Controlling Switching in Bistable [2]Catenanes by Combining Donor鈥揂cceptor and Radical鈥揜adical Interactions

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• 作者：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
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：July 18, 2012
• 年：2012
• 卷：134
• 期：28
• 页码：11709-11720
• 全文大小：689K
• 年卷期：v.134,no.28(July 18, 2012)
• ISSN：1520-5126

文摘

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鈥?bipyridinium (BIPY²⁺) units, interlocked mechanically with the tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺), were obtained by donor鈥揳cceptor template-directed syntheses in a threading-followed-by-cyclization protocol employing Cu(I)-catalyzed azide鈥揳lkyne 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鈥揳cceptor interactions between the CBPQT⁴⁺ ring and DNP, or (ii) radical鈥搑adical interactions between CBPQT^2(鈥?) and BIPY^鈥?, respectively. The switching processes, as well as the nature of the donor鈥揳cceptor interactions in the ground states and the radical鈥搑adical 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鈥搑adical 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^鈥? for the larger catenane and 17.0 kcal mol^鈥? 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鈥揳cceptor type, by the observation of a 530 nm charge-transfer band in the ground state, and (ii) of the radical鈥搑adical 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^鈥? radical cations are in a fast exchange regime. In general, the findings lay the foundations for future investigations where this radical鈥搑adical 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.