Synthesis And Photoelectric Performance Of Thermally Activated Delayed Fluorescence Materials With Multiple Charge-Transition States

In the past decade,organic thermally activated delayed fluorescence（TADF）materials have attracted wide attention and achieved rapid development due to their great prospects in the field of commercial organic light emitting diodes（OLEDs）.Organic TADF molecules can fully utilize both singlet and triplet excitons through reverse intersystem crossing（RISC）process to achieve highly efficient emission.In general,the molecules with intramolecular charge transfer（ICT）,namely TBCT-TADF type molecules,can be designed via single bonds orπ-conjugated bridges.As an alternating approach,molecules with through-space charge transfer（TSCT）can be constructed by non-conjugated bridges connecting closely spatial stacked donor（D）and acceptor（A）units.Besides,multiple-resonance（MR）induced TADF molecules can be designed via introducing opposite resonance boron and nitrogen atoms into polycyclic aromatic framework,without introducing D and A segments.This work is mainly focused on designing and synthesizing three types of organic TADF emitters with TBCT,TSCT and MR characteristics.Additionally,the luminescent mechanisms are explored deeply and these three kinds of TADF emitters are applied to OLEDs with high efficiency.In chapter 1,we briefly describe the emissive mechanisms of organic luminescent materials,the strategies of designing organic TBCT-,TSCT-and MR-TADF-type molecules,and their applications in the fields of OLEDs,pressure sensors,biological imaging and X-ray scintillator imaging.Finally,the main ideas of developing new efficient TBCT-,TSCT-and MR-TADF materials and the research content of this paper are described.In chapter 2,two new TADF molecules,t Bu In Pz-BO and In Pz-BO,are developed through linking new rigid polycyclic phenazine-derived donors with boron acceptor.Both emitters show fast reverse intersystem crossing rates and high PLQYs,ascribed to their high molecular structural rigidity.Consequently,the orange OLED fabricated with t Bu In Pz-BO exhibits a EQE_max of 22.0%with the electroluminescence peak at 585nm.Besides,the device displays extremely low efficiency roll-off.At high luminance of 1000 and 10000 cd/m²,the EQE values still maintain a high level of 20.2%and14.8%,respectively.This work reveals the great potential of the new phenazine-derived donors for constructing highly efficient TADF emitters.In chapter 3,the isomers with multifunctional features,TRZ-c-BPXZ and TRZ-a-BPXZ,with benzophenoxazin c-BPXZ and a-BPXZ as electron-donating cores,and triphenyltriazine as electron-withdrawing acceptor,are designed and developed.Significantly,TRZ-c-BPXZ not only displays on-off exchangeable crystal-dependent TADF character,but the on-off TADF switchable behavior in diverse aggregation states is able to reversibly controlled by multiple channels,such as grinding,heating,recrystallizing,fuming,vacuum evaporation and sublimation.Through the comprehensive theoretical simulation,as well as thoroughly research of stacking arrangements and molecular conformations of single crystals,the results unveil that not only is the complanate donor essential to devising TADF emitters,but also the quasi-equatorial conformation of emitters is crucial.And the properπ···πpacking in dimers also lengthens the delayed lifetime.These results shed light on the underlying mechanism of the switchable photophysical characteristics and on-off tunable TADF performance.To the best of our knowledge,TRZ-c-BPXZ is the first example of realizing on-off controllable TADF behavior by multiple convenient methods.In chapter 4,two novel TADF emitters,10-（acenaphtho[1’,2’:5,6]pyrazino[2,3-f][1,10]phenanthrolin-12-yl）-10H-phenoxazine（APPT-PXZ）and7-（acenaphtho[1’,2’:5,6]pyrazino[2,3-f][1,10]phenanthrolin-12-yl）-7H-benzo[c]phenoxazine（APPT-BPXZ）,are developed via combining the strong electron-donating and electron-withdrawing units.Both target compounds exhibit orthogonal configurations to decrease singlet-triplet splitting energy(ΔE_ST)for the remarkable TADF property.Furthermore,APPT-PXZ and APPT-BPXZ-based devices realize the EQEmaxs of 7.2%and 2.0%with the electroluminescence peaks at 640 nm and 665 nm,respectively.This work facilitates the development of the deep-red OLEDs.In chapter 5,a series of TBCT-TADF-type molecules were synthesized by introducing nitrogen atoms into dipyridinequinoxalin acceptors with diphenylacridine and phenoxazine donors,respectively.The emitters show polymorph,multicolor mechanochromic（MCL）,acidochromic,aggregation-induced luminescence（AIE）and TADF characteristics.Polymorph-dependent TADF behavior and multicolor MCL mechanisms are well explained via connecting thoroughly analyzing crystal staking modes and intermolecular interactions,and detailed theoretical calculations.Their applications in ink-free rewritable paper and OLEDs are demonstrated.In chapter 6,two novel TSCT-TADF emitters with the combination of two position isomeric polycyclic oxygen-bridged triphenylamine donors,oxygen-bridged triphenylborane acceptor and triptycene bridge,are developed.The space-confined face-to-face D-A alignment with improved parallelism and short distance enhanced the spatial D-A electronic interactions and improved PLQYs.This is the first example that the TSCT-based molecules not only realize 90%PLQYs in neat films,but also achieve100%PLQY in doped films.Meanwhile,both TSCT emitters are successfully applicated in the field of OLEDs.In chapter 7,helicene-based emitters with unique inherent circularly polarized luminescence（CPL）are promising yet remain a formidable challenge for highly efficient circularly polarized organic light emitting diodes（CP-OLEDs）,ascribed to their tough synthesis,low luminescent efficiency,and readily racemization in thermal deposition process.Herein,a pair of helicene-based enantiomers,namely（P）-helicene-BN and（M）-helicene-BN,were designed and synthesized,which merge helical chirality and the B/N/S inserted polycyclic aromatic framework to concurrently feature CPL and narrow TADF characteristics.Benefiting from the excellent thermal/photophysical/chiroptical properties,the narrowband green CP-OLEDs based on the enantiomers achieved maximum EQE_max of up to 31.5%,and dissymmetry factor(|g _EL|)of 2.2×10^-3.This work reveals the great potential of helicene-based emitters in CP-OLEDs.