Photoluminescent Metal–Organic Frameworks for Gas Sensing

Luminescence of porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) is sensitive to the type and concentration of chemical species in the surrounding environment, because these materials combine the advantages of the highly regular porous structures and various luminescence mechanisms, as well as diversified host‐guest interactions. In the past few years, luminescent MOFs have attracted more and more attention for chemical sensing of gas‐phase analytes, including common gases and vapors of solids/liquids. While liquid‐phase and gas‐phase luminescence sensing by MOFs share similar mechanisms such as host‐guest electron and/or energy transfer, exiplex formation, and guest‐perturbing of excited‐state energy level and radiation pathways, via various types of host‐guest interactions, gas‐phase sensing has its unique advantages and challenges, such as easy utilization of encapsulated guest luminophores and difficulty for accurate measurement of the intensity change. This review summarizes recent progresses by using luminescent MOFs as reusable sensing materials for detection of gases and vapors of solids/liquids especially for O2, highlighting various strategies for improving the sensitivity, selectivity, stability, and accuracy, reducing the materials cost, and developing related devices.


Introduction
The most common and typical methods that have been developed for the purpose of chemical analyses are mainly based on chromatography, optical absorption spectroscopy, and electrochemistry. However, most of these techniques require complexed instruments and time-consuming pretreatment steps, restricting their applications for in-fi eld and real-time detection. Compared with other transduction techniques, luminescence is the preferred signal for sensing, because of its visibility to the naked eye, well-developed technique, extremely low detection limits, and also simple sample preparation. There are great

Flexibility of MOFs
Framework fl exibility is a unique advantage of MOFs, [ 65,66 ] which is not only interesting for the notable structural variations under external stimuli, but also important for their usefulness in a wide range of applications such as storage, [ 67,68 ] separation, [69][70][71][72][73][74] and sensing. [75][76][77][78][79] Actually, even trivial (unnoticeable or hardly detectable) framework fl exibility has fundamental importance for luminescence, because the transient motion (vibration, torsion, etc.) of organic luminophores at the excited state is the main cause of non-radiative relaxation of the excited-state energy.
Obviously, large structural variations of MOFs at the ground state can have great infl uence on the luminescence properties. [ 80 ] For example, we showed that the fl exibility of a porous metal azolate framework [Zn 7 (ip) 12 ](OH) 2 (MAF-34, Hip = 1 H -Imidazo [4,5f ] [1,10]phenanthroline) is fundamental for its drastic fl uorescence responses toward a variety of solvent vapors and CO 2 ( Figure 1 ). [ 81 ] MAF-34 is an ultramicroporous C 3 N 4 -type ( ctn ) network interconnected by [Ru(22bpy) 3 ] 2+ (22bpy = 2,2'-bipyridine) like {Zn(ip) 3 }and tetradedral zincimidazolate {Zn(ip) 4 } 2fragments. Because the three-coordinated ipligands cannot fulfi ll the trigonal-planar geometry for the 3-connected node in the ctn net, {Zn(ip) 3 }adopts an obviously distorted octahedral coordination geometry which leads to signifi cant framework tension. Therefore, MAF-34 exhibited reversible crystal-to-amorphous structural transformations during guest adsorption, desorption, and exchange. The ip − ligands in the as-synthesized crystalline framework are well separated by the Zn(II) ions to avoid the typical face-to-face π-π interactions of this type of large planar aromatic molecules, although they are still close to each other with notable edge-toedge interactions. After removal of the guest solvent molecules, the adjacent ip − ligands become closer when the framework is distorted in the quasi-amorphous phase. Therefore, the assynthesized MOF exhibited strong cyan fl uorescence maximum at 487 nm, and the guest-free phase exhibited orange emission centered at 554 nm. For comparison, the diluted solution of Hip shows blue fl uorescence at 440 nm and crystalline Hip has no observable luminescence due to the serious π-π interactions. Guest-free MAF-34 showed reversible luminescence responses toward saturated MeOH, EtOH, H 2 O, benzene and nitrobenzene vapors. While electrons transfer from host framework to nitrobenzene accounted for the fl uorescence quenching, the luminescence responses for other vapors should arise from the guest-dependent network distortion, where the more crystalline ones (MeOH and EtOH) showed shorter emission wavelengths, and the more amorphous ones (H 2 O and benzene) showed longer emission wavelengths, because of the different extents of excimer formation controlled by the ligand-ligand distances. Interestingly, when CO 2 pressure changed from 4 to 90 Pa, desolvated MAF-34 showed gradually blue-shifted (from 554 to 540 nm) and enhanced fl uorescence (ca. 60%). Microcrystalline thin fi lm of MAF-34 can be directly grown on zinc slice by using the substrate instead of Zn(II) salt as a reactant. To quantitative monitor the intensity change, in situ de-gas and introduction of CO 2 were conducted in a sealed chamber equipped with quartz windows and a three-way valve which connects the chamber to a vacuum pump and a CO 2 cylinder, and all luminescent response processes could be repeated for at least three cycles. The strong CO 2 adsorption affi nity of the uncoordinated imidazolate N donors and hydroxide anions should have played important roles, which enable adsorption of large amounts of CO 2 to induce the amorphous-to-crystal structural transformation.
