# Structural Features Guiding the Design of Liquid-Crystalline Elastomeric Fluorescent Force Sensors
## 2 Discussion
### Structural Factors That Impact the Mechanofluorescent Behavior
#### 2.2.5 Type of Mesophase Exhibited by the LSCE  
Besides connecting the carbazole fluorophores to the main polymer backbone via an end-on connection, endowing side-on elastomeric materials with a different mesogenic arrangement leads also to the conception of very efficient optical force sensors [59]. In this section, four smectic liquid single crystal elastomers will be considered, namely, EM10_CBZN2_10, EM10_CBZN6_10, EM10_CBZN10_10 and EM10_CBZN11_10 (in Figure 6a). In this case, 4-methoxyphenyl 4-(9-decenyloxy)-benzoate (M10) has been used as the mesogenic monomer. Although this low molar mass liquid crystal shows a nematic mesophase within a very narrow temperature range, it leads to stable hexatic smectic B (SmB, mesogens spacing \(\approx 4.3\) A, layers spacing \(\approx 31\) A) elastomeric materials at room temperature when incorporated into polysiloxane networks. Specifically, these elastomeric systems also display a smectic A mesophase (SmA, mesogens spacing \(\approx 4.3\) A, layers spacing \(\approx 31\) A) at higher temperatures (between ca. 306 and 363 K).  
As a main difference with the nematic LSCEs that have been discussed previously in this review, smectic B elastomers show a mechanoluminescent response that is practically independent of the length of the carbazole flexible spacer (\(\Delta I_{\text{Max}}\) around \(-85\%\), in Figure 6b) [59]. In addition, it should be remarked that the \(\Delta I_{\text{Max}}\) value of the best performing smectic B LSCE (EM10_CBZ_N2_10, \(\Delta I_{\text{Max}}\) = \(-91\%\)) is almost threefold higher than the one obtained for the best nematic counterpart (\(\Delta I_{\text{Max}}=-34\%\), see Figures 3 and 4). The mechanofluorescent behavior of elastomer EM10_CBZ_N6_10 has also  
Figure 6: Chemical composition (**a**) and mechanofluorescent response (**b**) for the smectic LSCEs EM10_CBZN2_10, EM10_CBZN6_10, EM10_CBZN10_10 and EM10_CBZN11_10. The red empty circles indicate the mechanofluorescent response of elastomer EM10_CBZN10_10 at 328 K, i.e., within the thermal range of stability of its smectic A mesophase [59]. (Copyright from Wiley-VCH Verlag GmbH & Co. KGaA 2018).  
been investigated within the thermal range of stability of its SmA mesophase, thereby obtaining the very same \(\Delta I_{\rm Max}\) value than the one obtained under ambient conditions (\(-72\%\) vs. \(-70\%\)).  
These results reveal that the characteristic stratified nature of smectic phases is a main structural factor controlling the efficiency of LSCE-based force sensors. In the case of carbazole-containing smectic LSCEs, the carbazole fluorophores are located within the lamellar structure of the host mesophase, forcing a stronger interaction between both platforms than the one occurring in the case of their nematic analogs. Indeed, this is also the reason by which the observed mechano-fluorescent response is independent of the length of the flexible spacer used to connect the carbazole fluorophores to the main polymeric backbone.