Substituent Effects On The Spectra Properties Of Aryl Schiff Bases With Elongated Chains

Understanding the quantitative structure-property relationship (QSPR) helps to predict the optical spectra properties of the target compounds. Therefore, much attention has been focused on the QSPR of the spectra behaviors of conjugated organic compounds. Substituent effect on spectra behavior is an important factor for understanding the quantitative structure-property relationship.Schiff bases have been extensively used because of their liquid crystal and nonlinear optical behaviors. It is a meaningful work to study their substituent effects.In this thesis, systematic research was carried out on the spectra properties of aryl Schiff bases with different extent of conjugation:The13C NMR chemical shifts of eight kinds of substituted benzylidene anilines, with different backbone conjugation length, have been used as a probe to investigate substituent effects and the long-range transmission of substituent effects. A quantitative model was established to predict the δC(C=N) of the conjugated benzylidene anilines; Insights into the substituent effects on the UV energy of Schiff base with p-substituted styryl groups substituted on the aniline ring have been gained, and a quantitative model was established to correlate the UV maximum absorption wavenumbers of the model compounds and substituent parameters; the relationship between the molecular conformation and spectroscopic properties of symmetrical bis-Schiff bases was explored experimentally.In all, this thesis involves four aspects as follows:(1) A series of substituted Schiff bases, with different backbone conjugation length, have been synthesized. Their λmax values in ethanol were measured, and their13C NMR chemical shifts δC(C=N) in chloroform-d were determined.(2) Substituted Schiff bases with different backbone conjugation length have been used as a probe to investigate substituent effects and the long-range transmission of substituent effects. In this context, It was observed that the inductive and resonance effects of substituents Y on δC(C=N) attenuated with the increasing distance between Y and the imine carbon center, the transmission of the inductive and conjugative effects depended on the chemical bond numbers n(Y) between Y and the imine carbon, and the parameters n(Y)-2δF(Y) and n(Y)-2δR(Y) were suitable to express the corrected inductive and conjugative effects, respectively. However, as the length of the conjugated chain between X and the imine carbon center was elongated, the inductive effects of substituents X on δC(C=N) decreased, the change of the resonance effects was nearly ignorable, the chemical bond numbers n(X) between X and the imine carbon influenced only the transmission of inductive effects of X, and the n(X)-2σF(X) item was appropriate to evaluate the modified inductive effects of X. It is interesting that the correlations improve when△σ2is employed to quantify the effect of X or Y on δC(C=N), and the correlations are much better in the studying of the effect of X than that of Y on δC(C=N), and that△σ2plays more important role as X or Y become more electron-donating or electron-withdrawing. This shows that the interaction between X and Y cannot be neglected in case one of substituents X and Y is fixed. It was confirmed that△σ2was also suitable to evaluate the substituent cross-interaction effect on δc(C=N) in case both X and Y were varied, and the calculated δC(C=N) was more close to the experimental values when the difference between the electronic effect of X and Y increased. Similarly,△σ(cor) was proposed to describe the transmitted effect of the cross-interaction effect. With the parameters n(X)-2σF(X), σR(X), n(Y)-2σF(Y), n(Y)-2σR(Y),△σ(cor), and δc(parent), the δC(C=N) values of181samples can be well correlated. The correlation coefficient is0.9957, and the standard derivation is only0.23ppm. Moreover, the multi-parameter correlation equation is predicted well the δC(C=N) of other25samples of designed conjugated benzylidene anilines.(3) Schiff bases with p-substituted styryl groups substituted on the aniline ring, p-X-PhCH=NPhCH=CHPh-p-Y have been studied photochemically in ethanol. The substituent effects in the compounds are more similar to p-X-PhCH=NPh-p-Y than p-X-PhCH=CHPh-p-Y. The styryl group on the aniline ring extends the π-conjugation of the molecules, and leads to an increase in the maximum absorption wavelength, when compared to p-X-PhCH=NPh-p-Y. This can be described as a "styryl conjugation effect". Furthermore, a quantitative model has been established to correlate the UV maximum absorption wavenumbers of the title compounds and substituent parameters. The results help understand the substituent effects of conjugated compounds consisting of both benzylidene aniline and stilbene moieties.(4) The relationship between the molecular conformation and spectroscopic properties of symmetrical bis-Schiff bases was explored experimentally. The crystal structures of twelve samples of compounds were measured by X-ray diffraction. The results show that the vmax is dependent on the substituents at the benzylidene ring and the dihedral angle τ of the molecules, and the term sin(τ) is suitable to modify the substituent effects on the vmax. However, experimental investigations indicate that the dihedral angle τ has a limited effect on the values of δC(C=N) for Schiff bases derived from1,4-Phenylenediamine, while experimental investigations indicate that the dihedral angle τ has an obvious effect on the values of δC(C=N) for Schiff bases derived from1,4-Phthalaldehyde. This study provides a new understanding for the molecular conformation on spectroscopic properties of symmetrical Schiff bases.