Recent Progress in Metal-Organic Frameworks for White-Light Emission

LI Qing-QingDONG Ya-WenMao Fei-FeiWANG Kuai-Bing＊，，3WU Hua＊，ZHANG Qi-Chun

(1College of Resources and Environmental Sciences,Nanjing Agricultural University,Nanjing 210095,China)

(2Jiangsu Key Laboratory of Pesticide Sciences,Department of Chemistry,

College of Science,Nanjing Agricultural University,Nanjing 210095,China)

(3School of Materials Science&Engineering,Nanyang Technological University,Singapore 639678,Singapore)

Abstract:Metal-organic frameworks (MOFs)are a high-profile sort of multifunctional hybrid materials and display various luminescent behaviors due to the diversities of their architectures.Extraordinarily，the emergence of white-light-emitting (WLE)MOFs offers golden opportunities for designing and constructing new luminescent MOFs materials owing to their environmental friendliness，long service lives and high efficiencies.In this manuscript，we focus on recent developments of the WLE MOFs with particular emphases on their synthetic methods and properties，which mainly include co-doping of Ln3+in multifarious MOFs materials，lanthanide encapsulation or organic molecules capture in MOF pores to obtain tunable WLE MOFs，and several potential sensing applications including temperature，molecular and metal ions sensors.In addition，imminent challenges and future developments of WLE MOFs materials are both proposed.This review may arouse the interest of researchers who will design and construct new luminescent MOFs.

Keywords:metal-organic frameworks;white-light-emitting;synthetic methods;sensors

0 Introduction

Solid-state white-light-emitting (WLE)materials have caused tremendous concern in the fields of lighting or display devices due to their exciting properties such as environmental friendliness，excellent energy-saving，long operation lifetime，and high efficiency[1-5].Generally，the existing white light is mainly obtained by regulating the proportion of lightemitting diodes with fundamental blue，red and green emission.Nevertheless，the method gradually shows its intrinsic shortcomings such as high cost，thermal properties，phase separation，and poor color stability[6].In this situation，some scientists are trying to switch their research to light-emitting metal-organic framework(MOFs)materials，and the characteristic advantages of MOFs in solid-state luminescence are gradually revealed.Particularly，MOFs based on luminescent trivalent lanthanides (LnMOFs)have been widely considered as new multicolor luminescent materials because the emission of MOFs can be tuned through the rational choices of lanthanide ions or clusters(as nodes)as well as functional organic ligands(as spokes).Other available methods to gain WLE MOFs are through the co-doping of various emissive Ln3+ions into the isostructural MOFs materials.In general，Ln3+ions with high luminescence quantum yield，large Stokes shift，long-lived luminescence lifetimes，and narrowband emission[7-10]can be adopted as excellent candidates to construct a series of photoluminescence MOFs materials.

The remarkable structural of MOFs[11-14]also conduce to the design and preparation of photoluminescence materials[15].In the case of pore structure and surface area，MOFs obviously outperform traditional materials such as molecular sieve，mesoporous silica，macroporous polymer resin，and activated carbon.What′s more，MOFs could possess the world-record highest surface areas up to about 10 000 m2·g-1[16]，which can be fitted well in the fields of gas storage[17]，drug delivery[18-19]，opticalsensing/detection[20]，and energy storage[21-25].Taking advantage of these features，tunable colors and white light emissions are achieved by inserting luminescent guest molecules into MOF-based materials through host-guest supramolecular interactions[26-28].Thus，these functionalized MOFs not only possess traditional luminescence features but also exhibit some distinctive optical behaviors.To our knowledge，the first report concerning luminescent MOFs appeared in 2002.Since then，a large number of researches associated with the luminescent MOFs have been reported，whilst a few reviews covering the synthesis and applications of MOFs with WLE have been published.In this review，we summarized the design and synthetic methods of WLE MOFs as well as their potential applications in several fields including ratiometric luminescent thermometers and optical sensors for small molecules or metal ions in recent five years.

1 Design strategies for tunable WLE

In recent years，WLE MOFs materials have been well studied and extensively employed in the fields of illumination and display due to their unique superiority[29-32].Nevertheless，improving the quality of white lightgenerated from these MOFsisstill considered as a key issue and is very challenging.High-quality white-light illumination requires three explicit parameters[33]:Firstly，Commission Internationale de l′Eclairage (CIE)，usually representing chromaticity，is the quality of the color.The coordinate(0.33，0.33)indicates the equal energy point or the white point on the CIE chart;Secondly，correlated color temperature (CCT)is a measure of light source color appearance ranging from 2 500 to 6 500 K，and finally，color rendering index (CRI)is served as an evaluation criterion to assess the ability of a light source and displays the true colors of an object.CRI greater than 80 manifests that the true color of the light source can be accurately reflected.

