Excitated State Regulation And Properties Of Multiple Resonance Thermally Activated Delayed Fluorescence Materials

Organic light-emitting diodes（OLEDs）have been widely utilized in various electronic devices such as smartphones,computers,televisions,and watches due to their numerous advantages including high contrast,wide viewing angles,low power consumption,fast response times,and foldability.In 2012,the International Telecommunication Union（ITU）radiocommunication sector introduced the Broadcast Television 2020（B.T.2020）standard for the next generation of ultrahigh-definition（UHD）displays,which set higher requirements for the performance parameters of displays.Consequently,UHD display has become the future pursuit of OLEDs,posing greater challenges to the performance of organic light-emitting materials.Currently,commercially produced OLEDs extensively use heavy-metal phosphorescent materials.However,the expensive production costs and pollution issues associated with heavy metals have hindered further development.Subsequently,the emergence of thermally activated delayed fluorescence（TADF）materials,known as the third-generation light-emitting materials,brought hope for addressing the shortcomings of phosphorescent materials.TADF materials can effectively utilize triplet excitons,and their device performance can rival that of phosphorescent materials.However,traditional TADF materials typically require a strong donor-acceptor（D-A）type structure to induce complete separation of the highest occupied molecular orbital（HOMO）and the lowest unoccupied molecular orbital（LUMO）,resulting in long-range charge transfer（LRCT）excited states,thus effectively reducing the singlet-triplet energy splitting(ΔE_ST).However,this structure design often leads to excessive broadening of the emission spectrum,which is not conducive to achieving the goals of future UHD displays.Fortunately,multiple resonance induced thermally activated delayed fluorescence（MR-TADF）materials not only inherit the high efficiency advantages of TADF but also maintain narrowband emission characteristics through the design of rigid molecular skeletons,quickly becoming a hot topic in scientific research and industrial applications.Currently,commercialization of blue MR-TADF materials is quite mature,but the development of long wavelength materials such as green,yellow,and red light is still limited.Based on these issues,in this paper,the authors have enriched and expanded the design concepts of frontier molecular orbital engineering（FMOE）.Under this strategy,a series of blue-green,green,and yellow light-emitting materials with high efficiency and narrowband emission have been synthesized successively,and corresponding OLED devices have been prepared,demonstrating excellent device performance.The main research content is as follows:In Chapter II,the author continued the design concept of frontier molecular orbital engineering by introducing donors at the the meta-position of the B-substituted benzene ring（meta-HOMO）in the MR framework,resulting in a certain degree of red-shift in the spectrum.Furthermore,to prevent excessive broadening of the full-width at half-maximum（FWHM）due to the enhanced charge transfer（CT）excited states,the author introduced aπ-bridge between the MR core and the donor,weakening the CT state and maintaining narrowband emission properties.To realize the introduction of donor more conveniently,the author proposed a method of precise bromine functionalization.By applying bromination reactions to the MR framework and introducing bromine atoms at meta-HOMO position,various functional groups can be introduced into the MR framework.To comprehensively analyze the effects of group introduction on molecular performance,multifarious functional groups have been introduced into the MR framework,such as donors,acceptors and moieties without obvious push-pull electron properties.Among them,the molecule m-DPAc P-BNCz,containing a 9,9-diphenylacridin substituent,exhibits blue-green light emission with a peak of 491 nm and a FWHM of only 26 nm in toluene solution.The OLED device prepared using it as an emissive material also exhibits bright blue-green emission,with a FWHM of only28 nm,and the average maximum external quantum efficiency（EQE）of multiple parallel devices also reaches 40.6%,demonstrating excellent device performance.In Chapter III,to further induce red-shift in the spectrum,the author extended theπ-conjugated system at the meta-HOMO and the para-position of the B-substituted benzene ring（para-LUMO）in the MR parent framework.This induces further expansion of the frontier molecular orbitals（FMO）,forming a mixture of short-range charge transfer（SRCT）and long-range charge transfer（LRCT）excited states,thereby producing a red-shifted emission spectrum.Additionally,the author tuned the molecular excited-state properties by doping nitrogen atoms at different positions,achieving precise regulation of the photophysical properties.To realize the aforementioned design concept,the author applies the classic Scholl oxidative coupling reaction in synthesis,enabling the MR framework to undergo polycyclization,and constructing a series of organic light-emitting materials with high efficiency and narrowband emission properties.Among them,OLED device fabricated using BN-TP-N3 exhibits pure green emission,with an emission peak at 524 nm,a FWHM of only 33 nm and CIE（Commission Internationale de L’Eclairage）coordinates of（0.23,0.71）,which is extremely adjacent to the standard green light CIE coordinates of（0.21,0.71）stipulated by the National Television Standards Committee（NTSC）.Furthermore,the devices also demonstrate a maximum EQE of up to 37.3%and relatively low efficiency roll-off.In Chapter IV,following the design concept proposed in Chapter III,the author introduced chiral hetero[6]helicene fragments into the MR skeleton,and successfully constructed the MR-TADF emitter BN-Py with circularly polarized luminescence（CPL）property.The enantiomers（P）-BN-Py and（M）-BN-Py show good stability and there is no racemization phenomenon under vacuum heating,which provides a possibility for the preparation of CPL-OLED devices.Finally,OLED devices prepared based on（P）-BN-Py and（M）-BN-Py show bright green emission,with an emission peak of 532 nm,a FWHM of38 nm,and CIE coordinate of（0.30,0.67）.In addition,the corresponding devices also exhibit clear CPL signals,with electroluminescence dissymmetry factors(g _ELs)of-4.37×10^-4/+4.35×10^-4for（P）-BN-Py and（M）-BN-Py.In Chapter V,to construct luminescent materials with longer wavelengths,the author directly introduced strong aromatic amine donors into the meta-HOMO position of the MR framework,forming strong charge transfer excited states.Simultaneously,merging the aromatic amine donors into the entire MR framework further expanded theπ-conjugated system,enhancing the CT state,ultimately achieving emission spectra into the yellow region.To implement this design strategy,the author first utilized the previously reported key intermediate,Dt Cz B-Bpin,to introduce nitro-containing groups at para-LUMO position.Subsequently,the nitro groups were reduced to aromatic amines using the Cadogan reduction reaction,forming aπ-conjugated system,and followed by the introduction of a normal-butyl protected imine group.Molecule BN-Cz synthesized based on this strategy exhibits bright yellow emission in toluene solution,with an emission peak of 551 nm and a FWHM of 49 nm.Additionally,through nitrogen atom doping,the author achieved spectral narrowing of BN-Cz.Another molecule,BN-Cb,has a FWHM of 41 nm.Finally,the sensitized OLED devices fabricated using BN-Cz and BN-Cb as emitters also exhibit excellent performance,with emission peaks at 560 and 556 nm,FWHMs of 49 and 45 nm,and maximum EQE of 32.9%and 29.7%respectively.