Organic Boron-containing Optoelectronic Functional Materials:Design,Synthesis And Applications

Organic?-conjugated materials have attracted widespread attention owing to the promising applications in the fields of organic electroluminescent devices,organic field-effect transistors,organic photovoltaic cells and organic sensors.Heteroatoms play an important role in regulating the molecular electronic structure for realization of high performance and multifunctionality by their special orbital interaction with the?-conjugated system and spatial structure characteristics.Organic boron-doped?-conjugated materials are representative.Boron atom exhibits strong electron-withdrawing ability because of the natural vacant p-orbital for atom-level acceptor and effective conjugation with attached?-conjugated structures for generating many interesting optoelectronic properties.In addition,it also exhibits strong Lewis acidity due to the vacant p-orbital,which is easy to complex with the Lewis base for modification of optoelectronic properties.The lone pair of electrons of oxygen or nitrogen atoms can undergo a complexation with boron atom,forming a stabilized four-coordinated structure and fixing the entire molecular skeleton in a plane and effectively enhancing the interaction of the system.In this thesis,boron element is used to tune the optoelectronic properties of organic photoelectric functional materials for improving the solution-processible ability,luminescent efficiency and afterglow properities.The specific contents are as follows:(1)Generally,the boron-containing derivatives exhibit poor solubilities because of the vacant p-orbitals of boron atom should be protected by the large hindrance steric and rigid groups.To improve the solubility of boron-containing derivatives and explore their application potential in solution-processed organic light-emitting diodes,four boron-containing molecules were successfully prepared by using three-coordinated structure and star-shaped and asymmetric molecular design strategy in this chapter.With a combined method of quantum chemistry calculations and experiment,the thermal,photophysical and electrochemical properties of these newly prepared compounds have been investigated systematically.They exhibit good solubilities,stabilities and high triplet energy levels(>2.8 e V).The solution-processed blue and white Ph OLEDs hosted by this kind of boron-containing molecules displayed high device efficiencies.The asymmetric materials-based devices achieved the highest performance with maximum current efficiencies(CEs)of 38.9 and 37.0 cd A^-1,power efficiencies(PEs)of 30.5 and 19.4 lm W^-1,and external quantum efficiencies(EQEs)of 18.5%and14.5%,respectively.These results demonstrate that the star-shaped and asymmetric molecular design strategy can effectively improve solubilities of the boron-contaning molecules,promoting the development of boron-containing compounds in solution-processed organic light-emitting diodes.(2)Developing high-performance red TADF materials still remain a great challenge due to the dominated nonradiative transition according to the energy gap law.To simultaneously improve the luminescent efficiency and solubility,a series of boron-containing red TADF materials were constructed by introducing the electron-withdrawing difluoroboron?-diketonate structure,which can form strong charge transfer with electron-donors to reduce E_g and?E_ST.These molecules exihibt good solubilities and film-forming properties and high luminescent efficiency up to 86%in the doped film state due to the strong hydrogen bond between the difluoroboron groups to suppress the non-radiation transition.The solution-processed devices using these materials as emitters show maximum EQE of 8.2%with red emission peak at 605 nm and a turn-on voltage of 4.5 V.These performace are the highest results among the reported solution-processed red OLEDs.(3)In this chapter,the four-coordinated boron structure has been introduced into organic afterglow molecules to improve the afterglow efficiency.A thermally activated afterglow(TAA)molecule(DCz B)was designed by directly connecting the electron-donating carbazole and electron-withdrawing?-diketone difluoroboron to reduce the?E_ST and E_TD for effective excitons transformation between S₁,T₁ and T₁^*with the aid of the thermal energy fluctuation.More importantly,spin-forbidden triplet excited state excitons can be transformed to the spin-allowed singlet state,resulting in dramatically improvement of organic afterglow efficiency.Difluoroboron can form strong intermolecular hydrogen bond,which can suppresses the non-radiative transition effectively.Therefore,the exciton transformation and the suppression of the non-radiative transition can significantly improve the organic afterglow efficiency.The afterglow efficiency is up to 45%,which is among the highest efficiencies of organic single-component afterglow molecules reported so far.The TAA molecule with high afterglow efficiency was successfully applied in afterglow lifetime imaging and visual temperature detection.(4)The organic afterglow molecules with excited-state proton-transfer properties exhibit high-efficiency afterglow because the promoted ISC process that can realize the efficient accumulation of triplet excitons.The excited-state proton-transfer is a photochemical process,which can produce the tautomerism of enol and ketone structure under illumination with different electronic structures of the excited states to promote ISC process.Tri-mode afterglow with TADF,RTP and OURTP is achieved by the strong hydrogen bonding of difluoroboron groups.These materials show long lifetime up to0.65 s with high afterglow efficiency over 30%.In addition,the methyl-substituted molecule of Cz BNHPh CH₃ exhibits obviously photo-activated afterglow property with the lifetime from 94 ms to 0.61 s after photoactivation,and white afterglow with the CIE coordinate of(0.31,0.31),which is very close to the standard white emission with the CIE coordinate of(0.33,0.33).This study enriches the organic long afterglow systems and provides an important reaserch guideline for designing white afterglow materials.