# Progress of Organic/Inorganic Luminescent Materials for Optical Wireless Communication Systems
## 2 Luminescent Materials for VLC Systems  
### Metal Organic Frameworks (MOFs)  
Recently, the scientific community focused its efforts on the design of novel materials which show luminescence functionality with applicability in areas such as OLEDs, luminescence-based sensors, etc. They present very interesting properties in the application of functional electronic/optical devices, since these compounds exhibit outstanding mechanical and electronic features; these materials are known as MOFs [24; 25]. MOFs are hybrid inorganic-organic crystalline materials that consist of a union between metal ions and one or more organic bridging ligands with different components and coordination modes. This fact means that MOFs display applications in a considerable variety of fields [24]. A MOF-based material with potential applicability in VLC and light-emitting devices was designed by [PERSON] et al. [26], by whom the encapsulation of rhodamine was performed in an adenine-based bio-MOF. The authors realized that an increment in rhodamine concentration permitted color emissions from green to red, with emission lifetimes ranging from 3 to 7 ns. Here, the best CRI value of \(\sim\)94% was obtained using the material based on bio-MOF, BSSO, Rh@bioMOF-1, and CASN [BSSO-(Ba,Sr)\({}_{2}\)SiO\({}_{4}\):Eu\({}^{2+}\), CASN-CaAlSiN\({}_{3}\):Eu\({}^{2+}\)] [26]. Then, the possible use of dye-encapsulated MOFs in OLEDs was evaluated with a rhodamine-B/aluminum-MOF system (Al-DBA MOF). The combination of emissions from the dye (yellow) and linker (blue) allowed the fabrication of a WLED, which was known as MOF-WLED, exhibiting a frequency bandwidth of 3.6 MHz and a data rate of 3.6 Mbps [27]. Recently, a zeolitic imidazolate framework nanocrystal (PAN@ZIF-8) was also synthesized and successfully used as a self-powered VLC system for wireless human-machine interactions [28]. Additionally, mixed-matrix membranes (MMMs) based on luminescent MOFs and emissive polymers were proved in high-speed VLC. For instance, [PERSON] et al. [29] exhibited the preparation of MMMs based on the combination of emissive and stable zirconium-based MOF (NU-1000) and a luminescent PIM-1 (denominated as polymers of intrinsic microporosity-1), where an efficient energy transfer from the MOF to the polymer was promoted. The corresponding structures of the Zr-MOF and the polymer are shown in Figure 2a. The resultant MMMs revealed an exceptional modulation bandwidth of \(\sim\)80 MHz, affording a light communication rate of 215 Mbps, which represents a significant increase compared to the value displayed using the pure polymer (132 Mbps), as shown in Figure 2b. On the other hand, a promising luminescent MOF based on Zn\({}_{2}\)(tcbpe) (H\({}_{4}\)tcbpe = 1,1,2,2-tetrakis(4-(4-carboxyphenyl)phenyl)ethene) were shown to be an efficient converter of blue-light to yellow emission, using PLQY \(\sim\)76%, which can be improved by changing some of the organic ligands linked to the central metal with fluorinated tools [30]. Here, a PLQY of \(\sim\)88% can be achieved, with this rate being the actual record for yellow-emitting, blue-excitable MOF phosphors. At this point, the introduction of fluor into the main MOF structure produces a redshift in the optical properties, mainly in PL emission, and accelerates the radiative recombination pathway, generating lifetimes between 2 and 4 ns. Next, porous Zr-MOFs crystals based on benzenedicarboxylic and triethylenediamine were synthesized, with rhodamine molecules being encapsulated, thus producing warm-yellow emission with a PL lifetime of \(\sim\)6 ns [31]. Even if it is not reported, for any configuration of a VLC system for these types of MOFs, the deposition of the luminescent material on blue-LED to generate white emission with high color purity in function of the density of the added halide or the luminescent molecule could provide a novel alternative to accelerate data transmission.