Green emission band induced by crystal defects in halogenated (-Br, -Cl, -F) chiral imines with a benzo[b]thiophene-based moiety

Abstract

Four different chiral imines were analyzed in solid state, by changing systematically the halogen atom –X (-F, -Cl and -Br) in the para-position of the benzene ring in the chiral moiety of benzo[b]thiophene-based imine compounds. The molecular packings showed a significant distortion induced by the effect of the stereogenic carbon atoms and the corresponding halogen atom, resulting in nonplanar angular-shaped small molecules. Morphological studies by Scanning Electron Microscopy (SEM) were performed and changes in aspect and shape of the crystals can be clearly seen. The absorbance was registered in the UV-region (~200–400 nm), and showed typical transitions that are commonly displayed in organic molecules with π→π* and n→π* electronic transitions. The band gap was quantified by the Tauc model, and four electronic transitions were found, associated with those observed in experimental results by absorbance, located in the ~3.34–5.86 eV range. By X-ray diffraction (XRD) studies, a monoclinic→orthorhombic variation in the crystals was observed. Photoluminescence (PL) signals were examined in the Vis-region at room temperature and a green emission (GE) band located in the ~450–750 nm range was observed, which was associated with crystalline defects and the existing chromophores. Likewise, a PL signal located in the ~526–583 nm (~2.35–2.12 eV) range showing the shift of the green-yellow emission was noted. By using Raman spectroscopy, the vibrational modes displayed in these crystals were analyzed. A theoretical support of HOMO-LUMO analysis for these organic crystals is also reported.