Aggregation induced emission switching and electrical properties of chain length dependent π-gels derived from phenylenedivinylene bis-pyridinium salts in alcohol–water mixtures

Abstract

Supramolecular π-gels were formed in a mixture of aliphatic alcohols and water for a series of chromophoric phenylenedivinylene bis-N-alkyl pyridinium salts (PPV) appended with terminal aliphatic hydrocarbon chains of different lengths. Gelation could be controlled either by altering the ratio of various alcohol–water mixtures or by changing the aliphatic chain length of the gelator. The temperature- and the ratio-variation in the ethanol–water mixtures exhibited a tunable emission behavior depending on the extent of aggregation which was promoted by aromatic π-stacking, van der Waals and electrostatic interactions among the individual PPV units. Thus, a light-blue emission at higher temperature (>40 °C), a reddish-orange emission at low temperature (<20 °C) and a white-light emission at room temperature (25–30 °C) were observed in solution. The gelators possessing longer aliphatic chains exhibited a higher gel-melting temperature, increased viscoelasticity and shorter fiber diameter based on a delicate hydrophobic/hydrophilic balance. A semiconducting nature of the electrical conductivity was observed for the individual compounds and the magnitude of the current increased with increasing width of the gel fibers upon decreasing the aliphatic chain length. A reversible one-electron redox behavior was observed for the chromophore and the redox potential decreased with the increase in the chain length. A diffusion-controlled redox behavior was observed for the gelators with shorter aliphatic chains. However, the compounds with longer chains made the process diffusion-limited.