Pure Organic Room Temperature Phosphorescent Electroluminescent Materials Based On Phenothiazine Derivatives

Nowadays,organic light emitting devices(OLEDs)have been commercialized after decades of vigorous development.Organic light-emitting materials are crucial to the development of OLEDs.Up to now,there have been three generations of well-developed organic light-emitting materials.The first generation is fluorescent light-emitting materials.Devices based on fluorescent light-emitting materials have good stability and low efficiency roll-offs.But the maximum exciton utilization of fluorescent emitter is only 25%.In the second generation of phosphorescent heavy metal complexes,the exciton utilization limit is promoted to 100%.However,the preparation of these heavy metal complexes is both difficult and expensive.And the devices can not reach the same stability as those based on fluorescent materials.The third generation of thermally activated delayed fluorescent(TADF)materials,as pure organic small molecules,can reach 100%exciton utilization through the reverse intersystem crossing process.TADF materials have made a great breakthrough in the development of organic light-emitting materials.In addition to these three generations of materials,pure organic room-temperature phosphorescent(RTP)materials have also attracted increasing attention as promising luminescent materials.Back in the early days,most RTP molecules in film state suffered from low quantum yields due to their long lifetimes of milliseconds or even seconds.In this circumstance,the triplet excitons could be easily quenched by the environment.Therefore,they were usually used in data recording,bioimaging,chemo-and biosensors,optical storage,anti-counterfeiting encryption,and so on.In recent years,many OLEDs based on organic RTP materials have also been reported,but the performance of which was usually unsatisfactory.We have focused on this potential area and made a lot of efforts to prepare pure organic RTP devices.In this thesis,a series of phenothiazine derivatives with prominent RTP properties were synthesized.Based on these phenothiazine derivatives,a type of phosphorescent electroluminescent doped films was designed and corresponding highly efficient pure organic OLED devices were prepared.The main contents are as follows:1.In Chapter II,Ph-DPTZ,DPh-DPTZ,and DPTZN were synthesized by double-substitution of phenothiazine units on benzene,biphenyl,and naphthalene rings,respectively.As an electron-rich aromatic heterocyclic compound,phenothiazine has a strong electron-giving ability.In the molecule,the sp³ hybridized sulfur and nitrogen atoms made the two adjacent benzene rings form a large arc dihedral angle.The whole molecule showed a special non-planar butterfly-like conformation.Therefore,the backbone of phenothiazine was rigid and flexible simultaneously.The non-planar butterfly-like conformation could efficiently suppress the strong ?…? interactions during molecule stacking,which reduced the triplet-triplet annihilation(TTA)and facilitated the enhancement of phosphorescence emission.The three compounds were obtained by a very simple one-step Buchwald–Hartwig coupling reaction,separately.In this thesis,the host molecule triazine-benzimidazole derivative(PIM-TRZ)plays a very important role in the proposed electroluminescent film,so its synthesis process is also described in this chapter.2.In Chapter III,the properties of DPh-DPTZ and Ph-DPTZ were investigated.The two compounds had good thermal stability and could be used to fabricate OLED devices by vacuum evaporation.In the solid powder states,besides the fluorescence peak at around 460nm,DPh-DPTZ and Ph-DPTZ also exhibited phosphorescence at around 560 nm,with transient decay lifetimes of 260.0?s and 124.3?s,respectively.Two kinds of single crystals(crystal-B and crystal-Y)for DPh-DPTZ and one kind of single crystal for Ph-DPTZ were obtained.In single-crystal states,the phosphorescent emission intensity was enhanced for both emitters.And the phosphorescence emission intensity of the Ph-DPTZ crystal was weaker compared to DPh-DPTZ crystals.The photoluminescence spectra of the 10%DPh-DPTZ:PIM-TRZ doped film and the 10%Ph-DPTZ:PIM-TRZ doped film both had only one emission peak at 560 nm.And the long lifetimes were 17.8?s and 19.5?s for these two doped films,separately.The quantum yields of the 10%DPh-DPTZ:PIM-TRZ and 10%Ph-DPTZ:PIM-TRZ doped films were 46%and 