Excited State Behaviors And Photoelectric Properties Of Highly Efficient Fluorescent Materials Based On Triphenylamine And Anthracene Groups

Organic light emitting diode(OLED)has become a research hotspot in the field of lighting and displaying due to its unique properties such as wide viewing angle,ultra-thin,flexibility and so on.Organic small-molecule light-emitting materials have the advantages of easy synthesis,low cost and adjustable light color.However,it is an important challenge how to improve its exciton utilization efficiency(EUE)by 100%and improve the external quantum efficiency(EQE)of the device to break the limit of 5%.To state this problem,it is the effective way to make the 75%triplet excitons emit through the suitable reverse intersystem crossing process(RISC).The special RISC from T_m-S₁ can be achevied for the organic molecule with hybridized local and charge-transfer(HLCT)excited state.Generally,the charge-transfer(CT)excited state in HLCT molecules with the weak bonding energy of the exciton facilitates to efficient reverse intersystem-crossing(RISC)from triplet to singlet,consequently EUE can reach to 100%.Simultaneously,the localized excited(LE)state with a high oscillator strength can enhance photoluminescent quantum efficiency owing to the overlap of the highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO).Therefore,triphenylamine and anthracene were selected to design and synthesize a series of new blue and orange-red small-molecule light-emitting materials with HLCT state.Consequently,the non-doped and doped devices were fabricated.The main research contents of this paper are as follows:1.The blue-emitting materials are the requisite for the full-color displaying and white lighting in OLEDs.It is a task to obtain the excellent blue materials due to its lager energy gap.Two blue fluorescent materials with D-A structure was designed and synthesized by using spacial propeller triphenylamine(TPA)as donor(D)and rigid anthracene derivative as acceptor(A),namely TPA-AN-NA and TPA-AN-TFP.Their photophysical,excited state and electroluminescence properties were characterized.The maximum emission peaks of TPA-AN-NA and TPA-AN-TFP in the thin film state are at468 and 471 nm,which located in the blue light range.Two materials exhibited the HLCT excited state,facilated the improvement of EUE.The blue OLEDs devices were fabricated using TPA-AN-NA and TPA-AN-TFP as the light-emitting layer.The devices based TPA-AN-NA displayed the blue emission with maximum emission peak at 468 nm and the color coordinate(Commission International de L?Eclairage,CIE)of(0.14,0.16).The maximum external quantum efficiency(EQE)of 5.44%was achieved,which broken the theoretical value of 5%of traditional fluorescent materials.Radiative exciton yield was estimated to be 56.58%,which was higher than the limite of 25%,indicating that the triplet excition can upconverse to singlet ones and participate in emiting.2.It is still a formidable challenge to develop efficient red non-doped emitters with the excellent stability and high efficiency due to the serious aggregation-caused emission quenching(ACQ).To address this issue,the red aggregation-induced HLCT materials will be exploited in this paper.Thus,benzothiadiazole(BT)is introduced as a stong electon-accepting group to design the red materials.By regulating the linkages between the D and the A groups,four novel orang and red materials were synthesized with the aggregation-induced emission(AIE)effect,namely TBAN,TBT,TBAT and TABAT.The photophysical properties,excited state properties,aggregation state luminescence properties and the performances on devices of these four materials were studied in detail.The results indicated that the maximum emission peaks of TBAN,TBT,TBAT and TABAT in dichloromethane solution were located at 615,656,615 and 607 nm,respectively.The compound TBT presented the dominated LE excited state in the low-polarity solvents,the dominated CT state in the high-polarity solvents and the HLCT state in medium-polarity solvents.The doped and nondoped OLED devices were fabricated by using TBAN,TBT,TBAT and TABAT as the light-emitting layer respectively.The non-doped devices showed the better performances than that of doped ones owing to the AIE natures of emitting materials.The lower turn-on voltage of 2.7 V,the maximum brightness of74820 cd/m² and EQE of 5.64%were obtained form non-doped OLED based on TBAN with the low roll-off efficiency.