The structural transformation of a luminophore is not restricted in the host framework. Uemura and Kitagawa et al. reported an interesting strategy for luminescence detection of CO 2 and C 2 H 2 by utilizing the coupled structural transformations of a fl uorescent guest and the host framework in [Zn 2 (bdc) 2 (dabco)]·DSB (DSB@MOF, H 2 bdc = 1,4-benzenediarboxylic acid, dabco = 1,4-diazabicyclo-[2.2.2]octane, DSB = distyrylbenzene) ( Figure 2 ). [ 82 ] The host framework is a fl exible jungle-gym like structure, which possesses square 1D channels in its open form and squashed rhombic 1D channels in both the guest-free and DSB-included forms. DSB@MOF could selectively adsorb C 2 H 2 and CO 2 over other atmospheric gases, such as N 2 , O 2 , and Ar, inducing a host framework expanding accompanied by alteration of the DSB conformation. At 195 K, upon exposure to C 2 H 2 above 7.0 kPa or CO 2 above 30 kPa for a few minutes, the weak fl uorescence of DSB@MOF (centered at about 485 nm) changed to a stronger blue fl uorescence centered at about 425 nm, which can be ascribed to the conformational variations of DSB, leading to large changes in its fl uorescence properties. The structural transformation of the MOF, although seemingly has no effect on the fl uorescence change, should play an important role for selective adsorption of C 2 H 2 over CO 2 , as well as stabilization of the specifi c conformations of the DSB molecules. Such fl uorescence response is selective and reversible, which relies on a fl uorescence change of   organic molecules without any chemical interaction or energy transfer.
More accurate structure-property relationship can be obtained when single-crystal structures of the guestincluded MOFs are available. [ 83 ] Li et al. reported a luminescent MOF [(CuCN) 3 (H 2 bdpzmpy)]·guest (bdpzmpy = 2,6-bis((3,5-di-methyl-1 H -pyrazol-4-yl)methyl)pyridine) showing reversible solvent-responsive luminescence and real-time response toward acetonitrile vapor ( Figure 3 ). [ 84 ] This MOF is a two-fold interpenetrated srs network consisting of 1D CuCN helical chains linked by bdpzmpy, retaining 1D open channels (ca. 5.5-7.8 Å). The as-synthesized compound showed cyan luminescence centered at 490 nm, while its desolvated form exhibited blue luminescence centered at 450 nm. After immersion of the desolvated MOF in various organic solvents, the emission maximum showed 30-80 nm red-shift. The guestdependent luminescence responses were attributed to different extents of metal-to-ligand or intra/interligand charge transfers as well as different Cu···Cu contacts induced by this fl exible MOF, which were structurally confi rmed by single-crystal X-ray diffraction study. Although desolvation and/or solvent-exchange led to damage of single-crystallinity, solvent-inclusion singlecrystals can be obtained by direct syntheses from the corresponding solvents. For example, the intermolecular Cu···Cu separation changes 0.05 Å and the emission maximum changes 40 nm from the as-synthesized to the acetonitrile-saturated form. Upon exposing a thin fi lm compressed from mixed powders of the desovlated MOF and KBr in acetonitrile vapor with continuously increasing concentration, the emission at 450 nm gradually decreased, and a new peak at around 530 nm appeared as a shoulder.
Some insignifi cant transformation of the host framework can also result in totally different sensing performance. Dong et al. reported a microporous [Cu 2 (bimbpyb) 2 I 2 ]·4H 2 O (bimbpyb = 1-benzimidazolyl-3,5-bis(4-pyridyl)benzene) showing highly sensitive luminescence enhancement toward HCHO vapor, occurred in a single-crystal-to-single-crystal fashion ( Figure 4 ). [ 85 ] Upon exposed [Cu 2 (bimbpyb) 2 I 2 ]·4H 2 O to air containing trace HCHO, a luminescence enhancement with slightly blueshifted emission was observed, which was attributed to the structural rigidity enhancement imposed by the host-guest interactions and the structural variations of the Cu 2 I 2 core.   The HCHO-adsorbed crystal possesses a composition of [Cu 2 (bimbpyb) 2 I 2 ]·2HCHO·H 2 O, with multiple hydrogen bonding between HCHO and the host framework, accounting for its high sensitivity. In contrast, much weaker interactions between H 2 O and the host framework were found in [Cu 2 (bimbpyb) 2 I 2 ]·4H 2 O. Nevertheless, very small structural alterations of the host framework (<0.03 Å) were observed after the guest replacement. Therefore, the luminescence enhancement and blue shift was attributed to the strengthening of the host framework by HCHO.
In addition, useful information of structural transformation can also be obtained by using powder X-ray diffraction (PXRD), being favorable for understand the sensing behavior. Wang et al. reported a 3D MOF [Zn(dpe)(bdc)]·4H 2 O (dpe = 1,2-bis(4-pyridyl)ethane) which can show H 2 O-dependent luminescence properties ( Figure 5 ). [ 86 ] This MOF is 5-fold interpenetrated diamondoid network fi lled with 2D water layer containing hydrogen-bonded (H 2 O) 16 rings. The as-synthesized MOF showed a two-step water removal with a reversible desolvated/resolvated process from 30 to 86 °C, giving a partially dehydrated phase [Zn(dpe)(bdc)]⋅2H 2 O, and an irreversible one from 195 to 250 °C. It showed reversible luminescent response to humidity upon cycling between room temperature (emission maxium at 470 nm) and 86 °C (emission maxium at 510 nm), giving a large red shift and signifi cant intensity increase of the emission. The mechanism is considered to be shortening (0.03 Å) of the inter-channel distance and enhancing the π-π stacking between two dpe moieties due to desorption of the intra-channel water molecules, which was supported by crystal structure measurement from PXRD data of the partially dehydrated phase [Zn(dpe)(bdc)]·2H 2 O.