The existing MOFs are obtained by means of the combination of hundreds of organic ligands and metal ions，which endows MOFs with multifarious functions and structures.On the contrary，some desired MOFs with specific functions can be designed and acquired through adjusting organic ligands or metal ions used in preparation processes.Therefore，a good deal of attractive luminescence natures from MOFs can be produced，which are expected to provide a unique platform for the design and synthesis of new luminescent materials in solid phase.To be specific，organic linkers，metalions，adsorbed luminophores， and exciplex formation bound to the MOF cavities are all important factors that influence the generation of luminescence.Various optical and photonic MOFs materials can be obtained through doping different amount of Ln3+ions，encapsulating dye molecules，and absorbing exciplex into the resulting MOFs，which have been also confirmed as valid approaches to regulating the luminescent properties[34-38].

1.1 Lanthanide MOFs as WLE materials

Based on the emitting chromophore and mechanism，the photoluminescence (PL)of metalorganic frameworks can be achieved by means of different origins:intralig and charge transfer(ILCT)，metal-centered (MC)transitions (d-d，d-f，f-f，etc.)，ligand centered (LC)，π→π*or n→π*transitions，charge transfer between metal and ligand (MLCT or LMCT)， etc.Therefore， WLE materialscan be obtained by controlling the types and relative concentrations of Ln3+ions.

Experiments conducted by Pan et al.[39]showed that two series of Ln3+complexes with either 2D(6，3)-hcb network (LIFM-24(Ln)，Ln=Pr，Nd，and Sm;LIFM=Lehn Institute of Functional Materials)or 1D loop-and-chain(LIFM-25(Ln)，Ln=Eu，Gd，Tb，Dy，Ho，Er，Tm，and Yb)polymeric structures were designed by using a 1，1′-(1，4-phenylenebis(methylene))dipyridin-4(1H)-one zwitterionic ligand.Moreover，these Ln3+ions were coordinated through ligands and NO3-ions.Polymeric structures of these compounds are highly dependent on the radius of the lanthanide ions.Among the Ln-complexes，LIFM-24(Pr)and LIFM-25(Dy) exhibited white-light emission by varying excitation wavelengths(Fig.1).

Fig.1 Tunable and white-light emission of LIFM-24(Pr)at room temperature;Photo-excited images(a)and emission spectra;(b)CIE coordinates of the emission spectra at an excitation wavelength from 300～360 nm(the excitation wavelength interval is 3 nm，and the inset manifests the dependence of CIE-x with the excitation wavelength);(c)Excited at 290(orange)，330(pink)，345(white)，and 355(blue)nm，respectively[39]

When excited with 345 nm light，LIFM-24(Pr)emitted white light and the CIE coordinates were(0.34，0.30)with a correlated color temperature(CCT)and a color rendering index(CRI)of 5 468 and 98 K，respectively.Similarly，when excited with 339 nm light，LIFM-25(Dy)displayed white light with CIE coordinates closer to the white-light point(Fig.2).The CCT and CRI were 5 461 and 97 K，respectively.These two Ln-complexes showed WLE from the combination of ligand-based and lanthanide-based emissions when excited at an appropriate wavelength.

The study investigated by Song et al.[40]showed that an Eu-Tb-codoped La3+complex with a blueemitting 1，3-bis(4-carboxyphenyl)imidazolium as an organic ligand displayed white light with CIE coordinates of(0.323，0.358)，(0.323，0.337)，(0.314，0.327)，and(0.339，0.323)under the excitation wavelengths at 380，385，390，395 nm light，respectively.The emission probably stemmed from ILCT，MC and LMCT excited states.Some porous anionic Ln-MOFs with 1D hydrophilic channels were synthesized by the self-assembly of 5-(4-carboxyphenyl)picolinic acid (H2CPA)and lanthanide ions[41].Li and co-workers adjusted appropriately the proportion of the lanthanide ions in this framework to achieve dichromatic emission colors and three primary colors even white light.While the molar ratios of Gd3+∶Tb3+∶Eu3+were 0.99 ∶0.005 ∶0.005，0.98 ∶0.005∶0.015，and 0.98∶0.015∶0.005，white-light emissions with CIE coordinates of (0.292 1，0.281 5)，(0.3526，0.2843)，and(0.3315，0.331 7)were obtained separately.When 0.5%～1.5%(n/n)Tb3+was mixed into the Eu-Gd system，the as-obtained CCT was 5 531 K，falling into the white-light region.

