Organic Donor-acceptor Fluorescent Compounds: Synthesis, Photophysical Properties And New Mechanism For Organic Electroluminescence

Currently, the high cost of organic light-emitting diodes (OLEDs) is more andmore urgent to be resolved due to the wide application of OLEDs in flat-paneldisplays and solid-state lightings. The researchers recently developed somedonor-acceptor (D-A) fluorescent molecules based on different mechanism, towardincreasing the singlet exciton ratio in fluorescent OLEDs and realizing highperformance OLEDs with low cost. Among them, thermally activated delayedfluorescence (TADF) and triplet-triplet annealing (TTA) are two main mechanism.Recently, our group observed the reverse intersystem crossing (RISC) at high-lyingstates (Tn (n>1)→S1) in some D-A compounds, and further proposed a novelmechanism—hot exciton mechanism to harvest100%excitons through the RISCprocess of high-lying triplet excitons. Therefore, in principle, the hot exciton materialscan combine the high efficiency of phosphorescent OLEDs and the high stability offluorescent OLEDs well, and hot exciton mechanism provides a novel approach forthe molecular design of next generation organic ELmaterials.According to Kasha’s Rule, molecules in high-lying states will rapidly reach thelowest excited states, so it is very hard to employ high-lying excitons. Since hotexciton mechanism is based on the RISC process of high-lying triplet excitons, it isgreat challenging to break through the limit of traditional photochemistry theory and obtain high exciton utilization efficiency (ηr) and high PL efficiency (ηPL)simultaneously. There are still a great deal of fundamental works need to bedeveloped, such as the dynamic process of Tn→S1RISC, the relationship betweenmolecular structure and Tn→S1RISC rate, hot exciton materials with high deviceperformance and so on.In this thesis, we carried out the following studies based on the photochemicalprinciples toward realizing high performance OLED through new mechanism (hotexcition mechanism):1. Using phenothiazine (PTZ) as donor and anthracene (An) as acceptor, threedonor-acceptor (D-A) compounds were designed and synthesized through thedifferent PTZ-An connections. Upon excitation, locally excited (LE) state and CTstate are simultaneously existed in these D-A molecules. On the basis of thephotophysical data and DFT calculations, we found the photophysical properties ofD-A molecules were determined by the relative location of CT state and LE state inthese D-A molecules. Under the specific D-A geometry, CT state and LE state havesimilar energy level, and could be mixed or hybridized into a new state—HLCT state.The electroluminescence (EL) results indicated the HLCT material (PTZ-3-AnP)could achieve high ηr(62%), and its PL and EL performance was further enhancedthrough the molecule modification (decreasing the D-A torsion angle and increasingthe D-A conjugation length). The ηPLof PTZ-2An doped film reached46%. PTZ-2Andoped device exhibited the maximum current efficiency of13.06cd A-1, and themaximum brightness of51145cd m-2. More importantly, the device possessed highefficiency stability. The current efficiency of13.02cd A-1and12.03cd A-1was stillretained at high brightness of10000cd m-2and50000cd m-2, respectively.2. A near-infrared (NIR) compound, PTZ-BZP, was designed and synthesizedbased on phenothiazine and benzothiadiazole. The PTZ-BZP film displayed strongNIR fluorescence with an emission peak at700nm, and the corresponding quantumefficiency reached16%. The EQE of the undoped PTZ-BZP device was1.54%, whichwas among the highest results of undoped NIR fluorescent OLEDs reported so far.Notably, in the device, a high radiative exciton ratio of48%was observed. The study could provide new ideas for the design of efficient NIR-fluorescent molecules byemphasizing the full use of both singlet and triplet excitons.3. The high-lying RISC process in hot exciton mechanism was investigatedthrough various photophysical characterization methods. Firstly, the energy level ofT1state was determined through phosphorescence-sensitized technique, and the largeS1-T1energy gap in hot exciton materials demonstrated RISC process could not occurbetween lowest excited states. Secondly, the transient PL spectra results confirmedthere is a nano-second timescale delayed fluorescence in hot exciton materials,implying Tn→S1RISC is a fast kinetics process. In addition, we calculated the ΦRISC(Tn→S1) and ΦTIC(Tn→T1), and in some hot exciton materials ΦRISC/ΦISCcould reach91%. The calculation of each quantum efficiency further proved the rationality of hotexciton mechanism.