As shown by the above discussed examples, regardless of the host or guest luminescent centers, structural fl exibility can usually result in signifi cant luminescence changes. The luminescence emission wavelengths of fl exible MOFs usually shift upon exposure to guest analytes (note that the emission intensities generally change simultaneously), because the guest-induced structural transformations alter the separations and interactions between multiple luminophores.

Coordinative Guests
Many small molecules possessing O/N/S donor atoms can coordinate at the open metal sites (OMSs) on the pore surface of MOFs, which can facilitate non-radiative relaxation of the excited state and lead to luminescence quench. The most representative examples are water induced luminescence quenching of lanthanide metal based MOFs, [ 87 ] because their luminescence directly emit from the oxophilic lanthanide ions which have strong coordination ability for H 2 O, [88][89][90] and the highenergy O-H oscillators can effectively match electronic energy gaps of the lanthanide ions. [ 91 ] The luminescence quenching effect of water coordination can be utilized for detection of other coordinative guests, in which the coordinated water molecules are replaced by other guest with weaker quenching effect, leading to a luminescence enhancement.  ( Figure 6 ). [ 92 ] This MOF is a 3D framework structure consisting of 1D Eu 2 (RCOO) 6 (H 2 O) 4 infi nite rods and dicarboxylate pillars, with 3D channel occupied by the DMF guest molecules. The characteristic Eu(III) luminescence of the as-synthesized MOF reduces ca. 32% after the guest DMF molecules were replaced by water upon solvent-exchanged treatment. When the H 2 O-exchanged MOF was exposed in saturated vapors of various organic solvents, a more than 8-fold enhancement of luminescence was observed for DMF, while most of the other solvents gave only around 1-fold enhancement. To obtain reliable luminescence intensities, the sample holder was quickly transferred between the vapor container and spectrometer, and each measurement  3 ]· x DMF (ITQMOF-1-Eu, H 2 hfi pbb = 4,4′-(hexafl uoroisopropylidene)bis(benzoic acid)) ( Figure 7 ). [ 93 ] The structure of this compound was not directly determined because the crystals exhibit serious twinning. Instead, another related structure possessing the same chemi cal composition was determined by single-crystal X-ray diffraction as infi nite zigzag rod-shape Eu 2 (RCOO) 6 chains linking by hfi pbb 2− . Nevertheless, the N 2 isotherm at 77 K showed that the pore volume of guest-free ITQMOF-1-Eu is 0.14 cm 3 g −1 , and the Brunauer-Emmett-Teller (BET) surface area is 207 m 2 g −1 . Under alternating streams of ethanolsaturated and ethanol-free air, the characteristic emission of ITQMOF-1-Eu showed a reversible and rapid luminescence quenching toward ethanol. The sensing mechanism was rationalized by considering that ethanol may coordinate to the Eu(III) ions, through coupling with the vibrational states of the O-H   oscillators. More interestingly, the luminescence of this material was not quenched by water, so that it can effi ciently sense ethanol in the presence of water, which was ascribed to the hydrophobic ligand hfi pbb 2and pore surface.
Other coordinative gas can have stronger quenching effect compared with water vapor. [ 94 ] For example, Humphrey et al. reported that [Tb(tctpo)(H 2 O)]·2DMF·H 2 O (PCM-15, H 3 tctpo = tris( p -carboxyl)triphenylphosphine oxide) can be used to quantitatively detect trace amounts of NH 3 . [ 87 ] To obtain reliable luminescence intensity changes, a custom cell was designed to be directly interchangeable between the gas adsorption analyser and the spectrophotometer, which allowed each sample of PCM-15 to be activated under vacuum, directly exposed to adsorbates in situ, and studied spectrophotometrically over many cycles without physical manipulation or exposure to air, and each measurement was repeated three or more times using freshly prepared samples to provide error ranges. Fifteen gases (1 atm) and vapors (saturated) were used to study their abilities to quench the Tb(III) luminescence of the fully desolvated MOF, among which NH 3 was found to show the highest quenching effi ciency of 60%, while H 2 O showed only 44% quenching. The quenching mechanism was attributed to the coordination of NH 3 ( Figure 8 ). [ 95 ] These MOFs contain ellipse-like channels with the dimensions of 8.9 × 7.5 Å 2 , being fi lled by (H 2 O) 4 clusters that interacting with the coordinated H 2 O on Ln(III). Interestingly, when the MOFs were heated at 200 °C for 2 hours, only two guest water molecules were removed, while the coordinated water molecule was retained. After that, the characteristic luminescence of these MOFs showed a decrease in luminescence emission intensity compared with their assynthesized state. Therefore, these partially dehydrated MOFs can be used to sense water vapor with a turn-on mechanism, in which hydrogen bonding interactions between the O-H moieties of the coordinated H 2 O molecule and the electron lone pair of the analyte H 2 O to reduce the O-H vibration frequency and quenching effect of the H 2 O ligand.
