A fluorophoric-axle-based, nonfluororescent, metallo anti-[3]pseudorotaxane: recovery of fluorescence by means of an axle substitution reaction

Abstract

A Cu²⁺-templated, multinuclear, nonfluorescent, anti-[3]pseudorotaxane was synthesized on a fluorophoric axle. The Cu²⁺-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 CH₃CN showed a characteristic d–d band of a Cu²⁺ complex at 650 nm. Further, the X-band in the EPR spectrum of [3]pseudorotaxane suggested a distorted square-pyramidal geometry of Cu²⁺. 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 Cu²⁺ macrocyclic wheels (MC-Cu²⁺) 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-Cu²⁺ and fluorophoric axle in CH₃CN. In both studies, the sigmoidal curve fit supported the formation of 1:2 complex of the fluorophoric axle and MC-Cu²⁺ 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-Cu²⁺ 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.