nanomaterials

Review

Unconventional Approaches to Prepare Triazine-Based LiquidCrystal Dendrimers

Yao-Chih Lu 1, Hsiu-Fu Hsu 2 and Long-Li Lai 1,*

�����������������

Citation: Lu, Y.-C.; Hsu, H.-F.;

Lai, L.-L. Unconventional

Approaches to Prepare

Triazine-Based Liquid Crystal

Dendrimers. Nanomaterials 2021, 11,

2112. https://doi.org/

10.3390/nano11082112

Academic Editors: Paola Astolfi,

Michela Pisani and Francesco Vita

Received: 21 July 2021

Accepted: 17 August 2021

Published: 19 August 2021

Publisher’s Note: MDPI stays neutral

with regard to jurisdictional claims in

published maps and institutional affil-

iations.

Copyright: © 2021 by the authors.

Licensee MDPI, Basel, Switzerland.

This article is an open access article

distributed under the terms and

conditions of the Creative Commons

Attribution (CC BY) license (https://

creativecommons.org/licenses/by/

4.0/).

1 Department of Applied Chemistry, National Chi Nan University, No. 1 University Rd., Puli,Nantou 545, Taiwan; s103324901@mail1.ncnu.edu.tw

2 Department of Chemistry, Tamkang University, No. 151, Yingzhuan Rd., New Taipei City 251, Taiwan;hhsu@mail.tku.edu.tw

* Correspondence: lilai@ncnu.edu.tw

Abstract: Most triazine-based liquid crystalline (LC) dendrimers reported thus far are the main-chain LC macromolecules with long flexible chains at their periphery and attached to internal rigidor semi-rigid frameworks. Their formation of mesogenic phases often depends on the intermolecularface-to-face π–π interactions between dendritic molecules, which are unusual. Their mesogenicphases can also be formed by incorporation of mesogenic units to the dendritic skeletons throughlong flexible chains, as most side-chain LC dendrimers, in which the peripheral mesogenic unitsgenerally play the important roles. For main-chain triazine-based dendrimers, their morphologyis maintained by restricted freedom of rigid or semi-rigid connecting units, and their formationsof LC phases are therefore not straightforward to be controlled. In this review, we thus describemodulating of the intermolecular face-to-face π–π interactions between the triazine-based dendriticmolecules, with the aim of forming LC phases through molecular design.

Keywords: triazine; dendrimer; liquid crystal

1. Introduction

Since the discovery of dendrimers in the late 1970s, [1] there has been substantial de-velopment in their synthesis and study of their physical properties. Dendrimers generallyconsist of a central core, linking bridges and peripheral groups. They can be efficiently pre-pared by iterative processes through convergent, divergent, and combined approaches asdemonstrated in Scheme 1 [2]. Although dendrimers may have a branched architecture andthree-dimensional spatial arrangement, their morphology is still predictable. Formed bycontrollable step-by-step and iterative processes, dendrimers are a class of macromoleculeswith several characteristics that polymers do not have, such as monodispersity, controllablesizes, and internal cavities [2]. In particular, various types of functionally designed groupscan be attached at the periphery of dendrimers to give desired properties for this typeof macromolecule. Therefore, dendrimers have been extensively investigated and arecurrently applied in various fields, including drug delivery [3–9], catalysis [10–16], lightharvesting [17–22], and porous materials [23–27].

With mesogenic units in the dendritic framework, dendrimers have also been observedto exhibit liquid crystal (LC) behaviors on thermal treatment. Compared with traditional LCusually with low molecular weight, LC dendrimers often have a higher molecular weightand exhibit columnar mesophases on thermal treatment and are 2-D regular materials, thusyielding better self-assembling ability in a long-range domain and which have usefulnessas solvating candidates in opto-electronic devices [28–34]. LC dendrimers can generallybe divided into side-chain dendrimers in which the mesogenic units are incorporated atthe periphery of molecules and main-chain dendrimers in which the mesogenic moietiesare designed as linking moieties within the internal framework. Most LC dendrimers areside-chain LCs with flexible cores or linking units and peripheral mesogenic units linked