The gas coordination effect can not only quench the luminescence of the lanthanide metal ions but also perturb the excited state energy of the organic fl uorophore. [ 96,97 ] showing shifts of emission wavelengths upon interaction with ammonia at 100 °C ( Figure 9 ). [ 96 ] [Zn 2 (tcpe)] shows staggered 2D layers constructed from paddle-wheel shaped Zn 2 (RCOO) 4  Exposure of activated [Zn 2 (tcpe)] to triethylamine, ethylenediamine, DEF, and water vapors, as well as ammonia gas at room temperature shifted its emission maximum by 0−23 nm. Interestingly, only ammonia exposure (1% ammonia in nitrogen) is able to shift the emission maximum at 100 °C. Computational simulation revealed that such temperature-dependent luminescence response may be attributed to the stronger binding of NH 3 to the OMSs. While the NH 3 sensing of [Zn 2 (tcpe)] is irreversible, [Mg(H 2 dhbdc)] showed a reversible but smaller fl uorescence response to ammonia at 100 °C, which could be attributed to the weaker coordination ability of Mg(II) toward NH 3 .
The coordination effects of guests can be rationally used to modify the luminescence property and guest-responsive   [ 98 ] on a glass substrate, and then immersed it in a DMF solution of AgNO 3 to obtained a Ag(I) functionalized LMOF-111 thin fi lm. [ 99 ] LMOF-111 is a 3D coordination network consisting of undulating Zn 2 (bpdc) 2 layers and bpee pillars, as well as roughly rectangular 1D channels with pore-size distribution around 7.5 Å. [ 100 ] AgNO 3 @LMOF-111 showed weaker fl uorescence than the pristine material LMOF-111, which was attributed to the coordination of Ag(I) with the C=C bond of the bpee linker. Upon exposure to propylene, the emission intensity of AgNO 3 @ LMOF-111 was enhanced by ca. 43%, while a fl uorescence quenching of 12% was observed when exposed to propane gas. Such a fl uorescence enhancement was attributed to the weakening of the Ag(I)-bpee coordination after the formation of additional coordination bonds between the alkene analyte and Ag(I). The sensing response to propylene was reversible after heating at 60 °C under vacuum followed by nitrogen purging.
MOFs containing OMSs can bind strongly with coordinative guests, being favorable for improving the sensitivity but disadvantageous for improving the selectivity (diffi cult to distinguish different coordinated guests). In this context, water vapor which commonly exists in air should be the main concern because of its small size and strong coordination ability with OMSs. Fortunately, as shown by the above discussed examples, coordinative guests can change the luminescence properties of MOFs by many ways, and the water effect could be avoided or even utilized by rational consideration of the pore structures and luminescence mechanisms of MOFs.

Nitro-Containing and Aromatic Compounds
Organic molecules containing nitro groups are well known for their electron-defi cient nature and strong luminescence quenching effect, which can be benefi cial for detection of many nitro-containing explosives. [101][102][103][104][105][106][107][108][109] Li et al. demonstrated the fi rst application of luminescent MOFs in detection of nitro explosives. [ 100 ] The guest-free LMOF-111 (   fl uorescence emission centered at 420 nm upon excitation at 320 nm, which was assigned to the organic linkers. To study the luminescence sensing behavior of LMOF-111 toward nitro explosive 2,4-dinitrotoluene (DNT) and the plastics explosive taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB), ground powders of LMOF-111 were glued onto glass slides. The fl uorescence emission was quenched more than 80% within 10 seconds, accompanying with 42 nm red-shift of the emission maximum when the MOF was exposed in saturated vapors of DNT (0.019 Pa) or DMNB (0.29 Pa) in air, which shows a higher sensitivity and faster response than conventional luminescence sensing materials such as conjugated polymers (20-40% quench after 10-20 seconds exposure). In addition, the original fl uorescence of the MOF can be recovered by heating at 150 °C. Similar with other fl uorescence probes, the fl uorescence responses of LMOF-111 were attributed to electron transfer from the excited host framework to the electron defi cient DNT and DMNB molecules. Because DMNB is generally diffi cult to detect due to the lack of π-π interactions with the host framework, the remarkable fl uorescence sensitivity of LMOF-111 was further attributed to the pore confi nement effect of the molecular-sized channel which facilitates stronger interactions between the explosives and the host framework.
The fl uorescence quenching ability of a guest may depend not only on its electron withdrawing ability but also other factors.  ( Figure 11 ). [ 110 ] LMOF-121 is a 2-fold interpenetrated 3D network containing 1D channels with size ca. 5.8 × 8.3 Å 2 . Upon excitation at 280 nm, the guest-free compound emits ligand centered fl uorescence with maximum at 420 nm. Ground powders of LMOF-121 were glued onto glass slides for luminescence sensing. When exposed to saturated vapors of nitro-containing aromatic analytes, the fl uorescence was quenched to varying degrees following an order of nitrobenzene > m -dinitrobenzene > nitrotoluene ≈ p -dinitrobenzene > DNT, and can be fully recovered by heating at 150 °C for a few minutes. Besides the different electron-withdrawing abilities of the analytes, other factors must be involved for the observed trend. For example, the strongest and relatively poor quenching effects of nitrobenzene (32 Pa) and m -/ p -dinitrobenzene (0.12/0.0032 Pa), respectively, which violated their electron withdrawing abilities, was attributed to their signifi cantly different saturation vapor pressures. Notably, LMOF-121 can selectively detect aromatic DNT over aliphatic DMNB (quenching effi ciency < 1%). Besides the weak ability for electron acceptance, DMNB (7.1 × 7.3 × 7.7 Å 3 ) is also too large to enter the smaller pore of the MOF.
