Mechanochromism

Conformational transitions and electron redistribntion on pressure results in changes of the ion-radical properties and this is essential for optoelectronic devices. Pressure is the thrust distributed over a surface. The most important results of compression are reduction of molecular volumes and conformational changes of organic componnds. Spectral changes of solids are also termed piezo-chromism. Let ns consider several relevant examples of piezochromism. 

Pressure provokes transition of the linear (extended) conformation into the bent (V-like) one. (The V-like form is more compact and occupies a smaller volume.) It is obvious that the V-like form is favorable in respect of intramolecular electron transfer from the donor (the aniline part) to the acceptor (the pyrene part). In the utmost level of the phenomenon, the donor part transforms into the cation-radical moiety, whereas the acceptor part passes into the anion-radical moiety. Such transformation is impossible in the case of the extended conformation because of the large distance between the donor and acceptor moieties. The spectral changes observed reflect this conformational transition at elevated pressures. 

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Abstract We describe mechanochromic and thermochromic photoluminescent liquid crystals. In particular, mechanochromic photoluminescent liquid crystals found recently, which are new stimuli-responsive materials are reported. For example, photoluminescent liquid crystals having bulky dendritic moieties with long alkyl chains change their photoluminescent colors by mechanical stimuli associated with isothermal phase transitions. The photoluminescent properties of molecular assemblies depend on their assembled structures. Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Mechanochromic photoluminescent properties are also observed for a photoluminescent metallomesogen and a liquid-crystalline polymer. We also show thermochromic photoluminescent liquid crystals based on origo-(/ -phenylenevinylene) and anthracene moieties and a thermochromic photoluminescent metallocomplex. 

Fig. 2 Molecular structures of compounds 1-3 and observed mechanochromic photoluminescent behavior on the cubic-columnar liquid-crystalline phase transitions...

Kozhevnikov, Bruce, and coworkers described an LC N,C,N-coordinated plati-num(II) derivative 4 showing mechanochromic luminescence (Fig. 4) [31]. Compound 4 shows a hexagonal columnar phase up to 145 °C and another hexagonal columnar phase up to 220 °C. A pure film of compound 4 shows a red excimer-like emission at room temperature. Interestingly, the photoluminescent color of compound 4 changes from red to yellow by annealing at 110 °C and the yellow photoluminescence is preserved after cooling to room temperature. The red... 

Fig. 4 Molecular structure of compound 4 and observed mechanochromic photoluminescent behavior before and after applying mechanical stimuli...

Recently, Lee, Kwak, and coworkers have reported on mechanochromic photoluminescent LC polymer 5 (Fig. 5) [32]. This poly(diphenylacetylene) derivative has flexible dimethyloctadecylsilyl side chains. A solvent cast film made from toluene solution of compound 5 shows a smectic LC phase above 5 °C and phase transition to the isotropic phase is not observed until 200 °C. The cast film of compound 5 shows a blue emission. An orange emission can be observed when mechanical stimulus is induced to the cast film of compound 5. In the emission spectra, an additional shoulder appears after the mechanical stimulus is induced. The longer wavelength component is attributed to forming intermolecular excimers. The sky blue emission is recovered by annealing of compound 5 in hexane. 

Thermochromic photoluminescent liquid crystals have been reported recently as well as the mechanochromic photoluminescent liquid crystals. Weder and coworkers reported on oligo(p-phenylene vinylene) derivative 6 showing thermochromic... 

These are two closely related phenomena that may not warrant individual definitions. Mechanochromism describes a change in colour resulting from a physical perturbation... 

Crenshaw, B. R., Bumworth, M., Khariwala, D., Hiltner, P. A., Mather, P. T., Simha, R., and Weder, C., Deformation-induced color changes in mechanochromic polyethylene hlends. Macromolecules, 40, 2400-2408 (2007). 

Abstract The past 10 years have seen a resurgence of interest in the field of polymer mechanochemistry. Whilst the destructive effects of mechanical force on polymer chains have been known for decades, it was only recently that researchers tapped into these forces to realize more useful chemical transformations. The current review discusses the strategic incorporation of weak covalent bonds in polymers to create materials with stress-sensing and damage-repairing properties. Firstly, the development of mechanochromism and mechanoluminescence as stress reporters is considered. The second half focuses on the net formation of covalent bonds as a response to mechanical force, via mechanocatalysis and mechanically unmasked chemical reactivity, and concludes with perspectives for the field. 

Although the property of interest in all of the above-mentioned cyano-OPV-containing materials was a mechanically induced change of the photoluminescence color (i.e., a mechanically induced transformation from an excimer-rich to a monomer-dominated emission), certain cyano-OPVs also display a changed of their absorption color upon (dis)assembly. Thus, Kunzelman et al. reported mechanochromic blend films based on C18-RG and either poly(ethylene... 

Fig. 7 (a) Picture of a mechanochromic elastomer made by integrating C120H-RG into a thermoplastic polyurethane backbone in the unstretched state, (b) Picture of the same material in the stretched state. Both pictures were taken under illumination with ultraviolet light, (c) Ratio of monomer to excimer emission 7m//e (circles) and tensile stress (solid line) under a triangular strain cycle between 0% and 500% at a frequency of 0.0125 Hz. Adapted with permission from [41]. Copyright 2006 American Chemical Society... 

Fig. 13 (a) Molecular structures of PI and P2, building blocks used to assemble a healable mechanochromic supramolecular polymer, (b) Schematic illustration of the healable supramolecular polymers using donor-acceptor interactions between PI and P2. (b) reprinted with permission from [60], Copyright 2014 The Royal Society of Chemistry... 

In recent years, an increasing amount of attention has been paid to organic compounds whose emission colors change in response to mechanical force, because of their potential applications in mechanical sensing, optical recording, and memory devices. This is also the case for coordination compounds, and a number of mechanochromic coordination compounds have been synthesized, particularly compounds containing d ° metal ion(s). 

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