Precise Design and Deliberate Tuning of Turn-On Fluorescence in Tetraphenylpyrazine-Based Metal-Organic Frameworks

The manipulation on turn-on fluorescence in solid state materials attracts increasing interests owing to their widespread applications. Herein we report how the nonradiative pathways of tetraphenylpyrazine (TPP) units in metal-organic frameworks (MOFs) systems could be hindered through a topological design approach. Two MOFs single crystals of different topology were constructed via the solvothermal reaction of a TPP-based 4,4',4^″,4^‴-(pyrazine-2,3,5,6-tetrayl) tetrabenzoic acid (H₄TCPP) ligand and metal cations, and their mechanisms of formation have been explored. Compared with the innate low-frequency vibrational modes of flu net Tb-TCPP-1, such as phenyl ring torsions and pyrazine twists, Tb-TCPP-2 adopts a shp net, so the dihedral angle of pyrazine ring and phenyl arms is larger, and the center pyrazine ring in TPP unit is coplanar, which hinders the radiationless decay of TPP moieties in Tb-TCPP-2. Thereby Tb-TCPP-2 exhibits a larger blue-shifted fluorescence and a higher fluorescence quantum yield than Tb-TCPP-1, which is consistent with the reduced nonradiative pathways. Furthermore, Density functional theory (DFT) studies also confirmed aforementioned tunable turn-on fluorescence mechanism. Our work constructed TPP-type MOFs based on a deliberately topological design approach, and the precise design of turn-on fluorescence holds promise as a strategy for controlling nonradiative pathways.