Blue Glow Sticks: Cinnamic Acids and Arylacrylonitriles with Liquid-Crystalline Properties and Highly Fluorescent

Five new π-conjugated chromophores derived from acrylic acid and acrylonitrile were synthesized and their thermal and photophysical behaviors were analyzed. They were designed and synthesized through Knoevenagel condensations between arylacetylenebenzaldehydes and a methylene group activated from cyanoacetic and malonic acids. The characterization of the target compounds was performed by Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (NMR), 13C NMR, thermogravimetric analysis (TGA), absorption and emission spectroscopy techniques. Arylcinnamic acids were obtained as pure E-isomers. On the other hand, arylacrylonitrile was obtained as a mixture of E/Z-isomers and the pure E-isomer was isolated in chromatographic columns. The liquid crystal properties were investigated through differential scanning calorimetry and polarized optical microscopy. All chromophores displayed smectic and nematic mesophases. These materials displayed intense fluorescence under UV-Vis light excitation. The maximum absorption peaks were observed between 342-356 nm while emission varied from 428 to 460 nm in chloroform solution. Higher values for the quantum fluorescence yield were associated with acrylic acids compared to arylacrylonitriles. The solvatochromism studies of the materials showed a significant increase in the red-shift of the fluorescence spectra as the polarity of the solvent increased. In some cases, a variation in the emission wavelength from the blue to the green region of the spectrum was observed.