# Structural Features Guiding the Design of Liquid-Crystalline Elastomeric Fluorescent Force Sensors
## 2 Discussion  
### Design, Synthesis and Characterization of the Mechanofluorescent Elastomers  
This study will deal with the mechanofluorescent behavior of nematic and smectic LSCEs (smectic LSCEs will be described in detail in Section 2.2.5, vide infra). The different elastomers will be denoted as follows: EM_m_CBZX_n_z_, where M_m_ indicates the mesogenic monomer (M4 or M10) used, X_n_ states the atom (N or O) and the length of the flexible spacer through which the carbazole fluorophore is attached to the elastomeric network, and z informs about the carbazole content (see Scheme 2). All nematic LSCEs are composed of a main polymer backbone of polyhydrogeno-methylsiloxane (PHMS in Scheme 2) functionalized with the nematic mesogen 4-methoxyphenylyl 4-(3-butenyloxy)benzoate (M4 in Scheme 2). The use of this organic molecule in high proportions (70%-85% mol) endows the resulting elastomer with a stable nematic mesophase over a wide range of temperatures. In addition, 1,4-di-(10-undecenyoxy)benzene (CL in Scheme 2) is used as a cross-linking agent (10% mol) to connect the distinct polymeric chains and confer the required elastic properties to the final material. Finally, as we have already mentioned, it is our main aim to transduce mechanical forces into optical signals. For this task, carbazole derivatives (CBZX_n_ in Scheme 2) have been introduced as fluorogenic components in the elastomeric network at small concentrations (from 5% to 20% mol). Carbazole,  
Figure 1: Schematic representation of the different features that govern the mechanoluminescent behavior of LSCE-based optical force sensors. a well-known organic emitter, has been selected as a fluorophore not only due to its high fluorescence quantum yield and stability, but also because it can be easily functionalized at the required positions by means of well-established procedures. LSCEs have been synthesized according to the procedure described by [PERSON] and [PERSON] in 1991 [52]. According to this methodology, LSCEs can be prepared via a three-step protocol. In the first step, a partial Pt-catalyzed hydrosilylation reaction is carried out, where the distinct monomers react with the PHMS backbone through their terminal olefins. It should be highlighted that this reaction is stopped at its early stages (only after 2 h at 70 \({}^{\circ}\)C), since the main aim of this part of the process is to obtain a stable but tunable network. In the second step, the elastomeric sample is stretched along its longest axis in order to align the different nematic directors. After the second step, a monodomain liquid-crystalline elastomer, i.e., a LSCE, is obtained. In the third and final step, the sample is left to react at 70 \({}^{\circ}\)C for 2 days, without removing the applied force, to fix the anisotropy generated previously. Afterwards, the non-reacted monomers and the catalyst are removed from the material by a swelling-deswelling procedure, using toluene and hexane, respectively. A detailed description of the synthetic procedure can be found in [53]. The mesophase structure and the mesomorphic behavior of the prepared samples were analyzed by X-Ray diffraction (XRD) and differential scanning calorimetry (DSC). XRD was used to establish unambiguously the type of liquid-crystalline phase exhibited by the LSCE. Nematic liquid-crystalline elastomers reveal a broad reflex in the wide-angle region of their X-ray scattering pattern (20 \(\approx\) 20\({}^{\circ}\)-21\({}^{\circ}\)). From this reflex, the mesogens spacing is calculated by applying Bragg's law. On the other hand, the presence of a series of additional reflexes in the small-angle region of the X-ray scattering pattern is indicative of a smectic-like organization. In this case, the mesogens and layers spacing is calculated from the wide-angle and the first-order small-angle reflex, respectively.