Theoretical Study On Luminescent Covalent Organic Frameworks To Recognize Pollutant Molecules

Photochemical sensors have attracted much attention due to their advantages such as high sensitivity,good selectivity,simplicity and rapidity.They detect substances by the changes of luminescent properties?fluorescence intensity or color?,which are caused by the interaction between the detected substance and the sensitive layer.In this paper,density functional theory?DFT?and time-dependent density functional theory?TDDFT?were used to study the effects of NH₃,formaldehyde and nitrobenzene on the luminescent properties of luminescent covalent organic frameworks?LCOFs?.The structural units representing the periodic structure of covalent organic frameworks?COFs?are intercepted for subsequent study.Structural units and pollutant molecules?NH₃,formaldehyde and nitrobenzene?will form various hydrogen bond isomers and the most reasonable one was selected to clarify the effect of intermolecular hydrogen bonding on the luminescence mechanism of LCOFs.The hydrogen bond interaction between LCOF-1?Ph-An-COF?and NH₃ was studied.It was found that the combination of Ph-An-COF and NH₃ changed the luminescence mechanism of Ph-An-COF.The comparison of hydrogen bond length,infrared spectra and ¹H NMR between S₀ and S₁ states showed that hydrogen bond became stronger in the S₁ state,which was consistent with the prediction of electronic excitation energy.The enhancement of hydrogen bond in the electronic excited state decreases the fluorescence rate coefficient and increases the internal conversion rate coefficient of Ph-An-COF,which means that Ph-An-COF can detect NH₃.The hydrogen bond interaction between LCOF-2?sp²-COF-2?and formaldehyde was studied.It was found that the luminescence mechanism of sp²-COF-2 changed after formaldehyde was introduced.The electronic excitation energy indicated that the hydrogen bond was enhanced in the excited state,which leads to the red shift of the electron spectrum.Bond lengths,frequencies and ¹H NMR shifts indicated that hydrogen bonds in the S₁ state were enhanced.The enhancement of hydrogen bond decreases the energy gap between S₀ and S₁ states.Thus,the radiation transition and the fluorescence rate coefficient will also decrease.The fluorescence of sp²-COF-2 decreases after the addition of formaldehyde indicating that sp²-COF-2 can be used as a photochemical sensor to detect formaldehyde.The hydrogen bond interaction between LCOF-3?TAT-COF-2?and nitrobenzene was studied.After the formation of hydrogen bond,the luminescence mechanism of the TAT-COF-2 is nitrobenzene to ligand charge transfer,which is totally different from the originally mechanism of the ligand to ligand charge transfer?LLCT?.The geometry,IR spectra and ¹H NMR of the hydrogen-bonded complexe in the S₀ and S₁ states showed that the hydrogen bond was stronger in the S₁ states than its S₀ states.The enhancement of hydrogen bond is not conducive to the luminescence of TAT-COF-2,so the fluorescence coefficient rate of TAT-COF-2 decreases.It can be concluded that TAT-COF-2 is a fluorescent sensor for the detection of nitrobenzene.