- Email: [email protected]
Spectrally flat white light emission based on red-yellow-green-blue dye-loaded metal-organic frameworks
Optical Materials 89 (2019) 209–213
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Spectrally flat white light emission based on red-yellow-green-blue dyeloaded metal-organic frameworks
T
Jin Wanga,∗, Yunchao Zhanga, Ying Yua, Feihong Yea, Zeyan Fenga, Zhenjiang Huanga, Xiaoli Liua, Xinhui Zhoub a b
School of Telecommunication and Information Engineering, Nanjing University of Posts and Telecommunications, Nanjing, 210003, China Key Laboratory for Organic Electronics and Information Displays, Nanjing University of Posts & Telecommunications, Nanjing, 210023, China
A R T I C LE I N FO
A B S T R A C T
Keywords: White light emission Metal organic framework Fluorescent dye
We realize a white light emission based on metal-organic frameworks (MOFs), which are loaded with red/ yellow/green/blue (R/Y/G/B) primary-colour fluorescent dye molecules. The MOF⊃dye powders emitting red/ green/blue light are synthesized via an activated adsorption reaction, whilst the MOF⊃dye powder emitting yellow light is prepared via an in-situ reaction. By grinding these R/Y/G/B MOF⊃dye powders and adjusting the mass proportions of each primary-colour component, a white light emission with a quasi-flat spectrum is obtained. Experimentally, under excitation with an ultraviolet light (at a wavelength of 365 nm), the emission peaks of the R/Y/G/B MOF⊃dye powders are located at 621.0 nm, 560.0 nm, 522.5 nm and 438.5 nm, respectively. When the mass proportions of R/Y/G/B components are 35.3%, 21.2%, 14.7% and 28.8%, the fullwidth at half-maximum bandwidth of the light emission is 251.5 nm (from 391.5 nm to 643.0 nm), and the 1-dB bandwidth is 201.5 nm. The measured Commission Internationale de L'Eclairage (CIE) coordinates are (0.319, 0.341). The flat and wide spectrum of this material makes it a promising candidate in applications such as lighting, display and visible-light communication systems.
1. Introductions White light emitting diodes (LEDs) are widely used in lighting, display and visible-light communication (VLC) systems [1–3]. Commonly, a white LED is realized by mixing various primary-colour emissions from inorganic or organic phosphors excited with an ultraviolet InGaN light source. Primary-colour components from inorganic phosphors have a longer emission life, which leads to a low intrinsic modulation frequency [4,5]. By contrast, LEDs based on organic phosphors, also known as OLEDs, have a much shorter emission lifetime than their inorganic counterparts, and can operate at a higher intrinsic modulation frequency [6–8]. On the other hand, OLEDs have the advantages of ultra-thin structure, high responsivity, high contrast, and low power consumption, making them the focus of next-generation display systems [2]. Amongst the luminescent materials that realize white light and fast OLEDs, organic dye molecules are good choices owing to their high quantum yield, simple chemical tuning capability and rapid radiation emission rate [8–10]. However, organic dyes often suffer from aggregation-induced quenching in their solid state and secondary absorption caused by the self-quenching effect [11,12],
∗
thereby preventing their use as solid fluorophores. In recent years, metal organic framework (MOF) based luminescent materials that adsorb and encapsulate organic dye molecules have emerged as very promising candidates to realize new types of OLEDs [7,13–20]. MOFs are usually highly porous with modifiable pore size, and can effectively encapsulate organic dye molecules [21]. In fact, the dye molecules in the MOF channels are separated from each other. This effectively prevents aggregation-induced quenching and inhibits nonradiative relaxation related to the internal molecular motion. As a result, MOF-based OLEDs are very effective with respect to the emission quantum yield and photochemical stability [16,22]. In addition, by encapsulating different fluorescent dyes into the MOF pores, OLEDs emitting various colours can be easily obtained and they exhibit interesting luminescent properties (e.g. we recently reported linearly polarized warm-yellow light emission based on orientated dyes in rodlike MOF crystals [20]). Furthermore, by mixing different dyes fluorescing at their respective primary colours in MOF composites, a white light-emission can be obtained. In Ref. [18], Wen et al. introduced three dye molecules with respective red/green/blue (R/G/B) luminescence into the MOF channel simultaneously, and formed white light emissions
Corresponding author. E-mail address: [email protected] (J. Wang).
https://doi.org/10.1016/j.optmat.2019.01.019 Received 17 November 2018; Received in revised form 13 January 2019; Accepted 16 January 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 89 (2019) 209–213
J. Wang et al.
ambient temperature raised up to 250 cm3/g at 0.1 relative pressure and showed a typical type I isotherm (Fig. S1), which proved that this MOF crystal was a microporous material. 2.2. Preparation of MOF⊃dye crystals emitting the four primary colours Fig. 1. Scheme for realizing white light emission based on MOFs incorporated with fluorescent dyes of red/yellow/green/blue primary-colours.
