# Structural Features Guiding the Design of Liquid-Crystalline Elastomeric Fluorescent Force Sensors
## 2 Discussion
### Design, Synthesis and Characterization of the Mechanofluorescent Elastomers
Furthermore, the azimuthal intensity maxima for the first-order small-angle reflex, located at 0\({}^{\circ}\) and 180\({}^{\circ}\), evidence that the mesogens are not tilted and their longest axis is parallel to the layer normal. It should be also mentioned that the presence of a single and sharp wide-angle reflex at 20 \(\approx\) 20\({}^{\circ}\)-21\({}^{\circ}\) together with the absence of additional sharp reflections in the wide-angle region points out the existence of a hexatic smectic B mesophase. At higher temperatures, the broadening of this reflex reflects the transformation of the hexatic smectic B phase into a smectic A mesophase. The order parameter S for the average orientation of the long molecular axis of the mesogenic side groups with respect to the main axis of the LSCE is measured from the azimuthal dependence of the intensity at a scattering vector corresponding to the maximum of the wide-angle reflex (20 \(\approx\) 20\({}^{\circ}\)-21\({}^{\circ}\)). 20\({}^{\circ}\)-21\({}^{\circ}\)). The value of the order parameter is determined by the method reported by [PERSON] and [PERSON] [54; 55]. In all the cases, the highly anisotropic azimuthal intensity distribution of this reflex, showing intensity maxima at 90\({}^{\circ}\) and 270\({}^{\circ}\), indicates that the mesogens are homogeneously organized all over the elastomer, i.e., that LSCEs have been obtained. The thermal range of stability of the liquid-crystalline mesophase/s is assessed by differential scanning calorimetry. DSC thermograms reveal that the system adopts an enantiotropic liquid-crystalline mesophase (nematic or smectic B) between a glass transition temperature (\(T_{\mathrm{g}}\)) and a liquid crystal-to-isotropic phase transition, with the exception of the smectic elastomers, where a smectic B-to-smectic A phase transition was also detected before the isotropization of the LSCE. All LSCEs exhibit a liquid-crystalline nature under ambient conditions. The mechanofluorescent behavior of the different elastomers, which is directly related with their sensing abilities, was evaluated from the variation observed in the LSCE luminescence upon deformation (in Figure 1a) at room temperature. These analyses were carried out by registering the emission spectrum of the elastomeric material before and after applying a uniaxial deformation (\(\varepsilon\) in Figure 1) along the sample longest axis (coincident with \(n\)) in a stepped fashion (in Figure 1b). Excitation of the carbazole fluorophores at \(\lambda_{\mathrm{Ex}}\) = 290-315 nm results in a strong emission within the UV region of the electromagnetic spectrum. Mechanofluorescence experiments imply the monitoring of the amount of light emitted by the LSCE in the form of fluorescence at the wavelength of maximum emission (\(\lambda_{\mathrm{Em}}\) = 350-360 nm) while it is stretched along the anisotropic direction of the sample. As a whole, fluorescence intensity, \(I_{\mathrm{F}}\) (or in relative terms, \(\Delta I_{\varepsilon}\) = (\(I_{\varepsilon}-I_{0}/I_{0}\)) \(\times\) 100, where \(I_{\varepsilon}\) and \(I_{0}\) correspond to the fluorescence intensity of the stretched and unstretched sample, respectively) drops dramatically upon deformation (in Figure 1b,c) until a plateau is eventually reached. This constant value (\(\Delta I_{\mathrm{Max}}\) in Figure 1), which quantifies the force sensor efficiency, corresponds to the maximum variation in the material luminescence that can be induced upon mechanical stimulation and it is the benchmarking criterion of the performance of optical force sensors.