Fig.2 Tunable and white-light emission of LIFM-25(Dy)at room temperature;Emission spectra(a)excited at 290，320，339，and 360 nm，respectively;(b)CIE coordinates of the emission spectra at the excitation wavelength from 300～360 nm(the excitation wavelength interval is 3 nm，and the inset manifests the dependence of CIE-x with the excitation wavelength);(c)Schematic model for an excitation-dependent fluorescent plate made from LIFM-24(Pr)and LIFM-25(Dy)[39]

Similarly，Li group[42]synthesized three isomorphic red-，blue-，or green-light-emitting Ln-MOFs using H3dcpcpt(3-(3，5-dicarboxylphenyl)-5-(4-carboxylphenl)-1H-1，2，4-triazole)as the ligand by solvothermal reactions.A target complex，[Eu0.41Gd0.42Tb0.17(dcpcpt)(H2O)]n，was in-situ synthesized by meticulously tuning the stoichiometric ratio of Eu3+，Gd3+，and Tb3+ions.If excited with 320 nm light，the as-obtained MOF emitted white light with CIE coordinates of(0.330，0.338).Xiong et al.[43]synthesized three isostructural Ln-MOFs containing p-terphenyl-2，2″，4，4″-tetracarboxylate ligand (H4L)with red，green，and blue luminescent materials using Eu，Tb，or Dy dopants，respectively.Moreover，a series of mixed Ln-MOFs were synthesized by accurately regulating amounts of the doped ions and studied their luminescence properties.With the excitation wavelength at 336 nm，the CIE coordinates being (0.32，0.25)at 100 K and(0.30，0.32)at 50 K resulted in the emission of white light by the variation of energy transfer efficiency between lanthanide ions (Fig.3).Similarly，when the temperature reached 200 and/or 150 K，the emission colors became white with the CIE coordinates being(0.31，0.25)and/or (0.30，0.30)when the excitation wavelength was changed into 363 nm(Fig.4).

Fig.3 (a)Solid-state emission spectra of(Eu0.0667Tb0.0667Dy0.8666)(HL)(H2O)(DEF)excited at 336 nm under temperature from 300 to 50 K;(b)CIE chromaticity diagram of(Eu0.0667Tb0.0667Dy0.8666)(HL)(H2O)(DEF)excited at 336 nm as the temperature changing from 300 to 50 K[43]

Fig.4 (a)Solid-state emission spectra of(Eu0.0666Tb0.4667Dy0.4667)(HL)(H2O)(DEF)excited at 363 nm under different temperatures from 300 to 50 K;(b)CIE chromaticity diagram of(Eu0.0666Tb0.4667Dy0.4667)(HL)(H2O)(DEF)excited at 363 nm as the temperature changes from 300 to 50 K[43]

1.2 Ln3+modified MOFs as WLE materials

The encapsulation of the appropriate ratios of Ln3+ions into the porous structure of transition-metalbased MOFs provides new and alternate methods to realize WLE.In such systems，the coordination framework possesses dual functions as both a host and an antenna for protecting and sensitizing the Ln3+cations[44-46].Generally，transition metals used for the synthesis of WLE MOFs mainly include Cd2+and Zn2+.A feature article reported by Yan et al.[47]mentioned a new direction to build luminescent barcoded systems with anionic[Cd3(5-tbip)4]2-frameworks containing a series of luminescent Ln3+ions(Ln=Eu，Tb，Sm，Dy).It can be seen from the study that Dy3+@Cd-MOF exhibited white light output and the emission bands locating at about 480 and 574 nm，which could be assigned to4F9/2→6F15/2and4F9/2→6F15/2transitions of Dy3+respectively.Meanwhile，luminescence modulation was also achieved by co-doping Sm3+and Tb3+ions into the Cd-MOF，and the optimal proportion of the codopants for WLE was 98%(n/n)Sm3+and 2%(n/n)Tb3+.Similar white-light emission results were also gained in the Ln3+-incorporated microporous MOF materials，namely， HPU-14 ({[Zn3(L)2·2(Me2NH)]·3(CH3OH)·5(H2O)}n[48].The 1D nanotube channels of the blueemitting HPU-14 were occupied by Tb3+and Eu3+ions when soaked in the H2O solutions containing Tb3+and Eu3+ions(Fig.5).

The complexes with encapsulated Tb3+(HPU-14@Tb3+)and Eu3+(HPU-14@Eu3+)emitted green and red light(λex=365 nm)，respectively.The concurrent incorporation of Tb3+and Eu3+into the channel(HPU-14@Tb3+@Eu3+)and the adjustment of their relative concentration were carried out to produce WLE (Fig.6).When the concentration ratio of Tb3+and Eu3+was tuned to 5∶4，the blue emission of the host framework in concurrence with the green emission of Tb3+and the red emission of Eu3+resulted in white light(0.325，0.315)(λex=365 nm)(Fig.7).This material had a quite high quantum yield(7.1%)and the excited state lifetime was determined to be 0.733 ms at room temperature.

