Self-assembly of a photoluminescent metal-organic cage and its spontaneous aggregation in dilute solutions enabling timedependent emission enhancement

Charged metal-organic cages generally produce aggregates with various morphologies and different properties through the multiple supramolecular interactions in solution. Herein, a luminescent hexahedral metal-organic cage containing pyrene chromophores is successfully constructed through coordination-driven subcomponent self-assembly. The cage exhibits novel spontaneous aggregation in a dilute solution and time-dependent luminescence enhancement behavior during the subsequent incubation process. Dynamic light scatter(DLS) and transmission electron microscopy(TEM) results prove that the metalorganic cages can form blackberry-like aggregates in methanol dilute solution. Unexpectedly, the luminescent intensity of this system shows a linear increase with the extension of the incubation time in methanol, and this process is also reflected in the change in the quantum yield of the system(2% to over 80% after 5 days incubation time). Ultraviolet-visible(UV-vis),1 H nuclear magnetic resonance(1 H NMR) and mass spectra show that metal-organic cages can stably exist in dilute solution. Timedepended DLS and TEM data reveal that the aggregates of metal-organic cages are gradually changed from the dense state to the loose one, which may involve the transition of the system from an energy unstable state to a stable one, probably leading to the unusual time-dependent luminescent property. This unique time-dependent luminescent cage aggregate can be potentially applied as a “supramolecular time meter”.