Investigation Of Organic Fluorescent Moleculars With Rigid Structure And Their Photophysical Properties In Acid Condition

Since the inception of organic fluorescent materials,which has attracted a great deal of attention from researchers.These materials have been applied in many aspects of life,including the dyes,textile and printing industries,organic light emitting device industries and other biological industries.Compared with traditional inorganic luminescent materials,there are a variety of organic molecule luminescent materials that are easy to modify,and their unique and flexible structure can greatly improve their photoelectric properties,making them have more excellent photoelectric properties.Therefore,they have been highly valued by researchers in the field of organic luminescent materials.Meanwhile,with the rapid development of modern industry,energy shortages and environmental pollution have followed.Currently,fossil fuels are still the main sources of energy.These types of energy are not only non-renewable,but also emit large amounts of toxic and harmful gases after combustion,which seriously endanger human health and pollute the atmosphere,leading to acid rain.Nowadays,frequent acid rain will cause damage to external photoelectric materials and buildings,causing them to lose their original physical and chemical properties or optical characteristics.Therefore,it is urgent to design and synthesis acid-resistant organic fluorescent materials.Combing the research of organic fluorescent molecules,this paper design and synthesis the three types of organic fluorescent molecules with unique rigid conjugated structures,and focuses on studying their optical properties under acidic conditions,it is expected that they can maintain good optical characteristics under acidic environment,as follows:（1）Flavanthrone compounds with unique rigid conjugated structure:A new type of flavanthrone（A-6）with rigid conjugated structure was designed and synthesized.The molecular uses aminoanthraquinone as raw material and introduces 1-bromo-3,5-di-tert-butylbenzene through a series of coupling reactions,which increased the steric hindrance of the whole molecular and reduced the intermolecular interaction,making the two-dimensional conjugated structure more stable.Thereby,it prevented aggregation-induced quenching and increasing the probability of fluorescence transition,making the photophysical properties of A-6 more excellent.In addition,the UV absorption and fluorescence emission characteristics of A-6 in different acidic solutions（CH₃SO₃H,H₂SO₄,CF₃SO₃H,HCl,HF,H₃PO₄,HNO₃）were studied,the relative fluorescence quantum yield and fluorescence stability were characterized.Furthermore,in order to better understand the electronic properties of A-6,density functional theory（DFT）calculations were carried out at the B3LYP/6-311G**level,and the energy levels of the highest occupied molecular orbital（HOMO）and the lowest unoccupied molecular orbital（LUMO）and band gap value were discussed.The results show that this novel compound can maintain good optical properties,high fluorescence stability and quantum yield under acidic conditions.In conclusion,this paper proposes a design strategy for a novel organic fluorescent molecule,and expects this compound to be a material for acid-resistant organic luminescent devices,such as external solar panels and other photoelectric materials.（2）Carbazoles compounds with intramolecular charge transfer:Two kinds of compounds（B1-2,B1-3）with unique conjugated rigid structure were designed by the carbazole as the main structure,connecting multiple electron-rich fluoride ions through theπ-electron system,and the amino branch connected at the extremity of the tetrafluorobenzene group.The strong electronegativity of the fluorine ion groups on the benzene ring greatly improves the electrochemical properties of the entire molecule and has effect on the electron cloud density distribution of the conjugated system.Furthermore,the amino group at the end of the compound is prone to form intermolecular nitrogen/oxygen saturated hydrogen bonds,which can affect the optical characteristics.Therefore,the optical properties of the compound were explored under different acidic conditions（H₂SO₄,CH₃SO₃H,HF,H₃PO₄,HCl,HNO₃）and different solvents（n-Hexane,Tol,DCM,THF,Me OH,ACN）.In addition,the DFT was used to calculate the HOMO-LUMO gap value,and X-ray diffraction（XRD）was used to observe their crystal structures,while cyclic voltammetry（CV）was used to characterize the electronic properties of compounds.Multiple characterization results indicate that both B1-2 and B1-3 emit blue light and exhibit excellent photophysical properties and good electrochemical stability.They are sensitive to strong acid environments and can maintain high stability in acidic solutions.It proposes a strategy to develop novel stable organic blue-light-emitting molecules and are expected to be applied to materials for organic blue-light devices.In addition,this paper also designed compounds（B2-2,B3-2）with a triazine group as the electron acceptor center（A）and carbazole group（D）as the donating electron.Meanwhile,the triazine center is connected to the carbazole conjugated system through a benzene ring,forming a dendritic D-π-A type molecule with a unique rigid conjugated structure.Due to the different substituents（R=F,Ph）on the benzene ring,which effected on the electron cloud density distribution of the entire molecule,resulting in a higher fluorescence quantum yield and better optical stability.Meanwhile,the photophysical properties of the triazine-carbazole molecule（B2-2,B3-2）were also studied and discussed,and the changes in optical properties（UV-visible absorption and fluorescence emission）under different acidic conditions（CH₃SO₃H,H₂SO₄,CF₃SO₃H,HF,HNO₃）,and characterizing the fluorescence emission and stability under different p H conditions.In terms of electronic properties,theoretical calculations were used by DFT,and the energy levels of HOMO-LUMO and the energy band gaps value were discussed.Multiple characterization results show that B2-2 exhibits significant redshift and fluorescence enhancement under various acidic conditions,and can absorb a wider range of wavelengths under different acidic conditions.B3-2 is more stable under acidic conditions,and its optical characteristics are hardly affected by acidic solutions.Both compounds can be used as potential acid-resistant fluorescent molecular materials,and provide methods and ideas for designing acid-resistant D-A type organic fluorescent molecules.（3）Pyrimidine-acridine compounds containing nitrogen heterocycles:The methyl pyrimidine as the electron-accepter central and dimethyl-acridine group as electron-donor,which connected by a conjugated benzyl cyanide ring,formation of D-A type molecule（M-2）.It has a longπ-conjugated system and twisted molecular structure of multiple methyl substituents,which increase the steric hindrance of overall molecular and exhibited exceptional photophysical properties.Meanwhile,the optical properties in different solvents（n-hexane,dimethyl sulfoxide,dichloromethane,ethyl acetate,methanol,and acetonitrile）and in protonic acids（HF,CH₃SO₃H,H₂SO₄）were systematically investigated,which possessed intramolecular charge transfer（ICT）ability and shown obviously solvatochromic properties.Due to the three nitrogen atoms with different electron arrangements,two nitrogen atoms at pyrimidine were sp² hybridized,the nitrogen atoms on benzyl cyanide were sp hybridized and the nitrogen atoms at acridine were sp³hybridized.It will cause the hydrogen protons interact with the three types of nitrogen atoms in different degrees and leading to a distribution of the electron cloud density.Meanwhile,the values of the HOMOs-LUMOs gap and the DFT were performed and calculated.Moreover,its crystal structure was exhibited the distinct orthorhombic system（a=8.9160（4）?,b=44.289（3）?,c=9.6131（7）?,α=90°,β=90°,γ=90°,V=3796.0（4）?3,z=4,Dc=1.244 g/cm3）with stronger non-homogeneity and without high sub-symmetry axis,showing regular and stable crystal configuration.Multiple characterization results prove that M-1 has good optical characteristics and fluorescence emission enhancement,and can be used as acid-resistant organic luminescent materials.As typical D-A molecule,M-2 has significant solvatochromic,good optical and electrochemical characteristics,and can be used as TADF material.