Design,Synthesis And Photoelectric Properties Of Deep Blue Light Emitting Materials Based On Phenanthroimidazole Derivatives

Organic light-emitting diode,or OLED for short,owns the advantages of self-illumination,high sensitivity,low cost,wide viewing angle and light weight,and it have shown broad application prospects in the field of future-oriented lighting and display technology.As a technology that uses RGB three primary colors light mixing to achieve color reproduction,the development of OLED is highly dependent on the technological progress of red,green and blue light-emitting materials.In the full-color display of OLED,the display gamut is mixed by the light emitted by the RGB pixel level devices,and the color purity of the three primary colors directly determines the size of the gamut.With the continuous update and iteration of display technology,the market demand for ultra-high definition and ultra-wide color gamut display is also increasing.In 2012,the International Telecommunication Union（ITU）proposed a new standard for the CIE coordinates of the three primary colors of light,of which the CIE coordinates of the standard blue light are（0.131,0.046）.Since the band gap of blue light emitting materials is inherently high,achieving blue light emission with high electroluminescence efficiency and high color saturation is a very challenging task.At the same time,because the sensitivity of the human eye to blue light is lower than that of green light and red light,it also puts higher requirement on the luminance of blue light devices.Phenanthroimidazole group is composed of phenanthrene and imidazole ring and has a rigid planar structure.Phenanthroimidazole possesses high luminescence efficiency and short conjugation length,which makes it a good group for constructing high-efficiency deep blue materials.There is a large energy level difference between the lowest singlet（S₁）and triplet（T₁）energy levels of phenanthroimidazole,and the reverse intersystem crossing process at high energy levels could occur through the hot exciton mechanism,which could effectively inhibit the accumulation of triplet excitons in the device and obtain efficient exciton utilization.Although researchers have developed some phenanthroimidazole derivatives with blue-deep blue emission,there are still limited materials which could meet the requirements of BT.2020.The aim of this paper is to develop deep blue materials with high efficiency and color purity.A series of deep blue light-emitting materials were designed and synthesized by using phenanthroimidazole as the basic molecular building block and introducing relatively weak electron-withdrawing groups;the effective conjugation length,intramolecular charge transfer property and luminescence efficiency of the molecules were regulated by changing the different substituent linking positions;with further research on their aggregation structure and device performance,the electroluminescence device that meets the requirements of BT.2020 was obtained,which provided a new design strategy for the realization of deep blue light-emitting devices with high efficiency and high color purity.The research content of this paper could be summarized as follows:1.Using phenanthroimidazole as donor and dibenzothiophene as relatively weak electron acceptor,the compounds P2DBT and P3DBT with different conjugation lengths were constructed by regulating the connection between donor and acceptor;by introducing cyanide group into the phenyl ring at N1 position of phenanthroimidazole group in P2DBT and P3DBT,CP2DBT and CP3DBT were obtained respectively.The results of theoretical calculation and experimental characterization showed that the large torsion angle between phenyl ring at N1 position of phenanthroimidazole and imidazole ring（>70^o）,the introduction of cyan into the N1 benzene ring made LUMO of CP2DBT and CP3DBT mainly distributed in cyanobenzene units,which resulted in a low overlap of the frontier orbitals of these two molecules and a decline in photoluminescence efficiency;owning to the moderate torsion angle（～35^o）between the benzene ring at C2 position and imidazole ring in P2DBT and P3DBT,and the weak electron-withdrawing strength of dibenzothiophene,they both showed higher radiation transition rate in solid state than CP2DBT and CP3DBT.Compared with P3DBT,P2DBT possessed shorter conjugations on the C2 