Nanomaterials 2021, 11, 2112. https://doi.org/10.3390/nano11082112 https://www.mdpi.com/journal/nanomaterials

Nanomaterials 2021, 11, 2112 2 of 14

to the internal dendritic structures through long flexible chains. The mesogenic units areimportant in inducing their LC behavior, and this type of LC dendrimer mostly consists ofPMAMA (polyamidoamine), PPI (polypropylimine), Si-derivatives (siloxanes, carbosilanes,and carbosilazanes), and polyester as their dendritic network, all of which have beenreviewed extensively [2,35–38]. Special functional moieties, such as fullerene and ferrocene,may be incorporated in side-chain LC dendrimers for particular properties as reported inliteratures [2,36,39]. There are fewer main-chain LC dendrimers [40–45] than side-chainLC dendrimers because mesogenic moieties are used as bridging units in their internalstructures, leading to a rigid or semi-rigid dendritic framework in the main-chain LCdendrimers in which flexible alkyl chains are frequently grafted at the periphery to increasetheir solubility in organic solvents. Therefore, in contrast to side-chain LC dendrimersin which the LC phases are mostly induced by the peripheral mesogenic units, both therigid or semi-rigid internal frameworks and flexible peripheral chains are all importantin inducing mesophases for the main-chain LC dendrimers [2,36,37], and this leads theformation of mesophases of the main-chain LC dendrimers to be difficult to control. As inthe rod-like liquid crystals, the ionic, H-bond, and metallo interactions are also extensivelyemployed in LC dendrimers, which also consist of side-chain and main-chain types of LCmaterials, depending on whether their mesogenic units are incorporated in the internalframework or at the periphery of dendritic network, which have been reviewed in theliterature [46–48].

Nanomaterials 2021, 11, x FOR PEER REVIEW 2 of 15

Scheme 1. Preparation of dendrimers by the convergent, divergent, and combined approaches.

With mesogenic units in the dendritic framework, dendrimers have also been ob-served to exhibit liquid crystal (LC) behaviors on thermal treatment. Compared with tra-ditional LC usually with low molecular weight, LC dendrimers often have a higher mo-lecular weight and exhibit columnar mesophases on thermal treatment and are 2-D regu-lar materials, thus yielding better self-assembling ability in a long-range domain and which have usefulness as solvating candidates in opto-electronic devices [28–34]. LC den-drimers can generally be divided into side-chain dendrimers in which the mesogenic units are incorporated at the periphery of molecules and main-chain dendrimers in which the mesogenic moieties are designed as linking moieties within the internal framework. Most LC dendrimers are side-chain LCs with flexible cores or linking units and peripheral mesogenic units linked to the internal dendritic structures through long flexible chains. The mesogenic units are important in inducing their LC behavior, and this type of LC dendrimer mostly consists of PMAMA (polyamidoamine), PPI (polypropylimine), Si-de-rivatives (siloxanes, carbosilanes, and carbosilazanes), and polyester as their dendritic net-work, all of which have been reviewed extensively [2,35–38]. Special functional moieties, such as fullerene and ferrocene, may be incorporated in side-chain LC dendrimers for particular properties as reported in literatures [2,36,39]. There are fewer main-chain LC dendrimers [40–45] than side-chain LC dendrimers because mesogenic moieties are used as bridging units in their internal structures, leading to a rigid or semi-rigid dendritic framework in the main-chain LC dendrimers in which flexible alkyl chains are frequently grafted at the periphery to increase their solubility in organic solvents. Therefore, in con-trast to side-chain LC dendrimers in which the LC phases are mostly induced by the pe-ripheral mesogenic units, both the rigid or semi-rigid internal frameworks and flexible peripheral chains are all important in inducing mesophases for the main-chain LC den-drimers [2,36,37], and this leads the formation of mesophases of the main-chain LC den-drimers to be difficult to control. As in the rod-like liquid crystals, the ionic, H-bond, and metallo interactions are also extensively employed in LC dendrimers, which also consist of side-chain and main-chain types of LC materials, depending on whether their meso-genic units are incorporated in the internal framework or at the periphery of dendritic network, which have been reviewed in the literature [46–48].