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Fig.5 Schematic illustration of Ln3+incorporation into HPU-14[48]

Fig.6 Single crystals of HPU-14，HPU-14@Tb3+，HPU-14@Eu3+，and HPU-14@Tb3+@Eu3+illuminated with 365 nm laboratory UV light[48]

Fig.7 CIE coordinates of HPU-14@Tb3+@Eu3+and(a)the emission spectra of HPU-14，(b)HPU-14@Tb3+，(c)HPU-14@Eu3+and(d)HPU-14@Tb3+@Eu3+[48]

1.3 MOFs-dyes/complex composites as WLE materials

Besides the above-mentioned methods，the luminescent dye molecules encapsulated in the pores of MOFs have also received much attention due to their high quantum yields，facile chemical tunability，and fast radiative emission rates[49-50].Wang et al.[51]prepared WLE diodes by encapsulating yellowemitting rhodamine B(RhB)molecules in the cavity of a blue-emitting Al-DBA MOF (DBA=9，10-dibenzoate anthracene).Blue-emitting MOFs were immersed into an ethanol solution containing RhB(2.9 μmol·L-1)for approximately 28 h and finally obtained a composite hosting 0.09%(w/w)RhB molecules.This postsynthesized composite emitted bright white light with CIE coordinates，CCT，and quantum efficiency(QE)of(0.32，0.30)，6 085 K，and 12%，respectively.Similarly，some host-guest compositing materials were synthesized by Qian and co-workers[52]，which were also applied for WLE(Fig.8).

The resulting ZJU-28@DSM/AF (0.02%(w/w)DSM，0.06%(w/w)AF;DSM=4-(p-dimethylaminostyryl)-1-methylpyridinium;AF=acriflavine)complex emitted bright white light upon the excitation of 365 nm，which displayed three well-differentiated peaks.The blue emission band arose from MOF ZJU-28，the green band originated from AF，and the red one was attributed to DSM.The combination displayed a white light with CIE coordinates of(0.34，0.32)，which was rather close to those of ideal white light(0.33，0.33)(Fig.9).Furthermore，the CRI，CCT，and QE values of ZJU-28@DSM/AF reached 91，5 327 K，and 17.4%，respectively.Besides dyes，metal complexes can also be regarded as luminescent species.In 2013，Sun et al.mentioned a new approach to prepare white-light MOF[53].They combined a blue-emission MOF with a yellow-emission Ir-complex through an ion-exchange process to obtain a WLE MOF.Afterward，the cheaper Al-complex was also employed as a guest molecule to realize white light with CIE，CCT，QE，and lifetimes of(0.291，0.327)，7 796 K，11.4%，and 1.06 ns，respectively[54].

Fig.8 Schematic illustration of the encapsulation of cationic dyes into ZJU-28 via ion-exchange process[52]

Fig.9 (a)Emission spectrum;(b)emission colors in the CIE 1931 chromaticity diagram of ZJU-28@DSM/AF(0.02%(w/w)DSM，0.06%(w/w)AF)excited at 365 nm[52]

2 Potential applications for WLE MOFs

Luminescent MOFs，that represent one of the most attractive research topics，are widely used for sensing applications，especially with respect to the monochrome light-emitting such as red，green，blue，pink and yellow light，etc.This type of MOFs has been extensively reported and reviewed in recent years[32，55-57].Albeit the inspiring progress has been achieved，the potential applications of WLE MOFs as sensing materials in the environment are rarely mentioned.Hence，several recent researches of WLE MOFs on photoluminescence-based sensing of small molecules，temperature，and metal ions are presented.

2.1 Temperature sensors

Temperature，a fundamental physical parameter，exists in the natural environment，and we can feel it all the time.Compared with traditional thermometers，the fluorescence-based thermometry technique reveals more obvious advantages.For example，accuracy，sensitivity，and even the case of electromagnetic field fluctuations，this kind of thermometers are still noninterfering[58].In most applications，the sensing responses to the variations in emission intensity of one transition，which can be influenced by the excitation power，the drifts within the optoelectronic devices and whatnot.Furthermore，owing to the time-limited metrics，a certain number of fluorescent samples that only depend on energy and lifetime are not regarded as luminescent thermometers.On the contrary，dualemitting ratiometric，which utilizes the intensity ratio between two transitions of a single luminescent material rather than one transition，can circumvent the drawbacks of poor accuracy of the one transition intensity-based techniques.

