Fluorescent Dye-Based Metal–Organic Framework Piezochromic and Multicolor-Emitting Two-Dimensional Materials for Light-Emitting Devices

The dye encapsulated metal–organic frameworks (MOFs) are of interest for their practical applications such as light-emitting diodes, temperature sensing, and microlasers. However, very few MOFs have been reported to date where the dye moiety is utilized as one of the components of the framework. Fluorescein dye (FSD) doped materials have an ability to emit various colors such as red, green, and yellow depending upon their tautomeric form and external stimuli. In anticipation of light-emitting devices, three FSD-based novel MOFs, namely, {[Co2(FSD)2(H2O)4]·H2O·MeOH}n, FSDCo; {[Mn2(FSD)2(H2O)4]·H2O·MeOH}n, FSDMn; and {[Cd2(FSD)2(H2O)3(MeOH)]·2(H2O)}n, FSDCd, have been designed and synthesized. These crystalline materials were shown to exhibit multicolor light-emitting behavior and reversible green to brown piezochromism, implying their potential to act as fluorescent switches, mechanosensors, and security papers. Further, these crystalline MOFs were also shown to exhibit a rare optical phenomenon called pleochroism. The green colored crystals displayed multiple colors under polarized light due to the agglomeration of nano- and microcrystals at different angles on the surface of macrocrystals.