Novel Aggregation-Induced Emission Luminogens:Synthesis,Mechanism And Application

With the development of society and science,organic luminescent materials have played an important role in various fields such as energy,environment,health and national defense,making human life more colorful.Generally,organic luminescent materials are used in aggregate or solid state.However,most conventional luminescent materials possess large planar?-conjugated structures and suffer from serious aggregation-caused quenching?ACQ?effects,which greatly limit their practical applications.Aggregation-induced emission?AIE?,as another important photophysical phenomenon,can essentially solve the ACQ problem.This breakthrough will open new avenues towards highly efficient organic luminescent materials,and holds great scientific value and technological importance for practical applications.With nearly 20-year booming development,AIE research has made great achievements,becoming a cutting-edge scientific field pioneered by Chinese scientists.Even so,there are still some new challenges and basic scientific problems in AIE field.For example,the mechanisms of nonconventional AIE luminogens?AIEgens?need to be further improved;multi-functional AIE materials need to be further developed;the applications of AIEgens in biological imaging,organic light-emitting diode?OLED?and stimulus response still have a wider room for development.Based on the above scientific background,the studies presented in this thesis are as follows:In Chapter 2,we report a novel AIE system based on benzo[b]thiophene S,S-dioxide.The steric and electronic effects on the AIE property are elucidated through tailored structural management,spectroscopy and crystallography analyses,and theoretical calculations.The development of new AIEgens is conductive to gaining deep insight into AIE mechanism and creating efficient functional materials.These new AIEgens show excellent biocompatibility and low cytotoxicity,and can specifically stain lipid droplets in living cells,with unique enhanced fluorescence intensity as the increase of scan numbers.These novel AIEgens will be promising candidates for high-tech applications in materials science and biotechnology.In Chapter 3,the photodimerization reactions of benzo[b]thiophene S,S-dioxide derivatives are studied,based on which a new type of dynamic crystal photoactuator is constructed.Interestingly,obvious hopping,bending and splitting mechanical behaviors accompanied by significant fluorescence enhancement are observed in P-BTO crystals under UV light irradiation,which is closely related to the formation of photodimer 2P-BTO.Even though 2P-BTO is poorly?-conjugated due to the central cyclobutane ring,it still emits brightly in solid state with a high fluorescence quantum yield of 96.2%,exhibiting typical AIE property.By combining the photophysical measurements,crystal structure analysis and theoretical calculations,it is found that the excellent solid-state emission of 2P-BTO is mainly attributed to the synergistic effect of intramolecular through-space conjugation and the restriction of intramolecular motions.This work provides a new method for constructing highly efficient nonconventional AIEgens.Moreover,the AIE effect is considered to contribute significantly to visually monitor the transformation process of the molecular motions into the macroscopic scale in real-time,which has great application prospects in smart devices and bionic science.In Chapter 4,we endow AIEgens with thermally activated delayed fluorescence?TADF?characteristic,which should be a feasible strategy to fully utilize the electrogenerated triplet excitons and fabricate high-performance OLED.Two new luminogens?DBT-BZ-PXZ and DBT-BZ-PTZ?with an unsymmetrical D-A-D?structure based on the central benzoyl acceptor core are synthesized.They simultaneously display AIE and TADF characteristics with excellent solid-state fluorescence quantum yields and high exciton utilization,which can effectively alleviate the exciton annihilation processes.Then,using DBT-BZ-PXZ and DBT-BZ-PTZ as emitting layers,we fabricate a series of doped and nondoped OLEDs.Thereinto,the doped device based on DBT-BZ-PXZ shows excellent electroluminescence performance with high external quantum(?_ext),current??_C?,and power??_P?efficiencies of up to 19.2%,60.6 cd A^?1and 59.2 lm W^?1,respectively.The nondoped OLED based on DBT-BZ-PTZ is turned on at low voltage of 2.7 V and affords?_ext as high as 9.7%with very small efficiency roll-off at 1000 cd m^?2.These new luminogens with AIE and TADF properties possess great potential in developing high-performance OLED.In Chapter 5,we obtain another unsymmetrical molecule,DBT-BP-DMAC,exhibiting AIE,TADF and mechanoluminescence properties.By deeply analyzing the transient fluorescence spectra of DBT-BP-DMAC in dilute tetrahydrofuran solution,aggregate and solid film,it is found that the AIE and TADF characteristics are not absolutely independent and the formation of molecular aggregates plays a key role in endowing luminogens with delayed fluorescence.Actually,DBT-BP-DMAC shows unique aggregation-induced delayed fluorescence?AIDF?feature.The nondoped and doped OLEDs based on DBT-BP-DMAC have been fabricated,in which 4,4?-bis?carbazol-9-yl?biphenyl?CBP?is selected as the suitable host material.With the increase of doping concentrations,the turn-on voltage decreases slightly and the maximum luminance advances significantly.Meanwhile,the maximum external quantum effciencies are lowered moderately,but the efficiency roll-off can be redcued to some extent at high luminance.The nondoped OLED based on DBT-BP-DMAC exhibits a low turn-on voltage of 2.7 V,giving the maximum luminance of 27270 cd m^?2 and excellent?_ext,?_C,?_P of up to19.2%,60.6 cd A^?1 and 59.2 lm W^?1,respectively,with nearly null efficiency roll-off at 1000cd m^?2.These results further demonstrate that the AIDF materials have become promising candidates for fabricating efficient nondoped OLED with small efficiency roll-off.In Chapter 6,we systematically explore the underlying mechanism of AIDF phenomenon.DMF-BP-PXZ,DPF-BP-PXZ and SBF-BP-PXZ molecules with unsymmetrical D-A-D?configurations are synthesized.They exhibit typical AIDF effects,that is,these molecules show weak emissions and negligible delayed fluorescence in dilute solutions but strong emissions with prominent delayed components upon aggregate formation.Based on the photophysical measurements and theoretical calculations,it is found that the AIDF phenomenon can be mainly ascribed to the suppression of the internal conversion channel and the promotion of intersystem crossing process in solid state,which is conducive to generating the triplet excitons.Based on the small singlet-triplet energy splitting,triplet state can efficiently transfer to the radiative singlet state through reverse intersystem crossing process,yielding significant delayed fluorescence.In addition,theoretical calculations reveal that the strong emission of DMF-BP-PXZ is mainly derived from the higher energy electronic excited state?S₂?,exhibiting anti-Kasha behavior.Owing to the excellent AIDF property,the maximum?_ext,?_C,?_P of 14.3%,41.6cd A^?1 and 45.0 lm W^?1,respectively,with small efficiency roll-off of 1.4%at 1000 cd m^?2 are attained in nondoped OLED.The comprehensive insights into AIDF phenomenon in this work will rationally provide the design principles for novel AIDF materials,effectively promoting the development of OLED field.