Two families of bis(imido) symmetrical Schiff bases: X-ray crystal structure and optical properties

• Published in: Journal of molecular structure 2017-06, Vol.1137, p.681-691
• Main Authors: Ma, Xiao-Na, Tan, Xue-Jie, Xing, Dian-Xiang, Sui, Qi-Cheng, Liu, Jian-Min, Xu, Chen-Xing, Liu, Yun
• Format: Article
• Language: English
• Subjects: Alkyl-bridge; Bis(imido) symmetrical Schiff bases; Crystal structure; DFT calculations; Optical properties
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/j.molstruc.2017.02.082

Two families of bis(imido) symmetrical Schiff bases based on 4-methoxybenzaldehyde and thiophene-2-carbaldehyde linked by three lengths of alkyl groups (propane → butane → dodecane) have been prepared and characterized by 1H and 13C NMR, Q-TOF mass spectroscopy, IR and elemental analysis. The structures and conformations of four of them have been established by X-ray single-crystal diffraction analysis. It is interesting that different linking groups and different conformations hardly have effect on several properties. For example, they have nearly the same melting points, the same UV–Vis absorption spectra and the same fluorescence emission spectra. Spectral interpretations were guided by time-dependent DFT calculated transition energies and oscillator strengths, which agree well with the experimental UV–Vis spectra. TD-DFT calculations reveal that they share nearly the same transition mechanism, i. e. both bands are largely originated from the π → π* transitions and have little or nothing to do with the alkyl bridge. However, there are also important differences based on the identity of the terminal groups. For example, similar dimers 1 and 2 crystallize in very different space groups P43212 (No. 96) and P21/n (No. 14), and another set of similar dimers 4 and 5 crystallize in P21/n (No. 14) and P212121 (No. 16), respectively. This illustrates the flexible nature of these dimers in forming a variety of different packing motifs. The results of this research suggest that 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. Two families of dimers provide longer alkyl bridges between two methoxy benzene rings and two thiophene rings (propane → butane → dodecane), while maintaining the same optical properties and the same spectroscopy mechanisms. Single crystal structures and DFT calculations were used to interpret their similarities and differences. [Display omitted] •Single crystal structures of 4 out of 6 dimers were determined.•UV–Vis and luminescent properties have been evaluated.•DFT/B3LYP/6-311 + G(d,p) method was employed to interpret optical properties.•The effects of varying bridging length appended on dimers were explored.