As an opposite effect, electron-rich aromatics enhanced the luminescence with an order of toluene > benzene > chlorobenzene. The fl uorescence quenching/enhance mechanism was explained by the excited state electron transfer process, according to the reduction potential measurements and computational studies. For an electron defi cient analyte, electrons transferred from the conduction band of LMOF-121 to the lowest unoccupied molecular orbital (LUMO) of the analyte upon excitation, followed by a non-radiative relaxation. [ 110 ] For an electron-rich analyte, an opposite electron transfer process was proposed. However, a complete electron transfer is not likely, because this behavior would instead cause non-radiative relaxation of the excited state. Therefore, a more plausible explanation is that the analyte inhibits linker motions (at the excited state), [ 2 ] or formation of linker-analyte exciplex.
The direct detection of some nitro explosives can become enormously diffi cult due to their extremely low vapor pressures. The fl uorescent MOF [Zn 2 (hfdc) 2 (44bpy)]· x DMA (LMOF-202, H 2 hfdc = 9 H -fl uorene-2,7-dicarboxylic acid) is a 2-fold interpenetrated pcu network containing 3D intersecting channels. [ 111 ] Desolvation of LMOF-202 leads to a structural change as indicated by PXRD, and the guest-free MOF [Zn 2 (hfdc) 2 (44bpy)] is porous with BET surface area of 136 m 2 g −1 and displays signifi cantly red-shifted (60 nm) fl uorescence centered at around 490 nm. Exposure to the saturated vapors of various ketones,  Figure 11. Structure of [Zn 2 (oba) 2 (44bpy)] and its luminescence quenching and enhancement mechanisms toward electron defi cient and electron rich molecules exemplifi ed by nitrobenzene and toluene, due to electron transfer and exciplex formation, respectively. the emission intensity of guest-free [Zn 2 (hfdc) 2 (44bpy)] obviously enhanced, accompanying with small blue-shifts in emission wavelength. These fl uorescence responses were ascribed to exciplex formation between ketones and MOF. However, considering that the π-conjugation systems of ketones are too small, and the guest-modulated fl uorescence occurs at shorter wavelengths, a steric hindrance effect, i.e., the guest restricts the fl exibility/motion of the host framework which reduces non-radiative relaxation, might be more plausible. [ 2,112 ] Importantly, the fl uorescence sensing of cyclohexanone can be used to indirectly detect 1,3,5-trinitroperhydro-1,3-5-triazine (RDX), because the vapor pressure of RDX (6 × 10 −7 Pa) is too low to detect and cyclohexanone coexists in RDX as recrystallization solvent. [ 111 ] Further exposure to a sample of RDX recrystallized in cyclohexanone resulted in a >12% enhancement of emission intensity within 15 minutes, demonstrating the feasibility of such indirect detecting of RDX.
Electron-rich aromatic compounds can usually form exciplex with the fl uorophores in MOFs to enhance fl uorescence intensity. [ 107 ] For example, 1,4,5,8-naphthalenediimide (ndi) is known to generate exciplex emission when interacting with aromatic molecules. Kitagawa et al. reported an ndi-based luminescent MOF [Zn 2 (bdc) 2 (dpndi)]·4DMF (dpndi = N,N'-di(4pyridyl)-1,4,5,8-naphthalenediimide), [ 113 ] which is a fl exible 2-fold interpenetrated pcu network with changeable pore sizes. This MOF showed different emission wavelengths after the accommodation of a range of benzene derivatives with different ionization potentials, which was attributed to the formation of exciplex between the ndi core and the aromatic guest with different charge-transfer degrees. However, it is worth noting that after a complete charge transfer, such as using N , N -dimethylaniline as a guest, the pale-yellow MOF crystal turned deep purple with completely quenched fl uorescence due to the formation of a radical ion pair state between N , Ndimethylaniline and ndi. While the as-synthesized and guest-free forms of the MOF both showed very weak fl uorescence (quantum yield < 0.01, lifetime: 150 ps), the toluenesaturated sample showed very high fl uorescent intensity centered at 476 nm (quantum yield = 0.22, lifetime: 14.8 ns). Single-crystal X-ray structure of the toluene-saturated phase [Zn 2 (bdc) 2 (dpndi)]·2.5C 7 H 8 confi rmed strong face-to-face π-π stacking interaction (3.6 Å) between the guest and the ndi core ( Figure 12 ). The luminescence response of this MOF was further tested in different toluene vapor pressures (0-90% of the saturation vapor pressure) by using a fl uorescent microscope combining with a vapor-control system (by mixing helium fl ow saturated with toluene and pure helium fl ow in different ratios), for which the powdered sample was packed in a gas-fl owing glass capillary. A nonlinear fl uorescence enhancement effect against toluene vapor pressure was observed, which was attributed to the nonlinear adsorption amount against pressure and cooperative structural transition of the host.