In the research investigated by Hu and coworkers[59]，a series of new lanthanide compounds based on HL ligand((2-(2-sulfophenyl)-imidazo(4，5-f)(1，10)-phenanthroline)with various coordination modes of the sulfonic group，that could achieve luminescence WLE by energy transfer，were first synthesized.By adjusting the molar ratio of Eu3+/Tb3+from 1∶9 to 9∶1，the light-emission of four compounds changed from green to orange-pink and red，and the as-obtained white light upon the excitation of 370 nm.When the molar ratio of[EuL(glu)]n·2nH2O and [TbL(glu)]n·2nH2O(H2glu=glutaric acid)was 4∶6，pure white light could be basically realized at the excitation of 405 nm and the CIE color coordinate of(0.323，0.339)is close to those of pure white light.Besides，the as-obtained WLE material had high photoluminescence efficiency with a QE reaching up to 6.03%(Fig.10).

Meanwhile，to enhance the accuracy of most luminescence-based thermometers of a single emission，the mixing component of the molar ratio of 1Eu/2Tb (n1Eu∶n2Tb)of 4∶6 was utilized to meet the needs of temperature sensing performance from 50 to 225 K upon the excitation of 370 nm，and the emission spectra simultaneously exhibited the5D0→7F0～4transitions of the Eu3+and the5D4→7F6～2transitions of Tb3+ions.These two cations displayed similar temperaturedependent luminescence behaviors as demonstrated in Fig.11.The luminescent intensities between the Eu3+emission at 618 nm and the Tb3+emission at 546 nm decreased gradually with the increasing temperature because of the thermal activation of the nonradiative energy-transfer pathways.In addition，the temperature can be linearly related to the emission intensity ratio(ITb/IEu)of the5D4→7F5(Tb3+，546 nm)to5D4→7F5(Eu3+，618 nm)transition.The temperature sensitivity of n1Eu∶n2Tb=4∶6 was 0.68% per K，indicating that the n1Eu∶n2Tbcoordination polymer was a good luminescent thermometer in the range of 50 ～225 K.Similarly，Wang et al.[60]synthesized a set of mixed LnMOFs，[(EuxTb1-x)2(TDC)3(CH3OH)2(CH3OH)] (x=0.5，0.067，0.05，0.01，0.008 33，0.006 67，0.005，H2TDC=2，5-thiophenedicarboxylic acid)under solvothermal condition.The luminescence colors of these compounds could be modulated from red to orange，yellow，white and bluegreen by changing the excitation wavelength and the mixed molar ratio of Eu3+and Tb3+ions.As expected，the WLE Eu0.00667Tb0.99333-MOF was achieved with CIE(0.333 3，0.339 4)under the excitation wavelength at 350 nm through LMCT.The WLE material exhibited fascinating luminescence responsive to the temperature in the range of 288～353 K，which could be utilized as a temperature sensor.Interestingly，the intensity radio of IEu/ITDCcorrelated linearly very well to the temperature，enabling it as a ratiometric luminescent thermometer in the physiological temperature range (288～353 K).Therefore，the Tb3+/Eu3+mixed-MOF could be employed as an ideal candidate for self-referencing luminescent thermometers.

Fig.10 (a)Emission spectra of 1Eu-mixed 2Tb components in different mole ratios(λex=370 nm);(b)CIE x-y chromaticity diagram of 1Eu-mixed 2Tb components in different mole ratios(λex=370 nm)[CIE coordinates of(0.323，0.339)are called point l(n1Eu∶n2Tb=4∶6，λex=405 nm)];(c)Emission spectra of 3Eu-mixed 4Tb components in different molar ratios(λex=370 nm);(d)CIE x-y chromaticity diagram of 3Eu-mixed 4Tb components in different molar ratios(λex=370 nm)[59]

Fig.11 (a)Temperature-dependent PL spectra for the mixing components of n1Eu∶n2Tb=4∶6 in the temperature range of 25～300 K upon excitation at 370 nm;(b)Curves of the intensity for EuⅢ/5D0→7F2(618 nm)and TbⅢ/5D4→7F5(546 nm)with increasing temperature[59]

2.2 Molecule sensing

Benzene and its derived aromatic compounds are usually classified as volatile organic molecules(VOCs)[61].VOCs，a large group of species in the vapor and aqueous phase，can be potentially sensed by luminescence MOFs.The evaporation of a mass of VOCs will aggravate soil and water pollution，and even cause people some diseases，posing a serious threat to the environment and humans[62].Hence，the sensitive and selective detection of similar molecules is of great importance[63].The MOFs，serving as sensors，can mainly be broadly classified into two categories:solvatochromic MOF sensors[64]and vapochromic MOF sensors[65].Several obvious characteristics for the different VOC-exposed phases are a shift of emission spectra and an alteration of the luminescent intensity.