Cyanuric chloride, a simple heterocycle with three reactive sites on its ring, can be used as a branching unit for efficiently preparing dendrimers. For instance, the first chloro substituent can be replaced by alkylamines at 0 °C, and subsequently, the second and the

Scheme 1. Preparation of dendrimers by the convergent, divergent, and combined approaches.

Cyanuric chloride, a simple heterocycle with three reactive sites on its ring, can beused as a branching unit for efficiently preparing dendrimers. For instance, the first chlorosubstituent can be replaced by alkylamines at 0 ◦C, and subsequently, the second andthe third substituents can undergo similar substitutions at room temperature and above60 ◦C, respectively (Scheme 2) [49,50]. The triazine-based dendrimers were first preparedby Takagi [51] and later investigated by others [52,53]. Little progress on LC dendrimersbased on the triazine unit has been reported since then. To the best of our knowledge,in 2008, Lai and co-workers were the first to report LC dendrimers on the basis of the1,3,5-triamino-s-triazine unit [54]. Kotha and colleagues also prepared 1,3,5-triphenyl-s-triazine-based dendrimers, which exhibited a columnar phase after being mixed with1:1 molar ratio of trinitrofluorenone [55]. However, there has been less study of triazine-

Nanomaterials 2021, 11, 2112 3 of 14

based LC dendrimers, and most are main-chain types of LC dendrimers. Induction of theLC phases of triazine-based dendrimers can be achieved by manipulating the molecularmorphology of the dendrimer themselves or by adding extra components to form an LCdendritic mixture by intermolecular H-bond or face-to-face π–π interactions. By reviewingthis system, we hope to guide future work in this field.

Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 15

third substituents can undergo similar substitutions at room temperature and above 60 °C, respectively (Scheme 2) [49,50]. The triazine-based dendrimers were first prepared by Takagi [51] and later investigated by others [52,53]. Little progress on LC dendrimers based on the triazine unit has been reported since then. To the best of our knowledge, in 2008, Lai and co-workers were the first to report LC dendrimers on the basis of the 1,3,5-triamino-s-triazine unit [54]. Kotha and colleagues also prepared 1,3,5-triphenyl-s-tria-zine-based dendrimers, which exhibited a columnar phase after being mixed with 1:1 mo-lar ratio of trinitrofluorenone [55]. However, there has been less study of triazine-based LC dendrimers, and most are main-chain types of LC dendrimers. Induction of the LC phases of triazine-based dendrimers can be achieved by manipulating the molecular mor-phology of the dendrimer themselves or by adding extra components to form an LC den-dritic mixture by intermolecular H-bond or face-to-face π–π interactions. By reviewing this system, we hope to guide future work in this field.

Scheme 2. Reaction of amines with cyanuric chlorides at different temperatures.

2. Formation of Triazine-Based LC Dendrimers by Manipulating Their Molecular Morphology 2.1. LC Dendrimers with C2 Symmetry

The LC dendrimer with a C2-symmetry on the basis of a 1,3,5-triamino-s-triazine unit was first reported by Lai and colleagues [54]. They prepared the fourth generation LC dendrimer 1 by the convergent approach (Figure 1), with a single molecular weight of ~12,479. Compound 1 is a monotropic material, showing no mesophase during heating but a columnar hexagonal phase between ~160 and ~143 °C on cooling. Additionally, com-pound 1 did not show any significant absorbance beyond 280 nm in the UV-vis spectrum and started to decompose at 300 °C under nitrogen by TGA analysis, which is an im-portant characteristic required for opto- and electro-applications. However, the third gen-eration triazine-based dendrimer 2 did not exhibit mesogenic behavior on thermal treat-ment, isotropic at ~133 °C on heating, and crystallized at ~98 °C on cooling. Computer simulations for the conformations of dendrimers 1 and 2 showed that the steric effect from the peripheral skeleton of 1 is more significant than that of 2 [54], i.e., the dendritic con-formation of 1 is less coplanar due to congestion between the branched mainframe, thus reducing the face-to-face π–π interactions between dendritic molecules and forming the columnar phase on thermal treatment. Compared with traditional disk molecules, den-drimer 1 with a C2-symmetry having a columnar LC phase on cooling, is unusual. To ex-hibit columnar mesophases, the traditional C2-symmetry molecules may consist of a mac-rocyclic core [56,57]. Introducing a chiral core to result in helical stacking columns by modulating the intermolecular π–π interaction between molecules is another approach [58].

