Theoretical Studies On The Structures And Optical Properties Of BODIPY Fluorescent Compounds

Fluorescent compounds of boron-dipyrromethene（4,4-difluoro-4-bora-3a,4a-diaza-s-indacene,BODIPY）have attracted much attention due to their remarkable optical properties in many aspects.However,it has disadvantages of short wavelength and low fluorescence quantum yield,which limits its application.Therefore,it is crucial to design and develop novel BODIPY-like fluorescent materials with excellent spectral properties.In this thesis,density functional theory is used to study and analyze the regulation mechanism of substituents and heteroatoms on the electronic characteristics and fluorescence properties of BODIPY monomer/dimer fluorescent molecules in the expandedπ-conjugated system.It is generally believed that electron-withdrawing groups cause a red-shift in the spectrum,while electron-donating groups cause a blue-shift.However,the calculation results of monomer molecules in this study show that when electron-withdrawing groups already exist in the molecular structure,the introduction of stronger electron-donating groups can greatly increase the energy of HOMO orbital,thus significantly reducing the HOMO-LUMO energy gap(ΔE_H-L).This results in more red shifts in the absorption/emission wavelengths of the fluorescent molecules.Besides,the research results show that too strong push-pull effects can affect the balanced transmission of electrons and holes in the materials,making them unable to be used as raw materials for OLED devices.When materials have the above characteristics,they are more suitable for biological imaging,fluorescent ion probe and other fields.The results of studies on dimer compounds show that the push-pull effects on the absorption/emission wavelengths of dimers are basically the same as those of monomer molecules.In addition,the dimers molecules’bulks increase obviously,and there are too manyπ-electrons in the conjugated system,resulting in the inability of balanced electron and hole transmission,so they are not suitable in the electroluminescence field.It is worth noting that the dimers are harder to be excited,but the absorption/emission wavelengths increase significantly,which make them near-infrared（NIR）fluorescent dyes with ideal application prospects.In addition,the polarizable continuum model（PCM）method is used to simulate the influence of the polarity of the external solution on the fluorescence properties of BODIPY fluorescent molecules,verifying that the studied molecules are intramolecular charge transfer types.The obvious feature of this type of dyes is that as the polarity of the solvent increases,the absorption wavelength changes little,while the emission wavelength will undergo a significant red shift.In this thesis,theoretical studies on the structure and optical properties of BODIPY molecules have laid a theoretical foundation for understanding the structure-activity relationship of fluorescent materials.And it provided a theoretical basis for the laboratory to further develop and design efficient and stable organic luminescent materials.