Molecular Design With Small Singlet-triplet Gap For Thioxanthone Derivatives As Thermally Activated Delayed Fluorescence Materials

As the next-generation display technology, the advantages of organic light-emitting diode are fast response, wide viewing angle, color fidelity, high definition, beautiful appearance. It is recognized as the next generation display technology and new lighting. As the the core technology of organic electroluminescence material. Organic electroluminescence material become the focus of competition in the field. Although the first generation of fluorescence material have relatively high stability, but its internal quantum efficiency is only 25% due to the limitation of spin statistics. In spite of the second generation of phosphorescence material achieve 100% internal quantum efficiency, but the resources of iridium and platinum is limited, at the same time, it may cause pollution. To take advantage of the fluorescent material emitting triplet excitons, In recent years, researchers put forward three main mechanisms to improve the utilization rate of triplet excitons, such as,triplet-triplet annihilation, hot exciton and hybrided local charge-transfer, thermally activated delayed fluorescence. Especially, TADF attract scientists attention and become a hot topic in the field recently. As the molecule of thermally activated delayed fluorescence materials, small energy gap between lowest singlet and triplet excited states is able to promote exciton to realize reverse intersystem crossing from T1 state to S1 state, then fluorescence emits by exciton which jump from S1 state to V the ground state, so as to realize a 100% internal quantum efficiency, compared with phosphorescent material. As a third-generation electroluminescence fluorescence materials, it has the advantage of high effiency and low-cost, which has developing potential and application prospect. Although it has made great progress in blue and green for TADF materials, but red material is still very scarce.The 9H-thioxanthen-9-one-10,10-dioxide molecular contain a sulfone and a carbonyl group, it is a strong electron acceptor material. Its derivatives are usually applied in biological and medical fields, such as anti-tumor, anti-allergic and DNA repair, but it is rarely used in the photophysical and photochemical field. Recently,Wang et al. regard 9H-thioxanthen-9-one-10,10-dioxide as a acceptor unit,respectively connected to carbazole and triphenylamine unit. soon afterwards, they synthesis TXO-Ph Cz and TXO-TPA, the former is a green TADF molecular which achieves maximum external quantum efficiency of 21.5%, the latter is a yellow TADF material which realizes maximum external quantum efficiency of 18.5%. Given an excellent performance for TXO-Ph Cz and TXO-TPA, we further optimize the molecular design of such materials. For example, changing the connection from the site, increasing the distance between donor and receptor, changing the number of donor, changing the substituted bases on the donor and acceptor and so on, further reducing energy gap between lowest singlet and triplet excite state, thus triplet excitons realize reverse intersystem crossing, so as to improving the utilization of excitons.In this paper, densitiy functional theory(DFT) and time-dependent density functional theory(TDDFT) are used for the study of molecules. We obtain the following data, the highest occupied molecular orbitals and lowest unoccupied molecular orbitals and their electron density distribution, electronic absorption spectra and fluorescence emission spectra, natural transition orbitals, ionization potential,electron affinity, reorganization energy, etc. The purpose is to explore the effects of optical properties owing to changing the molecular structure, which also provides theoretical support for experimental chemists. The main contents cover the following two aspects:(1) Theoretical design small energy gap of carbazole and9H-thioxanthen-9-one-10,10-dioxide derivatives as thermally activated delayed fluorescence molecular. we have designed six different connection ways according to different active sites between donor and acceptor. Through the theoretical analysis,the molecular have smaller energy gap where the ninth site for carbazole connet the third site for acceptor, smaller gap is beneficial to realize reverse intersystem crossing(RISC). From the ionization potential and electron affinity of view, the molecule has the ability to inject hole and accept electron well. From the spectral data, the moleculars whose the ninth site of carbazole connect the second and third site of acceptor may be used as red TADF materials; on the basis of six kinds of different connections, by introducing a benzene ring, a π bridge, increasing the distance between the donor and acceptor, the energy gap which the ninth site of carbazole connect the third site of acceptor further reduce; on the basis of six kinds of different connection, we transform single structure into a double structure. According to the theoretical calculations, Compared double-vessel structure with single structure,energy gap does not change.(2) Theoretical design small energy gap of triphenylamine and9H-thioxanthen-9-one-10,10-dioxide derivatives as thermally activated delayed fluorescence molecular. We obtain three different connection moleculars by connecting the para and meta position of triphenylamine with the second and third sites of acceptor, the energy gap of molecular where the meta position for donor is connected with the third site for acceptor. The moleculars who connect the third position for acceptor possess a good hole injection and electron acceptance; By changing the donor and the substituents on the body under the second site substituted unchanged circumstances, carbazolyl group has a large steric hindrance compared to diphenylamine derivatives, the degree of overlap between the HOMO and LUMO decreases, the energy gap also become small; By fixing the para position of triphenylamine with the second site of acceptor, we introduce different substituents with electron-withdrawing ability on the sixth site for acceptor,-F,-CN. Through the calculational analysis, the electron-withdrawing ability more stronger, the energy gap more smaller. Additionally, the moleculars substituted via a cyano group is likely to be red TADF materials.