 2, 3, 4, 5, and 7), are named PEOn-OTF. These materials exhibit a fluorescence response and possess high thermal stability, with processing windows up to approximately 300 °C, as confirmed by thermogravimetric analysis. While samples with short chain lengths (n = 1, 2, and 3) undergo crystallization observed via differential scanning calorimetry, longer chains seem to restrict molecular packing, resulting in amorphous materials (n ≥ 4). Conductivity was studied via dielectric analysis, including an innovative approach to monitor dielectric relaxations through the imaginary component of the complex conductivity. Amorphous samples with long oxyethylene terminal chains display the highest direct current conductivity (σdc ∼ 10–3.5 S·cm–1 at 110 °C), which is of merit for organic media, but lower than that reported for trifluoromethylsulfonylimide (TFSI) analogues. Our detailed study of relaxations hints at the existence of strong correlations between morphology, dielectric response, and ion transport dynamics, which will be further explored in the near future.