The central linkers in triazine-based dendrimers significantly influence their morphology and consequently affect the formation of the mesophase during the thermal process. Instead of N-alkylation of an even-number carbon central linker, this strategy was also undertaken to an odd-number carbon central linker. As dendrimer
11 has already been observed to exhibit a columnar phase on thermal treatment, a strong CN polar group was incorporated in the central linker to investigate its influence on the mesogenic behavior. Dendrimers
15 and
16 were therefore further prepared for studying their LC properties (
Figure 10)
[65]. Dendrimer
15 exhibited a columnar phase between ~102 and ~118 °C on heating and between ~115 and ~82 °C on cooling. Dendrimer
16 showed a columnar phase from ~88 to ~144 °C on heating and from ~138 to ~68 °C on cooling. Compared with the DSC behaviors of
11, the solid-to-iso transition temperature of
15 (~118 °C) was lower than that of
11 (~144 °C) on heating, indicating that the alkylation on N in the central linker increased the steric congestion of triazine-based dendrimer and somewhat loosened their solid stacking as shown in the study of dendrimers
10,
12,
13 and
14. The solid-to-iso transition temperature of
16 (~144 °C) was similar to that of
11 (~144 °C), and the mesogenic range of
16 on cooling (from ~138 to ~68 °C) was broader than that of
11 (from ~115 to ~81 °C). The CN group in the central linker not only lowered the solidifying temperature but also raised the isotropic-phase-to-columnar-mesophase (iso-to-col) transition temperature on cooling. In principle, the steric congestion from benzyl moiety in the central linker of dendrimers
15 should not differ from and 4-cyanobenzyl moiety in
16, but their mesogenic behaviors on thermal treatment are significantly different. The strong polarity of the CN group strengthens intermolecular interactions between face-to-face dendritic molecules. For further understanding, the
Vm/
Vcell ratios of
15 and
16 were estimated on the basis of their powder-XRD data as ~0.80 and ~0.98, respectively. The
Vm/
Vcell ratio of dendrimer
16 was larger than that of
11 (~0.95), but that of dendrimer
15 was smaller, indicating that the loose order of their stacking is
16 >
11 >
15. As dendrimers
11,
15, and
16 all show a columnar phase in the mesogenic range, the face-to-face distances between two dendritic molecules (
dslice) for
11,
15, and
16 can be obtained from the powder-XRD data in a similar manner and are calculated to be 13.14, 13.28, and 13.68 Å, respectively. The
Vm/
Vcell ratios and
dslice distances show that the strong polarity of CN group significantly increased the face-to-face distance between dendritic molecules within columns. This reasonably explains that the col-to-solid transition temperature on cooling of
16 is lower than those of
11 and
15. However, as indicated previously, the solid-to-iso transition temperature of
16 (~144 °C) is similar to that of
11 (~144 °C) on heating, which can also be ascribed to the strong polarity of the CN group. During the heating process, the strong intermolecular polar-to-polar interactions within columns led dendrimer
16 to have the same solid-to-iso transition temperature as
11, although the stacking of
16 was looser. Neither dendrimer
11 nor
15 contains any functional group in the central linker, but the
dslice distance of
15 within columns is 13.28 Å, only slightly larger than that of
11 (13.14 Å). The more void space between dendritic molecules of
15 is occupied by the benzyl group, which results in the lower
Vm/
Vcell ratio of
15 (0.80) versus that of
11 (0.95). Since the
dslice distances of
11 and
15 do not differ from each other, the mesophase ranges of both dendrimer are also similar to each other.