Revealing the supramolecular interactions of the bis(azopyrenyl) dibenzo-18-crown-6-ether system

Abstract

Understanding self-assemble process of molecules is crucial for developing synthetic analogues and improving our knowledge of natural systems. Compounds with crown-ethers and pyrene sequences in their structure have caught the attention lately due to their self-assembly abilities. A new molecular system, bis(azopyrenyl) dibenzo-18-crown-6-ether (AzoPy-18C6), was designed and obtained by sidechain modifications of both amino groups from the dibenzo-crown-ether with pyrene moieties, resulting in the formation of two azopyrene groups on each side. The compound was characterized by physico-chemical techniques such as: 1H-NMR, 13C-NMR, TGA, and DSC. Our work aimed to explore the supramolecular arrangement of the AzoPy-18C6 molecular system from a fundamental point of view based on the theoretical and experimental determinations. The Wide-Angle X-ray Diffraction (WAXD) analysis revealed a supramolecular arrangement through the appearance of a sharp peak at 2θ = 1.914°. By investigating the photophysical properties in the excited state, by UV–vis analysis and fluorescence spectroscopy, the π-π interactions among the aromatic pyrene moieties within the ordered stacks of the AzoPy-18C6 were observed. This was confirmed also by the molecular dynamics and DFT calculations. The interplay between electronic densities of molecular orbitals was discussed in order to prove the dimer/excimer association. Theoretical calculations revealed that the dihedral angle of C-N=N-C, specific to the azopyrene units, and the Van der Waals interactions play a key role in the π-π stacking of the bis(azopyrenyl) dibenzo-18-crown-6-ether system.