A Small Change in Central Linker Has a Profound Effect in Inducing Columnar Phases of Triazine‐Based Unconventional Dendrimers

Abstract

Four unconventional triazine-based dendrimers have been prepared and characterized by (1) H and (13) C NMR spectroscopies, mass spectrometry, and elemental analysis. Based on DSC studies, polarizing microscopy, and powder XRD, two of these dendrimers, containing linkers with an odd number of carbon atoms, were observed to display columnar liquid-crystalline phases during thermal treatment. However, the other two dendritic analogues, containing linkers with an even number of carbon atoms, were not observed to behave correspondingly. Based on computer simulation, we reasonably assume that the dendrimers with an odd number of carbon atoms in their linkers distort their molecular shape and adopt two isomeric structures due to asymmetrical congestion. This reduces the molecular π-π face-to-face interaction, which in turn causes the dendrimers to form columnar LC phases during thermal treatment. However, the dendrimers with an even number of carbon atoms in their linkers have more symmetrical skeletons and do not display any liquid-crystalline phase upon thermal treatment. This new strategy should be applicable for eliciting the columnar liquid-crystalline properties of other types of unconventional dendrimers with rigid frameworks.

Unconventional Approaches to Prepare Triazine-Based Liquid Crystal Dendrimers

“…Molecular weights for dendrimers 9-12 are similar: ~6050, ~603 respectively. A computer simulation for molecular conformations of showed that 9 is asymmetrical and 10 is symmetrical along with the c diagram can show their differences [63]. As shown in Figure 6, den carbon linker in the dendritic core, allows the dendronic halves to b Molecular weights for dendrimers 9-12 are similar:~6050,~6036,~6022, and~6008, respectively.…”

“…This strategy can also be applied to central linkers of dendrimers. With the peripheral group fixed by the dialkylamino units, the central linker is modulated as C 5 H 10 , C 4 H 8 , C 3 H 6 , and C 2 H 4 ( Figure 5 ) [ 63 ]. Dendrimers 9 and 11 , with the odd-number carbon linker in the dendritic core, are LCs.…”

“…Because of the isomeric effect, the intermolecular face-to-face π–π interactions are reduced in the solid stacking, and the columnar phases of dendrimers 9 and 11 are thus formed on thermal treatment. To further understand the isomeric effect on influencing their stackings in LC state, their molecular volumes ( V m ) and cell volumes ( V cell ) were calculated on the basis of powder-XRD data of dendrimers 9 and 11 [ 63 ]. Since the calculated volumes are temperature-dependent, and the XRD data for 9 and 11 were collected at different temperatures, the V m / V cell ratios, instead of V m or V cell , were compared to remove the temperature factor.…”

“…Intermolecular H-bond interactions can also be applied to prepare 1,3,5-triamino- s -triazine-based dendrimers with the columnar phase. Figure 13 shows an example of a central core with a C 2 symmetry that can be efficiently prepared according to the literatures [ 62 , 63 , 64 , 65 , 66 , 67 ]. Incorporating special functional groups at the periphery of dendrons can give desired physical properties to the LC triazine-based dendritic mixtures.…”

Connector type‐controlled mesophase structures in poly(propyl ether imine) dendritic liquid crystals of identical dendrimer generations

“…In this context, nanophase segregation of distinct segments within the molecules having dendritic structural features hold a promise . Studies of the order and complexity, leading to distinct mesophase structures, were studied systematically using preformed dendrimers by modifying the peripheries with chosen mesogenic moieties . Elegant examples are (i) amphiphilic ionic second generation poly(amido amine) (PAMAM) dendrimer functionalized with differing amounts of myristic acid that exhibit a smectic A mesophase; (ii) second‐generation poly(propyleneimine) dendrimers peripherally functionalized with 4‐methoxybiphenyl, 4‐methoxyphenyl benzoate, trans ‐4‐(4‐pentylcyclohexyl)phenyl, 4‐fluorobiphenyl, and 3,4,5‐trifluorobiphenyl that exhibit a spontaneous homeotropic arrangement and (iii) azobenzene‐functionalized carbosilane dendrimers of first to fourth generations that prefers a smectic layer arrangement, whereas the fifth generation dendrimer of the same series prefers a rectangular columnar arrangement …”