Revealing Luminescent Detection Mechanism For Indoor Volatile Organic Compounds Via Non-covalent Interaction:A Computational Study

This thesis has presented theoretical studies via density functional theory(DFT)and time-dependent(TD-DFT)methods on noncovalent interactions,in particular,the intermolecular hydrogen bonding between luminescent covalent organic frameworks(COFs)and hydrogen acceptor and donor volatile organic compounds(VOCs).The research about air quality has gained rigorous importance in the modern era.Therefore,we took fluorescent probes,the COFs for the detection of indoor pollutants,i.e.,the formaldehyde and methanol to elucidate the sensing mechanism through the hydrogen bonding.The contents and results are as follows:(1)Initially,theoretical investigation of the sensing potential was carried out by considering electronically excited intermolecular hydrogen bonding dynamics for the luminescent polypyrene covalent organic framework and formaldehyde(PPy-COF-HCHO).The elevating strengthened hydrogen bond C=O---H-C was corroborated through geometric structures,electronic transition energies,binding energies,UV-Vis and infrared spectra comparison in both the ground state and the excited state.Frontier molecular orbitals inspection,noncovalent interaction(NCI)method,natural population analysis(NPA),and electron density difference(EDD)map established the idea about the stronger hydrogen bond from the distributed density on H-donor and H-acceptor moieties,which should account for charge transfer and eventual fluorescence quenching.Moreover,the determined radiative rate constant by molecular material property prediction package(MOMAP)program was a lower value for the PPy-COF-HCHO complex than a free PPy-COF molecule.This study significantly revealed the high sensitivity of the PPy-COF towards organic analyte,e.g.,the indoor air pollutant formaldehyde.(2)Secondly,the new prospective investigation was performed mainly for the luminescent polypyrene covalent organic framework and methanol as an indoor pollutant.The strengthening of hydrogen bonds C-H---O-H and B-O---H-O was confirmed via comparison of geometric structures,electronic transition energies,1-NMR,binding energies,UV-Vis and infrared spectra of both S0 and S1 states.From the MOs analysis,electronic configuration and Mulliken charge analysis;demonstrated the charge density variation in hydrogen bonding proximity that facilitates the nonradiative path,which may lead to the luminescence quenching.Then,from the MOMAP program,the decline in fluorescence was verified based on PPy-COF-MeOH complex lower fluorescent rate constant compared to isolated PPy-COF fragment.Above results significantly highlighted the role of the hydrogen bonding to describe the sensing potency and mechanism for the PPy-COF.(3)Finally,the sensing mechanism was theoretically examined for the luminescent thieno[2,3-b]thiophene-based covalent organic framework(TT-COF)towards the volatile organic contaminant the formaldehyde.The exploration of hydrogen bonds' dynamics was described through geometries,electronic transition energies,binding energies,1H-NMR,and IR spectra comparison.Frontier molecular orbitals examination,NCI and NPA methods,and plotted EDD map demonstrated the excited-state hydrogen bond strength explicitly affected electron density distribution,which may cause the luminescence decrement.The MOMAP program illuminated the decline in the luminescence owing to TT-COF-HCHO complex radiative rate constant.Above results elaborated,the TT-COF's potential to sense the formaldehyde.