From Molecules to Polymers—Harnessing Inter‐ and Intramolecular Interactions to Create Mechanochromic Materials

Abstract

tion occurs to avoid catastrophic materials failure and to possibly redirect degradation processes into useful responses. Consequently, research efforts directed toward the investigation and development of mechanoresponsive materials has drawn ever increasing attention. [2,3,5-8] Polymers that translate mechanical stresses into a defined response are thought to be useful for many different applications, including as tamper-proof packaging materials, [9] as degradable plastics, [10,11] or for structural health monitoring. [12] One possibility to render polymers mechanoresponsive is the integration of so-called mechanophores. The latter generally feature a weak covalent bond that undergoes either homo-or heterolytic cleavage upon experiencing a force that exceeds a certain threshold. [1,5,7,13,14] These motifs are covalently incorporated into a polymer and serve as predefined weak links that preferentially break or transform in response to an applied mechanical force. [1,6,14-16] Widely investigated mechanophores include spiropyrans, 1,2-dioxetanes, and Diels-Alder adducts. [16-18] The rate constants, threshold forces, and transition state bond-lengths and energies of many mechanophores have been well-characterized via techniques such as single-molecule force spectroscopy (SMFS), [19-22] force-modified potential energy surface modeling, [23] and in situ activation using sonication. [2,22,24] The insights gained in such studies have driven advancements in the field of mechanoresponsive materials and the mechanical activation has, for example, been harnessed to promote thermodynamically unfavorable reactions, [19] affect the intra-or intermolecular transfer of protons, [20,25,26] release small molecules, [27] initiate the depolymerization of the surrounding material, [10,28] or elicit changes in the material's color or fluorescence. [16,17,29] The fact that the active bond or functional group of a mechanophore is typically cleaved irreversibly can limit its utility. Some mechanophores can be returned to their original state through an additional stimulus such as light or heat, but unless the mechanophore undergoes a nonscissile cleavage (e.g., a ring-opening reaction, such as in spiropyrans), the reversal is generally hampered by kinetic factors that leave the mechanophore in its cleaved state. [30] An irreversible response can be desirable for some

Folded Perylene Diimide Loops as Mechanoresponsive Motifs

“…[14] However,t he fact that the active bonds in mechanophores are typically irreversibly cleaved can limit their utility.M echanoresponsive motifs that operate in an on-sacrificial and potentially reversible manner are thus attractive alternatives. [15] Mechanisms that can be exploited in this context include conformational changes in individual (macro)molecules,t he disassembly of associated non-covalent binding motifs,a nd the disruption of aggregates of dye molecules whose optical properties are influenced by such aggregation. Fore xample,b lending excimer-forming dyes into ap olymer matrix can be as traightforward means to prepare mechanochromic materials.…”

Halogen bonding in polymer science: towards new smart materials

“…For example, mechanoresponsive materials could be one field of interest since many other supramolecular interactions were already studied in this context. 76 Furthermore, the defined preparation of XB systems featuring different binding strength can improve the investigation of the stimuli-responsive behavior. This knowledge will later enable more intelligent materials applicable for different proposes including shape-memory, sensors as well as self-healing.…”

The Mechanochemical Release of Naphthalimide Fluorophores from β-Carbonate and β-Carbamate Disulfide-Centered Polymers

“…21,26 In the field of polymer mechanochemistry, mechanical force induces chemical transformations using mechanoresponsive molecules (mechanophores) embedded in polymer structures. [27][28][29] Mechanophores were, for example, used to force-activate or -modulate optical properties, [30][31][32][33] catalysis, [34][35][36] or to release small molecules. [37][38][39][40] However, employing mechanophores for the controlled release and activation of drugs by ultrasound is an area of research that was only recently established.…”