Morphological, structural and optical analysis of green, red and yellow emission bands in new chiral imines with a benzothiophene moiety

Abstract

Four enantiopure imines in solid state with different functional groups added in the para-position of the aromatic ring in the chiral moiety were analysed. The molecular packing was investigated with the aim of correlating this fact with the surface morphology, structural and optical changes of these imines. By applying Scanning Electron Microscopy (SEM) the morphology showed dense plates and smooth surface. X-ray diffraction studies exhibited a Monoclinic→Orthorhombic variation. The absorbance of these organic materials showed absorption bands associated with the π→π* and n→π* electronic transitions located in the UV-region, typical of organic molecules. By applying the Tauc model, the band gap energy was quantified. An electronic transition located at ∼2.5 eV and another well-defined band situated at ∼3.3 eV can be observed. Photoluminescence (PL) spectroscopy performed at room temperature with the 450 nm laser line showed green, yellow and red emission bands located in the ∼450–700 nm range of the Vis-region with different intensities. The experimental results indicated that the photoluminescence properties of the crystals strongly depend on molecular stacking in the crystal arrays and this optical behavior can be associated with different crystalline effects. The Raman spectra was examined to identify the vibrational modes in a qualitative way and the experimental results were confirmed with the reported values obtained through theoretical-experimental models.