Determination And Application Of Excited-state Substituent Constants Of Pyridyl

Pyridyl is a common organic group,and organic compounds containing pyridyl have been widely studied and applied,especially in the fields of photochemistry and photophysics.It needs excited-state substituent constants to quantify the excited-state properties of organic compounds so that to obtain good results.However,there is currently no available values of the excited-state substituent constant of pyridyl.In this paper,a series of 1-pyridyl-2-arylethylene compounds XCH=CHArY（abbreviated XEBY,here X is 2-pyridyl,3-pyridyl,4-pyridyl）were designed and synthesized,and their molecular structures were characterized by nuclear magnetic resonance spectroscopy and mass spectrometry.The excited-state substituent constants σCC（p）ex of the pyridyl and substituted phenyl were obtained,and their reliability were confirmed via using the σCC（p）ex values to quantify the fluorescence emission wave number vem and the chemical shift δC of the carbon atoms in bridging bond CH=CH and the reduction potential Ered of compound XEBY.The main contents of this work are as follows:（1）Thirty-six samples of pyridyl-CH=CHArY compounds were synthesized to obtain the excited-state substituent constant of pyridyl,and nine samples of pyridyl-CH=NArY compounds were synthesized to verify the reliability of σCC（p）ex values.（2）Using anhydrous ethanol as the solvent,the ultraviolet（UV）absorption maximum wavelength λmax of the XEBY were measured and converted to the wave number vmax（vmax=1/λmax）.In addition,the UV absorption vmax values of 234 samples of 1-substituted phenyl-2-arylethylene compounds reported in the literature were collected.The excited-state substituent constant σCC（p）ex of three pyridyl and twenty-three substituted phenyl were extracted（total of 26 groups）through curve-fitting method.The obtained excited-state substituent constant of pyridyl and substituted phenyl σCC（p）ex were further applied to the quantitative correlation of UV absorption vmax of the 358 samples of 1,2-disubstituted ethylene and 126 samples of diaryl schiff bases（9 samples of schiff bases were synthesized in this work）,in which good results were obtained and the reliability of the σCC（p）ex were verified.（3）Using anhydrous ethanol as the solvent,the solution fluorescence emission of the XEBY were measured.A six-parameter quantitative correlation equation was obtained to express the change regularity of vem.The results show that the main factor affecting vem is the excited-state substituent constant σCC（p）ex（X）of pyridyl,and the rest factors are σR（X）,σR（Y）,σF（Y）,σCC（p）ex（Y）and the substituent cross-interaction effect ΔσCCex2 between X and Y,in which the effects of substituents X and Y on vem are opposite;The solution fluorescence emission wavelength EM and solid fluorescence emission wavelength EMg of the XEBY were compared.The results show that EMg are larger than EM,and there is no obvious quantitative relationship between solid fluorescence emission wave number vemg and substituent effects.（4）The effects regularity of substituents on the chemical shifts of the two carbon atomsδC（X）and δC（Y）of the bridge bond CH=CH in compound XEBY were studied separately,and good results were obtained.The result indicate:the factors affecting δC（X）and δC（Y）are different.Among them,the substituent X and Y have an alternating effect on the electron cloud density on the bridging carbon,that is,these effect on δC（X）and δC（Y）are opposite;The excited-state substituent constant σCC（p）ex of pyridyl has the greatest contribution to δC（X）,however,its influence on δC（Y）is reflected in the form of ΔσCCex2;the push-pull effect between the substituents on δC in the XEBY was studied.The δC（Y/X）is used to quantitatively express the push-pull effect,and its value is affected by the two parameters of σ and σCCex.（5）The reduction potential Ered of compound XEBY were measured and quantitatively correlated.It is observed that the Ered are mainly affected by σCC（p）ex and σ（p）of the pyridyl,and the rest factors are σ（Y）、σCCex（Y）and ΔσCCex2.The difference of Ered of furan/thienyl-EBY versus pyridyl-EBY were compared and analyzed.The results show that it is easier to get an electron to form a radical anion in pyridyl-EBY due to pyridyl being an electron-deficient aromatic heterocyclic ring,which makes the compound pyridyl-EBY to be easier reduced than furan/thienyl-EBY.