Luminescent Mechanochromic Sensors Based on Poly(vinylidene fluoride) and Excimer‐Forming p‐Phenylene Vinylene Dyes

Abstract

Cyano‐substituted excimer‐forming oligo(phenylene vinylene) dyes (cyano‐OPVs) with terminal alkyl tails of different length were blended with two fluorinated host polymers with similar chemical composition but differing crystallinity. These blends were used to fabricate luminogenic mechanochromic thin films, which change their emission color upon deformation. The alkyl tails affect the solubility of the chromophores in the polymer matrix and lead to different aggregation properties; this is of importance because the mechanochromic fluorescence color change of the blends is related to the self‐assembly of the excimer‐forming dye in the unperturbed polymer matrix, and the dispersion of the dye aggregates upon deformation. Besides the length of the solubilizing tails, the dye concentration has an important influence on the aggregate size, which is crucial to creating a mechanochromic response, since the dye aggregates must be small enough to be dispersed during the deformation process. In‐situ opto‐mechanical measurements have shown that the mechanochromic effect occurs primarily during plastic deformation and that the mechanically induced dispersion of the dye aggregates becomes more pronounced as the crystallinity of the matrix polymer increases.magnified image

A Thermo‐ and Mechanoresponsive Cyano‐Substituted Oligo(p‐phenylene vinylene) Derivative with Five Emissive States

“…Cyano-OPV derivatives have as trong tendency to form excimers [9] in the solid state and often exhibit ah igh emission quantum yield, which makes them attractive for mechano-sensing. [10][11][12] As was demonstrated during our investigationo fo ther cyano-OPVd erivatives, [10][11][12][13] their self-assembly is governed by p-p stacking of the cyano-OPV cores on the one hand and by van der Waals interactions among auxiliary substituents on the other.T wocolor MRL behavior was achieved by balancing these competing interactions, whichc aused the formation of two (meta)stable polymorphs. [11e] This general concept hasalso been used to induce(meta)stableassembly states in afew other MRL materials.…”

“…[11e] This general concept hasalso been used to induce(meta)stableassembly states in afew other MRL materials. [3d, 5a, i, 7, 8] We show herein that the introductiono ft olyl groups, whichw ee xpected to complicate intermoleculari nteractions on account of steric (repulsion) and p-p (attraction) in- teractions, at the terminio fs olubilizing side chains of ap reviously investigated cyano-OPV core, [10][11][12][13] leads to an MRL material (1)w ith five different (meta)stable assembly states,w hich translate in as many different emission colors. Taken more broadly,o ur results show that the introduction of competing intermolecular interactions into excimer-forming chromophores is ap romising design strategy for multicolored stimuliresponsive luminescentm aterials.…”

“…As imilars tructure has been reported for ac yano-OPV derivative featuring methoxy groupsi nstead of the tolyloxyalkoxy moiety as peripheral substituents. [11e] The methoxy-substituted cyano-OPV also exhibitedo range emission (associated with static excimers,s ee below), and its solid-state emission characteristics are similar to the Of orm of compound 1.F igure 3c and ds hows that the peripheralt ails interdigitate each other so that two tolyloxy groups "sandwich" the outer rings of the cyano-OPV core, althought heir orientation is orthogonal to the latter.A ll cyano-OPVs knowntou s, [10][11][12][13] with the same core and general chemical substitution pattern (i.e.,t wo alkyloxy groups attached to the central phenyl ring, and one to each of the peripheral rings) display similaro range red emission in the solid state, which reflects very similar assemblies that promote excimer emission. Although the detailed structures remain unresolved, such excimer emission is absent in the Gand Yf orms, suggesting that the bulkiness and/orc onjugated nature of lateral tolyl rings play an important role, because they make other polymorphs accessible, unlike longeralkyl chains.…”

Cascade Polyannulation of Diyne and Benzoylacetonitrile: A New Strategy for Synthesizing Functional Substituted Poly(naphthopyran)s

“…22 These mechanical stimuli-responsive materials can be used as smart mechanosensors, luminescence switches, security, data storage media, etc. 23 Generally, the common methods of fabricating mechanochromic polymers are physical dispersion of specific dyes into pristine polymers 24 or covalent bonding of chromophoric units to macromolecules. 25 Yet, dyes physically dispersed into polymer substrates may suffer from migrationcaused aggregation, and consequently lower the thermal stability and processability of the polymer.…”

Mechanically Induced Multicolor Change of Luminescent Materials

“…Until now,v ariousc ompounds have been reported to show force-induced luminescent color changes in the solid state, including liquid crystals, [6] organic molecules, [7] organometallic compounds, [8] polymers, [9] dye-doped polymers, [10] ands o forth. However,m ost reported mechanochromic materials display ar eversible change between two colors, which arises from either ac hange in the molecular arrangement or at ransformation in the chemical structure.…”