The strong abilities of nitro-containing and aromatic compounds for involving in electron or energy transfer processes at the excited states are not only benefi cial for effective detection of these molecules, but also challenges for identifi cation of a specifi c analyte from a mixture of similar compounds. Thus, for highly selective and specifi c explosive detection, MOFs with more accurate molecular recognition are required.

Radical Gases (O 2 )
Besides electron defi cient molecules, unpaired electrons also have strong luminescence quenching effect by accepting energy from the excited-state luminophores. [114][115][116] For gases, NO and O 2 are typical radical/paramagnetic molecules with a triplet ground state. These light gases, even when interact weakly with other substances in the structural point of view, can be efficiently detected by luminescence quenching.
For luminescence O 2 sensing, the sensitivity is mainly determined by the O 2 permeability, the original luminescence lifetime, and the collision radius of the luminescent dye. [117][118][119][120] Precious metal (Pt, Ru, Au, Ir, Re, etc.) complexes are widely used for O 2 sensing due to their long phosphorescence lifetimes, being suitable for detecting O 2 with extremely low concentrations. [ 121 ] On the other hand, other luminescent probes with low sensitivities are useful for measuring high concentrations of oxygen. Because air pressure and wind speed change the partial pressure of O 2 , real-time two-dimensional image of air-pressure or wind-speed distribution can be obtained based on luminescence quenching of low-sensitivity probes such as pyrene. However, these materials must be dispersed in gas permeable porous matrixes, usually organic polymers, to allow  effective interaction with oxygen molecules. [ 120 ] Because MOFs can have large porosity and various luminescence properties, they are very attractive for O 2 sensing, and a number of advances have been reported in the past few years ( Table 1 ). It is worth noting that it is relatively easy to control the oxygen pressure, thus it is possible to measure the fl uorescence intensity in situ at a range of oxygen pressures, giving a lot of useful information.
Lin et al. reported the fi rst luminescent MOFs for O 2 -sensing, by using bridging ligands derived from classic phosphorescent precious metal complexes. By connecting Zn(II) with two metalloligands derived from a common O 2 sensitive complex Ir(ppy) 3 (ppy = 2-phenylpyridine), three phosphorescent MOFs were synthesized. [ 122 ] Among the three MOFs, a 2D bilayer structure containing the classic Zn 4 O(RCOO) 6 clusters and open channels of 7.9 × 4.3 Å 2 is relatively robust with a large BET surface area of 958 m 2 g −1 after desolvation ( Figure 13 ), while the other two containing both mononuclear and dinuclear Zn connecting nodes are nonporous toward N 2 adsorption after activation though they both contain guest mole cules at the as-synthesized states. At 1 bar O 2 , the luminescence quenching effi ciencies for the three MOFs and the two ligands are in the range of 32-59% and 8-16%, respectively, in which the tetranuclear-based structure showing the highest effi ciency of 59%. Moreover, only the tetranuclear-based bilayer structure showed some cycling stability of luminescence upon repeated O 2 dosing and removal. The distinct performance of this structure was ascribed to the permanent porosity which allows the collision of framework and O 2 molecule.
Later, several similar precious metalloligands were also used to connect d 10 transition metal ions or main group metal ions to form many other low-dimensional coordination polymers containing solvent accessible voids for O 2 sensing, in which their phosphorescence can be quenched by 46-83% at 1 bar O 2 or I 0 / I 100 = 1. 85-5.88. [123][124][125] Sun et al. also showed that [Ru(22bpy) 3 ] 2+ can be encapsulated as counter cation in an anionic coordination framework to give an O 2 -sensitive (ca. 75% quenching at 1 bar O 2 ) phosphorescent MOF (Me 2 NH 2 ) 5 (Ru-(22bpy) 3 ) 2 [In 9 (pydc) 18 ]·4DMF·18H 2 O (H 2 pydc = pyridine-2,5-dicarboxylic acid). [ 126 ] Interestingly, they can precisely control the hydrolysis of DMF by reaction time to obtain an isostructural non-luminescent MOF with only Me 2 NH 2 + counter cation, and even obtain a core-shell structure with the phosphorescent one covered by the non-luminescent analog. The core-shell structure was insensitive to O 2 because the outer shell prevents the core crystal to directly contact with O 2 .
Although MOFs constructing by precious metalloligands can be used directly for oxygen detection, they generally contain large amounts of precious metal (ca. 20 wt%), which is not economically benefi cial. Actually, high concentration of these complexes may induce the self-quenching effect because the complexes are too close to each other, and MOFs doped with small amounts of precious metal complex are effective enough to detect oxygen. Lin et al. reported that phosphorescent Ir and Ru complexes decorated with two carboxylate groups can be doped into the framework of a remarkably stable MOF [Zr 6 O 4 (OH) 4 (bpdc) 6 ] (UiO-67) to achieve high oxygen sensitivity with much reduced use of precious metal ( Figure 14 ). [ 127 ] UiO-67 consists of 12-connected Zr 6 O 4 (OH) 4 clusters linking by bpdc 2− ligands into an fcu topology network. The M(ppy) x (22bpy) 3− x (M = Ir(III) or Ru(II), x = 1, 2, 3) derived linear dicarboxylate ligands were used to partially (0.48-2.98%) replace the bpdc 2− ligands in UiO-67. The luminescence quenching experiments were conducted by pressing desolvated samples onto the surface of KCl pellets, then placing into quartz cuvettes equipped with vacuum/O 2 port system. Upon exposure to 0.8 bar O 2 , the phosphorescence intensities of these MOFs were instantly quenched by 29-65%. The sensing effi ciency of these MOFs with very small amounts of precious metal (0.3-0.9 wt%) are comparable with those of completely connected by precious metalloligands. The emission intensities were found to decrease with non-linear responses to increasing oxygen pressure, which was attributed to the inhomogeneous distribution of phosphorescent sites in the solid samples.
