Synthesis And Characterization Of Asymmetric Thermally Activated Delayed Fluorescence Materials Based On Phenylcarbonyl And Phenylsulfone Groups

Since its inception,organic light-emitting diodes have had many advantages over traditional LCD display and LED lighting,making OLED materials show a huge potential.Phosphorescent materials with high efficiencies have attracted our attention instead of traditional fluorescent materials as the first-generation organic light emitting materials,because of their low efficiencies.However,the development of phosphorescent materials with heavy atoms was limited owe to high cost.Then it come the thermally activated delayed fluorescence?TADF?materials considered as one of the third-generation organic light emitting materials.TADF materials have attracted much attention because of their simple preparation and low price.An idea TADF material needs to have at least two important features:one is a small energy gap(?E_ST)between singlet excited state?S₁?and triplet excited state?T₁?,and the other is high fluorescence quantum yield.Achieving a small?E_STT requires separation of HOMO and LUMO as much as possible,but a larger separation will result in decreasing fluorescence quantum yield.Thus it is difficult to designing TADF materials with high efficiency.Especially for blue TADF materials,more requirements need to be considered,such as high singlet state,high triplet state and wide band gap.Therefore it is a still challenge to design and develop blue TADF materials with high efficiencies.In this communication,a series of small molecule and polymer TADF materials with benzene carbonyl/sulfone group were synthesized.Their thermal stability,photophysical and electrochemical properties have been investigated,the specific research contents are as follow:?1?Based on the asymmetric structure of D-A-D',a phenyl ketone group with strong electron-withdrawing ability is selected as an acceptor?A?,and the N-phenyl carbazole with strong electron-donating ability is as a donor?D?,carbazole or diphenylamine is the other donor?D'?,and two small molecular sky blue materials CP-BP-CZ and CP-BP-DPA were designed and synthesized.At the same time,a methyl group was introduced into 3 position of benzene ring at the asymmetric end of CP-BP-CZ to obtain another sky blue material CP-BP-3M-CZ.The materials were determined by NMR and mass spectrum.The photophysical and electrochemical performance have been investigated as well as theoretical calculation.The results show that the?E_ST of CP-BP-CZ?CP-BP-DPA and CP-BP-3M-CZ were 0.47,0.66and 0.41 eV,respectively.Their fluorescence quantum yields were 0.73%,14.11%and3.57%,respectively.Among them,CP-BP-CZ and CP-BP-3M-CZ showed more obvious TADF characters,while CP-BP-DPA obtained higher fluorescence quantum yield.?2?Based on the asymmetric structure of D-A-D',a phenylene cyanide group which is stronger than the benzophenone group is selected as the A unit,and the D or D'donor at both ends is unchanged.Three small molecule blue-green TADF materials CP-MT-CZ,CP-MT-DPA and CP-MT-3M-CZ were designed and synthesized.The materials were determined by NMR and elemental analysis,Their photophysical and electrochemical properties have been investigated as well as theoretical calculation.The results showed that all of them showed?E_ST greater than 0.50 eV.Their fluorescence quantum yields of CP-MT-CZ?CP-MT-DPA and CP-MT-3M-CZ were5.02%,31.17%and 22.39%,respectively.Compared with above-mentioned materials with a ketone group as an A unit,the materials with the phenylene cyanide group as the acceptor material showed less obvious TADF characters,and fluorescence quantum yields were improved.?3?Based on the asymmetric structure of D-A-D',a benzene sulfone which is weaker than the benzene ketone is selected as the A unit,and the donors at both ends are unchanged.Two small-molecule TADF materials CP-PSO-CZ and CP-PSO-DPA are designed and synthesized.The results show that the maximum emission wavelengths of CP-PSO-CZ and CP-PSO-DPA are 412 nm and 428 nm,and emit deep blue light;?E_ST is 0.49 and 0.74 eV;their fluorescence quantum yields are29.54%and 18.25%,respectively.?4?We have designed and synthesized two blue TADF polymers Pm-CBZ-BP-DPA with different ratios?5%and 10%?,in which a poly?m-benzene?unit was considered as the main chain,a good hole transport carbazole was selected as the one-end side chain,a benzophenone-based small molecule TADF material with D-A-D'was used as the other side chain,and the main chain and side chain were directly connected by an oxygen and aliphatic chain.The structures of polymers were determined by ¹H NMR and GPC.The two polymers showed better thermal stability,photophysical and electrochemical properties.