Excited States Tailoring And Photophysical Properties Of D-?-A Molecules

Material energy conversion(light – electricity or electricity – light)usually undergo a high energy excited state,the understanding and regulation of the excited states is an important part of human scientific activities.Organic optoelectronic materials have been widely used,such as organic light emitting diodes(OLEDs)based on organic electroluminescent materials have been widely used in the filed of display and illumination,light absorption of organic dyes and polymers show great potential in the filed of new thin film solar cells.In organic(polymer)optoelectronic materials,the D-A(donor-acceptor)type molecule is the most important material system.For example,the D-A type fluorescent molecules with bipolar transported characteristic and regulable excited states have attracted much attention.In recent years,D-A fluorescent molecules based on TADF(thermally activated delayed fluorescence)mechanism and “hot exciton” mechanism have achieved high exciton utilization efficiency in OLED devices and improved the efficiency of the devices greatly.However,there are some basic problems in the design and excited states regulation of the D-A molecules: 1)the strength of the donor and the acceptor,as well as the twisted angle between the donor and the acceptor can affect the excited states properties;2)the strong intramolecular charge transfer state results in red shift in emission and reduces the the radiative decay rate.The D-?-A molecules,inserted ? bridge in D-A molecules,exhibit more abundant photophysical properties and excited states structure while maintaining the bioplor transported properties.Such D-?-A molecules is the focus of this thesis.In this thesis,we synthesis a series of D-?-A molecules and adjust their excited states.Their photophysical properties are studied,mainly in the following aspects:1.We adopt the D-?-A structural motif to design the HLCT materials,namely TPA-AN-BP and CZP-AN-BP,in which triphenylamine(TPA)and9-phenyl-9H-carbazole(CZP)are selected as the electron donors,benzophenone(BP)and anthracene(AN)are used as the electron acceptor and ?-bridge respectively.The comparison of photophysical properties and the DFT theoretical calculation of the two molecules revealed that TPA-AN-BP is fully hybrid with the LE state and CT state and possesses better HLCT state properties because TPA has stronger electron donating ability than CZP,resulting higher PLQY.What's more,the large ?ET2-T1 and small ?ES1-T2 can favor the RISC from T2 to S1.Therefore,a nondoped device with TPA-AN-BP as the emitter reached a maximum EQE of 4.39 % and a high EUE of 77 %.2.We have designed and synthesized two high efficiency fluorescent molecules,the standard blue molecule PPI-An-mCZP and the deep blue molecule mPPI-An-mCZP,in which the CZP acts as the donor,the anthracene acts as the ?-bridge and the 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole(PPI)acts as the acceptor.PPI-An-mCZP and mPPI-An-mCZP based devices exhibit great device performance,which achieve a high efficiency of exciton utilization of 82 % and 85 %,a maximum EQE of 7.87 % and 6.51 %,respectively.3.We have designed and synthesized two molecules(p-PPI-AnCN and m-PPI-AnCN)capable of photo-emission from the S2 and S1 state.The PPI acts as the donor,the the anthracene acts as the ?-bridge and the cyano group(CN)acts as the acceptor.The experimental results and theoretical calculations indicate that the larger ?ES2-S1 and molecular orbital separation decreases the internal conversion rate allowing the fluorescence from the S2 state possible.This study is of importance to investigate emission from high laying excited state in conjunction with the mechanisms of intramolecular electronic relaxation process.