Sometimes, precious metal complexes can be directly used as bridging ligands, without sophisticated organic syntheses. [Ru(Hip) 3 ] 2+ is a classic phosphorescent complex used for constructing luminescent complexes for interacting with DNA. By using [Ru(Hip) 3 ]Cl 2 to replace part of the reactant Zn(OH) 2 , we synthesized a series of solid-solution frameworks [Ru x Zn 7− x (ip) 12 ](OH) 2 ·guest ( x Ru:MAF-34, x = 0.10-0.16), in which Ru(II) partly replaced the octahedral Zn(II) ( Figure 15 ). [ 128 ] Synthetic experiments using different Ru(II) doping ratios showed that unknown crystalline impurities appeared at x > 0.16, which can be attributed to the smaller radius of Ru(II) and the framework tension of MAF-34. In other words, the higher Ru(II) doping ratios result in greater framework tension, fi nally leading to incompatibility between Ru(II) and the coordination network. Luminescence oxygen-quenching study revealed that the sensing effi ciencies of these materials increase along with the Ru(II) doping ratio, with a maximum of 88% quenching at 1 bar at x = 0.16, corresponding to I 0 / I 100 = 8.3. This phenomenon is opposite with the common trends that the higher concentration of the luminophore results in poorer quenching effi ciency due to self-quenching. Luminescence lifetime measurements demonstrated that the higher Ru(II) doping ratios results in longer lifetimes. Probably induced by the increasing framework distortion with higher doping ratios, the coordination geometry of [Ru(ip) 3 ] − approaches more to the normal octahedral one, facilitating its phosphorescence emission. Sorption experiment showed relatively high O 2 uptake of 0.20 mol L −1 at 1 bar and 298 K, which accounts for its relatively high oxygen-sensing effi ciency considering the relatively low precious metal contents (0.32-0.52 wt%) compared with full precious metalloligands. For comparison, the oxygen solubility in organic solvent C 29 HF 59 O 9 and polymers ethyl cellulose are only 0.012 and 0.008 mol L −1 , respectively. [ 129,130 ] By virtue of the suitable excitation and emission wavelengths of the Ru(II) complex, we fabricated a simple color-changing ratiometric oxygen sensor by spraying 0.16Ru:MAF-34 powder onto the outer surface of a blue light-emitting diode (LED). In the absence of O 2 , 0.16Ru:MAF-34 emits strong red light, which mixes with the leaked blue light to give a purple color. In the presence of O 2 , the red luminescence of 0.16Ru:MAF-34 was quenched, and the device only emits blue light.
Besides Ru, Ir and other precious metals, lanthanide ions can be also used to construct phosphorescent oxygen-sensing   MOFs. [ 131 ] ( Figure 16 ). [ 132 ] The bio-MOF-1 structure contains 1D channels of 7 × 7 and 10 × 10 Å 2 with counter cations. Upon excitation with 340 nm UV light, Yb 3+ @bio-MOF-1 showed NIR emission maximum at 970 nm, which is benefi t for sensitive detection in complex media such as biological samples since the background noise is very low. The luminescence of Yb 3+ @bio-MOF-1 at 970 nm was quenched by approximate 40% within 5 minutes upon exposure to O 2 gas under ambient pressure, which was reversible and stable after several cycles of exposure to O 2 and N 2 .
To increase the O 2 sensitivity of lanthanide luminescence, sensitization of the lanthanide luminescence can be helpful. Qian et al. compared the oxygen sensing properties of two lanthanide doped MOFs. [ 133 ]   quantum yield of only 1.1%. Therefore, the luminescence of Tb 3+ @CPM-5 fi lm at 544 nm was only quenched by 47% at 1 bar O 2 , corresponding to I 0 / I 100 = 1.89. While the luminescence quantum yields or lifetimes are critical for the different O 2 sensitivities of the two MOFs, the difference of their pore sizes/volumes might also play a role.
Although phosphorescent MOFs containing precious metal or rare earth metal have made considerable progresses for oxygen sensing, and their metal contents can be reduced to very lower level, completely avoiding the use of precious metal is always highly demanded. On the other hand, organic fl uorescent molecules with singlet excited state can also sense oxygen though their sensitivities are relatively low. [ 114,134,135 ] Actually, theoretically speaking, fl uorescence and phosphorescence have the same quenching probability when the luminophore is collided with oxygen. Their main difference for oxygen sensing could be the shorter lifetime of fl uorescence. However, according to the Stern-Volmer (SV) equation, high O 2 permeability of a MOF could compensate for the short luminescence lifetime. [ 120 ] We reported the fi rst base-metal MOF with singlet fl uorescence for oxygen sensing, for which [Zn 4 O(bpz) 2    (14.1 ns) is much shorter than for phosphorescent dyes. It was found that the fl uorescence of IRMOF-1, IRMOF-3, and MAF-X10 can be also quenched by O 2 , but their weak emission intensities and/or near ultraviolet emission colors make these O 2 quenching properties unnoticeable.
