Photoluminescent Green Emission Band Induced by the Systematic Change of -CH₃, -OCH₃ and Naphthyl Groups in Chiral Imines

Abstract

Herein, we report the morphological, optical, and structural modifications induced by the change of different functional groups in the para-position of the benzene ring in a series of chiral imines. These organic compounds were examined using scanning electron microscopy (SEM), optical absorption, X-ray diffraction, and photoluminescence (PL) techniques. SEM images showed drastic morphological changes, and the absorbance results showed significant changes in the bands located in the ∼200−400 nm range, associated with π → π*, δ → δ*, and n → π* transitions. An optical behavior similar to that of semiconductors (in the UV region), with two transitions in the ∼3.3−4.3 eV range was observed for the compounds. The results obtained by PL spectra exhibited changes in intensity, with gradually shifting increases in the green band emission. However, it is more intense with the crystals of the imine bearing the -OCH 3 functional group, considering that the aforementioned green band is associated with the different morphology in these crystals.

Influence of L-Tryptophan on Growth and Optical Properties of PbS Nanocrystalline Thin Films

“…XRD diffractograms for PbS-T15 and PbS-T20 films displayed a preferred orientation in the (200) reflection plane and has the highest intensity; this behaviour may be attributed to the L-tryptophan effect. This effect can be associated with molecular distribution, so the biomolecule shows a flat end that has a resonant effect correlated with π→π* electron transition, as well as a chiral carbon on the other moiety of the molecule bearing the amino acid functional group [20]. The organic molecule possibly undergoes a flattening in the crystalline growth and simultaneously is surrounded by small nanocrystals (nanoparticles that have a dipole moment different from zero, when approaching the charge density of L-tryptophan, and this is modified by the electrostatic interaction producing distortion in the growth of nanocrystals).…”

Chiral Mono- and α-Diimines and Their Pd(II) Complexes with Anticancer Activity

“…Likewise, in a series of chiral imines derived from 2-naphtaldehyde but with the halogen atoms in the para-position of the benzene ring of the amines replaced by other functional groups, such as ▬CH 3 , ▬OCH 3 and naphthyl groups, imines 45-48 ( Figure 14) exhibited green luminescence. As the previous results, the variations on the functional group as well as the molecular packing determined the morphological changes and consequently the luminescent properties of the imines [22].…”

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

“…The absorbance was examined in the UV-region (~200–400 nm or ~6.20–3.10 eV), which is associated with π→π* transitions that have been examined earlier [43]. These electronic transitions are generally assigned in this region of the electromagnetic spectra according to previous reports [15,46]. The absorbance vs. wavelength (nm) spectra of the I and I − X samples are shown in Figure 7.…”

“…The PL spectra at room temperature for I and I − X crystals is shown in Figure 10, displaying emission bands for I located at ~599 nm (~2.13 eV), I − F : ~568 nm (~2.18 eV), I − Cl : ~553 nm (~2.24 eV), and I − Br : ~599 nm (~2.07 eV), respectively. Generally, in inorganic materials, this range corresponds to green, yellow, and red emission bands [15,25]. In order to obtain more information about the PL spectra, it is possible to perform the deconvolution of the Gaussians, which in this case are asymmetric, usually involving two overlapping bands.…”

“…However, they have been also observed in rare earth oxides and these are associated with f → f transitions [14]. The introduction of functional groups in I crystals causes a PL signal increase of the characteristic emission peaks of I [15] by mesomeric and inductive effects, as the electron density of the aromatic ring is increased with the π→π* and n → π* electronic transitions, etc., along with hyperconjugation [16]. In summary, the objective of this paper is systematically synthesize four crystals and modify them by adding the − X halogen atom and to characterize the morphological, structural, and optical relationships associated with the change of − X atoms.…”