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

By Zhu, Zhixue; Fahrenbach, Albert C.; Li, Hao; Barnes, Jonathan C.; Liu, Zhichang; Dyar, Scott M.; Zhang, Huacheng; Lei, Juying; Carmieli, Raanan; Sarjeant, Amy A.; Stern, Charlotte L.; Wasielewski, Michael R. & Stoddart, J. Fraser
Published in Journal of the American Chemical Society NULL 2012

Abstract

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.

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