π-conjugated pyrenoimidazole derivatives: synthesis, spectroscopic and crystallographic studies

Abstract

This PhD thesis was aimed at the design and synthesis of new functional phenyl-pyrenoimidazole derivatives and the development of these compounds as novel chromophores/uorophores and supramolecular synthons for organic crystal engineering. An efficient and modular one-pot condensation reaction between 4,5-pyrenedione and various benzaldehydes in presence of ammonium acetate and acetic acid was used to prepare all the new phenyl-pyrenoimidazoles investigated in this thesis work. There are three major projects accomplished in this thesis. The first project investigated the properties of two new benzaldehyde-substituted pyrenoimidazoles, in which the electron-withdrawing formyl group is attached to the para and meta position of the phenyl group. These two isomers were characterized by various spectroscopic, X-ray single crystallographic analyses, in conjunction with density functional theory (DFT) calculations. The interactions of these two new compound with uoride anions through hydrogen bonds were examined as well, and the results demonstrated the applicability of the para-benzaldehyde substituted pyrenoimidazole as an efficient ratiometric uorescence probe for uoride anion. In the second project, a series of phenyl-pyrenoimidazole substituted with various organic functional groups (including CH₃, Cl, Br, OCH₃, CHO, and NO₂) were synthesized and their structure photophysical property relationships were subsequently studied by UV-Vis absorption and uorescence spectroscopy. Xray crystallographic and molecular electrostatic potential (MEP) analyses were undertaken to disclose the interplay of various non-covalent forces that govern the crystal packing of these pyrenoimidazole molecules. The results indicated that the substituent group is an important factor controlling the crystal structural properties, which attests to the possibility of using various phenylpyrenoimidazole derivatives as tunable supramolecular synthons for crystal engineering. In the third project, hydroxyl and alkoxyl-substituted phenyl-pyrenoimidazole derivatives were prepared and investigated for comparative studies of the effects of hydrogen bonding and π - π interactions on their crystallization behaviors. To construct different organic crystals, various aromatic carboxylic acids were used with these pyrenoimidazoles. The resulting crystal structures were analyzed by X-ray crystallography to show the formation of different supramolecular network structures in the solid state. Based on the crystallographic data obtained, Hirshfeld surface analysis and quantum theory of atoms in molecules (QTAIM) calculations were carried out to quantitatively interpret and comprehensively visualize the various intermolecular forces, such as hydrogen bonding, π - π, C-H...π, and H...H interactions, in the pyrenoimidazole crystals and co-crystals.