Though some plentiful examples of sensing reported in the literature to detect such organic molecules，most of them are non-WLE ratiometric MOF sensors[66].Furthermore，compared with temperature sensing，reports regarding molecule sensing using MOFs with WLE are still infrequent in recent years.Only a few documents regarding the sensing of molecules by white-light MOFs materials are discussed in the subsequent contents.The above-mentioned mixed Ln-MOF，marking as Eu0.00667Tb0.99333-MOF[60]，possesses white light emission under the excitation of 350 nm.It performed a sieving performance with respect to the studied phenolic compounds bisphenol A，quinol，phenol，β-naphthol，and 2，6-ditertbutylpcresol.The fluorescence intensities of Eu3+(617 nm)and Tb3+(545 nm)were almost completely quenched with the addition of β-naphthol(reaching 2.0 μmol)，while the ligand-based emission at 370 nm enhanced significantly.The observed phenomenon was assigned to the energy transfer between Ln3+ions and the ligand(Fig.12).

Interestingly，when the solutions of phenolic compounds such as bisphenol A，quinol，phenol and 2，6-ditertbutylpcresol were added into the same system，the highly sensitive selectivity of β-naphthol was basically not affected (Fig.13).The proper mechanism could be attributed to the competition of the excitation energies between β-naphthol and the Ln-MOF，thus resulting in the decreased luminescence intensity.

Fig.12 Percentage of fluorescencequenching of Eu0.00667Tb0.99333-MOF obtained in ethanol solution at room temperature;Reproduced according to the reported data[60]

Fig.13 Competitive binding studies of the other different phenolic compound(102 mol·L-1，100 μL)on Eu0.00667Tb0.99333-MOF in ethanol solution(based on 617 nm);Reproduced according to the reported data[60]

In another study，a dichromatic probe based on Ln3+@MOFs(denoted as D)was synthesized and used for chemical sensing[67].Researchers found that the color changes of D largely depended on the solvents used in the reaction process and DMF could be detected by color changes of the luminescence from D.In addition，when D was immersed in different solvents such as N，N-dimethylacetamide，methanol，ethanol，acetonitrile，acetone，CH2Cl2，ethyl acetate，tetrahydrofuran，and cyclohexane，the fluorescence color of D didn′t change under UV light irradiation(365 nm).A blue emission of the fluorescence was observed upon the dispersal of D powders in DMF，which could be ascribed to the changes in the efficiencies of energy transfer in the ligand-to-ELMs(ELMs means encapsulated luminescent modules).Furthermore，it could distinguish aromatic homologs and isomers，where the D exposed to fluorobenzene displayed the color from white to orange under the illumination of 365 nm due to the change of the emission intensity ratios，indicating probe D was expected to be promising materials for the sensing of organic molecules.

The above sections primarily focus on the detection of white light MOFs for the organic molecules in the liquid phase，however，some toxic gas molecules are also major targets.These small molecules could evaporate continuously into the air under some certain temperature and pressure，and then easily enter the porous structure of MOFs.The interaction between MOFs and analyte can change the luminescence color of WLE MOFs，which provides a new detection approach for gas recognition.The above-mentioned gas-sensing MOF(HPU-14@Tb3+@Eu3+)system could be a proper example(Fig.14)[48].

Upon the exposure to base-acid vapor，the fluorescence color of MOF material changed from white to blue upon the excitation at 365 nm，which could be observed by naked eyes.When the concentration of HCl gradually increased(Fig.15)，the emission intensities at 545 and 615 nm enhanced，while the emission intensity at 431 nm decreased due to the energy transfer from L4-to Zn2+.Interestingly，a rapid recovery of the photoluminescence from blue to white when the HCl-treated sample was exposed to Et3N，indicating the reversibility of the HPU-14@Tb3+@Eu3+.This selectivity for HCl was also observed in other volatile toxic gases such as DMF，DMA，ethylacetate，trichloromethane，acetonitrile，ethanol，methanol，acetone，styrene，methylbenzene，and nhexane gases (Fig.16).However，the emissive light colors were almostunchanged underthe same conditions.Based on the results mentioned above，the functional WLE MOF material with superb reversibility and stability characteristics can be employed as an outstanding optical sensor for rapid and specific recognition of HCl，which provides a new strategy for sensing application.

