Fluorescent Recognition Of Three Small Molecules By Covalent Polymers

The fluorescent sensors have unique advantages in detecting analytes.In this paper,the influence of the indoor pollutants(formaldehyde,ammonia)and aromatics(nitrobenzene,methylbenzene)on the luminescence properties of covalent polymers is studied from the perspective of hydrogen bonding applying density functional theory(DFT)and time-dependent(TD)DFT.Based on the experimental data,several reasonable motifs were established,and the hydrogen-bonded complexes were constructed by using the motifs and analytes.The most stable hydrogen-bonded complex was selected to investigate the behavior of hydrogen bonding in the excited state and its effect on the photophysical processes of covalent polymers.The aim of this study is to elucidate the nature of the fluorescence sensors sensing and analysis.The luminescent covalent organic framework 1(LCOF-1)can interact with formaldehyde through hydrogen bonding.After the combination with formaldehyde,the luminescence mechanism of the 2D COF is formaldehyde to ligand charge transfer,which is totally different from the originally mechanism of the ligand to ligand charge transfer(LLCT).The comparison of the bond length,infrared vibration modes and the NMR of proton in the ground state and excited state demonstrated that the behavior of hydrogen bonding in the excited S1 state is enhanced.At the same time,the fluorescence rate coefficients of the motif and hydrogen-bonded complex are calculated.And we quantitatively showed that the radiative transition process of LCOF-1 is reduced in the presence of formaldehyde,which results in a decreased fluorescence intensity.Ammonia can also combine with the LCOF-2 through hydrogen bonding.The frontier orbital of covalent polymers provide insight into the luminescence mechanism.The luminescence mechanism of the motif is photoinduced charge transfer(PET)in appearance of ammonia,which is different from the originally mechanism of LLCT.The analysis of the electron transition energy,bond length of the hydrogen bonding,1H-NMR associated with the hydrogen bonding formation as well as vibrational frequencies of proton donor and acceptor in the excited state demonstrated that the hydrogen bonding is enhanced in the excited state.And the calculation of the fluorescence rate constants indicated that ammonia can reduce the fluorescence intensity of LCOF-2.The hydrogen-bonded complexe can be formed between conjugated microporous polymer(CMP)and nitrobenzene(NB).The analysis of the frontier orbitals and electronic configuration showed that the electron-drawing group(NB)and electron-donating group(methylbenzene)have a different influence on the luminescence mechanism of CMP,respectively.The calculation of the fluorescence rate coefficients indicated that electron-drawing group can quench the fluorescence of CMP,whereas,electron-donating group can enhance its fluorescence,which are in accordance with the experimental phenomenon.We have theoretically confirmed that CMP has a different fluorescence response for methylbenzene and NB,respectively.