| White-light-emitting diodes(WLED)are considered as the fourth generation solid-state lighting sources as the advantages of high efficiency,long lifetime and environmental sustainability.So far,most of the phosphors used in WLED application are rare earth ions-doped inorganic materials.Unfortunately,rare earth elements are scarce and valuable,so it is urgent to carry out the research and development of rare earth-free phosphors.In this paper,we demonstrate a novel dye-encapsulated metal-organic-framework(MOF)strategy for the design of rare earth-free phosphors for WLED application.The fluorescent color of the dye-encapsulated MOF host-guest material can be tuned by changing the type and content of the loaded dye.And phosphors suit for ultraviolet chips and blue chips are developed.In order to study the luminescent properties and arrangement of dyes in MOFs,two series of hemicyanine dyes alkylated(E)-4-(4-(dimethylamino)styryl)-pyridine(DM-n)and alkylated(E)-4-(4-(diphenylamino)styryl)-pyridine(DP-n)were synthesized and encapsulated to three isostructural MOFs with different pore channel sizes.Fluorescent dyes maintained good luminescence properties in MOFs as they were effectively dispersed by one-dimensional channel of MOFs.The arrangement of dyes in one-dimensional channel were significantly affected by the matching between the size of dyes and channels.The size of channels in MOFs also affected the rate of dye encapsulation.The second harmonic signals of these host-guest materials were also measured,up to 0.4 times that of ?-quartz was detected.Luminescent property improvement of dye in MOFs was observed,and UV-excited phosphor for WLED was prepared.Cationic dye DSM((E)-4-(4-(dimethylamino)styryl)-1-methylpyridinium)could be encapsulated into an anionic MOF(Me2NH2)3[In3(BTB)4]ˇguests(ZJU-28,BTB=1,3,5-benzenetribenzoate)via ion exchange process by immersing ZJU-28 crystals in DMF solution of DSM,and the concentration of DSM in ZJU-28 enriched for more than 90 times.Meanwhile,the quantum yield of DSM in ZJU-28 enhanced for nearly 10 times compared with the DSM solution,which leaded to the significant enhancement for the fluorescence intensity of DSM in ZJU-28.DSM encapsulated ZJU-28 complex ZJU-28(?)DSM could exhibits both blue fluorescence from ZJU-28 and red fluorescence from DSM when excited by 365 nm ultraviolet.The emission colors of ZJU-28(?)DSM could be tuned by changing the content of DSM.Introducing green light emitting dye acriflavine(AF)to ZJU-28(?)DSM could prepare white light emitting materials by varying dye contents in MOF.ZJU-28(?)DSM/AF(0.02 wt%DSM,0.06 wt%AF)exhibited white light emission with the Commission International ed'Eclairage(CIE)coordinates of(0.34,0.32)under UV light excitation.The quantum yield of ZJU-28(?)DSM/AF(0.02 wt%DSM,0.06 wt%AF)was measured as 17.4%,quite high in MOF-based white light emitting materials.The color rendering index(CRI)value and correlated color temperature(CCT)value of ZJU-28(?)DSM/AF(0.02 wt%DSM,0.06 wt%AF)was 91 and 5327 K,respectively.Thus,we developed a new strategy to prepare phosphors for WLED application by encapsulating fluorescent dyes into MOFs.The combination of blue LED chips and yellow phosphors is the main way to fabricate WLED.Phosphors with high quantum yield were developed by encapsulating more efficiency dyes into ZJU-28,and they could be effectively excited by 460 nm blue LED chips.Yellow emission phosphor could be prepared by encapsulating green dye Cou 6(Cou-6 = Coumarin 6)and red dye DCM(DCM = 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran)into ZJU-28.ZJU-28(?)Cou-6/DCM(0.016 wt%Cou-6,0.0018 wt%DCM)phosphors exhibit high quantum yield of 74.34%excited at 460 nm.WLED made with ZJU-28(?)Cou-6/DCM phosphor were also fabricated,and the luminous efficacy of the WLED was 107 Im/W,CRI and CCT value are 67 and 5075 K,respectively.Another red dye Pyronin Y(Py)is more efficient in ZJU-28 than DCM,and ZJU-28(?)Cou-6/Py sample was further synthesized.ZJU-28(?)Cou-6/Py(0.016 wt%Cou-6,0.036 wt%Py)with quantum yield of 77.49%was used to fabricate WLED.At 20 mA drive current,the luminous efficacy of the WLED was 149 lm/W,CRI and CCT value are 79 and 4326 K,respectively.Dyes maintained excellent luminescent property in MOFs leading to high efficiency of these MOF-based phosphors.They have great potential for WLED applications.In the field of indoor lighting,high CRI value of WLED is required,typically higher than 80.In order to obtain a higher CRI value,phosphors with a ternary dye encapsulated system were prepared.Cou-6 and Rhodamine 6G(R6G)encapuslated material ZJU-28(?)Cou-6/R6G has high quantum yield above 90%.The luminous efficacy of fabricated WLED was as high as 179 lm/W,but with a low CRI value of 50.This phosphor was not appropriate to use in WLED application due to the poor CRI value.ZJU-28(?)Cou-6/R6G/DCM phosphor was prepared by adding DCM dye into ZJU-28(?)Cou-6/R6G to improve CRI value.WLED with ZJU-28(?)Cou-6/R6G/DCM(0.016 wt%Cou-6,0.008 wt%R6G,0.0017 wt%DCM)showed a higher but not ideal CRI value of 66.Further,we used another more efficient red emission dye Rhodamine 101(R101)to take place of DCM.ZJU-28(?)Cou-6/R6G/R101(0.016 wt%Cou-6,0.010 wt%R6G,0.014 wt%R101)was prepared with a quantum yield of 82.91%.A WLED was fabricated by combining ZJU-28(?)Cou-6/R6G/R101 phosphor with a 450-460 nm blue chip.At 20 mA drive current,the WLED showed a luminous efficacy of 126 lm/W,and a moderate CCT of 4446 K.the CRI value of this WLED reached up to 88,and the R9 value was 93.5.ZJU-28(?)Cou-6/R6G/R101 phosphor showed a good color rendering performance,and solved the high CCT,low CRI value problems of WLEDs made with commercial cerium-doped yttrium aluminum garnet(YAG:Ce)phosphors.These MOF-based phosphors are extremely promising used in indoor lighting applications. |
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