Scheme 2. Reaction of amines with cyanuric chlorides at different temperatures.

2. Formation of Triazine-Based LC Dendrimers by Manipulating Their MolecularMorphology2.1. LC Dendrimers with C2 Symmetry

The LC dendrimer with a C2-symmetry on the basis of a 1,3,5-triamino-s-triazine unitwas first reported by Lai and colleagues [54]. They prepared the fourth generation LCdendrimer 1 by the convergent approach (Figure 1), with a single molecular weight of~12,479. Compound 1 is a monotropic material, showing no mesophase during heatingbut a columnar hexagonal phase between ~160 and ~143 ◦C on cooling. Additionally,compound 1 did not show any significant absorbance beyond 280 nm in the UV-visspectrum and started to decompose at 300 ◦C under nitrogen by TGA analysis, which is animportant characteristic required for opto- and electro-applications. However, the thirdgeneration triazine-based dendrimer 2 did not exhibit mesogenic behavior on thermaltreatment, isotropic at ~133 ◦C on heating, and crystallized at ~98 ◦C on cooling. Computersimulations for the conformations of dendrimers 1 and 2 showed that the steric effectfrom the peripheral skeleton of 1 is more significant than that of 2 [54], i.e., the dendriticconformation of 1 is less coplanar due to congestion between the branched mainframe,thus reducing the face-to-face π–π interactions between dendritic molecules and formingthe columnar phase on thermal treatment. Compared with traditional disk molecules,dendrimer 1 with a C2-symmetry having a columnar LC phase on cooling, is unusual. Toexhibit columnar mesophases, the traditional C2-symmetry molecules may consist of amacrocyclic core [56,57]. Introducing a chiral core to result in helical stacking columns bymodulating the intermolecular π–π interaction between molecules is another approach [58].

2.2. LC Dendrimers with C3 Symmetry

In addition, deforming the planarity of trigonal disk molecules to reduce face-to-face π–π interactions in traditional molecules can also lead to the formation of columnarmesophases [59]. Therefore, changing the shape of the rigid cores can be an efficientapproach for preparing dendrimers with columnar LC phases, and thus two C3-symmetrydendrimers based on a triazine unit have been reported (Figure 2) [60]. Dendrimer 3, with amolecular weight of ~9248, is a monotropic material, which did not exhibit any mesogenicbehavior on heating but showed a columnar phase between ~137 and ~100 ◦C on cooling.Dendrimer 4 exhibited a columnar phase from ~141 to ~173 ◦C on heating and from ~168to ~136 ◦C on cooling.

As demonstrated by dendrimers 1 and 2, high molecular weight deforms the planarmainframe of dendritic molecules and leads to the formation of LC phases on thermaltreatment. However, the molecular weight of dendrimer 4 (~4423) is much smaller thanthat of dendrimer 2 (~6034). Apparently, the C2-to-C3 approach significantly changes

Nanomaterials 2021, 11, 2112 4 of 14

planarity of the internal mainframe of dendrimers. Thus 4 exhibits a columnar LC phaseduring the thermal process although there is less congestion from the dendritic mainframewhen compared with that of 2.

Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 15

NNNN

N

N

N

2

R =C8H17

NN

N

NN

NN

N N

NN

N

NN

NN

N NN NR2R2N

N

NN

NR2

NR2

N

N

N

N N

NN

N

NN

NR2

NR2

NN

NR2N NR2

N N NN

N

N

N

NN

NR2N NR2

N

NN

R2N

R2N

N

N

N

NN

N N

N

NN

R2N

R2N

N N

N NR2R2N

NNNN

N

N

N

1

N NN NNG1G1NN

N

NN

NNG1

NNG1

N

N

N

N N

NN

N

NN

NNG1

NNG1

NN

NG1NN NNG1

N N NN

N

N

N

NN

NG1NN NNG1

N

NN

G1NN

G1NN

N

N

N

NN

N N

N

NN

G1NN

G1NN

N N

N NNG1G1NN

NNG1=N N

NR2

N

NN

NR2

=

Figure 1. Structures of dendrimers (1) and (2) with C2-symmetry.

2.2. LC Dendrimers with C3 Symmetry In addition, deforming the planarity of trigonal disk molecules to reduce face-to-face

π–π interactions in traditional molecules can also lead to the formation of columnar mesophases [59]. Therefore, changing the shape of the rigid cores can be an efficient ap-proach for preparing dendrimers with columnar LC phases, and thus two C3-symmetry dendrimers based on a triazine unit have been reported (Figure 2) [60]. Dendrimer 3, with a molecular weight of ~9248, is a monotropic material, which did not exhibit any meso-genic behavior on heating but showed a columnar phase between ~137 and ~100 °C on cooling. Dendrimer 4 exhibited a columnar phase from ~141 to ~173 °C on heating and from ~168 to ~136 °C on cooling.

3

4

NN

NN

N

NNNN

R2N

N

NNN N

N

NN N

NN

NN

N

NN

N NR2N

N

NNNNNR2

N

N

NN

N

N

N

NN

NNR2

N N

N N

N

R2N

N

N NN N

NN

NN

NN NN

NR2

N

N NNN

NR2

NR2

N

NN

NN

N

N NNNNR2

N

NN

NN

NR2R2N

N

NN

N

N

NN

N

N

N

NNN

NR2R2N

NNNN

N

NR2

R2N

N

NR2

R2N

R2N

NR2

R2N

NR2R2N R2N

NR2

NN

NN

N

R2N

R2N

NN N

NN

NN

N

R2NNR2

N

NN

N

N

R2N

NR2

N NN N

NN

NN

NR2

NR2

NN

NN

N

NR2

R2N

NN

N

N

NN

N

N

NR2R2N

R=C8H17=

NN N

NN

N

N

N

N

Figure 2. Structures of dendrimers (3) and (4) with C3-symmetry.

As demonstrated by dendrimers 1 and 2, high molecular weight deforms the planar mainframe of dendritic molecules and leads to the formation of LC phases on thermal

Figure 1. Structures of dendrimers (1) and (2) with C2-symmetry.

Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 15

NNNN

N

N

N

2

R =C8H17

NN

N

NN

NN

N N

NN

N

NN

NN

N NN NR2R2N

N

NN

NR2

NR2

N

N

N

N N

NN

N

NN

NR2

NR2

NN

NR2N NR2

N N NN

N

N

N

NN

NR2N NR2

N

NN

R2N

R2N

N

N

N

NN

N N

N

NN

R2N

R2N

N N

N NR2R2N

NNNN

N

N

N

1

N NN NNG1G1NN

N

NN

NNG1

NNG1

N

N

N

N N

NN

N

NN

NNG1

NNG1

NN

NG1NN NNG1

N N NN

N

N

N

NN

NG1NN NNG1

N

NN

G1NN

G1NN

N

N

N

NN

N N

N

NN

G1NN

G1NN

N N

N NNG1G1NN

NNG1=N N

NR2

N

NN

NR2

=

Figure 1. Structures of dendrimers (1) and (2) with C2-symmetry.

2.2. LC Dendrimers with C3 Symmetry In addition, deforming the planarity of trigonal disk molecules to reduce face-to-face

π–π interactions in traditional molecules can also lead to the formation of columnar mesophases [59]. Therefore, changing the shape of the rigid cores can be an efficient ap-proach for preparing dendrimers with columnar LC phases, and thus two C3-symmetry dendrimers based on a triazine unit have been reported (Figure 2) [60]. Dendrimer 3, with a molecular weight of ~9248, is a monotropic material, which did not exhibit any meso-genic behavior on heating but showed a columnar phase between ~137 and ~100 °C on cooling. Dendrimer 4 exhibited a columnar phase from ~141 to ~173 °C on heating and from ~168 to ~136 °C on cooling.

