Abstract
Liquid single crystal elastomers (LSCEs) containing carbazole fluorogenic components alter their luminescence when they are stretched along the director direction. The differential luminescent behavior arises from the distinct interaction between the carbazole fluorophores and their local environment before and after the application of the mechanical input. Indeed, the uniaxial deformation of the material, along its anisotropic direction, forces a closer mesogen–fluorophore interaction, which leads to the quenching of the carbazole luminescence. Importantly, this intermolecular interaction is intimately related to the intrinsic order present in the LSCE. As a result, the amount of light emitted by the material in the form of fluorescence diminishes upon deformation. Thus, the application of mechanical stimuli to liquid-crystalline elastomers furnishes to two interconvertible states for the system with distinct optical properties (with either different emission color or fluorescence intensity). The initial state of the material is completely restored once the applied force is removed. In this way, this kind of macromolecular system can transduce mechanical events into detectable and processable optical signals, thus, having great potential as optical force sensors. In this context, the realization of the distinct structural factors that govern the interactions established between the mesogenic and fluorogenic units at the supramolecular level upon deformation is essential for the development of efficient LSCE-based force sensors. In fact, not only the density of carbazole units and their connection to the main polymer backbone, but also the presence of long range molecular order in the system and the type of mesophase exhibited by the LSCE are key factors for the conception of efficient force sensors based on these self-organized polymer networks. In this review, we present a comprehensive and systematic description of the different features that control the mechanoluminescent behavior of fluorescent liquid-crystalline elastomers and will guide the future design of LSCE-based force sensors with improved performances.
Highlights
Liquid single crystal elastomers (LSCEs) are weakly crosslinked polymer networks that combine the long range orientational order of liquid crystals, which is uniform along the whole sample, and the elasticity of conventional rubbers [1]
When LSCEs that incorporate azo derivatives within their polymeric network are illuminated with light of the appropriate wavelength, the linear trans form of the azo photochrome is transformed into its bent cis counterpart
The introduction of selected organic fluorophores into this type of self-organized polymeric system enables the modulation of the amount of light emitted in the form of fluorescence upon deformation, giving rise to LSCE-based optical force sensors [19]
Summary
Liquid single crystal elastomers (LSCEs) are weakly crosslinked polymer networks that combine the long range orientational order of liquid crystals, which is uniform along the whole sample, and the elasticity of conventional rubbers [1]. The intermolecular interactions established between the mesogenic molecules and rationally designed luminescent labels must be altered upon the application or release of a mechanical stimulus [11,12,13,14,15,16,17,18] In this way, the introduction of selected organic fluorophores into this type of self-organized polymeric system enables the modulation of the amount of light emitted in the form of fluorescence upon deformation, giving rise to LSCE-based optical force sensors [19]. Gaining insight into the mechanisms underlying mechanofluorescence in LSCEs is essential to pave the way to a new generation of mechanoluminescent elastomeric materials with novel functionalities and enhanced performances In this context, a careful design of the carbazole fluorophores at the molecular level and a proper engineering of the intermolecular interactions established with the surrounding mesogenic molecules is fundamental to improve the efficiency of the resulting force sensor. Elastomer EM4_CBZN6_5, which contains a 5% mol of carbazole units, exhibits a ∆IMax value of −2E7%las(tFoimguerreE3M).4_OCnBtZhNe 6o_th5,ewr hhaicnhdc, oLnStCaiEnss EaM5%4_mCBolZoNf 6c_a1rb0a, zwohleicuhncitosn, teaxihnisbiatstwa oΔ-IfMoalxdvhailguheeorf c−o2n7c%en(tFraigtiuorne o3f).caOrnbatzhoeleoflthueorrohpahndor,eLsSsChEoswsEMan4_inCcBrZeaNs6ed_1e0ffi, wcihenicchy coofn−ta3i4n%s a[1t9w].o-Ifnoltdhishiwghaeyr, aAcophpnilg.chSeynesrtt.rcIanatnrioobvna.z2oo02fle0c, ca3o,rxbnaFteOznoRtlPewEfEiltRuhoiRnrEoVtphIEheWoerlaesstoshmoewrisc annetwinocrrekalseeaddseftfoicmieonrceyeffiofc−ie3n4t%op[1ti9c]a.lIfnortcheissew7nasoyof r,1sa4. higher carbazole content within the elastomeric network leads to more efficient optical force sensors
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