Design Synthesis And Photoelectric Properties Of Excited State Proton Transfer Hot Exciton Materials

OLED technology has been widely used in flat panel displays and white light illumination since it was proposed 30 years ago.This technology is now in a critical period of industrial subversion whose key is fabricating a new generation of high-efficiency organic light-emitting materials.OLED light emitting materials have witnessed the development of the first generation of traditional fluorescent materials,the second generation of phosphorescent materials and the third generation of pure organic light emitting materials with high exciton utilization.In the first generation of fluorescent materials,only 25%of singlet excitons can be used for effective light emission,which critically limits the device efficiency.The second-generation phosphorescent materials are based on heavy metal complexes,due to the fact that heavy atoms will enhance the orbital coupling.Therefore,triplet excitons can flip to the ground state in the form of radiation transitions leading to their theoretical exciton utilization rate up to 100%.This type of materials greatly improves the device efficiency,and it is considered as the main material used in commercial OLEDs market.However,these materials suffer problems including high price,incomplete chromaticity?lack ofblue phosphorescent materials?,and shortage of heavy metal rethusurces,which prompts the further development of a new generation of organic light-emitting materials with high efficiency and high exciton utilization.In this backgroud,Pro.Yuguang Ma et al.proposed a hot exciton material with high-energy triplet reverse intersystem crossing characteristics.Through high-energy triplet reverse intersystem crossing process,100%exciton utilization rate was achieved,drawing wide attentions in this field.After numberous of researches,the hot exciton material system has been greatly developed,mainly based on regulating the properties of the excited state through the structural combination of the donor?D?and the acceptor?A?containing heteroatoms.Thus,the excited state characteristics required by the hot exciton process can be satisfied significantly.Although the D-A structure-based hot exciton material system has exhibited excellent performance,new structures of this material system are still needed to control the excited state characteristics.For this purpose,this paper mainly focuses on the development of excited state proton transfer luminescent material system with hot exciton characteristics,which can be shown as follows:1.Research on hot exciton materials with excited state proton transfer based on non-conjugated D-A structure:employing phenanthroimidazole as acceptors and triphenylamine as donors,organic light-emitting materials with excited state proton transfer properties m DPA-PPI-OH and m TPA-PPI-OH was designed and synthesized.Among them,the triphenylamine is connected through the non-conjugated nitrogen atom N1 of the phenanthroimidazole,resulting in non-conjugated relationship with the phenanthroimidazole.Photophysical studies have shown that the m DPA-PPI-OH and m TPA-PPI-OH exhibit the characteristics of excited state proton transfer,which causes their emission spectra are red-shifted relative to the corresponding comparative compounds?m DPA-PPI and m TPA-PPI?without proton transfer.This also leads to the decrease of fluorescence efficiency of the m DPA-PPI-OH and m TPA-PPI-OH.As a result,the maximum external quantum efficiency?EQE?of the devices based on m DPA-PPI-OH and m TPA-PPI-OH reached up to 2.72%and2.82%,respectively,while the exciton utilization efficiency?EUE?was 65%and 78%,both of which were better than m DPA-PPI(EQE_max:1.24%,EUE:20%)and m TPA-PPI(EQE_max:2.56%,EUE:56%)without the excited state proton transfer properties.In addition,theoretical calculation results have shown that the energy levels of m DPA-PPI-OH and m TPA-PPI-OH are more in line with the energy level arrangement of the hot exciton with high-energy triplet reverse intersystem crossing process,which induces effectively increasing of the excitation utilization.2.Research on the hot exciton material with excited state proton transfer based on the conjugated D-A structure:using phenanthroimidazole as acceptors,the triphenylamine group is connected to the C2 substituted benzene ring at different positions,designing and synthesizing organic light-emitting materials 3-TPA-PPI-OH and 5-TPA-PPI-OH with excited state proton transfer properties.Due to the conjugated nature of the C2 attachment site,there is a red shift of the fluorescence emission of 3-TPA-PPI-OH and 5-TPA-PPI-OH,which is shown as green light emission,compared to the non-conjugated m DPA-PPI-OH and m TPA-PPI-OH as mentioned in the previous chapter.And the fluorescence efficiency is slightly improved.Besides,both of 3-TPA-PPI-OH and 5-TPA-PPI-OH also exhibit the characteristics of excited state proton transfer.The maximum external quantum efficiency?EQE?of devices based on 3-TPA-PPI-OH and 5-TPA-PPI-OH reached up to 4.86%and6.14%,respectively,and the exciton utilization rate?EUE?was 76%and?100%.Theoretical calculation results also show that the energy level of 5-TPA-PPI-OH is more in line with the energy level arrangement of the hot exciton with high-energy triplet reverse intersystem crossing process,contributing to higher excition utilization than 3-TPA-PPI-OH.