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Controlling Switching in Bistable [2]Catenanes by Combining Donor–Acceptor and Radical–Radical Interactions

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posted on 18.07.2012, 00:00 by Zhixue Zhu, Albert C. Fahrenbach, Hao Li, Jonathan C. Barnes, Zhichang Liu, Scott M. Dyar, Huacheng Zhang, Juying Lei, Raanan Carmieli, Amy A. Sarjeant, Charlotte L. Stern, Michael R. Wasielewski, J. Fraser Stoddart
Two redox-active bistable [2]­catenanes composed of macrocyclic polyethers of different sizes incorporating both electron-rich 1,5-dioxynaphthalene (DNP) and electron-deficient 4,4′-bipyridinium (BIPY2+) units, interlocked mechanically with the tetracationic cyclophane cyclobis­(paraquat-p-phenylene) (CBPQT4+), were obtained by donor–acceptor template-directed syntheses in a threading-followed-by-cyclization protocol employing Cu­(I)-catalyzed azide–alkyne 1,3-dipolar cycloadditions in the final mechanical-bond forming steps. These bistable [2]­catenanes exemplify a design strategy for achieving redox-active switching between two translational isomers, which are driven (i) by donor–acceptor interactions between the CBPQT4+ ring and DNP, or (ii) radical–radical interactions between CBPQT2(•+) and BIPY•+, respectively. The switching processes, as well as the nature of the donor–acceptor interactions in the ground states and the radical–radical interactions in the reduced states, were investigated by single-crystal X-ray crystallography, dynamic 1H NMR spectroscopy, cyclic voltammetry, UV/vis spectroelectrochemistry, and electron paramagnetic resonance (EPR) spectroscopy. The crystal structure of one of the [2]­catenanes in its trisradical tricationic redox state provides direct evidence for the radical–radical interactions which drive the switching processes for these types of mechanically interlocked molecules (MIMs). Variable-temperature 1H NMR spectroscopy reveals a degenerate rotational motion of the BIPY2+ units in the CBPQT4+ ring for both of the two [2]­catenanes, that is governed by a free energy barrier of 14.4 kcal mol–1 for the larger catenane and 17.0 kcal mol–1 for the smaller one. Cyclic voltammetry provides evidence for the reversibility of the switching processes which occurs following a three-electron reduction of the three BIPY2+ units to their radical cationic forms. UV/vis spectroscopy confirms that the processes driving the switching are (i) of the donor–acceptor type, by the observation of a 530 nm charge-transfer band in the ground state, and (ii) of the radical–radical ilk in the switched state as indicated by an intense visible absorption (ca. 530 nm) and near-infrared (ca. 1100 nm) bands. EPR spectroscopic data reveal that, in the switched state, the interacting BIPY•+ radical cations are in a fast exchange regime. In general, the findings lay the foundations for future investigations where this radical–radical recognition motif is harnessed in bistable redox-active MIMs in order to achieve close to homogeneous populations of co-conformations in both the ground and switched states.