The rapidly growing number of luminescent coordination polymers (CPs) and metal–organic frameworks (MOFs) illustrates high interest as well as accessibility of such materials. The interplay between inorganic and organic components in these materials offers a large number of viable luminescence processes. The enormous variability of available ligand–metal combinations opens the possibility of creating luminescence “by design”, based on a proper understanding of the processes involved. Together with the multifunctionality of MOFs (e.g. porosity) further options like sensing become accessible. This tutorial review covers the main luminescence processes and focuses especially on metal-based luminescence of coordination polymers and MOFs.

This feature article focuses on tuning options of photoluminescence properties of lanthanide containing Metal–Organic Frameworks (MOFs) and Dense Frameworks by selection of an appropriate set of metal ions together with suitable ligands. In addition to lanthanide-only systems, frameworks with main group and transition metal ions that are heterometallic or co-doped with variable lanthanide content offer excellent tuning options for luminescence. The MOF feature porosity enables further applications such as sensors for a large number of chemical analytes by selective influences on the luminescence upon contact. The application of functional thin films marks the most recent development of this amazingly growing field, involving processing and structuring.

The series of anhydrous lanthanide chlorides LnCl₃, Ln = Pr–Tb, and 4,4′-bipyridine (bipy) constitute isotypic MOFs of the formula ² _∞[Ln₂Cl₆(bipy)₃]·2bipy. The europium and terbium containing compounds both exhibit luminescence of the referring trivalent lanthanide ions, giving a red luminescence for Eu³⁺ and a green luminescence for Tb³⁺ triggered by an efficient antenna effect of the 4,4′-bipyridine linkers. Mixing of different lanthanides in one MOF structure was undertaken to investigate the potential of this MOF system for colour tuning of the luminescence. Based on the gadolinium containing compound, co-doping with different amounts of europium and terbium proves successful and yields solid solutions of the formula ² _∞[Gd_2−x−yEu_xTb_yCl₆(bipy)₃]·2bipy (1–8), 0 ≤ x, y ≤ 0.5. The series of MOFs exhibits the opportunity of tuning the emission colour in-between green and red. Depending on the atomic ratio Gd:Eu:Tb, the yellow region was covered for the first time for an oxygen/carboxylate-free MOF system. In addition to a ligand to metal energy transfer (LMET) from the lowest ligand-centered triplet state of 4,4′-bipyridine, a metal to metal energy transfer (MMET) between 4f-levels from Tb³⁺ to Eu³⁺ is as well vital for the emission colour. However, no involvement of Gd³⁺ in energy transfers is observed rendering it a suitable host lattice ion and connectivity centre for diluting the other two rare earth ions in the solid state. The materials retain their luminescence during activation of the MOFs for microporosity.

Smart optical composite materials suitable for ratiometric sensing applications and harvesting options have been developed. The hybrid materials consist of core/shell particles with Fe₃O₄/SiO₂ as the core and different luminescent lanthanide-containing metal–organic frameworks (MOFs) as the shell. The magnetic properties enable collection of microparticles via an external magnetic field and, thus, a strong signal augmentation of the luminescence signal. Thereby, MOF luminescence functions as a read-out signal of the sensing that can be influenced by different chemical compounds, e.g. by quenching with low concentrations of water. The combination of MOFs, which contain different luminescence centers combined with a different sensitivity towards water, results in a system that can be exploited as a ratiometric sensor. We have utilized the MOFs ³ _∞[Eu₂(BDC)₃]·2H₂O·2DMF (BDC²⁻ = benzene dicarboxylate) and ² _∞[Ln₂Cl₆(bipy)₃]·2bipy (Ln = Eu, Tb; bipy = 4,4′-bipyridine) for functionalization of the microparticles, resulting in a color-tuned yellow-emitting mixed-MOF composite system together with a Fe₃O₄/SiO₂ core. Interaction with water decreases the luminescence unequally for both luminescence centers, which enables a quantitative determination of the water content by analysis of the ratio of the Tb³⁺ and Eu³⁺ luminescence bands. This process is supported by possible harvesting via superparamagnetism of the composite. Altogether, high sensitivity with a detection limit of 0.3% (20 μg) is achieved, equal to Karl-Fischer titration but also suitable for an “on-the-fly” analysis.

Homoleptic frameworks of the formula ³ _∞[Sr_1−xEu_x(Im)₂] (1) (x = 0.01–1.0; Im⁻ = imidazolate anion, C₃H₃N₂ ⁻) are hybrid materials that exhibit an intensive green luminescence. Tuning of both emission wavelength and quantum yield is achieved by europium/strontium substitution so that a QE of 80% is reached at a Eu content of 5%. Even 100% pure europium imidazolate still shows 60% absolute quantum efficiency. Substitution of Sr/Eu shows that doping with metal cations can also be utilized for coordination compounds to optimize materials properties. The emission is finely tuneable in the region 495–508 nm via variation of the europium content. The series of frameworks ³ _∞[Sr_1−xEu_x(Im)₂] presents dense MOFs with the highest quantum yields reported for MOFs so far.