Different dye molecules are introduced into the MOF pores by two methods. DCM, coumarin 6 (C6), and coumarin 120 (C120) were used respectively as the red, green, and blue dyes to prepare MOF⊃DCM, MOF⊃C6, and MOF⊃C120 via the activated adsorption reaction. The preparation strategy of R/G/B MOF⊃dyes is essentially the same as that used in Ref. [18]. Additionally, RhB was used as the yellow dye molecule for the in-situ synthesis of MOF⊃RhB. MOF⊃DCM/C6/C120 crystals were prepared via the activated adsorption reaction with the following steps: approximately 50 mg of the newly prepared colourless and transparent MOF crystals were measured and placed into a 25 ml clean beaker. The crystals were washed three times with 10 ml DMF solvent and filtered with medium speed filter paper. They were then placed in a 60 ml evaporating dish and warmed in a vacuum drying chamber (0.2 kPa pressure) at 100 °C for approximately 10 h, to obtain the colourless MOF crystals with the guest molecules eliminated. Ten mg of the solid dye was weighed and placed in a 20 mL sample vial, and 10 mL of DMF solvent was added. The mixture was shaken gently until the dye completely dissolved. As the change in solution volume is negligible before and after dissolution of the dye, the resulting concentration of the solution was simply 1 mg/mL. Subsequently, 20 mg of the activated MOF crystals were weighed and immersed in dye/DMF solution. The vial was sealed and stored undisturbed and away from light for two days, and the rudimentary MOF⊃dye product was obtained. In the final step, the rudimentary product was washed with DMF solvent until the liquid became clear and free of fluorescence under UV light. The MOF⊃dye was collected as the final product. It should be noted that MOF crystals exhibit different stabilities in different solvents. The solvent used for MOF preparation was also used as the solvent for the dyes to ensure the stability of MOF crystals during adsorption. Fig. 3(a), (c), and (d) show respectively the fluorescence microscopy images of MOF⊃DCM, MOF⊃C6, and MOF⊃C120 crystals under UV irradiation. MOF⊃RhB crystals were synthesised via the in-situ reaction as follows: an MOF precursor solution was first made by following the same steps as for the preparation of the MOF before heated. Then, 60 ml of this precursor solution was divided into two portions. Thirty mg of RhB were added into one portion (30 ml) and ultra-sonicated until the dye was completely dissolved, to produce the MOF precursor solution with RhB. The concentration of RhB in the solution was 1 mg/mL. To avoid the aggregation of RhB molecules in the solution, the concentration should remain below a certain value. In the third step, the solution was transferred to a polytetrafluoroethylene liner of 50 mL capacity and heated at 100 °C for 48 h. Then, after being cooled naturally to room temperature, the reaction was finished. Fig. 3 (b) presents the microscopy images of respective fluorescent MOF⊃RhB crystals under UV irradiation. X-ray powder diffraction (XRPD) experiments (Bruker D8 ADVANCE) were also performed for the MOF and MOF⊃dye crystals, and their XRPD patterns are shown in Fig. 4 The diffraction peaks of these crystals are basically the same, which means that their spatial framework structures should be basically same, i.e. the MOF structure is not affected by the presence of the dye molecules. The concentrations of DCM, C6 and C120 adsorbed inside the MOF can be evaluated from the concentration differences of these dyes in the solution before and after the adsorption procedure. The concentrations of these dyes in the solution were expressed via the relationships between the absorbance and the concentration of these dyes in the DMF solution, which were obtained via the UV-Vis absorbance measurement (Figs. S2, S3, S4 and S5). In this work, the concentration of DCM, RhB, C6 and C120 in MOFs were 0.098%wt, 0.068%wt, 0.039%wt and
with bit-tunable emission spectra by changing the contents of the three dye molecules or the excitation wavelength. It should be noted that the dye molecules used in Ref. [18] are linear molecules, which enter the MOF channel relatively easily via ion exchange, e.g. fluorescent brightener (CBS-127, KSN), coumarin 6 (C6), 4-(dicyanomethylene)-2methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), 4-(p-dimethylaminostyryl)-1-methylpyridinium (DSM), etc. However, as discussed in Ref. [18], those planar dye molecules including cationic Rhodamine-B (RhB), anionic methylene blue and neutral RhB-6G have difficulty to enter the MOF channel via ion exchange, owing to their relatively stronger covalent bonds. This limits the preparation of white OLEDs based on MOF composites. Also, the spectrum of the white OLED realized in Ref. [18] is not flat enough, where the 1 dB bandwidth of the flat top (commonly defined in optical filters [23]) of this OLED seems to be < 150 nm. This would limit the utilization of this type of white OLED e.g. in VLC. In this study, we realize spectrally-flat white light emission based on MOF⊃dyes, which are loaded with red/yellow/green/blue fluorescent dye molecules, respectively. The schematic of the strategy is shown in Fig. 1. MOFs are prepared by the solvent thermal reaction method, and the dye molecules are introduced into the MOF pores via two methods: the red, green and blue MOF⊃dyes are prepared via an activated adsorption reaction, and the yellow MOF⊃dye is prepared via an in-situ reaction. These red/yellow/green/blue MOF⊃dye powders are then ground and mixed. By adjusting the mass proportions of each primarycolour component, white light emission can be obtained. In the manuscript, the fluorescence properties of the four primary-colours and of white light emission are also characterized. 2. Preparation of MOF crystals and primary-colour MOF⊃dye 2.1. Synthesis of MOFs We first synthesize the MOF-BDC crystal, namely [Zn2(BDC)2TED]n (TED = triethylenediamine, BDC = 1,4-benzenedicarboxylic acid), by using the solvent thermal reaction method. The preparation of the MOF was as follows: a mixture of Zn(NO3)2·6(H2O) (2.8 mmol), H2BDC (2.9 mmol) and TED (1.95 mmol) was added to a 30 ml N,NDimethylformamide (DMF) solution; an ultrasonic treatment was applied to dissolve all chemicals until the solution was clear and then a certain amount of the nitric acid solution was added to adjust the pH value. The solution was then transferred into a 50 ml Teflon-lined autoclave and heated in an oven at 100 °C for 48 h. Finally, after being cooled naturally to room temperature, millimetre-size crystals of MOFBDC were obtained. A single MOF-BDC crystal was colourless and transparent. Fig. 2 shows the molecular structure diagrams of the MOF-BDC crystal obtained from single crystal X-ray diffraction (XRD) observations (Bruker D8 QUEST). From the side view of the MOF channel structure, shown in Fig. 2(a), the MOF crystal has a periodical pore channel structure along the growth direction of the TED ligand, which is considered as the direction of the MOF channel. Fig. 2(b) shows that this MOF crystal has a six-prism shape, which belongs to a hexagonal system. The diameter of the largest MOF pore is 12.9 Å. Also, to gain the porosity information of this MOF crystal, we performed the nitrogen adsorption measurement by using V-Sorb 2800P specific surface area and pore size analyser. Its nitrogen adsorption isotherm at 25 °C 210
Optical Materials 89 (2019) 209–213
J. Wang et al.
Fig. 2. Molecular structure diagrams of the MOF-BDC crystal obtained from single crystal X-ray diffraction observations. (a) Side view and (b) cross-sectional view of the MOF channel structure.
Fig. 3. Microscopy images of the four fluorescent MOF⊃dye crystals under UV excitation. a) The red-emitting MOF⊃DCM; b) yellow-emitting MOF⊃RhB; c) green-emitting MOF⊃C6; and d) blue-emitting MOF⊃C120. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 5. Photoluminescence spectra of MOF⊃DCM, MOF⊃RhB, MOF⊃C6, and MOF⊃C120 crystals by UV excitation (365 nm wavelength).
dyes were dispersed in solvent with concentrations from 0.2 mg/ml to 1 mg/ml, the emission peaks of DCM, RhB, C6 and C120 in the DMF solution were at 609–614 nm, 579–610 nm, 519–528 nm, and 437–438 nm (Fig. S6), similar to those values in Refs. [18,24,25], where the respective 1-dB bandwidths were about 33–38 nm, 15–20 nm, 22 nm and 29 nm. Therefore, the respective dyes have been incorporated in MOFs and were responsible for the luminescence. Also, by comparing the emission peaks before and after incorporating the respective dyes into MOFs, it can be seen that the emission peaks of most dyes did not change much, while that of MOF⊃RhB was shifted to shorter wavelength. Thus, it could be regarded that these dye molecules in the MOF channels were separated from each other, and the aggregation-induced quenching did not take place. Most interestingly, while 1-dB bandwidth of the in-situ synthesized MOF⊃RhB was not really changed, the 1-dB bandwidths of dyes adsorbed into the MOF channel increased. This is due to the charge transfer among the dyes closed together in the MOF channel, resulting in broadening of the emission. In fact, the increase of the 1-dB bandwidth of dyes incorporated inside the MOF is advantageous to realize a white light emission with a wide bandwidth. Also, we checked the MOF⊃dyes with different dye concentrations. As discussed above, the R/Y/G/B MOF⊃dye crystals were not saturated adsorbed and had low dye concentrations. When the activated MOF crystals were immersed in DCM/C6/C120 solution for 96h, the concentration of DCM, C6 and C120 in MOFs were 0.186%wt, 0.063%wt and 0.087%wt. It was found that the longer the MOFs were soaked in DCM/C6/C120 solution, the darker the MOF⊃dye crystals were. For
Fig. 4. XRPD patterns of MOF, MOF⊃C120, MOF⊃C6, MOF⊃RhB and MOF⊃DCM (from bottom to top).
