Force sensing and stress imaging with donor-acceptor torsional springs

能够对外部刺激做出响应的智能材料正在受到广泛的研究，因为它们在生物学，信息技术，自我修复，传感和能源相关主题中的应用几乎是无限的。机械响应聚合物在暴露于机械应力时急剧改变性质，其可用于成像应力或应变、切换反应性或偏置以其他方式难以进入的不同反应途径。最普遍的力致变色体系之一是螺吡喃/汞体系（SP/MC）。该系统以及其他系统依赖于均相或非均相的键断裂，因此显示出主要与它们的双态性质和两种形式之间的缔合势垒相关的内在限制。特别地，SP衍生物还具有温度不稳定的缺点，这阻碍了熔融加工，因此具有广泛的适用性。SP衍生物还显示出通常限制长期稳定性的光漂白。对于与生物系统、机器人、高性能材料的开发和基础研究相关的许多应用，具有快速和直接响应以及完全可逆性的无障碍机械致变色系统将具有显著的优势。该提案涉及开发用于力传感和应力成像的供体-受体（D-A）扭转弹簧的新概念。这样的扭转弹簧的中心组件是共价连接的供体-受体二元体，其平衡几何形状中表现出大的二面角。通过适当的共价键向聚合物链/基质施加力，然后允许D-A系统的力诱导平面化，导致增加的电子相互作用，从而导致电荷转移（CT）带的红移。这种力诱导的平面化是完全可逆的，并且另外允许根据所使用的D-A系统和相关扭转电势的力/颜色可调谐性。所提出的D-A扭转弹簧是完全可逆的无障碍系统。通过使用理论和实验相结合的方法，我们将研究和使用不同的扭转势的各种D-A弹簧，将它们纳入合适的聚合物矩阵，允许宏观应力的D/A弹簧，并最终研究由此产生的力依赖的光学性能的变化。通过建立机械变色的新概念及其所有优点，我们将推进机械响应聚合物的新领域，并开辟前所未有的新机会和应用。

Smart materials that are able to respond to external stimuli are under extensive investigation because of their almost unlimited range of applications in biology, information technology, self-healing, sensing and energy-related topics. Mechano-responsive polymers drastically change properties upon exposure to mechanical stress, which can be used to image stress or strain, switch reactivity or to bias distinct reaction pathways that are otherwise difficult to access. One of the most prevalent mechanochromic system is the spiropyran/ merocyanine system (SP/MC). This system, and also others, rely on homogeneous or heterogeneous bond scission and hence show intrinsic limitations that are mainly related to their two-state nature and the associate barrier between the two forms. Particularly SP derivatives have the further disadvantages of being temperature-labile which prohibits melt processing and hence broad applicability. SP derivatives also show photo bleaching which generally limits long-term stability. For many applications related to biological systems, robotics, the development of high performance materials and fundamental investigations, a barrierless mechanochromic system with fast and direct response and full reversibility would hold significant advantages. This proposal is concerned with the development of the new concept of donor-acceptor (D-A) torsional springs for force sensing and stress imaging. The central component of such torsional springs is a covalently linked donor-acceptor dyad exhibiting a large dihedral angle in its equilibrium geometry. The application of force by appropriate covalent linkage to a polymer chain/ matrix then allows force-induced planarization of the D-A system, leading to increased electronic interaction and thus a red-shift of the charge transfer (CT) band. Such a force-induced planarization is completely reversible and additionally allows force/color tunability according to the D-A system and related torsion potential used. The proposed D-A torsional spring is a barrierless system that is fully reversible. By using a combined theoretical and experimental approach, we will investigate and use different torsion potentials of various D-A springs, incorporate them into suitable polymer matrices that allow to transduce macroscopic stress to the D/A spring, and finally investigate the resulting force-dependent change of the optical properties. By establishing this new concept of mechanochromism with all its advantages mentioned, we will advance the new field of mechano-responsive polymers and open new opportunities and applications of unprecedented extent.