Mechanochromophores based on conformational changes of donor–acceptor–donor (DAD) springs allow sensing of forces acting on polymer chains by monotonic changes of absorbance or photoluminescence (PL) wavelength. Here, we identify a series of thiophene (D)-flanked quinoxalines (A) as molecular torsional springs for force sensing in bulk polymers at room temperature. The mode of DAD linkage to the polymer matrix and linker rigidity are key parameters that influence the efficacy of force transduction to the DAD spring and thus mechanochromic response, as probed by in situ PL spectroscopy of bulk films during stress–strain experiments. The largest shift of the PL maximum, and thus the highest sensitivity, is obtained from an ansa-DAD spring exhibiting bridged D units and a stiff A linker. Using detailed spectroscopy and density functional theory calculations, we reveal conformer redistribution in the form of a thiophene ring flip as the major part of the overall mechanochromic response. At forces as low as 27 pN at early stages of deformation, the ring flip precedes mechanically induced planarization of the ansa-DAD spring, the latter process producing a PL shift of 21 nm nN^–1. Within the stress–strain diagram, the thiophene ring flip and DAD planarization are thus two separated processes that also cause irreversible and reversible mechanochromic responses, respectively, upon sample failure. As the thiophene ring flip requires much smaller forces than planarization of the DAD spring, such micromechanical motion gives access to sensing of tiny forces and expands both sensitivity and the force range of conformational mechanochromophores.

中文翻译：

---

Conformer Ring Flip 增强了 ansa-Donor-Acceptor-Donor 机械变色扭转弹簧的机械变色性能

基于供体-受体-供体 (DAD) 弹簧构象变化的机械发色团允许通过吸光度或光致发光 (PL) 波长的单调变化来感测作用在聚合物链上的力。在这里，我们确定了一系列噻吩 (D)-侧翼喹喔啉 (A) 作为分子扭转弹簧，用于室温下本体聚合物中的力传感。DAD 连接到聚合物基质的模式和连接器刚度是影响力转换到 DAD 弹簧的功效的关键参数，从而影响机械变色响应，正如在应力-应变实验期间体膜的原位 PL 光谱所探测的那样。PL 最大值的最大偏移，因此是最高的灵敏度，是从ansa获得的-DAD 弹簧展示桥接 D 单元和刚性 A 连接器。使用详细的光谱学和密度泛函理论计算，我们揭示了噻吩环翻转形式的构象异构体再分布是整体机械变色响应的主要部分。在变形早期阶段的力低至 27 pN 时，环翻转先于 ansa -DAD 弹簧的机械诱导平面化，后一过程产生 21 nm nN ^{–1的 PL 偏移}. 在应力-应变图中，噻吩环翻转和 DAD 平面化因此是两个独立的过程，在样品失效时也会分别引起不可逆和可逆的机械变色响应。由于噻吩环翻转需要的力比 DAD 弹簧的平面化小得多，因此这种微机械运动可以感知微小的力，并扩大了构象机械发色团的灵敏度和力范围。