Theoretical Investigation On Narrowband Properties And Molecular Design Of Thermally Activated Delayed Fluorescence Materials

Thermally activated delayed fluorescence(TADF)materials have been widely studied and applied in display due to their ability to convert triplet excitons into singlet excitons through the process of reverse intersystem crossing(RISC),achieving a theoretical 100% exciton utilization rate.High-color purity luminescence requires that the materials have a narrow full width at half maximum(FWHM)in the spectrum,traditional donor-acceptor(D-A)TADF materials generally exhibit a width of over 70 nm at the FWHM,while multiple resonance(MR)TADF materials improve FWHM,with FWHM typically less than 40 nm,they are mainly based on Boron/Nitrogen(B/N)types and Carbonyl/Nitrogen(C=O/N)types.However,the narrowband emission mechanisms of MR materials are not yet complete,the descriptions of FWHM differences are not clear,and red-narrowband TADF materials are rare.In view of this,this article uses quantum chemistry method as a research tool to explore the relationship between structures and properties of MR-TADF materials,to explain the FWHM differences between materials and to construct red-narrowband molecules from the perspective of theoretical calculations,to provide theoretical guidance for the design of narrowband materials.The main research contents are as follows:(1)Based on MR-TADF materials of B/N types,addressing the question of how the number and position of B/N atoms affect the narrowband properties of materials,this work introduces B-π-N and B-π-B,N-π-N on the MR skeleton respectively,and establish corresponding molecular models.The effects of B/N atoms on the narrowband properties are explored through the analysis of the geometric deviation,the contribution of vibrational frequency,the frontier molecular orbital,and the mode of charge transfer,etc.Theoretical calculations show that these materials have the small reorganization energy.The B-π-N can reduce the donor strengths,the wavelength shows blue shift,and B-π-B and N-π-N can enhance the donor and acceptor strengths by local conjugation,the wavelength shows red shift.At the same time,the increase in the number of B/N atoms at the central benzene ring effectively enhances short-range charge transfer(SRCT).The work reveals the influence of the number and position of B/N atoms on the narrowband emission and luminescence color,which can provide theoretical guidance for the design of such narrowband materials.(2)Based on MR-TADF materials of C=O/N types,addressing the current problem of scarce red-narrowband materials,this work combines the advantages of the long-range charge transfer(LRCT)structure that favors spectral redshift and the MR core structure that favors the retention of narrowband emission,linking the donor substituent and the acceptor MR core to construct hybrid MR-CT type molecules.The design strategy of MR-CT type is explored by analyzing the geometric deviation,dihedral angle,degree of orbital overlap,and the mode of charge transfer,etc.Theoretical calculations indicate that frontier molecular orbital overlap and electron-hole overlap of the MR-CT types are inversely related to the dihedral angle of MR core and substituent(within the range of0-90°).The smaller the dihedral angle,the higher the orbital overlap,the more pronounced the MR degree and SRCT property,and the more conducive to achieving narrowband emission.The molecular series constructed by substituents a and b are MR-CT molecules,of which c1-a,c2-a,and c3-a exhibit superior red-narrowband emission.The work proposes MR-CT hybridization strategy to construct red-narrowband materials,which is of great significance for the development and design of such materials.(3)Based on fluorescence materials,traditional D-A TADF materials and MR-TADF materials containing C,N,O atoms,for how to describe the problem of property differences in the FWHM of the three types of materials,this work selects experimental molecules with emission wavelengths between 400-500 nm and large differences in FWHM as the research system.The correlation between structure and FWHM is explored through the analysis of the geometric deviation,the reorganization energy component,the frontier molecular orbital,the natural transition orbital,etc.Theoretical calculations find that differences in structural relaxation and charge transfer characteristics are responsible for the different FWHM performance of these three types of materials.Molecules with larger FWHM have large Huang-Rhys factors,while MR-TADF molecules exhibit small Huang-Rhys factors and strong SRCT characteristics.In addition,localized excitation(LE)characteristic caused by high orbital overlap is beneficial for improving FWHM.The work systematically illustrates the differences in FWHM properties of three different types of materials,and provides insight into the structure-property relationship of the materials.