0.048%wt respectively. Due to the low concentration of dyes inside the MOF, the MOF structures did not change, as proved in Fig. 4. It should be noticed that the single crystal X-Ray diffraction measurement was not applicable for such low dye concentrations. 2.3. Luminescence property of primary colour MOF⊃dye crystals The photoluminescence (PL) spectra of these MOF⊃dye crystals emitting primary colours were measured under UV excitation (at 365 nm wavelength), shown in Fig. 5. It can be seen under UV excitation that the emission peaks of the respective R/Y/G/B MOF⊃dye crystals are located at 621.0 nm, 560.0 nm, 522.5 nm and 438.5 nm, where 1-dB bandwidths of respective MOF⊃dyes were about 50.5 nm, 18 nm, 45 nm, and 34.5 nm. On the other hand, when these R/Y/G/B 211
Optical Materials 89 (2019) 209–213
J. Wang et al.
Fig. 6. The CIE coordinates of the MOF⊃dyes emitting the four primary colours. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 8. CIE coordinates of the MOF⊃dye composites obtained by varying mass proportions of the primary-colour fluorescent crystals. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 7. Time-resolved fluorescence of R/B/G/Y MOF⊃dyes, their 1/e−lifetimes were 1.00 ns, 3.59 ns, 1.06 ns and 2.50 ns respectively.
Fig. 9. Photoluminescence spectrum of white light-emitting MOF⊃dye.
Table 1 Compositions of the white light-emitting MOF⊃dye (No. 5 is the best so far). No.
MOF⊃DCM
MOF⊃RhB
MOF⊃C6
MOF⊃C120
1 2 3 4 5
2.0 mg 4.4 mg 5.5 mg 7.5 mg 7.5 mg
4.5 mg 4.5 mg 4.5 mg 4.5 mg 4.5 mg
3.1 mg 3.1 mg 3.1 mg 3.1 mg 3.1 mg
5.1 mg 5.1 mg 5.1 mg 5.1 mg 6.1 mg
The CIE coordinates of respective MOF⊃dye crystals were also measured and marked in Fig. 6. The CIE coordinates of R/Y/G/B MOF⊃dye crystals are (0.575, 0.414), (0.400, 0.588), (0.273, 0.417), and (0.156, 0.093) respectively. By comparing them with the CIE coordinates of respective saturated R/Y/G/B fluorescence, namely (0.66, 0.33), (0.48, 0.48), (0.21, 0.71), and (0.14, 0.08), it can also be concluded that synthesized R/Y/G/B MOF⊃dye crystals were not saturated adsorbed. Furthermore, we studied the PL lifetimes of respective R/Y/G/B MOF⊃dye crystals, via a time-resolved fluorescence measurement (Edinburgh FLS920P). The measurement results are shown in Fig. 7. The fitted PL lifetimes of respective R/Y/G/B MOF⊃dye crystals were 1.00 ns, 3.59 ns, 1.06 ns and 2.50 ns respectively, while the measured PL lifetimes of respective R/Y/G/B dyes were 2.53 ns, 3.00 ns, 2.26 ns and 3.38 ns. The PL lifetime of the MOF⊃RhB has a slight increase where that of most dye-MOFs has a significantly decrease which is benefit for improving the intrinsic modulation frequency for VLC applications. Last but not least, the quantum yields of the R/Y/G/B MOF⊃dye
the in-situ synthesized MOF⊃RhB, the concentration of RhB in product fell to 0.049%wt as the concentration of RhB in the precursor solution was decreased from 1 mg/ml to 0.75 mg/ml. For these MOF⊃dyes with different dye concentrations, we performed PL spectra, shown in Fig. S7. The emission peaks and bandwidths of MOF⊃DCM, MOF⊃C6 and MOF⊃C120 are basically same, while that of MOF⊃RhB had a slight change. It should be noticed that the spectra and the intensities of four MOF⊃dye crystals would be varied under excitation at different wavelengths, as proved in Ref. [18].
212
Optical Materials 89 (2019) 209–213
J. Wang et al.
crystals were measured to be 0.43%, 2.0%, 4.9% and 9.3%.
Declaration of interest
3. Preparation and characterisation of the white light-emitting MOF⊃dye composite
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The white light-emitting MOF⊃dye composite was prepared by mixing R/Y/G/B primary-colour MOF⊃dye crystals. At the beginning, the R/Y/G/B MOF⊃dye crystals with respective mass of 2.0 mg, 4.5 mg, 3.1 mg and 5.1 mg (corresponding mass proportions of 13.6%, 30.6%, 21.1%, and 34.7%) were firstly fully ground in a grinder to obtain the mixture No. 1. The CIE coordinates of this mixture were found to be (0.262, 0.312), which indicates that the white light has a bluish tint. To amend for the excess blue light and inadequate yellowred light given by the mixture No. 1, the proportion of constituents was adjusted step by step. As shown in Table 1, the quantity of the redfluorescent MOF⊃DCM crystals was increased and that of the bluefluorescent MOF⊃C120 crystals was varied. The change in the CIE coordinates of the adjusted mixtures is shown in Fig. 8. Material emitting warm white light (No. 5) was eventually obtained at mass proportions of 35.3% (R), 21.2% (Y), 14.7% (G), and 28.8% (B). The CIE coordinates of this material are (0.319, 0.341), and the colour temperature is approximately 5500 K. The resulting photoluminescence spectra of the MOF⊃dye composites (from No. 1 to No. 5) emitting warm white light is shown in Fig. 9. It can be seen in the photoluminescence spectrum of the first synthesized composite No. 1 that the red spectral part was not sufficient. Therefore, from No. 1 to No. 4, the mass of the red MOF⊃DCM crystal was increased from 2.0 mg to 7.5 mg. The composite No. 4 exhibited a sufficient red spectral part, however, a not sufficient blue spectral part. Thus, from No. 4 to No. 5, the mass of the red MOF⊃C120 crystal was increased from 5.1 mg to 6.1 mg. Finally, when the mass of DCM, RhB, C6, and C120 were 7.5 mg, 4.5 mg, 3.1 mg and 6.1 mg, a white light emission with flat spectrum was obtained. For this composite No. 5, the full-width at half-maximum (FWHM) bandwidth of the light-emission spectrum range is 251.5 nm (from 391.5 nm to 643.0 nm), and the 1-dB bandwidth is 201.5 nm, almost covering the entire visible light region. Compared with the 1 dB bandwidth of the materials shown in Refs. [7,18], ∼150 nm and < 150 nm respectively, this organic material is a better candidate for white light emission. Also, for these composites (No. 1 to 5), the quantum yields were between 2.0% and 4.0% (limited by the lowest quantum yield of MOF⊃DCM), while the PL lifetimes were 3–5 ns. Nevertheless, in comparison to commercial inorganic phosphors, this material has a good CIE coordinate and a short PL lifetime, resulting in a high intrinsic modulation bandwidth.
