Novel Organic Blue-Emitting Molecules: Design,Excited-State Modulation,and Optoelectronic Applications

Organic light-emitting diodes(OLEDs)shine in the field of display and lighting applications,due to their unique advantages such as light-weight,low power consumption,high contrast,self-luminescence,and flexibility.As the core component of OLED,organic electroluminescent materials have always been a research hotspot in both academia and industry.At present,improving the performance of blue OLED and developing pure organic electroluminescent materials are the innovation directions of OLED display technology,and are also important breakthroughs for our country to break the foreign technology monopoly in the new generation of display fieldFrom a material design perspective,the development of organic light-emitting materials with high photoluminescence efficiency(?_PL)in solid-state as well as high exciton utilization efficiency(?_r)is the key to achieving high-performance OLEDs.However,traditional organic light-emitting materials can emit strongly in dilute solutions,but in the solid-state,they will face serious aggregation-caused quenching(ACQ)problems,which greatly limits their application in practical scenarios.Aggregation-induced emission(AIE),proposed by Chinese scientist Prof.Tang Ben Zhong,can essentially solve the ACQ problem,and open new avenues towards organic light-emitting materials with high?_PL in solid-state.On the other hand,traditional fluorescent materials can only use singlet excitons to emit light,and the theoretical limit of?_r is only 25%.How to effectively use the non-radiative triplet excitons is the key to improving?_r.At present,fluorescent materials that can achieve 100%?_r mainly include thermally activated delayed fluorescence(TADF)materials,hot exciton materials,and stable radical luminescent materials using doublet state.Among them,the hot exciton mechanism,proposed by Prof Ma Yuguang,is to realize the triplet excitons utilization through the high-energy reverse intersystem crossing(h RISC)process,which has great potential in the blue OLED.Based on the above research background,this thesis attempts to combine AIE concept and hot exciton mechanism to give full play to their advantages,and thus develop new blue-emitting molecules with independent intellectual property rights,and finally achieve high-performance blue OLEDs with both high?_PL and high?_r.The specific research content is as follows:In Chapter 2,phenanthroimidazole(PI)is chosen as the building block of the organic blue-emitting molecules,and then benzonitrile group with electron-withdrawing ability and the tetraphenylethylene(TPE)conjugation group with AIE property are introduced into the N1 and C2 positions of PI,respectively.Further,by changing the meta/para connection mode of the substituted groups,six new blue-green/sky blue luminescent materials are successfully designed and synthesized.Based on the experimental and theoretical calculation,it is found that all molecules show high?_PL in solid-state due to their AIE properties,and the different spatial structures of N1 and C2 positions separate the charge transfer(CT)state component and the locally excited(LE)state components,which can suppress emission redshifts while raising the?_r.Finally,by maximizing the advantages of LE and CT state components,the non-doped OLEDs based on pp CTPI exhibit the best EL performance with maximum luminance(L),current efficiency(?_c),power efficiency(?_p)and external quantum efficiency(?_ext)of up to31070 cd m^?2,18.46 cd A^?1,16.32 lm W^–1 and 7.16%,respectively,and very small efficiency roll-off of 4.0%at 1000 cd m^?2 luminance,which is the best OLED performance result based on TPE-containing organic emitters.In Chapter 3,the replacement of C6/C9 positions of PI is investigated.The molecular excited-state is fine-tuned by introducing substitution groups with different conjugate degrees and electron-donating abilities.Finally,three blue/deep blue emitters with typical LE/CT state distributions(E_LE?E_CT,E_LE?E_CT,and E_LE?E_CT)are successfully designed and synthesized.Photophysical measurements show that all three emitters show prominent aggregation-enhanced emission(AEE)and hybridized local and charge-transfer(HLCT)properties.The“hot exciton”mechanism based on these emitters reveal that a balanced LE/CT distribution can promote the h RISC process by enhancing the SOC between the triplet and singlet,and thus improve the?_r.As a result,the?_r of 2CzPh-CNNPI is close to 100%,and the maximum?