Design,Synthesis And Photoelectric Properties Of Donor-acceptor Type Deep Red And Near Infrared Luminescent Molecules Based On Phenoxazine Unit

Organic deep red and near-infrared luminous materials have a good application prospect in the military,biological imaging,optical communication,organic light-emitting diode（OLED）,and other fields.However,due to the limitation of band-gap rules,the development of highly efficient deep-red photothermal exciton materials（λEL≥650 nm）is still a great challenge.The two key factors affecting organic deep red and near-infrared luminescent materials are the improvement of fluorescence quantum efficiency and exciton utilization.In this paper,we mainly solve the above two key problems by constructing hot exciton materials with HLCT characteristics.We used phenoxazine as the donor to construct efficient deep red and near-infrared luminescent molecules by changing the acceptor capacity and adding connecting units between the donor and acceptor and systematically studied their photophysics and electroluminescence properties of devices.The specific research content is as follows:1.Two bilaterally symmetric D-A-D type deep red and near-infrared molecules2PPOXBZ and 2PPOXNZ were constructed by using phenoxazine as donor and benzothiadiazole or naphthothiadiazole as acceptor,respectively.Both molecules showed good thermal and electrochemical stability when the thermal and electrochemical properties were tested.The lowest excited states（S1）of 2PPOXBZ and2PPOXNZ are both hybrid local and charge transfer（HLCT）states.Interestingly,2PPOXBZ also exhibited aggregation-induced luminescence（AIE）properties in tetrahydrofuran solutions with different water contents.Therefore,the fluorescence quantum efficiency is higher in the solution or film state.Before the preparation of the undoped device,the hole and electron mobility of the two molecules were tested,and both molecules showed excellent hole transport ability.Based on this characteristic,we design a simple undoped device structure with no hole transport layer.The undoped device based on 2PPOXBZ shows a maximum external quantum efficiency（EQEmax）of 3.38%with deep red emission at 678 nm,and the undoped device based on2PPOXNZ shows a maximum external quantum efficiency（EQemax）of 1.23%with near-infrared emission at 755 nm.Due to the introduction of naphthothiadiazole acceptor unit with a large energy gap of triplet states,the calculated exciton utilization of 2PPOXNZ reaches 63.4%,while that of 2PPOXBZ is only 44.5%.In addition,the two molecule-based doped devices also performed well,the doped device based on2PPOXBZ successfully achieving a maximum EQE of 8.85%at 636 nm,which is high in reported hot exciton red materials.The doped device based on 2PPOXNZ still achieves the maximum EQE of 4.49%at 700 nm.2.Bilateral asymmetric D-A-D near-infrared（NIR）molecule TPANZPOXP was constructed using naphthothiadiazole as the acceptor,trianiline,and phenoxazine as the donor.On this basis,another NIR molecule TNZ-3PPOXP was constructed by adding a benzene ring between the strong donor and acceptor.To explore the effect of the benzene bridge on the excited state and luminescence behavior of the molecule.Through the analysis of the natural transition orbit and solvation effect of the two molecules,the CT state characteristics of TNZ-3PPOXP were slightly reduced,and the fluorescence quantum efficiency was significantly increased.The performance of the undoped device based on TNZ-3PPOXP is not satisfactory,only reaching the maximum external quantum efficiency of 2.17%with near-infrared emission at 694 nm.The undoped device based on TPANZPOXP achieves a maximum external quantum efficiency of 2.66%at 736 nm.After doping the two molecules,the performance of the TNZ-3PPOXP device is significantly improved,achieving a maximum external quantum efficiency of 9.95%with deep red emission at 660 nm.The efficiency roll-off is also low,only 2.6%at 100 cd m^-2and 21%at 1000 cd m^-2.