The Effect Of Substituent Homoring Competition Effect On Spectroscopic Properties Of 2-phenylbenzooxazole Compounds

In a conjugated organic molecule with multiple substituents connected to the same aromatic ring,a certain substituent may have different positioning methods,thus showing different electronic effects,and there is a competition relationship,we call this phenomenon as"homoring competition effect".In this paper,56 substituted 2-phenylbenzoxazoles（XBZY for short）have been synthesized,and studied the influence of substituent homoring competition effect on ultraviolet absorption,fluorescence emission and NMR chemical shift,the quantitative correlation equation was obtained,and compared with the spectroscopic properties of the o-hydroxy disubstituted diaryl Schiff base（2-OH-XBAY）and the o-difluoroboroxy disubstituted diaryl Schiff bases（2-OBF₂-XBAY）and analyzed.The results are as follows:（1）The uv absorption wavenumberν_max of XBZY is mainly affected by 8 substituent parameters,and there is homoring competition effect,that is,the two kinds of electronic effects with N-terminal and O-terminal as the location reference group work simultaneously and compete with each other.The field/induction effect of substituent X with N-terminal as the reference groupσ_F（X）_Nand the ground state electronic effect of substituent X with O-terminal as the reference groupσ（X）_O are the main influencing factors,with the sum of their contribution reaching 73.27%.The maximum uv absorption wavelengthλ_max of XBZY containing the same X and Y groups is 29.1～48.6 nm smaller than that of 2-OH-XBAY.And theλ_max of XBZY is 59.2～76.8nm smaller than that of 2-OBF₂-XBAY.（2）The fluorescence emission wavenumberν_em of compounds XBZY is related to the effects of 6 parameters,and the excited-state substituent parameters of substituent X and Y are the most important factors.The contribution of the conjugate effect of the substituent X with N-terminal as the reference groupσ_R（X）_N is close to that of the conjugate effect with O-terminal as the reference groupσ_R（X）_O.By comparing the fluorescence emission wavelength Em of 2-OBF₂-XBAY and XBZY,it is found that the Em of XBZY is 96～131 nm smaller when the X and Y groups are the same.The comparison of the quantitative correlation equation of XBZY and 2-OBF₂-XBAY shows that the factors affecting the fluorescence emission are quite different.The Em of2-OBF₂-XBAY is mainly affected by the ground state polarity parameters of the substituent X,while the fluorescence emission of XBZY is greatly affected by the excited-state substituent parameters,which may be related to the existence of charge transfer in the molecule in the excited-state of 2-OBF₂-XBAY.（3）The chemical shift of the C atom in the bridge bondδ_C（C=N）of XBZY is mainly affected by 6 parameters.The electronic effect of X with N-terminus as the reference group is larger than that with O-terminal as the reference group.The field/induction effect of X with N-terminal as the reference groupσ_F（X）_N and the field/induction effect of substituent Yσ_F（Y）are the most important factors,and the sum of their contribution is more than 60%.The factors affecting the chemical shift of C in the bridge bond on XBZY and 2-OH-XBAY and2-OBF₂-XBAY have both the same parameters and some differences.By comparing XBZY and 2-OH-XBAY compounds with the same X and Y groups,it is found that the chemical shift of C in the bridge bond are close,and theδ_C（C=N）of XBZY is slightly larger in most cases.By comparing XBZY and 2-OBF₂-XBAY compounds with the same X and Y groups on both sides,it can be seen that the chemical shift of C in the bridge bond on the 2-OBF₂-XBAY is basically large,and the difference between the two is 1～5 ppm.