Acknowledgment This work was supported by the National Natural Science Foundation of China, (Grant No. 61575096). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.01.019. References [1] S. Tonzani, Time to change the bulb, Nature 459 (2009) 312–314. [2] P. Pust, P.J. Schmidt, W. Schnick, A revolution in lighting, Nat. Mater. 14 (2015) 454–458. [3] C. Lee, C. Shen, C. Cozzan, et al., Gigabit-per-second white light-based visible light communication using near-ultraviolet laser diode and red-, green-, and blue-emitting phosphors, Optic Express 25 (2017) 17480. [4] T. Komine, M. Nakagawa, Fundamental analysis for visible-light communication system using LED lights, IEEE Trans. Consum. Electron. 50 (2004) 100–107. [5] T. Koonen, Indoor optical wireless systems: technology, trends and applications, J. Lightwave Technol. 36 (2018) 1459–1467. [6] H. Chun, P. Manousiadis, S. Rajbhandari, et al., Visible light communication using a blue GaNμLED and fluorescent polymer color converter, IEEE Photon. Technol. Lett. 26 (2014) 2035–2038. [7] Z. Wang, Z. Wang, B. Lin, et al., Warm-white light-emitting diode based on a dyeloaded metal-organic framework for fast white-light communication, ACS Appl. Mater. Interfaces 9 (2017) 35253–35259. [8] Z. Zhao, H. Su, P. Zhang, et al., Polyyne bridged AIE luminogens with red emission: design, synthesis, properties and applications, J. Mater. Chem. B 5 (2017) 1650–1657. [9] B. W. D'Andrade, S.R. Forrest, White organic light-emitting devices for solid-state lighting, Adv. Mater. 16 (2004) 1193–1203. [10] N.H. Kim, Y.H. Kim, J.A. Yoon, et al., Color optimization of single emissive white OLEDs via energy transfer between RGB fluorescent dopants, J. Lumin. 143 (2013) 723–728. [11] Y. Sun, N.C. Giebink, H. Kanno, et al., Management of singlet and triplet excitons for efficient white organic light-emitting devices, Nature 440 (2006) 908–912. [12] H.H. Fang, Q.D. Chen, J. Yang, et al., Two-photon pumped amplified spontaneous emission from cyano-substituted oligo(p-phenylenevinylene) crystals with aggregation-induced emission enhancement, J. Phys. Chem. C 114 (2010) 11958–11961. [13] S. Jin, H.J. Son, O.K. Farha, et al., Energy transfer from quantum dots to metalorganic frameworks for enhanced light harvesting, J. Am. Chem. Soc. 135 (2013) 955–958. [14] H. Yang, White organic light-emitting devices based on blue fluorescent dye combined with dual sub-monolayer, J. Lumin. 142 (2013) 231–235. [15] M.J. Dong, M. Zhao, S. Ou, et al., A luminescent dye@ MOF platform: emission fingerprint relationships of volatile organic molecules, Angew. Chem. 53 (2014) 1575–1579. [16] Y. Cui, H. Xu, Y. Yue, et al., A luminescent mixed-lanthanide metal-organic framework thermometer, J. Am. Chem. Soc. 134 (2012) 3979–3982. [17] Y. Cui, R. Song, J. Yu, et al., Dual-emitting MOF⊃dye composite for ratiometric temperature sensing, Adv. Mater. 27 (2015) 1420–1425. [18] Y. Wen, T. Sheng, X. Zhu, et al., Introduction of red-green-blue fluorescent dyes into a metal-organic framework for tunable white light emission, Adv. Mater. 29 (2017) 1700778. [19] L.L. Chong, Z. Feng, L. Xing, et al., A luminescent Ln-MOF thin film for highly selective detection of nitroimidazoles in aqueous solutions based on inner filter effect, J. Lumin. (2019) 23–29. [20] J. Wang, F. Ye, Z.J. Huang, et al., Linearly polarized surface warm-yellow LED based on orientated organic dyes in rod-like metal-organic framework crystal arrays, Opt. Mater. Express 8 (2018) 2901–2909. [21] H.L. Jiang, Y. Tatsu, Z.H. Lu, et al., Non-, micro-, and mesoporous metal-organic framework isomers: reversible transformation, fluorescence sensing, and large molecule separation, J. Am. Chem. Soc. 132 (2010) 5586–5587. [22] J. Yu, Y. Cui, H. Xu, et al., Confinement of pyridinium hemicyanine dye within an anionic metal-organic framework for two-photon-pumped lasing, Nat. Commun. 4 (2013) 2719. [23] H. Venghaus (Ed.), Wavelength Filters in Fibre Optics, Springer Verlag, Berlin, 2006. [24] S.K. Sagoo, R.A. Jockusch, The fluorescence properties of cationic rhodamine B in the gas phase, J. Photochem. Photobiol., A 220 (2011) 173–178. [25] H.V. Demir, S. Nizamoglu, T. Erdem, et al., Quantum dot integrated LEDs using photonic and excitonic color conversion, Nano Today 6 (2011) 632–647.
4. Conclusion We achieve spectrally wide and flat white light emission based on MOF⊃dye composite, which consists of R/Y/G/B primary-colour MOF⊃dye powders. The MOF crystal is prepared by the solvent thermal reaction method, in which the R/G/B dye molecules are introduced into the MOF pores via an activated adsorption reaction, and the yellow dye molecules are loaded via an in-situ reaction. By mixing and grinding these R/Y/G/B MOF⊃dye powders and adjusting the mass proportions of each primary-colour component, white light emission is obtained under excitation by an ultraviolet light (at a wavelength of 365 nm). When the respective mass proportions of R/Y/G/B components are 35.3%, 21.2%, 14.7% and 28.8%, the FWHM of the light-emission range is 251.5 nm (from 391.5 nm to 643.0 nm), the 1-dB bandwidth is 201.5 nm, and the CIE coordinates are (0.319, 0.341). The spectrally flat white light emission characteristics show that this material could be a promising candidate in applications such as lighting, display and VLC systems, where this strategy has better repeatability, colour stability and requires a simple preparation process.
