Substituent Effects Control Spiropyran–Merocyanine Equilibria and Mechanochromic Utility

Abstract

From a microscopic point of view, CC bond cleavage produces radicals which may undergo secondary reactions that further destabilize polymeric materials. Thus, for a long time, such mechanicallyinduced bond scission and degradation has been treated in negative context, as these reactions occurred in an uncontrolled way, at undefined positions at the polymer chain and usually caused property profiles of lower quality. The view on mechanically-induced chemical reactions has meanwhile changed, with mechanochemistry [6,7] being nowadays a vital field that aims at exploiting mechanical force as a form of energy to trigger chemical reactions, which yield product distributions that may or may not differ compared to heat-and light-induced reactions. [8] A prime example in the area of polymer mechanochemistry includes mechanically induced metal-ligand bond scission [9] that gives rise to mechano-catalysis, as pioneered by the Sijbesma group. [10,11] Work by Hickenboth et al. has investigated a force-induced pericyclic ring opening reaction which furnished different product distributions compared to what is obtained according to the Woodward-Hoffman rules. [12] Polymers are ideal for this purpose as stress transduction to the molecular level is easily achieved using solution sonication or bulk mechanical testing. If a mechanically-induced reaction leads to changes in optical properties, the material is termed mechanochromic. The structural unit undergoing optical changes upon mechanical activation is called mechanophore. [13] In 2001, Tipikin et al. observed that grinding a spiropyran (SP) small molecule produces the corresponding merocyanine (MC) and thus a color change. [14] The force-induced SP→MC isomerization by using polymers with SP units covalently incorporated at their mid points was pioneered by Moore et al. using sonication and later on by Sottos et al. using classical stress-strain experiments of bulk specimens. [13,15] Their pioneering works initiated a large number of studies, not only in the area of SP containing polymers but also related to the development of other mechanophores, in the areas of mechanochemistry and mechanochromic polymers. Since then SPs are popular mechanophores despite their multiple isomers and conformers. [16-19] It is interesting to note that the two positions of polymer linkage at SP, which are meanwhile considered most efficient for mechanoactivation, have been used by Smeets et al. as early as 1974 to Spiropyran (SP) derivatives can be converted into the colored merocyanine (MC) form using a variety of triggers. Optical switching by light for memories and dynamic systems is long known. Recently, mechanical force has been reported as an additional stimulus that converts SP into MC. SP-based mechanochromic systems are especially interesting for polymer scientists, as the covalent attachment of polymer chains is ideal to transduce force to the SP level. Whether such materials are investigated to better understand fundamental processes or long standing questions in polymer science, to ...

Shear-activated chemisorption and association of cyclic organic molecules

“…15 Further, the mechanochromic properties of spiropyran could be tuned by inserting suitable substituents to the mechanophores. 16 These studies have shown that even subtle differences between reactants can affect how mechanical stress activates chemical reactions.…”

Tailoring the Properties of Optical Force Probes for Polymer Mechanochemistry

“…[ 32 , 33 ] Craig and coworkers have demonstrated that the force required for the isomerization of spiropyran can be tuned by varying the attachment points of the OFP to the polymer chain. [ 34 , 35 ] However, this has only a minimal effect on the mechanical activation in bulk polymeric materials due to the elusive nature of the interplay between molecular OFP and macroscopic polymer system. [ 36 , 37 ]…”

A Perspective on Cephalopods Mimicry and Bioinspired Technologies toward Proprioceptive Autonomous Soft Robots

“…Extensive reports about the theory and application of such systems can be found in refs. [165][166][167][168]. One of the most investigated mechanochromic species is spiropyran (SP).…”