Monitoring Hierarchical Assembly of Ring-in-Ring and Russian Doll Complexes Based on Carbon Nanoring by Förster Resonance Energy Transfer.

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We presented the construction of the ring-in-ring and Russian doll complexes on the basis of triptycene-derived carbon nanoring (TP-[12]CPP), which not only acts as a host for pillar[5]arene (P5A) but also serves as an energy donor for building Förster resonance energy transfer (FRET) systems. We also demonstrated that their hierarchical assembly processes could be efficiently monitored in real time using FRET. NMR, UV-vis and fluorescence, and mass spectroscopy analyses confirmed the successful encapsulation of the guests P5A/P5A-An by TP-[12]CPP, facilitated by C-H···π and ···π interactions, resulting in the formation of a distinct ring-in-ring complex with a binding constant of Ka = 2.23 × 104 M-1. The encapsulated P5A/P5A-An can further reverse its role to be a host for binding energy acceptors to form Russian doll complexes, as evidenced by the occurrence of FRET and mass spectroscopy analyses. The apparent binding constant of the Russian doll complexes was up to 3.6 × 104 M-1, thereby suggesting an enhanced synergistic effect. Importantly, the Russian doll complexes exhibited both intriguing one-step and sequential FRET dependent on the subcomponent P5A/P5A-An during hierarchical assembly, reminiscent of the structure and energy transfer of the light-harvesting system presented in purple bacteria.