Maxwell-Boltzmann statistics applied in the study of photoluminescent emission bands in the (S)-(-)-1-(4-bromophenyl)-N-1,2,3,4-(tetrahydro-1-naphthyl)methanimine organic crystals

Abstract

The morphological, structural and optical properties of a new crystalline enantiopure imine derived from p-bromobenzaldehyde were examined, namely the (S)-(−)-1-(4-bromophenyl)-N-1,2,3,4-(tetrahydro-1-naphthyl)methanimine compound. The analysis of the molecular packing associated with the surface morphology was carried out by means of Scanning Electron Microscopy (SEM). The absorbance vs. λ(nm) spectrum showed a relative maximum located in the ~350–650 nm (~3.54–1.90 eV) range assigned to π→π* and n→π electronic transitions characteristic of organic materials. Also, an inflection point located at ~559 nm (~2.21 eV) was tentatively associated with the behavior of π-resonance as well as the inductive effect generated by the halogen (-Br) atom and the stereogenic carbon atom. The band gap energy (Eg) was examined by applying the Tauc model, and presented four electronic transitions located at ~2.45 eV, ~2.66 eV, ~2.81 eV, and ~2.88 eV. A monoclinic crystal system and a P21 space group were established by single-crystal X-ray Diffraction (XRD) for the enantiopure imine. The Photoluminescence (PL) spectrum at room temperature showed two emission bands located in the Vis-region identified as a violet emission (VE) band located at ~412 nm and a yellow emission (YE) band at ~601 nm. These bands were associated with native crystalline defects. From the deconvolution of PL bands arose three different emission bands located at ~426, ~561, and ~631 nm, respectively. In order to investigate their origin, the statistical Maxwell-Boltzmann model was applied for the first time as an original approach, under the assumption that crystal defects correspond to vacancies and molecular interstices. The vibrational modes were investigated by Raman spectroscopy and compared with those obtained by theoretical and experimental analysis.