Fully π-conjugated Covalent Organic Framework Luminescent Materials:Synthesis And Application In LEDs

Light-emitting diodes（LEDs）are a kind of semiconductor devices that can directly convert electricity directly into light,due to their advantages of high efficiency,energy saving,long life and small size,they are widely used in general lighting,full-colour displays,backlights of liquid crystal displays（LCDs）,agricultural light sources and optical communications.Among them,white LEDs（WLEDs）are the most important and widely used,those are usually achieved by down-conversion luminescent materials（commonly known as phosphors）excited by ultraviolet（UV）or blue light chips.Up to now,most of commercialized phosphors contain expensive,scarce and non-renewable rare earth elements.In the long term,it is very significant for LEDs to develop down-conversion luminescent materials with excellent performances but without rare earth elements.Covalent organic framework（COF）materials are a class of porous crystalline polymers composed of light elements（e.g.,C,N,O,B,H,etc.）connected by covalent bonds.COF luminescent materials have the advantages of luminescent properties easily adjusted by organic groups,high stability and easy synthesis,etc.They have potential applications in the field of luminescence.Among them,fullyπ-conjugated COF luminescent materials have better luminescent property attributed to their betterπ-electron delocalization,moreover,they have higher stability,so they as down-conversion luminescence materials are very suitable for LEDs.In this dissertation,four fullyπ-conjugated COFs with good luminescent properties were designed and synthesised,after their optical and thermal properties were investigated,a series of WLEDs with good luminescent qualities were fabricated by using the COFs as down-conversion luminescent materials under the excitation of blue-emitting chips.The details are as follows.A fullyπ-conjugated yellow-emitting COF was prepared via Knoevenagel condensation by terephthalaldehyde（TPA）and 2,2’,2’’-（（1,3,5-triazine-2,4,6-triyl）tris（[1,1’-biphenyl]-4’,4-diyl））triacetonitrile（TTTA）.Its maximum excitation wavelength(λ_ex,max)was 452 nm,its light-emitting was located at 515～740 nm with the maximum emission wavelength(λ_em,max)of 585 nm.Its photoluminescence quantum efficiency（PLQY）was 26.0%.Its thermal decomposition temperature（T_d）was 495℃.A series of WLEDs with good luminescent qualities were prepared by using it as phosphors（blended into epoxy resin）under the excitation of 460 nm Ga N-based blue chips(25.0 lm·W^-1).When blending concentration was 3.0 wt.%,a WLED with the best luminescent performances was obtained,its luminous efficiency（LE）was 18.18lm·W^-1,its color rendering index（CRI）was up to 83.1,its correlated color temperature（CCT）was 5743 K,and its CIE value was（0.33,0.29）.A fullyπ-conjugated orange-emitting COF(λ_ex,max=464 nm,light-emitting at 550～740 nm,λ_em,max=610 nm,PLQY=22.0%,T_d=395℃)was prepared via Knoeve nagel condensation by 4,4’,4’’,4’’’-（ethene-1,1,2,2-tetrayl）tetrabenzaldehyde（ETTBA）and 4,4’-（pyrene-1,6-diyl）dibenzyl cyanide（PDBC）.A series of neutral/warm white L EDs were fabricated by using 460 nm Ga N-based blue chips as the excitation light sou rces and this COF（x wt.%,x=0.5,1.0,1.5,2.0,2.5）together with yellow-emitting Y₃Al₅O₁₂:Ce³⁺（4.0 wt.%）blended in epoxy resin as down-conversion luminescent mate rials.When blending concentration of the COF was 1.0 wt.%,a WLED with the best l uminescent performances was obtained,its LE was 17.60 lm·W^-1,its CRI was 80.1,it s CCT was 5239 K and its CIE was（0.34,0.31）.A fullyπ-conjugated yellow-emitting COF(λ_ex,max=456 nm,light-emitting at510～800 nm,λ_em,max=565 nm,PLQY=24.0%,T_d=460℃)was prepared via Knoevenagel condensation by 4,4’-（pyrene-1,6-diyl）dibenzaldehyde（PDBA）and1,3,5-tris（4-cynomethylphenyl）benzene（TCPB）.A series of WLEDs with good luminescent performances were prepared by using the COF as as phosphors（blended into epoxy resin）under the excitation of 460 nm Ga N-based blue chips.The best performances of the WLEDs was achieved at a blending concentration of 2.0 wt.%,its LE was 23.06 lm·W^-1,its CRI was 80.3,its CCT was 10444 K and its CIE was（0.29,0.26）.A fullyπ-conjugated green-emitting COF was prepared via Knoevenagel conden sation by 4’,4’’’,4’’’’’-（1,3,5-triazine-2,4,6-triyl）tris（（[1,1’-biphenyl]-4-carbaldehyde））（TTTC）and 2,2’,2’’-（（1,3,5-triazine-2,4,6-triyl）tris（[1,1’-biphenyl]-4’,4-diyl））triacetonit rile（TTTA）(λ_ex,max=456 nm,light-emitting at 490-750 nm,λ_em,max=550 nm,PLQY=22.0%,T_d=585℃).A series of WLEDs with high CRIs were prepared by using 460 nm Ga N-based blue chips as the excitation light sources and this COF（2.0 wt.%）to gether with SrAlSiN₃:Eu³⁺（x wt.%,x=0.1,0.4,0.7,1.0,1.3）blended in epoxy resin a s down-conversion luminescent materials.When doping concentration of SrAlSiN₃:E u³⁺was 0.7 wt.%,a WLED with the best luminescent performances was obtained,its LE was 17.60 lm·W^-1,its CRI was 90.3,its CCT was 5962 K,its CIE was（0.32,0.33）.The frameworks of fullyπ-conjugated COF luminescent materials are linked by sp²-C or sp²-N.These luminescent COFs have high stability in air,water and most of organic solvents,moreover,their goodπ-electron delocalization usually endows them with good luminescent property.Their luminescent property can be further improved via introducing functional organic groups with high luminescent efficiencies（such as carbazole,pyrene and fluorene）or AIE activity（such as tetraphenyl ethylene）,or via constructing units with intramolecular charge transfer（ICT）ability,which would endow luminescent COFs with significant advantages when they were used as down-conversion luminescent materials for LEDs.