Density Functional Theory Investigation On Electronic Structure And Properties Of Organoboron Compounds

Organoboron compounds are among the most versatile classes of heteroatom substituted organic molecules.Applications of organoboron compounds have seen a particularly dramatic surge in fluorescence sensors,fluorescent imaging materials,photoresponsive materials,and organic photoelectric materials due to their unique structure and excellent photoelectric properties.In particular,applications of organoboron compounds as fluorescent sensors and electrochemical sensors in molecular recognition fields have attracted more and more attention.Further theoretical insights into luminescence mechanisms,molecular recognition mechanisms,and the reaction mechanisms for organoboron compounds as fluorescent sensors are significant to design and development more effective fluorescent chemosensors.In this work,we conduct a theoretical study by employing the density functional theory(DFT)and time-dependent density functional theory(TD-DFT)methods to investigate the structures of organoboron compounds and their anion complexes,which would provide a theoretical references for the design and development of organoboron fluorescent sensors and electrochemical sensors by analyzing the electronic structure in the ground and the excited states,interaction types with anions,the fluorescence sensing mechanism and the electron transfer properities in applied electric fields.The main research contents are as follows.1.The electronic structure,excited state properties and fluorescence recognition mechanism of six D–?–A tridurylboron compounds were investigated using DFT and TD-DFT methods.It mainly included the geometric structures,electronic structures,excitation energy and emission energy,natural transition orbitals(NTO),and hole-electron analysis for ground and excited states.The results indicated that the CT degree of the first excited states mainly affects the fluorescence response mechanism for fluorine ions.The CT intensity depends on the structural differences caused by the types and number of electron-withdrawing and electron-donating groups in the D–?–A system.The hole–electron theory was used to quantify the CT degree to provide references for the design and synthesis of efficient fluoride anion sensors.2.The DFT and TD-DFT methods were used to theoretically study the structures,photophysical properties,types of interactions with iodide anions,and the fluorescence quenching mechanism for phenylboronic acid derivatives,which can selectively recognize iodine anion.The initial structures of phenylboronic acid derivatives with iodine ion were determined by conformation search.Interaction type was further confirmed by electron localization function(ELF)and reduced density gradient(RDG)analysis.The iodide anion fluorescence quenching mechanism was determined by frontier molecular orbital(FMO),hole electron analysis(hole-electron),and electron density difference(EDD)analysis.The results show that phenylboronic acid derivatives and iodide anions were based on hydrogen bonding.Fluorescence quenching was due to the intense CT transition between phenylboronic acid derivatives and iodide anions in excitation and de-excitation process.3.The structural,excited states properties and binding interaction with fluorine ions of N,C–chelated four coordinated organoboron compounds have been investigated utilizing DFT and TD-DFT methods.The initial structures of the organoboron compounds with fluorine ion for geometry optimizations were prescreened with the aid of the conformation search.The ground state and excited state properties were studied by analyzing the bond lengths,bond orders,excitation energies and emission energies,hole-electron,and frontier molecular orbital.The results confirmed in the theoretical perspective that a nucleophilic reaction occurred between organoboron compounds and fluorine anion,and fluorine anion attacks boron atom to cleavage boron–nitrogen bond.The results indicated that fluorescence quenching was induced by the intense CT transition in emission.The first excited state regard as a ?dark state?.Eight organoboron compounds with different ? conjugate sizes or donor and acceptor groups have been designed,and three candidates have been screened as sensors in fluoride anion recognition.The structure–property relationships were established to provide references for further investigations of the fluoride ion fluorescence sensors.4.The electronic structure of the binuclear Mn(II)cage metal complex in applied electric fields were theoretically investigated by DFT theory.The theoretical calculations indicated that the HOMO–LUMO gap of the cage metal complex is narrow and the gap decreases with the increased electric field,which reveals that the complex shows excellent electron transfer characteristics.The density of States(DOS)and frontier molecular orbital analysis further illustrated that the LUMO orbital energy reduction was the main reason for the energy gap reduction.Mayer bond order(MBO)analysis,electron localization function(ELF)analysis,atoms in molecules(AIM)analysis and charge density difference(CDD)analysis were carried out to study the electronic structure and the structural stability under applied electric fields.Theoretical calculations show that the binuclear Mn(II)cage metal complex had stable structure and electron transport properties,which had potential applications in the field of electrochemical sensor and molecular recognition device.