Radically Enhanced Molecular Switches

The mechanism governing the redox-stimulated switching behavior of a tristable [2]­rotaxane consisting of a cyclobis­(paraquat-p-phenylene) (CBPQT4+) ring encircling a dumbbell, containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units which are separated from each other along a polyether chain carrying 2,6-diisopropylphenyl stoppers by a 4,4′-bipyridinium (BIPY2+) unit, is described. The BIPY2+ unit acts to increase the lifetime of the metastable state coconformation (MSCC) significantly by restricting the shuttling motion of the CBPQT4+ ring to such an extent that the MSCC can be isolated in the solid state and is stable for weeks on end. As controls, the redox-induced mechanism of switching of two bistable [2]­rotaxanes and one bistable [2]­catenane composed of CBPQT4+ rings encircling dumbbells or macrocyclic polyethers, respectively, that contain a BIPY2+ unit with either a TTF or DNP unit, is investigated. Variable scan-rate cyclic voltammetry and digital simulations of the tristable and bistable [2]­rotaxanes and [2]­catenane reveal a mechanism which involves a bisradical state coconformation (BRCC) in which only one of the BIPY•+ units in the CBPQT2(•+) ring is oxidized to the BIPY2+ dication. This observation of the BRCC was further confirmed by theoretical calculations as well as by X-ray crystallography of the [2]­catenane in its bisradical tetracationic redox state. It is evident that the incorporation of a kinetic barrier between the donor recognition units in the tristable [2]­rotaxane can prolong the lifetime and stability of the MSCC, an observation which augurs well for the development of nonvolatile molecular flash memory devices.本文报道了一种三稳态[2]轮烷（tristable [2]rotaxane）的氧化还原触发开关行为的调控机制：该轮烷以环双(百草枯-对亚苯基)（cyclobis(paraquat-p-phenylene)，CBPQT4+）环作为大环组分，其环绕着哑铃型组分；哑铃型组分沿带有2,6-二异丙基苯基封端基团的聚醚链排布，包含四硫富瓦烯(tetrathiafulvalene, TTF)与1,5-二氧萘(1,5-dioxynaphthalene, DNP)两种识别单元，二者之间由4,4'-联吡啶鎓(4,4′-bipyridinium, BIPY2+)单元分隔。BIPY2+单元可通过限制CBPQT4+环的穿梭运动，显著延长亚稳态共构象(metastable state coconformation, MSCC)的寿命，使得该MSCC可在固态中被分离出来，并能稳定数周之久。作为对照研究，本文考察了两种双稳态[2]轮烷与一种双稳态[2]索烃([2]catenane)的氧化还原开关机制：这两类超分子组装体均由CBPQT4+环分别环绕哑铃型组分或大环聚醚构成，其组分中均包含带有TTF或DNP单元的BIPY2+单元。对该三稳态[2]轮烷、两种双稳态[2]轮烷以及[2]索烃开展变扫描速率循环伏安法与数字模拟研究，结果揭示了其调控机制涉及双自由基态共构象(bisradical state coconformation, BRCC)：在此状态下，CBPQT2(•+)环中仅一个BIPY•+单元被氧化为BIPY2+二价阳离子。该BRCC的观测结果进一步通过理论计算以及该[2]索烃处于双自由基四价阳离子氧化还原态的X射线晶体学(X-ray crystallography)分析得到了验证。显而易见，在该三稳态[2]轮烷的给体识别单元之间引入动力学势垒，能够延长MSCC的寿命与稳定性，这一发现为非易失性分子闪存器件的开发带来了良好前景。