Pyrene is a good oxygen-sensitive probe when dispersed in porous materials. [ 118,139,140 ] However, this carcinogen and environmental pollutant tends to aggregate and/or evaporate at high temperature/low pressure. [ 141 ] We encapsulated pyrene into the prototypi cal metal-organic zeolite SOD-[Zn(mim) 2 ] (Hmim = 2-methylimidazole, MAF-4 or ZIF-8) via an in situ loading strategy, because MAF-4 has large cavities with diameter of 11.4 Å and small apertures with diameter of 3.3 Å, which are ideal for immobilizing pyrene (3.4 × 7.2 × 11.6 Å 3 ). [ 142 ] Benefi ting from the size match, the motion of pyrene can be well restricted while the diffusion of oxygen into MAF-4 is allowed ( Figure 20 ). Indeed, thermogravimetry-mass spectroscopy and gas chromatography-mass spectroscopy analyses showed that no pyrene was released from MAF-4 even at high temperature. Upon excitation at 344 nm, pyrene@MAF-4 showed characteristic emissions of pyrene monomer and excimer, and the intensity ratio depends on the inclusion amount of pyrene. At    for samples loaded with 1.8-0.047 pyrene molecules per cage, respectively. Moreover, since the small apertures of MAF-4 prevent other large quencher molecules to pass through, the fl uorescence of pyrene@MAF-4 can not be interfered when exposed in the vapors of comment quenchers such as nitrobenzene, 2,6-dinitrotoluene and 2,4-dinitrotoluene. By virtue of the unique crystal growth behavior of MAF-4, several methods can be used for fabrication of thin fi lms of pyrene@MAF-4 on the surfaces of various materials useful for sensing air fl ows.
Although some fl uorescent materials have demonstrated high oxygen-sensing effi ciencies, phosphorescent dyes are still the most preferred choices for luminescence sensing, considering that phosphorescence based on triplet excited state shows high quantum yield and luminescent intensity, long lifetime and large Stokes shift, which can reduce the interference from scattered light and fl uorescence background, meanwhile improve the sensitivity and signal to noise ratio. [ 47,143,144 ] Many Cu(I) complexes are cheap phosphorescent materials, but they can be easily destroyed by air/moisture. [ 145,146 ] Based on the stable metal azolate framework system, [ 147 ] we demonstrated that [Cu(detz)] (MAF-2, Hdetz = 3,5-diethyl-1,2,4-trizole), a Cu(I) triazolate framework with 3D intersecting micropores, can serve as an extremely oxygen-sensitive material ( Figure 21 ). [ 148 ] Upon excitation at 292 nm in vacuum, MAF-2 showed broad   and structureless emission maximum at 508 nm with Stokes shift of 14562 cm −1 , which is larger than for most oxygensensing materials (ca. 4000-10000 cm −1 ). The phosphorescence intensity of MAF-2 were found to decrease linearly against the O 2 pressure, with an extraordinarily high quenching of 99.72% at 1 bar ( I 0 / I 100 = 355.8). Besides the very long phosphorescence lifetime of 115.9(2) µs, the relatively large O 2 solubility of 0.12 mol L −1 and diffusion coeffi cient of 1.4 × 10 −7 cm 2 s −1 were responsible for the high O 2 sensitivity. Since MAF-2 can be synthesized at room temperature by simply mixing the metal ion and the ligand, a counter-diffusion crystal-growth method was developed to grow microcystals on silicon rubber thin fi lms. The soft membrane not only maintained original luminescence and oxygen-sensing properties, but also further improved the chemical stability of MAF-2.
Although O 2 sensing based on luminescent MOFs have achieved numerous progresses, especially for improving/tuning the sensitivity and reducing the materials cost, these materials are still far from practical applications. Obviously, there are great needs and diffi culties for luminescence sensing materials to enhance the O 2 selectivity over other electron-defi cient and/ or radical molecules. On the other hand, thin fi lm and device fabrication are always great challenges for MOFs.

Conclusions
While MOFs have been extensively studied for gas storage, separation, and heterogeneous catalysis during the past decade, using luminescent MOFs for gas sensing is still developing and needs more attention. For practical applications, the ideal luminescent MOFs need to demonstrate high selectivity, high sensitivity, fast response, and full recyclability. Only a very small percentage of luminescent MOFs have been evaluated for gas sensing so far, especially for quantitative detection, considering the immense structural diversity of MOFs. To achieve high selectivity and sensitivity, the molecular sieving of MOFs, either by precise control over pore size or by framework fl exibility, should be the key strategy. So far, the majority of luminescent MOFs have been focused on using bulk microcrystalline powders, but an increased attention is being paid on the fabrication of thin fi lms and devices, because of their necessity for quantitative sensing of gas-phase analytes. Based on the versatile structures and chemistry, as well as many viable types of sensing mechanisms such as the distance change between multiple luminophores, coordination to luminescent metal center, and electron/energy transfer between host and analyte, luminescent MOFs and related devices are very promising. And obviously, they are experiencing very fast growth for not only gas-phase but also liquid-phase chemical sensing.