# Luminescence Tuning of Fluorinated Bistolanes via Electronic or Aggregated-Structure Control
## 3 Results and Discussion
### Phase Transition Behavior  
The phase transition behavior of fluorinated bistolanes **1 aB-D** and **1 bB-D** was investigated using POM and DSC. Figure 4 shows the phase transition behavior observed during the 2 nd heating cooling processes and the microphotographic images of the LC phases.  
Figure 4: Phase transition behavior of **(a) 1 aB–D** and **(b) 1 bB–D** during the 2 nd heating [H] and cooling [C] processes. Abbreviations: Cr, crystal; SmA, smectic A; N, nematic; and Iso, isotropic phases. **(c)** Microphotographic images recorded by POM during the cooling process.  
Figure 3: Molecular and crystal structures of **(a) 1 aB** and **(c) 1 aC**. Packing structures of **(b) 1 aB** and **(d) 1 aC**. The light-blue lines in the packing structures indicate close contacts shorter than the sum of van der Waals radii (C: 170 pm, H: 120 pm, F: 148 pm).  
Among **1 aB-D** with a methoxy group at one molecular terminal and an electron-withdrawing substituent at the other, AcO-substituted **1 aB** exhibited no LC behavior, as evidenced by the dramatic change in the POM images from nonfluidic bright-viewing to fluidic dark-viewing fields during both the heating and cooling processes. The steric hindrance of the AcO group and the dipole moment (\(\mu_{\perp}\)) along the short molecular axis induced by the carbonyl moiety in the AcO group likely make it difficult to order structures in the condensed phase, resulting in a direct phase transition to the isotropic (Iso) phase. In contrast, the POM images of **1 aC** with a CF\({}_{3}\) group and **1 aD** with a CN group displayed fluidic bright-viewing fields between the crystal (Cr) and Iso phases; POM and DSC measurements only revealed a single LC phase for **1 aC** and **1 aD**. As the microphotographic images of **1 aC** and **1 aD** display a four-brushed Schlieren texture, which is typical of nematic (N) LC phases, the single LC phases of **1 aC** and **1 aD** are assigned as N phases. The melting temperature (\(T_{\rm m}\), defined as the phase transition temperature between the Cr and LC phases) and the clearing temperature (\(T_{\rm c}\), defined as the phase transition temperature between the LC an Iso phases) are much higher for **1 aD** than for **1 aC** (Figure 3a). This behavior is consistent with the linear and rigid structure of the CN group, which induces tight packing in an antiparallel direction owing to the large \(\mu_{\parallel}\) value [37; 38; 39].  
Changing the length of the flexible chain at the molecular terminal by replacing CH\({}_{3}\)O with C\({}_{6}\)H\({}_{13}\)O affected the LC behavior. **1 bB-D** all showed LC phases, and the \(T_{\rm m}\) and \(T_{\rm c}\) values for **1 bB-D** were dramatically decreased compared with the corresponding values for **1 aB-D**. Typically, such stabilization of LC phases can be attributed to the introduction of a longer flexible unit [23]. The POM and DSC measurements revealed that AcO-substituted **1 bB** and CN-substituted **1 bD** displayed only a N LC phase owing to the steric bulkiness of AcO and the rigid linear rigid structure of CN with a large \(\mu_{\parallel}\) value. However, CF\({}_{3}\)-substituted **1 bC** exhibited both SmA and N LC phases, analogous to those previously reported for **1 bA**. Thus, a suitable choice of substituent at the molecular terminal is important for achieving precise control of the LC behavior, including the LC phase, phase transition temperature, and LC temperature range.