3

4

NN

NN

N

NNNN

R2N

N

NNN N

N

NN N

NN

NN

N

NN

N NR2N

N

NNNNNR2

N

N

NN

N

N

N

NN

NNR2

N N

N N

N

R2N

N

N NN N

NN

NN

NN NN

NR2

N

N NNN

NR2

NR2

N

NN

NN

N

N NNNNR2

N

NN

NN

NR2R2N

N

NN

N

N

NN

N

N

N

NNN

NR2R2N

NNNN

N

NR2

R2N

N

NR2

R2N

R2N

NR2

R2N

NR2R2N R2N

NR2

NN

NN

N

R2N

R2N

NN N

NN

NN

N

R2NNR2

N

NN

N

N

R2N

NR2

N NN N

NN

NN

NR2

NR2

NN

NN

N

NR2

R2N

NN

N

N

NN

N

N

NR2R2N

R=C8H17=

NN N

NN

N

N

N

N

Figure 2. Structures of dendrimers (3) and (4) with C3-symmetry.

As demonstrated by dendrimers 1 and 2, high molecular weight deforms the planar mainframe of dendritic molecules and leads to the formation of LC phases on thermal

Figure 2. Structures of dendrimers (3) and (4) with C3-symmetry.

2.3. LC Dendrimers with the Loss of C2 Symmetry by Changing the Peripheral Groups

Reducing the intermolecular face-to-face π–π interactions between the molecules oftriazine-based dendrimer seems to be a good strategy for forming mesogenic phases onthermal treatment. According to literature [61], mixing two pure components can reduce themolecular interaction in the solid state, and thus lowers the melting point of the mixture. Forexample, the m.p. of (R)-(−)-2-amino-1-propanol is ~24–26 ◦C, which is the same as that of(S)-(+)-2-amino-1-propanol, but the m.p. of the 1:1 mixture of these two components dropsto ~8–10 ◦C. Accordingly, four triazine-based LC dendrimers were prepared (Figure 3) [62].Dendrimer 5, with the loss of the C2-symmetry, exhibited a columnar phase from ~95 to~152 ◦C on heating and from ~147 to ~64 ◦C on cooling. Dendrimer 6, also with the lossof the C2-symmetry, is a monotropic material, only showing a columnar phase between

Nanomaterials 2021, 11, 2112 5 of 14

~123 and ~145 ◦C on heating. Dendrimers 7 and 8, with a C2-symmetry, did not exhibitany mesogenic phase on thermal treatment. Dendrimer 7 became isotropic at ~132 ◦Con heating and crystallized at ~117 ◦C on cooling; and dendrimer 8 became isotropic at~229 ◦C on heating and solidified at ~221 ◦C on cooling. Dendrimers 5, 6, 7, and 8 all havesimilar molecular weights of ~2368, ~2030, ~2813, and ~2363, respectively. Based on themolecular simulation investigations [62], they are almost co-planar and the congestionsbetween their dendritic frameworks are negligible. However, dendrimers 5 and 6 areisomeric. As shown in Figure 4, dendrimers 5 or 6, in principle, can be produced from thecombination of the corresponding Cl-dendron and piperazino-dendron, respectively. Eachdendron consists of at least four isomers, and therefore, dendrimers 5 and 6 have manyisomers in their solid stackings, reducing their intermolecular face-to-face π–π interactionsand forming LC phase during the thermal process.

Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 15

treatment. However, the molecular weight of dendrimer 4 (~4423) is much smaller than that of dendrimer 2 (~6034). Apparently, the C2-to-C3 approach significantly changes pla-narity of the internal mainframe of dendrimers. Thus 4 exhibits a columnar LC phase dur-ing the thermal process although there is less congestion from the dendritic mainframe when compared with that of 2.

