Mesomorphism of azomethine luminophores. X-ray diffraction and differential scanning calorimetry studies of 4′-butyl-4-propyloxysalicylideneaniline

The structure and photophysical properties of the luminophore 4′-butyl-4-propyloxysalicylideneaniline (1) were studied by X-ray diffraction analysis and differential scanning calorimetry. At room temperature, compound 1 does not luminesce. According to the DSC data, compound 1 undergoes a crystal—mesophase phase transition at 54.1 °C and it melts at 68.4 °C, being transformed into an isotropic melt. When cooled to 67.9 °C, the isotropic melt becomes turbid due to the formation of a mesophase. A yellow fluorescence of the sample is achieved under UV irradiation, it intensifies on further cooling, persists at room temperature for about a week, and then fades. The mechanical action triggers the growth of luminescent crystals (1′). The X-ray diffraction study of original non-luminescent crystal 1 and luminescent crystal 1′ demonstrated that they are structurally identical. The possible cause of the difference in the photophysical properties of crystals 1 and 1′ is discussed. It is based on dynamic processes leading to a shift of the prototropic equilibrium toward either the benzenoid or quinoid configuration of the bonds in the azomethine moiety.