Synthesis And Properties Of D-?-A Small Molecular And Polymeric Luminescent Materials Based On Pyrimidine-Carbazole

Organic light-emitting diodes?OLEDs?have been developed vigorously during the past decades and are widely used in lighting and display nowadays.Organic electroluminescent materials are essential factor to determine the performance of OLEDs.Among the three generations of OLED materials,thermally activated delayed fluorescence?TADF?emitters that are noble metal-free have both advantages of high emission efficiency like phosphors and low cost like traditional fluorophors,and become the hotspot of both fundamental research and industrial community.Blue TADF emitters are especially precious in comparison with green and red ones due to their higher excited state energies.In this dissertation,pyrimidine was selected as the electron acceptor?A?to design small molecular TADF emitters in combination with different donor?D?groups including carbazoel,acridine,and so on.Incorporating trifluoromethyl on the bridge between D and A or adjusting linking styles of D and A on bridge and D/A molar ratio was explored to tune the light-emitting properties of these compounds,with the goal to reach TADF feature and high performance.Furthermore,conjugated polymers were designed and prepared by using poly-carbazole as the main chain and pyrimidine-based TADF units as the side chains.The detailed contents are as follows:1.A group of small molecular emitters were designed and prepared using pyrimidine as acceptor and carbazole or derivatives as donors.Trifluoromethyl was incorporated on the phenylene bridge and close to donor.As revealed by density function theory?DFT?calculation,there is a noticeable change in the molecular configuration by introducing trifluoromethyl group which significantly increases the dihedral angle?85-87°?between the donor and the phenylene bridge,and reduces both the D-A conjugation and the overlap of HOMO and LUMO.As judged from the low-temperature fluorescence and phosphorescence spectra,the triplet energies of these molecules were calculated at about 2.9 eV,and the singlet and triplet energy difference(?E_ST)were determined as small as 0.21 eV for PmCF₃-tBuCz and almost zero eV for PmCF₃-CzDP.Such low?E_ST values along with highly twist molecular conformations indicate that many of these compounds may have TADF feature.2.A series of pyrimidine-carbazole based TADF emitters were designed and prepared by linking N-phenylcarbazole at 2-,4-,6-site of pyrimidine.At the same time,pyrimidine and carbazole were set at the ortho-position on phenylene bridge and the D/A molar ratio was adjusted between 1:1 for 2-CzPm and 4-CzPm and 2:1 for 4,6-CzPm.The time-resolved photoluminescence decay and temperature-depend PL decay measurements verified that these three emitters are TADF emitters,with delayed fluorescence lifetimes of 36.4,6.2 and 18.4?s,respectively.Blue OLEDs were fabricated using this series of materials as doped emitter and the 4,6-CzPm-based device exhibited best performance,with the maximum external quantum efficiency,current efficiency and power efficiency of 11.2%,11.4 cd/A and 8.0 lm/W,respectively.3.A group of conjugated polymers were designed and prepared using poly-carbazole as main chain and pyrimidine-based TADF unit as light-emitting sidechains.Carbazole,acridine,or phenoxazine was selected as D to tune the luminescent color or overall properties of polymers in combination with 5-?trifluoromethyl?pyrimidine as A.The DFT calculation found that when the trifluoromethyl group is connected to the 5-position of pyrimidine,the LUMO distribution is similar to those of triazine's derivatives.Different donors such as acridine and phenoxazine are connected to the 4-position of the pyrimidine acceptor to form an asymmetric light-emitting monomer.All monomers contain a twisted D-?-A structure where the donor and the phenylene bridge are almost perpendicular to each other with dihedral angle of 89.2°.Therefore,the singlet and triplet energy differences of these molecules are quite small,i.e.0.005-0.01 eV.This paper speculate that small-molecule monomers?i.e.the sidechains of polymers?have TADF property,and thus the obtained polymers are characterized by TADF feature as well.The detailed behaviors of these polymers are being studied now by experiments.