Rational Design And Theoretical Study Of Thermally Activated Delayed Fluorescent Materials

Thermally activated delayed fluorescent (TADF) molecules have been widely studied as the third generation of organic electroluminescent (EL) materials. The up-conversion from triplet to singlet exciton can be realized by TADF molecules through thermal excitation. In the absence of heavy metals, the internal quantum efficiency can theoretically reach 100%, substantially enhancing the efficiency of electroluminescence. Therefore, it is of great importance to design high efficient TADF organic molecules. In this thesis, the geometry structure, frontier molecular orbital, singlet-triplet energy gap, optical properties and charge transport properties of TADF molecules have been deeply studied by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The main research contents are showed as follows:We have successfully demonstrated and elucidated the broad range color tuning of spiro-acridine TADF materials via "CH"/N substitution and conjugation degree control on the acceptor fragment. All of the designed TADF materials possess small ?EST values, which may provide an efficient up-conversion from the T1 to S1. The calculated delayed emission wavelengths cover the whole visible region. The wavelength values for the "CH"/N substituted derivatives are bathochromically shifted compared with that for the parent compound. The derivative 2c exhibits a red emission and the highest reverse intersystem crossing rate (kRISC) value among all of the investigated derivatives. The derivative 2d exhibites a blue emission centered at 432 nm with the smallest stokes shift due to a decrease in the degree of ?-conjugation. Our theoretical studies provide hints for the design of efficient spiro-conjugated TADF candidates with high kRISC values as blue and red emission materials in the future.A series of donor-acceptor compounds, including asymmetric D-B-Ai-B and symmetric D-B-Ai-B-D topologies, have been designed and investigated using DFT and TD-DFT toward highly efficient TADF materials. Phenoxazine (PXZ) is adopted as a donor (D) fragment, while 1,3,4-oxadiazole (A1), benzo[c][1,2,5]-thiadiazole (A2), and quinoxaline (A3) are selected as acceptor fragments. A phenyl ring (B) is connected to Ai to extend the ?-conjugation, leading to strong electron-withdrawing ability. Our results indicate that the ?EST of symmetric D-B-Ai-B-D compounds are smaller than those of asymmetric D-B-Ai-B ones. For the same topologic series, the ?EST values decrease with increasing electron-withdrawing strength of B-Ai-B. The lowest AEST value has been obtained for D-B-A2-B-D among all these investigated compounds. Then the potential energy surface and normal mode analyses were applied to discuss the charge injection and transport characteristics. The designed D-B-Ai-B-D compounds exhibited more effective charge injection with lower ionization potential and higher electron affinity than D-B-Ai-B ones. Meanwhile, the temperature dependent mobility was predicted by Marcus theory, both hole and electron mobility of D-B-Ai-B-D increase with increasing temperature in the range of 5-200 K. However, hole mobility slightly decreases from 200 K to 300 K. The newly designed D-B-A2-B-D compounds demonstrate higher electron and hole mobility than D-B-A1-B-D, implying that the chemical modification of acceptors effectively improves the carrier transport ability.A series of benzo[1,2-b:4,5-b'] dithiophene (BDT) based TADF molecules have been designed and investigated using DFT and TD-DFT. The DFT studies indicated the 4,8-positions of BDT substituted molecules are favorable for interrupting the electronic communication between donor and acceptor fragments, contributing to the implementation of TADF phenomenon. The TD-DFT calculations showed that the designed BDT-mAn, where n and m refer to the number and series name of A fragments, exhibited a mixed states, which comprised a large proportion of charge transfer components and a small part of locally excited components. Combined with the energy gap rules, the triplet exciton may be converted into singlet exciton through RISC along the excited-state channel of T,, T2, and T4 to S1 for BDT-An, BDT-2An, and BDT-4An (n= 1 or 2), respectively. The calculated Aem of BDT-4An and BDT-2An exhibited a significantly hypochromatic shift compared with that of BDT-An due to the increase in the numbers of accepter subunits. BDT-2An and BDT-4An (711.45-766.61 nm) may exhibit long wavelength emission characters. Moreover, the calculated hole and electron reorganization energy values of all the designed molecules are in the range of 0.15-0.20 eV and 0.15-0.32 eV at the PBE0 theoretical level, respectively. For the BDT-2A2 and BDT-4A2 with small singlet-triplet energy gaps, the predicted carrier mobilities are ?h=3.43 cm2v-1 s-1 and ?e= 0.18 cm2 v-1 s-1 for BDT-2A2, and ?h= 0.67 cm2 v-1 s-1 and ?e=1.38 cm2 v-1 s-1 for BDT-4A2, respectively.The influences of the bridge connected donor and acceptor on ?Est and ?em have been extensively investigated using DFT and TD-DFT. Benzo[1,2-b:4,5-b'] dithiophene is employed as an excellent donor (D) unit, while [1,2,5]thiadiazolo [3,4-c]pyridine-7-carbonitrile serves as the acceptor (A) unit with strong electron-withdrawing ability. Benzene and naphthalene units are selected as the investigated bridges (B). We mainly analyzed the role of the B unit on tuning the ?Est value and emission wavelength. The introduction of (C6H4)n linkers interrupted the electronic communication between HOMO and LUMO, facilitating the effective separation between D and A units. As the numbers of n increase, the singlet energy level values decrease. However, the triplet energy level of al (n=1) is higher than those of the other two molecules a0 (n=0) and a2 (n=2), leading to the smallest ?ESt value among these three molecules. Molecule b1, with the D and A units substituted in the meta-position of the benzene bridge, shows effective intramolecular charge transfer characters. The calculated results verify that b1 has a small AEST value (0.0447 eV). As for molecule dl, naphthalene instead of benzene acts as a B connecting D and A units. The predicted AEST (0.0592 eV) is slightly larger than that of b1 due to the slight increase of overlap on the B unit. We also explore the role of the molecular topology. The symmetric A-B-D-B-A topologic molecule e exhibits a smaller AEST value than the asymmetric A-B-D molecule dl. The results indicate that the emission wavelength red-shift in the order of ?em (a0)<?em (al)< ?em(a2) with the increase of n values. The ?em value of b1 is larger than that of al. Compared to the Aem of asymmetric molecule dl, the wavelength of symmetry molecule e shows blue shift.The molecular matching issues between TADF dopant and host in organic light emitting diodes have been studied using the theoretical calculation method. We designed a series of outstanding molecules as the host materials. Compared with 3TPAPFP and 3CzPFP, the electron-donating group (-CH3) substituted molecules 1 and 3 exhibited a low singlet energy level (Es@S1), respectively. However, the electron-withdrawing group (-CN) substituted molecules 2 and 4 showed a high Es@S1, respectively. The triplet energy levels (ET@T1) of these substituted molecules are close to the corresponding parental molecules. The emission wavelengths of the designed molecules are shorter than the absorption wavelength of the TADF dopant 4CzIPN, contributing to the energy transfer from the host to dopant materials. The molecule 4 can be used as an excellent host candidate of the green TADF dopant 4CzIPN.