Fig.14 (a)Asymmetric coordination environment of Zn2+;(b)Trinuclear cluster;(c)Three-dimensional framework;(d)4，8-connected topology[48]

Fig.15 Emission spectra of HPU-14@Tb3+@Eu3+exposed to HCl gas with different concentrations(left);Emission spectra of HPU-14@Tb3+@Eu3+in HCl(g)and HCl-treated HPU-14@Tb3+@Eu3+in Et3N(g)(right)[48]

Fig.16 Luminescence image of HPU-14@Tb3+@Eu3+toward other common volatile toxic gaseous analogues[48]

2.3 Metal ions sensing

WLE materials can not only respond to temperature and small molecules but also can be used to detect metal ions，which are a series of indispensable elements in biological systems and exist in our daily lives[68-70].Especially，trace metal elements(Fe，Ag，Cu Mn，etc.)are closely related to human health.For example，metal ions with low concentration possess the antibacterial property and involve enzyme metabolism.Moreover，the absence of these metal ions will result in some symptoms and diseases[71].Nevertheless，the excess metal ions may also accumulate in the skin under certain conditions and cause damage to the liver or kidney[72].Once the concentrations of metal ions in the environment exceed the nationally permitted emission standards，it will cause serious pollution to soil，water as well as the atmosphere and destroy the ecological balance.Tunable luminescence materials by varying amounts of incorporated metal cations and excitation wavelength provide an effective approach for the identification of these metal ions[73].Thus，the highly efficient and straightforward detection of metal ions by WLE materials contributes to diagnose the diseases and control environmental pollution.Two factors can be used to explain the phenomenon:one is that immobilizing the unsaturated functional sites(Lewis basic sites)within porous MOFs can realize highly recognition for the analyte ions，and the other possible factor for sensing ions is the interaction between subject and object such as hydrogen bonding，or Lewis acid/base interactions，which results in the luminescence changes.As an example，Li and coworkers[74]reported three isomorphic 1D chain-like lanthanide coordination polymers (LnCPs)，namely，{Et3NH[Ln(L-DBTA)2(CH3OH)2(H2O)2]·2H2O}n(L-DBTA=L-O，O′-dibenzoyl tartaric acid，Ln=Sm 1，Eu 2，Tb 3，respectively).To construct WLE materials，they synthesized another complex 4({Et3NH[(Sm0.2Eu0.2Tb0.6)(L-DBTA)2(CH3OH)2(H2O)2]·2H2O}n)by exploring the stoichiometric ratio of Sm3+，Eu3+，Tb3+.The sample was excited at 322 nm and the CIE coordinate of(0.333，0.334)was achieved correspondingly，which was quite close to the pure white light CIE coordinate of(0.333，0.333)(Fig.17).

Especially，among all cations，only Cr3+，Cu2+，Fe2+，Fe3+，and Hg2+ions showed significant quenching phenomena due to dynamic and/or static factors，while other ions displayed no distinct changes in intensity.Interestingly，Ag+and Mn2+had an intense effect on the luminescence color of WLE materials from white to red and blue，respectively (Fig.18).This was attributed to the existence of the open oxygen sites and interaction with the phenyl group of the ligand.In addition，under dissolving，heating and even exposure to acidic and basic conditions，the luminescence intensity of complex 4 still remained unchanged，indicating that it possessed reusability and good stability.Therefore，complex 4 with unique higher selectivity and sensitivity was suitable for probing Ag+and Mn2+ions.

Fig.17 3D solid-state photoluminescent spectra of complex 4[74]

Fig.18 (a)Photographs of Mn+@4(M=Na+，Ca2+，K+，Al3+，Ba2+，Mg2+，Zn2+，Pb2+，Cr3+，Cu2+，Fe2+，Fe3+，Hg2+)excited at 322 nm;(b)Luminescence spectra of the sensor with distinct concentrations of Ag+ions，and photographs of Ag+@4 with Ag+ion concentrations of 5×10-8and 5×10-2mol·L-1;(c)Luminescence spectra of the sensor with distinct concentrations of Mn2+ions，and photographs of Mn2+@4 with Mn2+ion concentrations of 5×10-8and 5×10-8mol·L-1[74]

Another good metal-ion sensing example was reported by Zhang et al.[67]，where a 3D anionic framework with nanoscale channels[Me2NH2]2[Zn5(H2O)2(L)6]·32DMF(H2L=4-(5，7-dioxo-5，7-dihydroimidazo[4，5-f]isoindol-6(1H)-yl)benzoic acid)was synthesized through a facile hydrothermal condition.In this regard，a new trichromatic and WLE ratiometric luminescent probe(denoted as D)was obtained with the CIE coordinate of(0.33，0.34)by a blue-emitting MOF as the matrix，red-and green-emitting complex cations as encapsulated luminescent modules.According to the changes in luminescent color，it could be seen that the probe D exhibited prominent capabilities of sensing Fe3+and Al3+ions.As the material was stable in aqueous solution，it was used for sensing various metal cations.It was obvious to observe that some metal ions led to complete quenching or enhancing of the luminescence when probe D was immersed in a methanol solution containing Fe3+or Cd2+ions(Fig.19a).The luminescence color of the probe D after incorporating Al3+clearly changed from white to blue recognized by the naked eye (Fig.19b).With the increasing concentrations of Al3+from 0 to 1.5 mmol·L-1，the blue and green emissions were enhanced，whereas the red emission was decreased，and then the three emission intensities of D gradually decreased due to energy transfer or distribution，and finally the collapse of the construction(Fig.19c).Meanwhile，the luminescence intensity of the probe D was signifi-cantly depended on the concentration of Fe3+.When the addition concentration of Fe3+reached up to 1.50 mmol·L-1，the WLE of probe D was completely quenched under 365 nm of irradiation (Fig.19 (c，d)).This possible sensing mechanism could be assigned to the decomposition of the framework，a static mechanism，the energy transfer from the ligand to Fe3+ions，and the strong interaction between D and Fe3+ions.

