Luminescent Properties of MOFs

The possibilities of coordinatively unsaturated sites in the stracture of MOFs allows for unique detection of both Lewis acids and bases. Unsaturation of coordinating ligands allows for possible interactions with cations that may reversibly enter the framework. Cations typically exist in the framework stmcture of MOFs, and cation exchange may occur readily under appropriate conditions. Such interactions can have dramatic effects on the luminescent properties of MOFs and allows for unique sensing applications involving various cations in solution. 

The charge transfer between the organic linkers and guest species leads to the formation of exciplex state or results in luminescence quenching. In particular, guest species with nitro or thiol groups deprive the luminescent properties of MOFs and quench their luminescence. 

Quite a comprehensive review of the luminescent properties of MOFs is presented by Cui et al. [242], The outlook is broader than the sensor materials and covers the areas of lighting (MOF-based OLEDs), display materials, and optical devices. 

James (16), Kitigawa and co-workers (17-19), Wuest (20), and Chen and coworkers (21) explore the chemistry, structures, and properties of MOFs and CPs from a variety of perspectives. Indeed, a recent special issue of the Journal of Solid State Chemistry was dedicated to these very topics (22). One wiU notice almost immediately that this field is dominated by materials based on block transition metal compositions. Lanthanide (Ln)-containing materials have been much scarcer, perhaps for reasons to be discussed herein (e.g., a tendency to exhibit higher coordination numbers) (23). With this in mind, however, recent advances in polymeric Ln-containing materials suggest that these compounds are as structurally diverse and that the unique luminescence behavior of the /-elements may be harnessed for applications, such as sensing and molecular recognition (23-30). Such inherent properties may extend the applications of framework materials beyond those introduced above. 

The luminescent properties of lanthanide ions in MOFs are enhanced by coordinating organic molecnles... 

Recently, the use of nanoscale lanthanide MOFs for biological and biomedical applications has attracted increasing attentions. The interest of these functional materials relies on the combination of the chemical or biofunctional behavior of MOFs and the unique luminescence properties of lanthanide ions, such as high photostability, long decay rates, large Stokes shifts, and narrow emission bands. Besides their luminescent characteristics, lanthanide MOFs can possess paramagnetic properties which help to increase the relaxation rate of water protons in the... 

The approach based on lanthanide(III)-dopant is a successful strategy to impart luminescent properties into MOFs. Luo et al. have synthesized an MOF of [NH4]2[Zn(btc)l 6H20 (btc = 1,2,4,5-benzenetetracarboxylate) [6]. Theanionic microporous framework accommodates [NFU]" as a counter cations. The lanthanide ions can be introduced in the micropores by the replacement with the counter cation of The MOF was immersed in the solution of EuCls or Tb(C104)3 and... 

The study of Eu and Tb MOF coordination polymers showed that they are characterized by strong photoluminescence peaks at 580-695 nm for Eu and 490-620nm for Tb due to f -f transitions [234], The luminescent properties of Tb-, Yb, and Eu-MOFs were also reported in Ref. [235], and they were shown to be sensitive to the molecules present in the MOF cavities. 

The luminescent properties of Pb-pyridineacrylate MOF materials are studied [236], and two emission bands at 418 and 438 nm were found due to CT interactions in the MOF structure. 

Hsu et al. demonstrated the sensing property of an Na[Tb(OBA)2] (OBA = 4,4 -oxybis(benzoate)) MOF [29] the proposed MOF showed the ability to act as a stimuli responsive luminescent material towards a variety of solvents. The luminescence measurements of the MOF displayed that the D4 —> F4 emissions were the strongest intensities in BuOH and EtOH suspensions, but these intensities were significantly reduced in MeOH and H2O suspensions. The small molecules of H2O and MeOH get closer to the terbium ions of the framework that quenches the luminescence intensities effectively on the other hand, the alkyl sterical hindrance of EtOH and BuOH potentially protects the terbium ions from quenching by OH oscillators. 

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