Self-Assembly of Functionalized Oligothiophene into Hygroscopic Fibers: Fabrication of Highly Sensitive and Fast Humidity Sensors

A new symmetric oligothiophene exposing tetraethylene glycol (TEG)-based side-chains is designed and synthesized. This molecule is found to self-assemble in solution forming supramolecular fibers, via pi-pi stacking between the conjugated oligothiophene backbones, which are phase segregated on the sub-nanometer scale from the TEG side-groups. The delocalization of the charges through the oligothiophene pi-pi stack ensures efficient charge transport while the hygroscopic shell, decorating the surface of the fibrillar structures, determines a certain affinity for polar molecules. Upon exposure to humidity, under environmental conditions, such supramolecular architectures are capable of reversibly absorbing and desorbing water molecules. Absorption of water molecules, due to increased environmental humidity, causes a fast and reproducible increase of the electrical current through the fibers by a factor 100 from 15% to 90% relative humidity, as measured in 2-terminal devices. Such process is extremely fast, taking place in less than 45 ms. The humidity-responsive characteristics of the presented oligothiophene-based fibers can be exploited for the facile fabrication of high-performances and solution-processable electrical resistive humidity sensors.