Experimental and DFT Study of the Photoluminescent Green Emission Band of Halogenated (-F, -Cl, and -Br) Imines.

The morphological, optical, and structural changes in crystalline chiral imines derived from 2-naphthaldehyde as a result of changing the -F, -Cl, and -Br halogen (-X) atoms are reported. Scanning electron microscopy (SEM), optical absorption, photoluminescence (PL), and powder X-ray diffraction (XRD) studies were performed. Theoretical results of optical and structural properties were calculated using the PBE1PBE hybrid functional and compared with the experimental results. Differences in surface morphology, absorbance, XRD, and PL of crystals were due to the change of halogen atoms in the chiral moiety of the imine. Absorption spectra exhibited the typical bands of the naphthalene chromophore located in the ~200-350 nm range. Observed absorption bands in the UV region are associated with π→π* and n→π* electronic transitions. The band gap energy was calculated using the Tauc model. It showed a shift in the ~3.5-4.5 eV range and the crystals exhibited different electronic transitions associated with the results of absorbance in the UV region. XRD showed the monoclinic→orthorhombic crystalline phase transition. PL spectra displayed broad bands in the visible region and all the samples have an emission band (identified as a green emission band) in the ~400-750 nm range. This was associated with defects produced in the morphology, molecular packing, inductive effect and polarizability, crystalline phase transition, and increase in size of the corresponding halogen atoms; i.e., changes presumably induced by -C-X…X-, -C-X…N-, -C-N…π, and -C-X…π interactions in these crystalline materials were associated with morphological, optical, and structural changes.

2,5-Bis{[(-)-(S)-1-(4-bromo-phen-yl)eth-yl]imino-meth-yl}thio-phene.

The title compound, C22H20Br2N2S, was synthesized under solvent-free conditions. The mol-ecule shows crystallographic C 2 symmetry, with the S atom of the central thio-phene ring lying on a twofold rotation axis. Furthermore, as a consequence of the (S,S) stereochemistry, the mol-ecule has a twisted conformation. The dihedral angle between the thio-phene and benzene rings is 72.7 (2)° and that between the two benzene rings is 55.9 (2)°. In the crystal, mol-ecules are arranged in a chevron-like pattern, without any significant inter-molecular inter-actions.

2,5-Bis{[(-)-(S)-1-(4-methyl-phen-yl)eth-yl]imino-meth-yl}thio-phene.

The title chiral bis-aldimine, C24H26N2S, was synthesized using a solvent-free Schiff condensation. The mol-ecule displays crystallographic C 2 symmetry, with the S atom lying on the twofold axis parallel to [100]. As a consequence of the (S,S) stereochemistry, the tolyl groups are oriented towards opposite faces of the thiophene core, giving a twisted conformation for the whole mol-ecule. Mol-ecules are arranged in the crystal in a herringbone-like pattern, without any significant inter-molecular contacts.

(+)-(S)-N-[(1-Benzo-thio-phen-2-yl)methyl-idene]-1-(naphthalen-1-yl)ethyl-amine.

In the title compound, C21H17NS, the C=N double bond shows an E conformation. The dihedral angle between the mean planes of the naphthyl residue and the benzo-thio-phene residue is 89.14 (6)°. The crystal packing is stabilized by inter-molecular C-H⋯π inter-actions, building a ribbon structure along the a axis.