axis of phenanthroimidazole,larger band gap and bluer light color thanks to the different positions of dibenzothiophene connection.The non-doped device based on P2DBT achieved the highest efficiency and color purity,the maximum external quantum efficiency and CIE coordinate were8.9%and（0.16,0.09）,respectively,and the maximum luminance reached 40000 cd m^-2.2.In order to further regulate intramolecular CT and expand the material system,phthalide group was used as the blue light material acceptor unit for the first time.The D-A type molecule PPT and D-π-A type molecules PPPT and PAPT were obtained by connecting phthalein directly or through benzene bridge and anthracene bridge with the donor phenanthroimidazole.Benzene bridge and anthracene bridge in PPPT and PAPT could increase the overlap of frontier orbits,gave the emitters the properties of hybrid localized charge transfer excited state and promote the radiation transition rate.The torsion angles of anthracene unit in PAPT were relatively large（>75^o）,resulting in inhibition of the intramolecular CT of phenanthroimidazole to phthalide in this molecule.The analysis of single crystal structure showed that the hydrogen bond interaction in PPT and PPPT could alleviate the vibration relaxation and lead to high PLQY in solid state.The PLQY of PAPT-based non-doped and doped films reached72.1%and 80.8%,respectively.The maximum EQE of the PAPT-based non-doped device was 7.5%,the CIE coordinates were（0.162,0.188）,and the maximum luminance exceeded 30,000 cd m^-2.In the doped device,the maximum EQE of PAPT increased to 10.2%,FWHM was only 53 nm,and CIE coordinates were（0.151,0.085）.Theoretical calculation and experimental characterization showed that the devices could achieve relatively high exciton utilization through hot exciton mechanism.3.In order to achieve the blue shift of emission by reducing the intramolecular CT intensity and the limitation of conjugations,the electroneutral triphenylbenzene（TPB）with distorted structure was connected with phenanthroimidazole,and the compounds TPP and PTP were obtained.The twisted configuration around the tri-substituted benzene ring in TPB inhibited the molecular conjugation of TPP and increased the band gap.The PLQY of the non-doped films based on TPP and PTP reached 72.8%and83.5%,and the radiation transition rates k_r were as high as 7.00×10⁸ s^-1 and 6.68×10⁸s^-1,respectively.Owing to the larger band gap of TPP,non-doped device based on TPP possessed bluer luminescence,the luminescence peak was at 416 nm,the FWHM was55 nm,and the color coordinate were（0.16,0.05）,which was close to the blue light standard of BT.2020（0.131,0.046）.At the same time,the maximum EQE of the TPP-based non-doped device reached 7.2%,which could maintain as 4.1%at the luminance of 1000 cd m^-2,and the maximum luminance reached more than 10000 cd m^-2.In this work,by connecting neutral groups on C2 axis of phenanthroimidazole and increasing torsion angles,deep blue light-emitting materials with large band gap were obtained.4.After summarizing the previous three parts of work,the biphenyls connected to the phenzimidazole C2 long axis in TPP were retained,and methyl groups with different amounts and substitution positions were introduced into the biphenyl unit,and four blue light-emitting materials PP1M,P2MP,PP3M and P4MP were obtained.Among them,PP3M showed the bluest emission and large oscillator strength.The main luminescence peaks of PP3M in non-doped and doped films were at 439 nm and 400 nm,and the PLQY reached 85.3%and 86.3%,respectively.In the non-doped device,the maximum external quantum efficiency of PP3M reached 7.6%,and the CIE coordinates of electroluminescence spectrum were（0.16,0.08）,which indicated deep blue emission was achieved.In the doped device,the spectrum of PP3M was blue-shifted and narrowed,the emission wavelength was at 420 nm,while the FWHM was only 53 nm.The CIE coordinates of PP3M-based doped device were（0.16,0.04）,which met the BT.2020 definition criteria for high saturation blue light.The EQE of the device was as high as 7.2%,and the maximum luminance exceeded 6000 cd m^-2,which was at a relatively good level of deep blue devices with the same color purity.Combining theoretical calculation and experimental results,it was found that PP3M realized high device efficiency and low efficiency roll-off mainly through hot exciton mechanism.