_ext of2CzPh-CNNPI-based non-doped blue OLED(CIE_x,y=0.154,0.143)reaches 5.09%.On the other hand,the doped deep blue OLEDs based on 2CzPh-CNNPI with different doping concentrations achieves maximum?_extof 8.42%at CIE_x,y=0.153,0.080 and?_extof 9.02%at CIE_x,y=0.155,0.102,which are among the state-of-the-art blue OLEDs in the relative color gamut.In Chapter 4,continuing the structural regulation idea of 2CzPh-CNNPI,a sky-blue emitter 2TPA-CNNPI with balanced LE/CT state distribution is successfully designed and synthesized.The more accurate calculations of the insulation excitation energy-level distributions,the corresponding orbital properties and SOC values,and the excited-state dynamics parameters of 2TPA-CNNPI further demonstrate that,the balanced LE/CT state distribution can accelerate the h RISC process with an enlarged SOC between ³LE and ¹HLCT state.Thanks to the high?_r,the maximum?_ext values of non-doped sky-blue OLED(CIE_x,y=0.155,0.268)and doped sky-blue OLED(CIE_x,y=0.153,0.202)are as high as 5.04%and7.62%,respectively.In view of the blue emission and high?_r features of 2CzPh-CNNPI and2TPA-CNNPI,they are used as the host material for hot exciton-sensitized green and red fluorescent OLEDs,and the corresponding maximum?_ext reach 8.28%and 6.17%.In addition,using the incomplete energy transfer between the host and guest materials,the host-sensitized white OLED also achieves a high maximum?_ext of 8.57%.The maximum?_ext of all the sensitized OLEDs are more than 5%,which not only proves the existence of the h RISC process of hot exciton materials,but also provides a feasible strategy for high efficiency full-color host-sensitized fluorescence OLEDs.In Chapter 5,referring to the spatial structural characteristics of the N1 and C2 positions of PI,the Tri PE groups are introduced into the C5/C10 positions of PI for conjugation extension,and p-cyano and o-methyl groups are inserted to the phenyl at the C2 position for twisted D–A structure.Finally,blue emitter 2Tri PE-BPI-MCN is successfully designed and synthesized.Compared with its matrix framework 2Tri PE-BPI,the spatial structure of 2Tri PE-BPI-MCN can also separate the LE state component and CT state component.Based on the theoretical and experimental investigations,it is found that the LE and AIE characteristics of the S₁ of 2Tri PE-BPI-MCN make it exhibit blue emission and high?_PLin solid state,while the high-lying CT state enables it to achieve high?_rthrough the hot exciton process.As a consequence,the 2Tri PE-BPI-MCN-based non-doped blue OLED(CIE_x,y=0.153,0.147)shows a maximum L,?_c,?_pand?_ext of up to 6129 cd m^?2,4.72 cd A^–1,3.17 lm W^–1 and 4.60%,respectively,and extremity small efficiency roll-off of 4.0%.Further,based on the correspondence between the spatial structure characteristics of substitutes of PI at different positions and the LE/CT state distribution,and from the functional angle of the long-axis skeleton,the short-axis skeleton and the long and short axes connecting elements,a"crossed long and short axes"structural model is extracted for constructing high-performance organic blue electroluminescence materials.In Chapter 6,based on the proposed crossed long and short axes structural model,the carbazole with shorter conjugate degree and rigid skeleton is selected as the the building block,and the C2/C7 and N9 positions of carbazole are decorated to achieve the separation of LE and CT states.Finally,ultraviolet(UV)emitter 2Bu Cz-CNCz is successfully designed and synthesized,and its emission peak and?_PL in doped film are 395 nm and 70.6%,respectively.Theoretical calculations combined with experimental measurements show that the suitable conjugate degree,planarity and structural rigidity of 2Bu Cz-CNCz are the intrinsic causes of its high-quality UV emission.Further,the crossed long and short axes structures can not only improve the carrier injection capability by using the D–A structure at the short axis direction,but also maintain the LE dominant emission characteristic at the long axis direction.Moreover,the high-lying CT state component can activate the hot exciton channel to improve?_r.As a consequence,the maximum?_extof 2Bu Cz-CNCz-based doped UV OLED(CIE_x,y=0.161,0.031)is up to 10.79%,representing a record high efficiency of UV OLEDs.And the efficiency roll-off is also relatively small.On the other hand,the 2Bu Cz-CNCz-based non-doped near UV OLED(CIE_x,y=0.157,0.050)also exhibits a high?_extof 5.24%and a very flat efficiency roll-off.These results not only further verify the feasibility of crossed long and short axes molecular design strategy,but also show the great potential of a new generation of high-performance OLED based on hot exciton materials.