We report how the nanoconfined environment, introduced by the mechanical bonds within an electrochemically switchable bistable [2]rotaxane, controls the rotation of a fluorescent molecular rotor, namely an 8-phenyl-substituted boron dipyrromethene (BODIPY). The electrochemical switching of the bistable [2]rotaxane induces changes in the ground-state co-conformation and in the corresponding excited-state properties of the BODIPY rotor. In the starting redox state, when no external potential is applied, the cyclobis(paraquat-p-phenylene) (CBPQT4+) ring component encircles the tetrathiafulvalene (TTF) unit on the dumbbell component, leaving the BODIPY rotor unhindered and exhibiting low fluorescence. Upon oxidation of the TTF unit to a TTF2+ dication the CBPQT4+ ring is forced toward the molecular rotor leading to an increased energy barrier for the excited-state to rotate the rotor into the state with the high non-radiative rate constant, resulting in an overall 3.4-fold fluorescent enhancement. On the other hand, when the solvent polarity is high enough to stabilize the excited charge transfer state between the BODIPY rotor and the CBPQT4+ ring, the movement of the ring towards the BODIPY rotor produces an unexpectedly strong fluorescent signal decrease as the result of the photoinduced electron transfer from the BODIPY rotor to the CBPQT4+ ring. The nanoconfinement effect introduced by mechanical bonding can effectively lead to the modulation of the physicochemical properties as observed in this bistable [2]rotaxane. On account of the straightforward synthetic strategy and the facile modulation of switchable electrochromic behavior, our approach could pave the way for the development of new stimuli-responsive materials based on mechanically interlocked molecules for future electro-optical applications, such as sensors, molecular memories and molecular logic gates.

中文翻译：

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嵌入双稳态轮烷中的荧光分子转子的电化学开关

我们报告了由电化学可切换双稳态 [2] 轮烷内的机械键引入的纳米限制环境如何控制荧光分子转子的旋转，即 8-苯基取代的硼二吡咯亚甲基 (BODIPY)。双稳态 [2] 轮烷的电化学转换引起 BODIPY 转子的基态共构象和相应激发态性质的变化。在起始氧化还原状态下，当没有施加外部电位时，环双（百草枯-对亚苯基）（CBPQT4+）环组分环绕哑铃组分上的四硫富瓦烯（TTF）单元，使 BODIPY 转子不受阻碍并表现出低荧光。在 TTF 单元氧化为 TTF2+ 阳离子后，CBPQT4+ 环被迫朝向分子转子，导致激发态的能量势垒增加，从而使转子旋转到具有高非辐射速率常数的状态，从而导致整体3.4 倍荧光增强。另一方面，当溶剂极性足够高以稳定 BODIPY 转子和 CBPQT4+ 环之间的激发电荷转移状态时，环向 BODIPY 转子的运动会产生出乎意料的强荧光信号降低，这是光诱导的结果。电子从 BODIPY 转子转移到 CBPQT4+ 环。正如在双稳态 [2] 轮烷中观察到的那样，机械键合引入的纳米限制效应可以有效地调节物理化学性质。