Mix and Match Tuning of the Conformational and Multistate Redox Switching Properties of an Overcrowded Alkene

Overcrowded alkenes
have received considerable attention
as versatile
structural motifs in a range of optical switches and light-driven
unidirectional motors. In contrast, their actuation by electrochemical
stimuli remains underexplored, even though this alternative energy
input may be preferred in various applications and enables additional
control over molecular switching states and properties. While symmetric
bistricyclic overcrowded enes (BAEs) containing two identical halves
based on either thioxanthene (TX) or acridine (Acr) motifs are known
to be reversible conformational redox switches, their redox potentials
are generally too high or low, respectively, thereby preventing wider
applications. Herein, we demonstrate that the “mixed”
TX-Acr switch possesses redox properties that lie between those of
its parent symmetric analogs, enabling interconversion between three
stable redox and conformational states at mild potentials. This includes
the neutral anti-folded, the dicationic orthogonal,
and a unique twisted monoradical cation state, the latter of which
is only accessible in the case of the mixed TX-Acr switch and in a
pathway-dependent manner. Consequently, with this multistate redox
switch, a myriad of molecular properties, including geometry, polarity,
absorbance, and fluorescence, can be modulated with high fidelity
and reversibility between three distinct stable states. More generally,
this study highlights the versatility of the “mix and match”
approach in rationally designing redox switches with specific (redox)
properties, which in turn is expected to enable a myriad of applications
ranging from molecular logic and memory to actuators and energy storage
systems.

拥挤烯烃(Overcrowded alkenes)作为一类多功能结构基元，在各类光开关及光驱动单向分子马达领域受到了广泛关注。与之形成鲜明对比的是，尽管电化学刺激作为一种替代能源输入方式在诸多应用中更具优势，且可实现对分子开关状态与性质的额外调控，但基于拥挤烯烃的电化学驱动研究仍有待深入探索。目前已知，基于噻吨(TX)或吖啶(Acr)基元构建的对称双环拥挤烯烃(BAEs)，其分子骨架包含两个相同的半段，可作为可逆构象型氧化还原开关，但二者的氧化还原电位分别普遍偏高或偏低，从而限制了其更广泛的应用。在本研究中，我们证实“混合”型TX-Acr开关拥有介于其对称母体类似物之间的氧化还原特性，可在温和电位下实现三种稳定氧化还原与构象状态间的相互转化。这三种状态分别为中性反折叠构象、二阳离子正交构型，以及一种独特的扭曲单自由基阳离子态；其中后者仅可在混合TX-Acr开关中获得，且呈现路径依赖性。依托这种多状态氧化还原开关，诸多分子性质（包括几何构型、极性、吸光度与荧光性能）可在三种截然不同的稳定状态间以高保真度与可逆性进行调控。更广泛而言，本研究凸显了"mix and match"（混搭）策略在理性设计具备特定（氧化还原）特性的氧化还原开关中的通用性，而这类开关有望推动从分子逻辑与存储、驱动装置到储能系统等诸多应用的实现。