A Fluorophoric‐Axle‐Based, Nonfluororescent, Metallo anti ‐[3]Pseudorotaxane: Recovery of Fluorescence by Means of an Axle Substitution Reaction

A Cu 2+ ‐templated, multinuclear, nonfluorescent, anti ‐[3]pseudorotaxane was synthesized on a fluorophoric axle. The Cu 2+ ‐templated [3]pseudorotaxane was characterized by the electrospray ionization mass spectroscopy (ESI‐MS), UV/Vis and EPR spectroscopy, and single‐crystal X‐ray data. The ESI‐MS showed peaks that support the formation of [3]pseudorotaxane. The UV/Vis spectrum of [3]pseudorotaxane in CH 3 CN showed a characteristic d–d band of a Cu 2+ complex at 650 nm. Further, the X‐band in the EPR spectrum of [3]pseudorotaxane suggested a distorted square‐pyramidal geometry of Cu 2+ . Importantly, formation of the [3]pseudorotaxane was confirmed by the single‐crystal X‐ray structural analysis, which showed that one fluorophoric axle was threaded into two Cu 2+ macrocyclic wheels (MC‐Cu 2+ ) with an anti conformation. The UV/Vis and fluorescence titration experiments were carried out to follow the solution‐state formation of [3]pseudorotaxane by MC‐Cu 2+ and fluorophoric axle in CH 3 CN. In both studies, the sigmoidal curve fit supported the formation of 1:2 complex of the fluorophoric axle and MC‐Cu 2+ complex. Secondly, the release of the fluorophoric axle from the nonfluorescent [3]pseudorotaxane through the formation of a [2]pseudorotaxane was demonstrated by titrating a solution of the [3]pseudorotaxane with a stronger bidentate chelating ligand, such as 1,10‐phenanthroline (Phen). Substitution of the fluorophoric axle from the [3]pseudorotaxane with about 100 % efficiency was achieved by the addition of approximately two equivalents of Phen, and the formation of a Phen‐threaded [2]pseudorotaxane was established by ESI‐MS of the resulting solution and a single‐crystal X‐ray study. Axle substitution was also confirmed by a fluorescence titration experiment, which showed a step‐wise recovery of the fluorescence intensity of the fluorophoric axle. The association constants for the formation of the [3]‐ and [2]pseudrotaxanes were calculated from the fluorescence and UV/Vis data. In addition, 2,2′‐bipyridine (BPy), which is a relatively weaker bidendate chelating ligand compared to Phen, showed an inefficient and incomplete axle substitution of the [3]pseudorotaxane, although BPy previously showed the formation of [2]pseudrotaxane with the MC‐Cu 2+ wheel in solution and ESI‐MS studies. In this context, the formation of a BPy‐threaded [2]pseudrotaxane was further established by single‐crystal X‐ray diffraction study.