Synthesis,Characterization And Properties Of Schiff Base Dimmers Including Aromatic Rings

Symmetry plays an important role in a variety of biological processes and the dimer structure is ubiquitous in natural products.The dimeric molecules would be expected to show enhanced biological activity other properties relative to their corresponding monomeric counterparts.Schiff base dimers are widely used in many areas,such as medicinal&analytical chemistry,photochemical materials.As experimental evidence,the optical properties related to the electronic transition are important,useful yet simple indicator about the dimer structures.Herein,we report the synthesis,structures,and optical properties of three families of alkyl-linked schiff base dimmers,i.e.dimmers derived from 4-methoxyl-1-formylbenzene（MF series）,2-thiophene formaldehyde（TFD series）and 2-Acetylthiazole（AT series）.As new flexible models for better understanding of the relationship between bridging groups,supramolecular arrangements and optical properties,precise crystal structure investigations have been carried out and theoretical calculations were used to elucidate the electronic transition mechanisms.We have found that similar dimers belonging to the same series have nearly the same UV-Vis spectra and fluorescence properties.That is to say,the lengthening of the alkyl bridge has no effect on the optical properties.In order to further analyze the result from the point of molecular structure,and to determine the spectroscopy mechanisms,theoretical calculations were performed to make the assignment of these electronic transitions.All together our data,the absorption spectra reproduced from DFT calculations fit the experimental results well,so attempts to understand the nature of electronic transitions were carried out through comparison of the related molecular orbitals.All twelve dimers have two main absorption bands,which can be assigned as the?→?*transitions and have little or nothing to do with the alkyl bridges,so the lengthening and the conformations of alkyl bridges have small effect on UV-Vis spectra.Single crystal structures of five out of twelve dimers were determined.The results showed that no correlation occurs between the crystal structure and the mode of weak interaction patterns,even though they have similar molecule structures.For example,similar dimers MF-PDA and MF-BDA crystallize in the space groups P4₃2₁2（No.96）and P2₁/n（No.14）,which are very different in symmetry,even though they have similar non-covalent interactions（no classical hydrogen bonding,only weak C–H···πhydrogen bonds andπ–πstacking interactions）and similar molecule structures.That’s to say,the rational design and prediction of crystal structures are more difficult than optical properties,even though similar weak interactions can be controlled in assembling the molecules.Some interesting results have been obtained,but still more work need to be performed to better understand their biological activies and optical properties.