# Structural Features Guiding the Design of Liquid-Crystalline Elastomeric Fluorescent Force Sensors
## 3 Conclusions and Perspectives  
The elongation of carbazole-containing liquid single crystal elastomers, in the direction parallel to their longest axis, provokes a huge decrease in their emission intensity. When the elastomeric material is deformed along its anisotropic direction, i.e., parallel to the director, an increase in its intrinsic nematic  
Figure 7: Time-resolved mechanofluorescence experiment for LSCE EM4_CBZN6_10 [19]. (Copyright from Wiley-VCH Verlag GmbH & Co. KGaA 2015). ordering forces a closer interaction between the carbazole fluorophores and the mesogenic molecules. These differential interactions established at the nanoscale result in a quenching of the luminescence of the formers at the macroscopic level. In this way, two interconvertible states are generated mechanically for the system with excellent contrast, in other words, the luminescence of carbazole-containing LSCEs can be switched from on to off upon deformation. Therefore, carbazole-based LSCEs are valuable fluorescent flexible materials able to convert mechanical sensations into processable optical signals and, thus, they find great applicability as optical force sensors. This review presents to the reader the recent progress made in the design of these emerging materials, emphasizing the different structural factors that play a key role in their performance. Regarding the carbazole fluorophores, a higher concentration of the organic emitter into the elastomeric network leads to more efficient optical force sensors. Nevertheless, the presence of long range orientational order in the elastomeric material is essential to achieve optical force sensors with these macromolecular networks. The linkage (end-on or side-on) of the carbazole monomers to the elastomeric network also impacts significantly the sensing abilities of the resulting material, with the end-on attachment the one producing the most efficient sensors. In addition, the length of the alkyl chain that binds the organic fluorophores to the polymeric backbone fine-tunes the material performance. In the case of side-on systems, maximum efficiency is achieved when both fluorophores and mesogens have flexible spacers of a comparable length. When end-on fluorophores are considered instead, short spacers (up to 6 methylene units) are preferable since they transmit more efficiently the mechanical input from the polymer chains to the fluorogenic components. Finally, SmB LSCEs, with a layered structure, yield also very efficient optical force sensors independently of the length of the fluorophore flexible spacer. In SmB LSCEs, the fluorogenic platforms and mesogenic units are restricted within the smectic lamellae. Such a feature induces an effective coupling between the fluorogenic and mesogenic components, leading to more efficient optical force sensors than with their nematic analogs. On the basis of these considerations, mechanoluminescent decay is not only controlled by the molecular architecture of the carbazole platform and its connection to the polymeric backbone, but also by the inherent organized nature of the LSCE host mesophase. Importantly, the design schemes summarized in this review have evolved into a new family of efficient fluorescence-based force sensors capable of converting mechanical inputs into optical outputs under ambient conditions. Thus, these findings have undeniable implications in the future design of novel LSCE-based materials for transducing and sensing purposes. Further investigation in the field might involve the incorporation of side-on or end-on carbazole fluorophores into main-chain nematic or smectic LSCEs, where an even stronger transmission of the mechanical input from the polymeric backbone to the mesogenic and fluorogenic components should be expected. The design of force sensing elastomeric materials that emit light in the form of fluorescence within the visible region of the electromagnetic spectrum would also be an important step forward.