s-Triazine-Based Functional Discotic Liquid Crystals: Synthesis, Mesomorphism and Photoluminescence

A new series of C-3-symmetric, pi-conjugated molecules was designed, synthesized and characterized. The materials were derived from electron-accepting s-triazine, appended covalently to electron-donating styrylbenzene arms, and were readily prepared in excellent yield with high purity by means of threefold condensation of triphosphonate with n-alkoxybenzaldehydes under Horner-Wadsworth-Emmons reaction conditions. Examination of the phase transitional properties by several complementary techniques evidenced self-assembly into a hexagonal columnar phase, occurring over wide and reasonable thermal ranges. The photophysical properties were studied both in solution and in the fluid/frozen columnar states by UV/Vis absorption and photoluminescence spectroscopy. The emission spectra obtained as a function of the temperature rule out the breaking-up of larger columns and a non-radiative, thermally activated process. A study carried out on thin films of the glassy columnar state, which accounts for conserved fluorescence, defect-free orientation, and freezing ionic species, with the help of atomic force microscopy (AFM) images, suggested a homogeneous granular morphology comprising fibrillar structures. Dissimilarities in the surface morphology and birefringence of thin films of the solid and frozen columnar states were clearly shown by Raman spectroscopy. An electrochemical investigation revealed a LUMO energy of -4.0 eV. Thus, the discotic motifs presented herein meet certain criteria of organic materials, which are essential for developing electronic devices.