213
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Spectrally flat white light emission based on red-yellow-green-blue dyeloaded metal-organic frameworks
T
Jin Wanga,∗, Yunchao Zhanga, Ying Yua, Feihong Yea, Zeyan Fenga, Zhenjiang Huanga, Xiaoli Liua, Xinhui Zhoub a b
School of Telecommunication and Information Engineering, Nanjing University of Posts and Telecommunications, Nanjing, 210003, China Key Laboratory for Organic Electronics and Information Displays, Nanjing University of Posts & Telecommunications, Nanjing, 210023, China
A R T I C LE I N FO
A B S T R A C T
Keywords: White light emission Metal organic framework Fluorescent dye
We realize a white light emission based on metal-organic frameworks (MOFs), which are loaded with red/ yellow/green/blue (R/Y/G/B) primary-colour fluorescent dye molecules. The MOF⊃dye powders emitting red/ green/blue light are synthesized via an activated adsorption reaction, whilst the MOF⊃dye powder emitting yellow light is prepared via an in-situ reaction. By grinding these R/Y/G/B MOF⊃dye powders and adjusting the mass proportions of each primary-colour component, a white light emission with a quasi-flat spectrum is obtained. Experimentally, under excitation with an ultraviolet light (at a wavelength of 365 nm), the emission peaks of the R/Y/G/B MOF⊃dye powders are located at 621.0 nm, 560.0 nm, 522.5 nm and 438.5 nm, respectively. When the mass proportions of R/Y/G/B components are 35.3%, 21.2%, 14.7% and 28.8%, the fullwidth at half-maximum bandwidth of the light emission is 251.5 nm (from 391.5 nm to 643.0 nm), and the 1-dB bandwidth is 201.5 nm. The measured Commission Internationale de L'Eclairage (CIE) coordinates are (0.319, 0.341). The flat and wide spectrum of this material makes it a promising candidate in applications such as lighting, display and visible-light communication systems.
1. Introductions White light emitting diodes (LEDs) are widely used in lighting, display and visible-light communication (VLC) systems [1–3]. Commonly, a white LED is realized by mixing various primary-colour emissions from inorganic or organic phosphors excited with an ultraviolet InGaN light source. Primary-colour components from inorganic phosphors have a longer emission life, which leads to a low intrinsic modulation frequency [4,5]. By contrast, LEDs based on organic phosphors, also known as OLEDs, have a much shorter emission lifetime than their inorganic counterparts, and can operate at a higher intrinsic modulation frequency [6–8]. On the other hand, OLEDs have the advantages of ultra-thin structure, high responsivity, high contrast, and low power consumption, making them the focus of next-generation display systems [2]. Amongst the luminescent materials that realize white light and fast OLEDs, organic dye molecules are good choices owing to their high quantum yield, simple chemical tuning capability and rapid radiation emission rate [8–10]. However, organic dyes often suffer from aggregation-induced quenching in their solid state and secondary absorption caused by the self-quenching effect [11,12],
∗
thereby preventing their use as solid fluorophores. In recent years, metal organic framework (MOF) based luminescent materials that adsorb and encapsulate organic dye molecules have emerged as very promising candidates to realize new types of OLEDs [7,13–20]. MOFs are usually highly porous with modifiable pore size, and can effectively encapsulate organic dye molecules [21]. In fact, the dye molecules in the MOF channels are separated from each other. This effectively prevents aggregation-induced quenching and inhibits nonradiative relaxation related to the internal molecular motion. As a result, MOF-based OLEDs are very effective with respect to the emission quantum yield and photochemical stability [16,22]. In addition, by encapsulating different fluorescent dyes into the MOF pores, OLEDs emitting various colours can be easily obtained and they exhibit interesting luminescent properties (e.g. we recently reported linearly polarized warm-yellow light emission based on orientated dyes in rodlike MOF crystals [20]). Furthermore, by mixing different dyes fluorescing at their respective primary colours in MOF composites, a white light-emission can be obtained. In Ref. [18], Wen et al. introduced three dye molecules with respective red/green/blue (R/G/B) luminescence into the MOF channel simultaneously, and formed white light emissions
Corresponding author. E-mail address: [email protected] (J. Wang).
https://doi.org/10.1016/j.optmat.2019.01.019 Received 17 November 2018; Received in revised form 13 January 2019; Accepted 16 January 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 89 (2019) 209–213
J. Wang et al.
ambient temperature raised up to 250 cm3/g at 0.1 relative pressure and showed a typical type I isotherm (Fig. S1), which proved that this MOF crystal was a microporous material. 2.2. Preparation of MOF⊃dye crystals emitting the four primary colours Fig. 1. Scheme for realizing white light emission based on MOFs incorporated with fluorescent dyes of red/yellow/green/blue primary-colours.
