Molecular Spring‐like Triple‐Helix Coordination Polymers as Dual‐Stress and Thermally Responsive Crystalline Metal–Organic Materials

Abstract

Elastic metal–organic materials (MOMs) capable of multiple stimuli‐responsiveness based on dual‐stress and thermally responsive triple‐helix coordination polymers are presented. The strong metal‐coordination linkage and the flexibility of organic linkers in these MOMs, rather than the 4 Å stacking interactions observed in organic crystals, causes the helical chain to act like a molecular spring and thus accounts for their macroscopic elasticity. The thermosalient effect of elastic MOMs is reported for the first time. Crystal structure analyses at different temperatures reveal that this thermoresponsiveness is achieved by adaptive regulation of the triple‐helix chains by fine‐tuning the opening angle of flexible V‐shaped organic linkers and rotation of its lateral conjugated groups to resist possible expansion, thus demonstrating the vital role of adaptive reorganization of triple‐helix metal–organic chains as a molecular spring‐like motif in crystal jumping.

Reversible Thermosalience in a One-Dimensional Coordination Polymer Preceded by Anisotropic Thermal Expansion and the Shape Memory Effect

“…60 Recently, Mei et al described thermally actuated CPs with a molecular spring-like triple-helix building unit. 60 However, the TS property triggered by anisotropic strain due to thermal expansion during heating was irreversible, thereby limiting the property to a single time event. Reversible TS properties are reported in some organic crystals, 11,16,30,36,52,53 but such macroscopic mechanical motility is difficult to achieve after the incorporation of metal components as in the case of CPs.…”

“…This could possibly be due to thermoresponsive mechanical movements of the crystals which is also dependent on heating rates. 60 To get a deeper insight into the thermal phenomena, single crystals of the CP were analyzed extensively by differential scanning calorimetry (DSC) by heating the crystals to a temperature well below its decomposition temperature. When single crystals of 1 were heated at a rate of 5 K min −1 , an endothermic transition was observed between 401 and 410 K with a "sawtooth" profile which is a characteristic feature of the TS effects 11,30,31,49 (Figure 2a).…”

Jumping Crystal Based on an Organic Charge Transfer Complex with Reversible ON/OFF Switching of Luminescence by External Thermal Stimuli

“…6,19 Besides, most of them were discovered by accident. 20,21 How to design jumping crystals via a bottomup approach remains a huge challenge for both chemists and materials scientists.…”

“…Crystals with stimuli-switchable properties are broadly defined as dynamic crystals, whereas in recent years, the definition is gradually limited to the mechanical properties changing under external stimuli like light, heat, or force, macroscopically manifesting as crystal bending, twisting, jumping, or other motions. − Among all mechanical behaviors, jumping (in whole or in part) is the most exciting and impressive, because it is the only type in which crystals convert non-kinetic energy into tremendous kinetic energy for quick moving. − Crystals possessing the above features, academically specified as “jumping crystals”, not only provide a promising approach forward toward intelligent kinetic device, − but also amplify molecular-level events into the macro world, serving as an ideal probe for sensing molecule motions in crystals. − However, the number of reported jumping crystals is very limited for now, which is three dozen or so. , Besides, most of them were discovered by accident. , How to design jumping crystals via a bottom-up approach remains a huge challenge for both chemists and materials scientists.…”

“…A golden rule to design jumping crystals (Figure , top), based on the summarization of occasional phenomenon, as we have recognized, is trying to generate force through stimulating lattice deformation, ,, which is usually triggered by reactions (thermal decomposition of ammonium salts, photoinduced cis–trans isomerization of azos, photoswitchable ring opening and closing of diarylethenes, [2+2] photocycloaddition of olefins or [4+4] photocycloaddition of anthracenes, for example), ,,− conformational transition, ,, packing mode transition, − and molecular machine rotation (rotaxanes-containing or amphidynamic crystals). , Due to the not all-inclusive stimulation, the inhomogeneity caused by varying degrees of lattice deformation on different regions of a crystal provides the motive power for jumping. Nevertheless, there are some issues unsolved in the aforementioned design concept: (1) Specific functional groups and/or molecular backbones are always required, at the expense of time-consuming, complicated molecular design, and synthesis.…”