Synthesis And Fluorescence Properties Of Benzimidazole Derivatives

Heterocyclic compounds have important roles in the pharmaceutical and agrochemical industries.Among them,benzimidazoles a class of nitrogen-containing heterocyclic compounds fused with imidazole and benzene rings,and their derivatives are widely distributed in medicines,metal ligands,polymers and materials.Therefore,designing and constructing their derived structures and exploring their applications is an important research topic.In recent years,they have been committed to developing efficient and economical methods to synthesize new benzimidazole derivatives,with the expectation that their applications in many fields can continue to break through innovation.This dissertation revolved around the benzimidazole building block,and firstly developed a synthetic strategy without the involvement of transition metals.This strategy used cesium carbonate（Cs₂CO₃）as a green base to efficiently convert 2-chlorobenzimidazole and alcohol/phenol into the corresponding benzimidazole-containing ether derivatives under air atmosphere.Exploration of the substrate scope showed that the corresponding（hetero）aryl ethers can be synthesized from various（hetero）aryl chlorides and alcohols/phenol.In addition,the structures of the obtained compounds were confirmed by X-ray crystallography,nuclear magnetic resonance spectroscopy（NMR）,high resolution mass spectrometry（HRMS）and other characterization methods.It was worth mentioning that this method avoided the use of transition metals and solvents,and it was unnecesary to remove water from the reaction reagents.Furthermore,this new strategy can be applied to the efficient preparation of bioactive molecules（Fenthiaprop-ethyl and Diphenhydramine）.In addition,this synthetic approach was scalable,as evidenced by the gram-scale reactions of representative ether products（3b,2s,and Fenthiaprop-ethyl）.In order to elucidate the mechanism of this process,density functional theory（DFT）calculations and mechanistic experiments were also carried out in this chapter.Based on these results,a possible reaction pathway including four stages（binding,hydrogen activation,concerted attack and separation）was proposed,which revealed the role of Cs₂CO₃ in the whole process:providing an alkaline environment and stabilizing reaction intermediates.Finally,the counter-ion effects of Na₂CO₃,K₂CO₃ and Cs₂CO₃on this process were investigated,and the reaction pathways and energy diagrams of the three carbonates were calculated by means of DFT,which finally explained the reason why Cs₂CO₃ had the best performation in promoting this transformation.Next,a novel fluorescent molecule P1 was designed and developed by the combination of benzimidazole units and bridging Cl atoms.Its structure was not only characterized by conventional characterization methods such as NMR and HRMS,but also confirmed by X-ray crystallography.Furthermore,the role of bridging Cl atoms in photoluminescence was explained through a series of experiments.The photophysical properties of the two compounds before and after the introduction of the Cl bridges showed that the incorporation of Cl atoms resulted in the emission of blue fluorescence and a slight red shift of the absorption band.DFT calculations verified that the introduction of electronegative Cl atoms adjusted the HOMO and LUMO energy levels of the resulting compound and reduced the band gap（E_g）.Meanwhile,the introduction of bridging Cl atoms led to an obvious intramolecular charge transfer phenomenon in P1.This is the fundamental reason for its large Stokes shift,which justified its low fluorescence quantum yield and further rationalized the phenomenon that P1 emitted blue fluorescence.