## 3 Results and Discussion
### Thermotropic Liquid Crystalline Phase Behavior
Concentrating on the heating cycles, at least three transitions can be observed for all salts and some of the salts (**1d**, **1f**, **2e**, **2f**) show an additional transition at lower temperatures. The second transition is the most pronounced and does not shift much in temperature within the series. In contrast, the first and third transition shifts to higher temperatures with the increasing alkyl chain within the series. A small hysteresis is visible for all salts comparing the cooling and heating transitions. As these salts exhibit several transitions upon heating and cooling, it is reasonable to assume that they show at least one liquid crystalline phase. Thus, the first transition within the cooling cycle after heating for all salts is identified as the clearing point (T\({}_{\rm c}\)) from the isotropic melt into the mesophase and the subsequent transition as the melting point (T\({}_{\rm m}\)). The clearing and melting points are summarized in Table 1. Based on this data, an increase of the clearing points (the first transition) within each series of the 3-pyridinium salts (**1d-f**) and the 5-pyridinium derivatives (**2d-f**) is observed. Thus, increasing clearing points with increasing chain length are observed and could be explained by attractive van der Waals forces and the interactions between the hydrocarbon chains contributing to the local structure and thus, inducing micro-segregation between hydrophobic alkyl chains and charged ionic regions [22]. Besides this trend, by comparing the regioisomers with each other when bearing the same alkyl chain length, the clearing points are quite similar. Only the salts **1f** and **2f** differ in order of ~14 \({}^{\circ}\)C. On the other hand, the melting points (the second transitions) are quite similar within the series (except for **2d**) but differ depending of the reciprocal position of the perfluoroheptyl chain and the pyridinium moieties on the interspacing oxadiazole ring. In fact, the 3-pyridinium salts (**1d-f**) possess higher melting points than the corresponding 5-pyridinium derivatives. The following discussion about the liquid crystalline phase behavior will concentrate only on the transitions upon cooling. For this, the temperature of each transition, the corresponding enthalpy values and the suggested phase transitions are summarized in Table 2. Since the change in structural ordering of the phase is proportional to the magnitude of the enthalpy change, the enthalpies in Table 2 can be compared with literature values to determine the type of phase. The enthalpy for the melting transition from a crystalline solid-state to a liquid crystalline phase (Cr \(\longrightarrow\) LC) or an isotropic liquid (Cr \(\longrightarrow\) I) phase is ~30 to 50 kJ mol\({}^{-1}\), whereas the enthalpy for a liquid crystalline phase to a liquid crystalline  
Figure 1: Differential scanning calorimetry (DSC) recorded for **1d-f** and **2d-f** showing the second cooling and heating curves with 10 \({}^{\circ}\)C min\({}^{-1}\) under nitrogen atmosphere. Exothermic peaks point upward. phase (LC \(\longrightarrow\) LC) and a liquid crystalline phase to an isotropic liquid (LC \(\longrightarrow\) I) are only associated with small changes around \(\sim\)4-6 kJ mol\({}^{-1}\)[23]. Comparing the enthalpy values within the series with each other (**1d-f** and **2d-f**), they are quite alike for the second transitions, and no trend is visible. For the smaller peaks (first and third transition), the enthalpy values seem to rise, apart from a few exceptions (value of first transition of **1e** and third transition of **2e**). In general, the highest enthalpy values are calculated for the second transition of each salt. Since there are three relevant exothermic peaks in each DSC trace, it can be assumed that the samples exhibit LC phases or more than one crystalline phase.