Synthesis & optoelectroninc properties of perfluorinated columnar liquid crystals

Abstract

We describe the synthesis and characterization of supramolecules self-assembled into hexagonal columnar cylinder mesophases, [4-(1-Pyrenyl)-butyl] 3, 4, 5-tris-(12, 12, 12, 11, 11, 10, 10, 9, 9, 8, 8, 7, 7, 6, 6, 5, 5-heptadecafluoro-n-dodecan-1-yloxy)-benzoate (PF8B). Thermal and phase transition of PF8B was confirmed by differential scanning calorimetry (DSC) data, polarized optical microscopy (POM) and X-ray diffraction (XRD) image. This molecule contains a pyrene core and three perfluorinated ($-CF_3$) tails. And then, we show that a π-conjugated supramolecular columnar liquid crystal (PF8B) exhibits photoluminescence (PL) properties. In the center, the pyrene molecules are stacked in a column surrounded by aromatic and perfluorinated tails, which can be conjugated and play a part as chromophore. We found that the absorption and PL properties of the dendritic solid were affected by the orientation of π-stacks within the column. The orientation of the columns was controlled by cooling rates on glass substrate and investigated by using POM and scanning electron microscopy (SEM). Homeotropic alignment of columns was obtained by slow cooling from isotropic state in sandwich glass substrates, while planar alignment occurs by rapid cooling. Planar alignment of the cylinders causes a strong broad emission band and a second weaker shoulder which reveals the property of isolated molecule. However, vertically aligned supramolecular cylinders led to the red-shift of PL spectra and a decrease in property of isolated molecule. This behavior was further verified by comparing the optical properties of the isolated molecules with the π-conjugated supramolecular dendrimers.