Aggregation−induced emissions in columnar wedge−shaped pyridinium−based ionic liquid crystals

Abstract

Two series of ILCs derived from wedge−shaped N-phenylpyridiniums 1–2 were prepared and their mesomorphic properties were investigated. The X−ray crystal structure analyses of two single crystals 2-PF6 (n = 12) and 2-NTf2 (n = 12) revealed that bilayer structures induced by H−bonds in the solid state were observed. A complementary dimeric structure induced by H−bonds in such type of wedge−shaped cation (or anion) moiety was attributed to the formation of columnar phases. The type of counter anions has a significant influence on the formation of the mesophase; five counter anions (i.e. X = PF6‾, BF4‾, OTf‾, SCN‾, RSO3‾) induced columnar mesophase. In contrast, anion X = Tf2N‾ as a bulky moiety and less H−bond active sites formed crystal phase. The powder XRD data appeared that the diameter of the hard columnar part increased with the effective ionic radius for different counter ion. A linear correlation plot (R2 = 0.9203) was obtained. All ethanol solutions of compounds 1–2 were detected without any observable signals on photoluminescence. But compounds 2-X will aggregate in the aqueous mixture with high water fractions (ƒw) needed to turn on their light emission process. Weak PL signals are recorded at ƒw < 50% because the luminogens 2-OTf are fully dissolved and quenched in these solvent compositions. The PL intensity started to enhance at ƒw > 50% till it reached to the maximum intensity at ƒw ~70%.