44%,respectively.Therefore,two devices using 10%DPh-DPTZ:PIM-TRZ and 10%Ph-DPTZ:PIM-TRZ as the light-emitting layer were prepared,which achieved outstanding external quantum yields(EQEs)of 13.8%and13.0%,respectively.3.In Chapter IV,the aforementioned benzene and biphenyl rings were replaced with naphthalene rings to obtain DPTZN.DPTZN in the crystal state had not only a fluorescence emission peak at 465 nm but also a strong phosphorescence emission peak at 570 nm.A thin film of pure DPTZN was prepared by vacuum vapor deposition.A considerable percentage of phosphorescence emission at 570 nm was still observed in the thin-film state.The single crystal of DPTZN was also prepared,in which the DPTZN molecule had two different conformations,Cis-DPTZN and Trans-DPTZN.Through theoretical calculations,the T₁?S₀ transition of Cis-DPTZN possessed a charge transfer feature.The spin-orbit coupling(SOC)constant was 1.28 cm^-1for Cis-DPTZN.Therefore,this conformation was identified as the main source of DPTZN phosphorescence emission.Then[DPTZN/PIM-TRZ]-doped films with different doping concentrations were prepared.The lifetime of the doped films gradually decreased while increasing the DPTZN doping concentration.The emission peak of the 10%DPTZN:PIM-TRZ doped film coincided with the phosphorescence emission peak of DPTZN crystals,which was also located at 570 nm.The long lifetime of the 10%DPTZN:PIM-TRZ doped film was 87.0?s.The 10%DPTZN:PIM-TRZ based OLED achieved a maximum external quantum efficiency of 11.5%,along with a maximum power efficiency of 32.6 lm W^-1.4.In Chapter V,the devices were optimized further and high-performance OLEDs based on classic fluorescence emitters were constructed.Based on work in Chapter IV,Rubrene was introduced as a fluorescence emitter into the doped film with RTP properties to form a[PIM-TRZ/DPTZN/Rubrene]composite film system.Among them,PIM-TRZ was used as the host,DPTZN was adopted as the phosphor photosensitizer,and Rubrene served as the fluorescence emitter.The doping of Rubrene in the light-emitting layer resulted in a substantial increase in the device efficiency.The EQE of these devices decreased as the doping concentration of Rubrene increased,caused by the concentration quenching effect of Rubrene.The device showed the best performance when the Rubrene doping concentration was 0.3%.And the EQE of this device increased to 15.7%,compared to the phosphorescence OLED without Rubrene(11.5%).The EQE of the device without the sensitizer DPTZN was only 3.8%,indicating that DPTZN served as the bridge in the energy transfer from PIM-TRZ to Rubrene.The luminescence spectra and transient decay lifetimes of the[PIM-TRZ/DPTZN/Rubrene]doped films were measured.As the Rubrene doping concentration increased,the spectra gradually became narrower and eventually approached the emission peak shape of Rubrene.And the transient decay lifetime also gradually decreased.The UV absorption spectrum of Rubrene in the toluene solution and the steady-state luminescence spectrum of 10%DPTZN:PIM-TRZ had a large overlap,from which the F(?)rster radiuswas calculated to be 29.6 nm.A larger F(?)rster radius was beneficial for the energy transfer from DPTZN to Rubrene.Two single crystals of PIM-TRZ,crystal-Et OH and crystal-PE were prepared.Both of them had obvious phosphorescence emission peaks at 500nm and 550 nm.Crystal-PE had stronger phosphorescence emission than crystal-Et OH.The SOC matrix elements(SOCME)of crystal-Et OH and crystal-PE between T₁ and S₀ were0.703 cm^-1 and 0.785 cm^-1,respectively.The SOCME of crystal-Et OH and crystal-PE between S₁ and T₁ were 0.371 cm^-1 and 0.790 cm^-1,separately.These were consistent with the experimental results.PIM-TRZ also exhibited phosphorescent properties in both the thin film and the solid-state.The process of energy transfer in the electroluminescent films was summarized.The carrier injections into PIM-TRZ produced singlet and triplet excitons,which were transferred to DPTZN via F(?)rster and Dexter energy,respectively.The singlet excitons of DPTZN could be transferred to the triplet state by the intersystem crossing process.Subsequently,the triplet excitons were transferred to the singlet state of Rubrene by a long-range F(?)rster energy transfer process,resulting in a large number of singlet excitons in rubrene molecules.PIM-TRZ with RTP properties increased and stabilized the triplet excitons of DPTZN,thus enhancing the efficiency of the device.