Fig.19 (a)Histograms of the percentages of remaining emission intensities after suspensions of D were dispersed in 0.75 mm solution of metal ions in MeOH;(b)Photographic images of D toward the accommodation of metal ions under the illumination of 365 nm;(c)Change of CIE chromaticity coordinates of D(MeOH suspension)upon an incremental addition of the Al3+solution(1.5 mmol·L-1)(left)and Fe3+solution(1.5 mmol·L-1)(right);(d)Luminescent spectra;(e)Plots of the relationships between the Fe3+concentration and intensity ratios of IL/IELM of D/MeOH suspensions)when a total of 1.5 mm Fe3+was added incrementally[67]

In addition，linear dependence between the intensity ratios of IL/I[Pt]+and the concentration of Al3+ions as well as the relationship between the intensity ratios of IL/I[Ru]2+and the concentration of Fe3+ions was deduced (Fig.19e).The sensing platform achieved a low detection limit of 0.41 and 0.12 mg·L-1for Fe3+and Al3+ions，respectively.The luminescent sensor for Fe3+ions was not influenced by the coexistence of other metal ions in DMA solutions，which demonstrates high selectivity for detecting Fe3+ions.This result suggests the potential application of MOFs for the sensing of metal ions.

3 Conclusions and outlook

This review illustrates the recent progress of MOFs as a kind of multicolor materials on producing white light-emission，and mainly consists of two aspects(synthetic methods and applications).The section of synthetic methods includes the co-doping of Ln3+ions，lanthanide encapsulation and organic molecules capture，while the section of potential applications mainly discusses the multifunctional sensor performance of WLE MOFs materials as temperature，small molecule and metalions sensors.Despite the fabrication and potential applications of various WLE MOFs that have been achieved remarkable progress，some challenges in this field still demand to be further explored and addressed，such as profound structure optimization， intricate luminescence mechanisms，and practical applications.

Structure and components are essential factors to luminescent metal-organic frameworks.The luminescence performance from porous MOFs can be changed by encapsulating extraneous guest molecules，so it is necessary to increase surface areas or optimize the structures to improve their overall luminescence effect for the specific requirement of future applications.Changing the doping types of emission components such as metal nanoparticles and biological molecules in these porous materials or the excitation wavelength can also result in WLE materials.In addition，metal ions and organic linkers are usually used to adjust luminescence by antenna effects.Therefore，in synthetic methods，designing new organic ligands or the purposed selection of metal ions is also an advisable choice in terms of approaching desirable luminescent materials.

In most reports，the mechanisms of photoluminescent response between WLE MOFs and the analyte are only based on speculation，and the clear evidence is rare.Thus，revealing the underlying mechanism of WLE MOFs as sensors is still a research hotspot.For the detection applications，WLE MOFs materials have already been demonstrated to exhibit excellent performance for sensing.However，more efforts are required in other directions，such as anion sensors，pressure sensors，explosives sensors，drug sensors，and cellular sensors.Furthermore，the challenges posed by the weak stability of MOFs under aqueous conditions might influence the recognition effect of WLE MOFs towards analyte species，and restrict the development of WLE MOFs.Apart from water-tolerance，both the complicated synthesis and the high cost of raw materials become the coercive factors to develop the large-scale production of WLE MOFs for practical applications.Although the challenge is obvious，more and more scientists believe that WLE MOFs materials should have a bright future for real applications.

Acknowledgments:This work was supported by the Qing Lan Project of Jiangsu Province，the Natural Science Foundation of Jiangsu Province(Grant No.BK20180514，BK20131314)，the National Natural Science Foundation of China (Grant No.21371098)，the China Postdoctoral Science Foundation(Grant No.2015M570430)，the Jiangsu Postdoctoral Science Foundation(Grant No.1401007C)and the scholarship of China Scholarship Council (Grant No.201906855022).ZHANG Qi-Chun acknowledges AcRF Tier 1 (Grants RG 111/17，RG 2/17，RG 114/16，RG 113/18)and Tier 2 (Grants MOE 2017-T2-1-021 and MOE 2018-T2-1-070)，Singapore.