Different dye molecules are introduced into the MOF pores by two methods. DCM, coumarin 6 (C6), and coumarin 120 (C120) were used respectively as the red, green, and blue dyes to prepare MOF⊃DCM, MOF⊃C6, and MOF⊃C120 via the activated adsorption reaction. The preparation strategy of R/G/B MOF⊃dyes is essentially the same as that used in Ref. [18]. Additionally, RhB was used as the yellow dye molecule for the in-situ synthesis of MOF⊃RhB. MOF⊃DCM/C6/C120 crystals were prepared via the activated adsorption reaction with the following steps: approximately 50 mg of the newly prepared colourless and transparent MOF crystals were measured and placed into a 25 ml clean beaker. The crystals were washed three times with 10 ml DMF solvent and filtered with medium speed filter paper. They were then placed in a 60 ml evaporating dish and warmed in a vacuum drying chamber (0.2 kPa pressure) at 100 °C for approximately 10 h, to obtain the colourless MOF crystals with the guest molecules eliminated. Ten mg of the solid dye was weighed and placed in a 20 mL sample vial, and 10 mL of DMF solvent was added. The mixture was shaken gently until the dye completely dissolved. As the change in solution volume is negligible before and after dissolution of the dye, the resulting concentration of the solution was simply 1 mg/mL. Subsequently, 20 mg of the activated MOF crystals were weighed and immersed in dye/DMF solution. The vial was sealed and stored undisturbed and away from light for two days, and the rudimentary MOF⊃dye product was obtained. In the final step, the rudimentary product was washed with DMF solvent until the liquid became clear and free of fluorescence under UV light. The MOF⊃dye was collected as the final product. It should be noted that MOF crystals exhibit different stabilities in different solvents. The solvent used for MOF preparation was also used as the solvent for the dyes to ensure the stability of MOF crystals during adsorption. Fig. 3(a), (c), and (d) show respectively the fluorescence microscopy images of MOF⊃DCM, MOF⊃C6, and MOF⊃C120 crystals under UV irradiation. MOF⊃RhB crystals were synthesised via the in-situ reaction as follows: an MOF precursor solution was first made by following the same steps as for the preparation of the MOF before heated. Then, 60 ml of this precursor solution was divided into two portions. Thirty mg of RhB were added into one portion (30 ml) and ultra-sonicated until the dye was completely dissolved, to produce the MOF precursor solution with RhB. The concentration of RhB in the solution was 1 mg/mL. To avoid the aggregation of RhB molecules in the solution, the concentration should remain below a certain value. In the third step, the solution was transferred to a polytetrafluoroethylene liner of 50 mL capacity and heated at 100 °C for 48 h. Then, after being cooled naturally to room temperature, the reaction was finished. Fig. 3 (b) presents the microscopy images of respective fluorescent MOF⊃RhB crystals under UV irradiation. X-ray powder diffraction (XRPD) experiments (Bruker D8 ADVANCE) were also performed for the MOF and MOF⊃dye crystals, and their XRPD patterns are shown in Fig. 4 The diffraction peaks of these crystals are basically the same, which means that their spatial framework structures should be basically same, i.e. the MOF structure is not affected by the presence of the dye molecules. The concentrations of DCM, C6 and C120 adsorbed inside the MOF can be evaluated from the concentration differences of these dyes in the solution before and after the adsorption procedure. The concentrations of these dyes in the solution were expressed via the relationships between the absorbance and the concentration of these dyes in the DMF solution, which were obtained via the UV-Vis absorbance measurement (Figs. S2, S3, S4 and S5). In this work, the concentration of DCM, RhB, C6 and C120 in MOFs were 0.098%wt, 0.068%wt, 0.039%wt and
with bit-tunable emission spectra by changing the contents of the three dye molecules or the excitation wavelength. It should be noted that the dye molecules used in Ref. [18] are linear molecules, which enter the MOF channel relatively easily via ion exchange, e.g. fluorescent brightener (CBS-127, KSN), coumarin 6 (C6), 4-(dicyanomethylene)-2methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), 4-(p-dimethylaminostyryl)-1-methylpyridinium (DSM), etc. However, as discussed in Ref. [18], those planar dye molecules including cationic Rhodamine-B (RhB), anionic methylene blue and neutral RhB-6G have difficulty to enter the MOF channel via ion exchange, owing to their relatively stronger covalent bonds. This limits the preparation of white OLEDs based on MOF composites. Also, the spectrum of the white OLED realized in Ref. [18] is not flat enough, where the 1 dB bandwidth of the flat top (commonly defined in optical filters [23]) of this OLED seems to be < 150 nm. This would limit the utilization of this type of white OLED e.g. in VLC. In this study, we realize spectrally-flat white light emission based on MOF⊃dyes, which are loaded with red/yellow/green/blue fluorescent dye molecules, respectively. The schematic of the strategy is shown in Fig. 1. MOFs are prepared by the solvent thermal reaction method, and the dye molecules are introduced into the MOF pores via two methods: the red, green and blue MOF⊃dyes are prepared via an activated adsorption reaction, and the yellow MOF⊃dye is prepared via an in-situ reaction. These red/yellow/green/blue MOF⊃dye powders are then ground and mixed. By adjusting the mass proportions of each primarycolour component, white light emission can be obtained. In the manuscript, the fluorescence properties of the four primary-colours and of white light emission are also characterized. 2. Preparation of MOF crystals and primary-colour MOF⊃dye 2.1. Synthesis of MOFs We first synthesize the MOF-BDC crystal, namely [Zn2(BDC)2TED]n (TED = triethylenediamine, BDC = 1,4-benzenedicarboxylic acid), by using the solvent thermal reaction method. The preparation of the MOF was as follows: a mixture of Zn(NO3)2·6(H2O) (2.8 mmol), H2BDC (2.9 mmol) and TED (1.95 mmol) was added to a 30 ml N,NDimethylformamide (DMF) solution; an ultrasonic treatment was applied to dissolve all chemicals until the solution was clear and then a certain amount of the nitric acid solution was added to adjust the pH value. The solution was then transferred into a 50 ml Teflon-lined autoclave and heated in an oven at 100 °C for 48 h. Finally, after being cooled naturally to room temperature, millimetre-size crystals of MOFBDC were obtained. A single MOF-BDC crystal was colourless and transparent. Fig. 2 shows the molecular structure diagrams of the MOF-BDC crystal obtained from single crystal X-ray diffraction (XRD) observations (Bruker D8 QUEST). From the side view of the MOF channel structure, shown in Fig. 2(a), the MOF crystal has a periodical pore channel structure along the growth direction of the TED ligand, which is considered as the direction of the MOF channel. Fig. 2(b) shows that this MOF crystal has a six-prism shape, which belongs to a hexagonal system. The diameter of the largest MOF pore is 12.9 Å. Also, to gain the porosity information of this MOF crystal, we performed the nitrogen adsorption measurement by using V-Sorb 2800P specific surface area and pore size analyser. Its nitrogen adsorption isotherm at 25 °C 210
Optical Materials 89 (2019) 209–213
J. Wang et al.
Fig. 2. Molecular structure diagrams of the MOF-BDC crystal obtained from single crystal X-ray diffraction observations. (a) Side view and (b) cross-sectional view of the MOF channel structure.
Fig. 3. Microscopy images of the four fluorescent MOF⊃dye crystals under UV excitation. a) The red-emitting MOF⊃DCM; b) yellow-emitting MOF⊃RhB; c) green-emitting MOF⊃C6; and d) blue-emitting MOF⊃C120. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 5. Photoluminescence spectra of MOF⊃DCM, MOF⊃RhB, MOF⊃C6, and MOF⊃C120 crystals by UV excitation (365 nm wavelength).
dyes were dispersed in solvent with concentrations from 0.2 mg/ml to 1 mg/ml, the emission peaks of DCM, RhB, C6 and C120 in the DMF solution were at 609–614 nm, 579–610 nm, 519–528 nm, and 437–438 nm (Fig. S6), similar to those values in Refs. [18,24,25], where the respective 1-dB bandwidths were about 33–38 nm, 15–20 nm, 22 nm and 29 nm. Therefore, the respective dyes have been incorporated in MOFs and were responsible for the luminescence. Also, by comparing the emission peaks before and after incorporating the respective dyes into MOFs, it can be seen that the emission peaks of most dyes did not change much, while that of MOF⊃RhB was shifted to shorter wavelength. Thus, it could be regarded that these dye molecules in the MOF channels were separated from each other, and the aggregation-induced quenching did not take place. Most interestingly, while 1-dB bandwidth of the in-situ synthesized MOF⊃RhB was not really changed, the 1-dB bandwidths of dyes adsorbed into the MOF channel increased. This is due to the charge transfer among the dyes closed together in the MOF channel, resulting in broadening of the emission. In fact, the increase of the 1-dB bandwidth of dyes incorporated inside the MOF is advantageous to realize a white light emission with a wide bandwidth. Also, we checked the MOF⊃dyes with different dye concentrations. As discussed above, the R/Y/G/B MOF⊃dye crystals were not saturated adsorbed and had low dye concentrations. When the activated MOF crystals were immersed in DCM/C6/C120 solution for 96h, the concentration of DCM, C6 and C120 in MOFs were 0.186%wt, 0.063%wt and 0.087%wt. It was found that the longer the MOFs were soaked in DCM/C6/C120 solution, the darker the MOF⊃dye crystals were. For
Fig. 4. XRPD patterns of MOF, MOF⊃C120, MOF⊃C6, MOF⊃RhB and MOF⊃DCM (from bottom to top).
