Design,Synthesis And Properties Of Deep Red And Near-infrared Luminescent Molecules With D-A-D Structure

Organic NIR luminescent molecules have important applications in the fields of biomarkers,optical fiber communication,infrared detection,OLED and so on.However,there are fewer efficient deep-red/near-infrared(DR/NIR)materials because of the energy gap law(EGL),and the photoluminescence efficiency is low in this band.The main material systems for organic near-infrared luminescence include metal complexes,?-conjugated chromophores,free radical luminescence,thermally activated delayed fluorescent(TADF)materials,and D-A(donor-acceptor)type charge transfer state luminescent molecules.This paper focuses on D-A-D charge transfer NIR luminescent molecules.D-A structure is the basic strategy to realize low-energy near-infrared emitting materials.This type of molecule can achieve color adjustment and efficiency improvement by regulating the electron push-and-pull ability of D-A and the structure of D-A molecules.However,the charge transfer(CT)state characteristics formed between the donor and acceptor in the D-A structure cause the separation of the molecular frontier orbitals,resulting in a lower PLQY(fluorescence quantum yield).Therefore,how to use the CT properties of D-A molecules to obtain the ideal light color and improve the luminous efficiency of such materials is an important issue in this field.Through the accumulation of research on CT materials,our research group proposed a new molecular excited state design concept,that is,hybridized local and chargetransfer(HLCT)excited state.The molecular front orbit of HLCT state characteristics is not completely separated,and the lowest excited state still shows medium or large oscillator strength,resulting in larger radiation transition rate and higher photoluminescence efficiency(PLQY).The luminescence is the mixture of LE and CT states,the component of LE state ensures the fluorescence quantum yield of the molecule,and the CT state property regulates the spectrum range of the molecule.Due to the unique excited state characteristics of the HLCT state characteristic molecule,the molecule still has a high photoluminescence efficiency in the thin film state,showing superior OLED device performance.Although the radiation transition rate of deep red/near-infrared luminescent molecules with D-A structure in the HLCT state has been increased,but the non-radiative transition process is still the main influencing factor that reduces the photoluminescence efficiency of the molecular.Therefore,how to suppress the non-radiative transition process is an important issue for the D-A structure deep red/near infrared luminescent molecule with HLCT characteristics.In this thesis,the electron-deficient rigid heterocyclic naphththiadiazole(NZ),pyridinethiadiazole(PT)and benzodithiophene dione(BT)are used as the basic acceptor(A)building blocks,with triphenylamine,diphenylamine for donor(D)primitives designed and synthesized a series D-A-D type fluorescent molecules.Their emission ranges from deep red to near infrared.Systematically studied their photophysical and electroluminescence properties.The specific research content is as follows:1.Using triphenylamine as the donor,naphththiadiazole(NZ)as the acceptor,and pyrimidine ring and benzene ring as the connecting unit,designed and synthesized two D-A-D type deep red/near-infrared luminescent molecules 2TPA-NZN and 2TPA-BZN.Theoretical analysis and photophysical research show that 2TPA-NZN and 2TPA-BZN have hybridized local and chargetransfer(HLCT)excited state characteristics.Therefore,the two molecules have higher photoluminescence efficiency(PLQY),the PLQY in solution is 76%and 82%,respectively.Compared with 2TPA-NZN,the molecular structure of 2TPA-BZN is more rigid and has a lower non-radiative transition rate.The PLQY in the film is 64%.The low-temperature fluorescence spectroscopy experiments and theoretical settlement revealed the abnormal luminous efficiency-temperature-dependent characteristics,that is,the change of torsion angle has a greater impact on the LE and CT transition components of the HLCT state.In the high temperature region,the molecular structure is more twisted and the efficiency is high,which is derived from HLCT₁,the proportion of LE states accounted for the most HLCT₁;in the low temperature region,the rotation freedom of molecular becomes larger and the efficiency is low,which is derived from HLCT₂,the proportion of CT states accounted for the most HLCT₂.The deep red/near infrared OLED devices based on 2TPA-NZN and 2TPA-BZN achieved maximum external quantum efficiencies(EQE_max)of 5.35%and 5.06%,respectively.The emission peak is 665 nm and the spectrum of device is stable under different voltages.2.Using diphenylamine as the donor(D)unit,pyridinethiadiazole(PT)as the acceptor(A)unit,and thiophene as the connecting unit between D-A,designed and synthesized two D-A-D type deep red/near infrared luminescence DTd PS-PT and DTt PS-PT.They have hybridized local and chargetransfer(HLCT)excited state.Theoretical analysis and photophysical studies show that the molecule DTd PS-PT with two thiophene units has strong CT state characteristics.In dichloromethane solution,the fluorescence emission peak of DTd PS-PT is 810 nm,realizing near-infrared emission.The distance between D and A caused weak CT state of DTd PS-PT(which introduced three thiophene elements),so that its spectrum can only reach deep red light emission(680 nm).The EL emission of doped OLED devices based on DTd PS-PT and DTt PS-PT molecules is 756 nm and 671 nm,respectively,and the light color reaches near-infrared emission and deep-red light emission,respectively.Compared with our previous work of introducing a thiophene as a connecting unit between D-A(DTPS-PT),increasing the number of thiophene between D-A changes the planar conjugation and CT state characteristics of the molecule,making the molecule's photoluminescence efficiency decrease,the fluorescence spectrum of the molecule is blue shifted.3.Using diphenylamine as the donor(D)unit,naphththiadiazole(NZ)as the acceptor unit,and thiophene as the connecting unit,designed and synthesized two D-A-D type deep red/near infrared fluorescent molecules DTPS-NZ and DTd PS-NZ.Theoretical analysis and photophysical research show that both molecules have HLCT state characteristics.In order to understand the excited state properties of the molecule,the PL spectra of 2TPA-NZN and 2TPA-BZN were tested in different solvents.In the low-polar solvent,the excited state dipole moment of the DTPS-NZ(7.1 D)is larger than that of DTd PS-NZ(6.8 D),which indicates that the LE state of DTPS-NZ is stronger.In the highly polar solvent,the excited state dipole moment of DTd PS-NZ(15.8 D)is significantly greater than that of the DTPS-NZ(12.6 D),indicating that the CT state of DTd PS-NZ is stronger.The stronger CT state characteristics of DTd PS-NZ makes its fluorescence emission red-shifted to 796 nm,realizing near-infrared emission.The EL emission of doped OLED devices based on DTPS-NZ and DTd PS-NZ is 675 nm and 711nm,respectively.The device spectrum is stable under different voltages.4.Using triphenylamine as the donor unit,benzodithiophene dione(BT)as the acceptor unit,designed and synthesized a new type of D-A-D type NIR fluorescent molecule 2TPA-BTO.This molecule has good thermal stability and electrochemical characteristics,with a thermal decomposition temperature of 402?,and the electrochemical energy gap is 1.6 e V.Theoretical analysis and photophysical research show that the fluorescent molecule 2TPA-BTO has strong HLCT state characteristics.In order to explore the excited state characteristics of the molecule2TPA-BTO,the fluorescence spectra was tested in different solvents.With the increase of the polarity of the solvent,the fluorescence emission peak of 2TPA-BTO red-shifted to 167 nm,indicating that the polarity of the solvent had a great influence on the excited state properties of the molecule.In dichloromethane solution,the fluorescence emission of 2TPA-BTO is 830 nm.The EL emission of the doped OLED device based on 2TPA-BTO is 775 nm,which realizes near-infrared emission,and the device spectrum is stable under different voltages.