To demonstrate the sieving properties and molecular functionalization of the crystals, we suspended a batch of MOF-NiCl2 for one hour in an ethanol solution of sodium resorufin (SR, 8.9 × 10 -5 M). This chromophore has excellent fluorescent properties and can easily enter the channels of the structures (dimensions of SR: 4.1 Å × 9.6 Å; cross-section of the channel is 7.5 × 6.4). The negative charge of the SR dye facilitates its uptake by the cationic framework. The initial MOF-NiCl2 is colorless, also under polarized light (Figure , top). The optical images of isolated SR-functionalized crystals clearly show the homogeneous distribution of the dye, in the free bridge as well as in the fantails nanorods (Figure , bottom). This observation demonstrates that the channels are chemically accessible regardless of their different spatial orientation. The intensity of the purple color shows pronounced changes at different angular directions, under polarized light which strongly supports the alignment of dye molecules inside the channels. Interestingly, the polycrystalline nature of the crystals results in unique optical properties. The individual sheaf-like crystals have areas with different optical properties under polarized light. Confocal fluorescence lifetime imaging microscopy (FLIM) of crystals loaded with SR were obtained by exciting at λ = 570 nm and detecting the emission at λ = 650-700 nm (Figure ). The fluorescence lifetime of the functionalized sheafs and nanorods (= minor product) are similar (≈ 1.8 ns), indicating the same arrangement of the dyes inside the channels (Figure ). These nanorods also showed a homogeneous distribution of the dye and polarized light-dependent optical properties. The formation of the sheaf-like superstructures was followed by ex situ SEM measurements. The solvothermal reaction was stopped at different reaction times (t = 5 min to t = 5 days; Figure ). No precipitation was observed for t < 5 min. Structures having a rod-like morphology (length = 4.0 ± 0.7 μm) were observed at t = 5 min. Branches are present on some of the structures and have a smaller width than the rods. The branching is evident from the visible lines where the new structures grew. Electron diffraction showed that these rods already have crystalline packing similar to the sheafs that are formed after 5 days (Figure ). After t = 10 min, we observed the formation of hedrites; hyperbranching is clearly seen on both ends giving an appearance of fantail structures. The size of the fantails increases with time and well-defined sheaf-like superstructures develop from the hedrites at t = 2 h. As the reaction time increases (t = 5 days) no significant changes in the morphology of the crystals were observed, and no further branching occurred to form spherulites; under these conditions the sheafs are the thermodynamic product.