Influence of the side-group at C=N bridging bond of bis-aryl Schiff bases on the wavelength of absorption maximum of ultraviolet absorption spectra

Abstract

The compounds N-(benzylidene)-anilines XArCH=NArY (XBAY), N-(phenyl-ethylene)-anilines XArC(CH 3 )=NArY (XPEAY) and N-phenyl-α-phenylnitrones XArCH=N(O)ArY (XPNY) have bridging group CH=N, C(CH 3 )=N and CH=N(O) respectively, in which the C(CH 3 )=N has a side-group methyl CH 3 at carbon end and the CH=N(O) has a side-group O atom at nitrogen end. In this work, a series of XPEAY and XPNY were synthesized, and their longest wavelength maximum λ max (nm) of ultraviolet absorption spectra were measured. Then the change regularity of the ν max (cm -1 , ν max =1/λ max ) of XPEAY and XPNY were investigated, and they were compared with that of XBAY (reported by ref.26). The results indicate: (1) There are no good linear relationships between the ν max of XBAYs and XPEAYs or XPNYs. (2) In case of a same set of X-Y group couples, the distribution of λ max of XPEAYs is larger than that of XPNYs. (3) The side-group CH 3 makes the effect of σ(X) larger than that of σ(Y) on the ν max of XPEAYs, whereas the O atom makes the effect of σ(Y) larger than that of σ(X) on the ν max of XPNYs. (4) The cross-interaction between X and Y has important effect on the all ν max . However, the cross-interaction between CH 3 and X/Y has not important effect on the ν max of XPEAY, and the cross-interaction between O and X/Y has not important effect on the ν max of XPNY.

Substituent effects on the UV absorption energy of 2,5‐disubstituted pyrimidines

“…Cao group have systematically investigated the substituent effect on the UV absorption energy of substituted aryl Schiff bases with imine bridge group (C═N), and found that the influence rule of the substituents on 2 ends of the dipole were distinctive for different parent skeleton (summarized in Table ). This phenomenon was also confirmed by this study.…”

“…Hammett parameter ( σ ), proposed by Hammett in 1937, is a well‐known molecular descriptor characterizing the ground‐state substituent effect. It has been extensively applied to the quantitative structure‐property relationship studies and exhibited good performances . The Hammett parameter for para‐position substituent is expressed as σ p , which consists of 2 parts, the inductive effect σ F and the resonance effect σ R .…”

“…Because the molecular orbital energies are affected by the substituents in molecule, the substituent effects on the UV absorption energy have been widely investigated. For example, Cao has investigated the substituent effect on the UV absorption energies of 1,4‐disubstituted benzenes and benzylideneaniline derivatives with imine bridge group (C═N) . Praveen has studied the effect of substituent on the UV‐visible absorption and photostability of liquid crystals.…”

Influence of the methyl group at C=C bridging bond of stilbene on the longest wavelength maximum in ultraviolet absorption spectra

“…Hammett constant σ was widely used but was less successful in quantifying the UV absorption spectra of organic compounds. In 2008, Cao et al proposed excited‐state substituent constant to express the contribution of substituent electronic effect to the excited‐state molecule, and then they used the to quantify the UV absorption spectra of many kinds of compounds . They observed that the wave number ν max (cm ‐1 , ν max = 1/λ max ) of the longest wavelength maximum λ max (nm) of UV were mainly affected by the excited‐state substituent constant for substituted benzene, stilbene, and conjugated polyene .…”

“…They observed that the wave number ν max (cm ‐1 , ν max = 1/λ max ) of the longest wavelength maximum λ max (nm) of UV were mainly affected by the excited‐state substituent constant for substituted benzene, stilbene, and conjugated polyene . Whereas the ν max of Schiff bases were affected by both and σ because of having a polar bridging bond C=N . It should be noted that the bridging group CH=CH in stilbene XArCH=CHArY (abbreviated XSBY) is a symmetrical group (ie, its right and left ends all are the CH).…”