0.048%wt respectively. Due to the low concentration of dyes inside the MOF, the MOF structures did not change, as proved in Fig. 4. It should be noticed that the single crystal X-Ray diffraction measurement was not applicable for such low dye concentrations. 2.3. Luminescence property of primary colour MOF⊃dye crystals The photoluminescence (PL) spectra of these MOF⊃dye crystals emitting primary colours were measured under UV excitation (at 365 nm wavelength), shown in Fig. 5. It can be seen under UV excitation that the emission peaks of the respective R/Y/G/B MOF⊃dye crystals are located at 621.0 nm, 560.0 nm, 522.5 nm and 438.5 nm, where 1-dB bandwidths of respective MOF⊃dyes were about 50.5 nm, 18 nm, 45 nm, and 34.5 nm. On the other hand, when these R/Y/G/B 211
Optical Materials 89 (2019) 209–213
J. Wang et al.
Fig. 6. The CIE coordinates of the MOF⊃dyes emitting the four primary colours. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 8. CIE coordinates of the MOF⊃dye composites obtained by varying mass proportions of the primary-colour fluorescent crystals. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 7. Time-resolved fluorescence of R/B/G/Y MOF⊃dyes, their 1/e−lifetimes were 1.00 ns, 3.59 ns, 1.06 ns and 2.50 ns respectively.
Fig. 9. Photoluminescence spectrum of white light-emitting MOF⊃dye.
Table 1 Compositions of the white light-emitting MOF⊃dye (No. 5 is the best so far). No.
MOF⊃DCM
MOF⊃RhB
MOF⊃C6
MOF⊃C120
1 2 3 4 5
2.0 mg 4.4 mg 5.5 mg 7.5 mg 7.5 mg
4.5 mg 4.5 mg 4.5 mg 4.5 mg 4.5 mg
3.1 mg 3.1 mg 3.1 mg 3.1 mg 3.1 mg
5.1 mg 5.1 mg 5.1 mg 5.1 mg 6.1 mg
The CIE coordinates of respective MOF⊃dye crystals were also measured and marked in Fig. 6. The CIE coordinates of R/Y/G/B MOF⊃dye crystals are (0.575, 0.414), (0.400, 0.588), (0.273, 0.417), and (0.156, 0.093) respectively. By comparing them with the CIE coordinates of respective saturated R/Y/G/B fluorescence, namely (0.66, 0.33), (0.48, 0.48), (0.21, 0.71), and (0.14, 0.08), it can also be concluded that synthesized R/Y/G/B MOF⊃dye crystals were not saturated adsorbed. Furthermore, we studied the PL lifetimes of respective R/Y/G/B MOF⊃dye crystals, via a time-resolved fluorescence measurement (Edinburgh FLS920P). The measurement results are shown in Fig. 7. The fitted PL lifetimes of respective R/Y/G/B MOF⊃dye crystals were 1.00 ns, 3.59 ns, 1.06 ns and 2.50 ns respectively, while the measured PL lifetimes of respective R/Y/G/B dyes were 2.53 ns, 3.00 ns, 2.26 ns and 3.38 ns. The PL lifetime of the MOF⊃RhB has a slight increase where that of most dye-MOFs has a significantly decrease which is benefit for improving the intrinsic modulation frequency for VLC applications. Last but not least, the quantum yields of the R/Y/G/B MOF⊃dye
the in-situ synthesized MOF⊃RhB, the concentration of RhB in product fell to 0.049%wt as the concentration of RhB in the precursor solution was decreased from 1 mg/ml to 0.75 mg/ml. For these MOF⊃dyes with different dye concentrations, we performed PL spectra, shown in Fig. S7. The emission peaks and bandwidths of MOF⊃DCM, MOF⊃C6 and MOF⊃C120 are basically same, while that of MOF⊃RhB had a slight change. It should be noticed that the spectra and the intensities of four MOF⊃dye crystals would be varied under excitation at different wavelengths, as proved in Ref. [18].
212
Optical Materials 89 (2019) 209–213
J. Wang et al.
crystals were measured to be 0.43%, 2.0%, 4.9% and 9.3%.
Declaration of interest
3. Preparation and characterisation of the white light-emitting MOF⊃dye composite
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The white light-emitting MOF⊃dye composite was prepared by mixing R/Y/G/B primary-colour MOF⊃dye crystals. At the beginning, the R/Y/G/B MOF⊃dye crystals with respective mass of 2.0 mg, 4.5 mg, 3.1 mg and 5.1 mg (corresponding mass proportions of 13.6%, 30.6%, 21.1%, and 34.7%) were firstly fully ground in a grinder to obtain the mixture No. 1. The CIE coordinates of this mixture were found to be (0.262, 0.312), which indicates that the white light has a bluish tint. To amend for the excess blue light and inadequate yellowred light given by the mixture No. 1, the proportion of constituents was adjusted step by step. As shown in Table 1, the quantity of the redfluorescent MOF⊃DCM crystals was increased and that of the bluefluorescent MOF⊃C120 crystals was varied. The change in the CIE coordinates of the adjusted mixtures is shown in Fig. 8. Material emitting warm white light (No. 5) was eventually obtained at mass proportions of 35.3% (R), 21.2% (Y), 14.7% (G), and 28.8% (B). The CIE coordinates of this material are (0.319, 0.341), and the colour temperature is approximately 5500 K. The resulting photoluminescence spectra of the MOF⊃dye composites (from No. 1 to No. 5) emitting warm white light is shown in Fig. 9. It can be seen in the photoluminescence spectrum of the first synthesized composite No. 1 that the red spectral part was not sufficient. Therefore, from No. 1 to No. 4, the mass of the red MOF⊃DCM crystal was increased from 2.0 mg to 7.5 mg. The composite No. 4 exhibited a sufficient red spectral part, however, a not sufficient blue spectral part. Thus, from No. 4 to No. 5, the mass of the red MOF⊃C120 crystal was increased from 5.1 mg to 6.1 mg. Finally, when the mass of DCM, RhB, C6, and C120 were 7.5 mg, 4.5 mg, 3.1 mg and 6.1 mg, a white light emission with flat spectrum was obtained. For this composite No. 5, the full-width at half-maximum (FWHM) bandwidth of the light-emission spectrum range is 251.5 nm (from 391.5 nm to 643.0 nm), and the 1-dB bandwidth is 201.5 nm, almost covering the entire visible light region. Compared with the 1 dB bandwidth of the materials shown in Refs. [7,18], ∼150 nm and < 150 nm respectively, this organic material is a better candidate for white light emission. Also, for these composites (No. 1 to 5), the quantum yields were between 2.0% and 4.0% (limited by the lowest quantum yield of MOF⊃DCM), while the PL lifetimes were 3–5 ns. Nevertheless, in comparison to commercial inorganic phosphors, this material has a good CIE coordinate and a short PL lifetime, resulting in a high intrinsic modulation bandwidth.
Acknowledgment This work was supported by the National Natural Science Foundation of China, (Grant No. 61575096). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.01.019. References [1] S. Tonzani, Time to change the bulb, Nature 459 (2009) 312–314. [2] P. Pust, P.J. Schmidt, W. Schnick, A revolution in lighting, Nat. Mater. 14 (2015) 454–458. [3] C. Lee, C. Shen, C. Cozzan, et al., Gigabit-per-second white light-based visible light communication using near-ultraviolet laser diode and red-, green-, and blue-emitting phosphors, Optic Express 25 (2017) 17480. [4] T. Komine, M. Nakagawa, Fundamental analysis for visible-light communication system using LED lights, IEEE Trans. Consum. Electron. 50 (2004) 100–107. [5] T. Koonen, Indoor optical wireless systems: technology, trends and applications, J. Lightwave Technol. 36 (2018) 1459–1467. [6] H. Chun, P. Manousiadis, S. Rajbhandari, et al., Visible light communication using a blue GaNμLED and fluorescent polymer color converter, IEEE Photon. Technol. Lett. 26 (2014) 2035–2038. [7] Z. Wang, Z. Wang, B. Lin, et al., Warm-white light-emitting diode based on a dyeloaded metal-organic framework for fast white-light communication, ACS Appl. Mater. Interfaces 9 (2017) 35253–35259. [8] Z. Zhao, H. Su, P. Zhang, et al., Polyyne bridged AIE luminogens with red emission: design, synthesis, properties and applications, J. Mater. Chem. B 5 (2017) 1650–1657. [9] B. W. D'Andrade, S.R. Forrest, White organic light-emitting devices for solid-state lighting, Adv. Mater. 16 (2004) 1193–1203. [10] N.H. Kim, Y.H. Kim, J.A. Yoon, et al., Color optimization of single emissive white OLEDs via energy transfer between RGB fluorescent dopants, J. Lumin. 143 (2013) 723–728. [11] Y. Sun, N.C. Giebink, H. Kanno, et al., Management of singlet and triplet excitons for efficient white organic light-emitting devices, Nature 440 (2006) 908–912. [12] H.H. Fang, Q.D. Chen, J. Yang, et al., Two-photon pumped amplified spontaneous emission from cyano-substituted oligo(p-phenylenevinylene) crystals with aggregation-induced emission enhancement, J. Phys. Chem. C 114 (2010) 11958–11961. [13] S. Jin, H.J. Son, O.K. Farha, et al., Energy transfer from quantum dots to metalorganic frameworks for enhanced light harvesting, J. Am. Chem. Soc. 135 (2013) 955–958. [14] H. Yang, White organic light-emitting devices based on blue fluorescent dye combined with dual sub-monolayer, J. Lumin. 142 (2013) 231–235. [15] M.J. Dong, M. Zhao, S. Ou, et al., A luminescent dye@ MOF platform: emission fingerprint relationships of volatile organic molecules, Angew. Chem. 53 (2014) 1575–1579. [16] Y. Cui, H. Xu, Y. Yue, et al., A luminescent mixed-lanthanide metal-organic framework thermometer, J. Am. Chem. Soc. 134 (2012) 3979–3982. [17] Y. Cui, R. Song, J. Yu, et al., Dual-emitting MOF⊃dye composite for ratiometric temperature sensing, Adv. Mater. 27 (2015) 1420–1425. [18] Y. Wen, T. Sheng, X. Zhu, et al., Introduction of red-green-blue fluorescent dyes into a metal-organic framework for tunable white light emission, Adv. Mater. 29 (2017) 1700778. [19] L.L. Chong, Z. Feng, L. Xing, et al., A luminescent Ln-MOF thin film for highly selective detection of nitroimidazoles in aqueous solutions based on inner filter effect, J. Lumin. (2019) 23–29. [20] J. Wang, F. Ye, Z.J. Huang, et al., Linearly polarized surface warm-yellow LED based on orientated organic dyes in rod-like metal-organic framework crystal arrays, Opt. Mater. Express 8 (2018) 2901–2909. [21] H.L. Jiang, Y. Tatsu, Z.H. Lu, et al., Non-, micro-, and mesoporous metal-organic framework isomers: reversible transformation, fluorescence sensing, and large molecule separation, J. Am. Chem. Soc. 132 (2010) 5586–5587. [22] J. Yu, Y. Cui, H. Xu, et al., Confinement of pyridinium hemicyanine dye within an anionic metal-organic framework for two-photon-pumped lasing, Nat. Commun. 4 (2013) 2719. [23] H. Venghaus (Ed.), Wavelength Filters in Fibre Optics, Springer Verlag, Berlin, 2006. [24] S.K. Sagoo, R.A. Jockusch, The fluorescence properties of cationic rhodamine B in the gas phase, J. Photochem. Photobiol., A 220 (2011) 173–178. [25] H.V. Demir, S. Nizamoglu, T. Erdem, et al., Quantum dot integrated LEDs using photonic and excitonic color conversion, Nano Today 6 (2011) 632–647.
4. Conclusion We achieve spectrally wide and flat white light emission based on MOF⊃dye composite, which consists of R/Y/G/B primary-colour MOF⊃dye powders. The MOF crystal is prepared by the solvent thermal reaction method, in which the R/G/B dye molecules are introduced into the MOF pores via an activated adsorption reaction, and the yellow dye molecules are loaded via an in-situ reaction. By mixing and grinding these R/Y/G/B MOF⊃dye powders and adjusting the mass proportions of each primary-colour component, white light emission is obtained under excitation by an ultraviolet light (at a wavelength of 365 nm). When the respective mass proportions of R/Y/G/B components are 35.3%, 21.2%, 14.7% and 28.8%, the FWHM of the light-emission range is 251.5 nm (from 391.5 nm to 643.0 nm), the 1-dB bandwidth is 201.5 nm, and the CIE coordinates are (0.319, 0.341). The spectrally flat white light emission characteristics show that this material could be a promising candidate in applications such as lighting, display and VLC systems, where this strategy has better repeatability, colour stability and requires a simple preparation process.
213