2.3. LC Dendrimers with the Loss of C2 Symmetry by Changing the Peripheral Groups Reducing the intermolecular face-to-face π–π interactions between the molecules of

triazine-based dendrimer seems to be a good strategy for forming mesogenic phases on thermal treatment. According to literature [61], mixing two pure components can reduce the molecular interaction in the solid state, and thus lowers the melting point of the mix-ture. For example, the m.p. of (R)-(−)-2-amino-1-propanol is ~24–26 °C, which is the same as that of (S)-(+)-2-amino-1-propanol, but the m.p. of the 1:1 mixture of these two compo-nents drops to ~8–10 °C. Accordingly, four triazine-based LC dendrimers were prepared (Figure 3) [62]. Dendrimer 5, with the loss of the C2-symmetry, exhibited a columnar phase from ~95 to ~152 °C on heating and from ~147 to ~64 °C on cooling. Dendrimer 6, also with the loss of the C2-symmetry, is a monotropic material, only showing a columnar phase between ~123 and ~145 °C on heating. Dendrimers 7 and 8, with a C2-symmetry, did not exhibit any mesogenic phase on thermal treatment. Dendrimer 7 became isotropic at ~132 °C on heating and crystallized at ~117 °C on cooling; and dendrimer 8 became isotropic at ~229 °C on heating and solidified at ~221 °C on cooling. Dendrimers 5, 6, 7, and 8 all have similar molecular weights of ~2368, ~2030, ~2813, and ~2363, respectively. Based on the molecular simulation investigations [62], they are almost co-planar and the congestions between their dendritic frameworks are negligible. However, dendrimers 5 and 6 are iso-meric. As shown in Figure 4, dendrimers 5 or 6, in principle, can be produced from the combination of the corresponding Cl-dendron and piperazino-dendron, respectively. Each dendron consists of at least four isomers, and therefore, dendrimers 5 and 6 have many isomers in their solid stackings, reducing their intermolecular face-to-face π–π in-teractions and forming LC phase during the thermal process.

N

N

N

NN

NN

NN

NOR

R2N

NN

NRO

NR2

N

N

N

NN

NN

NN

NRO

NR2

NN

NOR

R2N

NN

N

N

N

NN

NN

NN

NNR2

R2N

NN

NR2N

NR2

N

N

N

NN

NN

NN

NR2N

NR2

NN

NNR2

R2N

NN

5: R =C8H15 , 6: R = C6H13 7: R =C8H15 , 8: R = C6H13 Figure 3. Structures of dendrimers (5) and (6) without a C2-symmetry and structures of dendrimers (7) and (8) with a C2-symmetry.

Figure 3. Structures of dendrimers (5) and (6) without a C2-symmetry and structures of dendrimers (7)and (8) with a C2-symmetry.

Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 15

Figure 4. Structural isomers of dendrons to form dendrimers (5) and (6).

2.4. LC Dendrimers with the Loss of C2 Symmetry by Changing the Central Cores Partially changing the peripheral group to break C2-symmetry of molecules can lead

dendrimers to contain isomers, and face-to-face π–π interactions between dendritic mol-ecules are thus modulated to form the LC phase with thermal treatment. 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 C5H10, C4H8, C3H6, and C2H4 (Figure 5) [63]. Dendrimers 9 and 11, with the odd-number carbon linker in the dendritic core, are LCs. Dendrimer 9 was observed to exhibit a columnar phase from ~109 to ~168 °C on heating and from ~146 to ~87 °C on cooling. Dendrimer 11 showed a columnar phase be-tween ~107 and ~144 °C on heating and between ~115 and ~81 °C on cooling. Dendrimers 10 and 12 with the even-number carbon linker in the dendritic core are not liquid crystal-line. The solid-phase-to-isotropic-phase (solid-to-iso) transition temperatures for den-drimers 10 and 12 are ~132 and ~147 °C, respectively, on heating, and the isotropic-phase-to-solid-phase (iso-to-solid) transition temperatures for 10 and 12 are ~100 and ~90 °C, re-spectively, on cooling.

Figure 5. Structures of dendrimers (9–12).

Molecular weights for dendrimers 9–12 are similar: ~6050, ~6036, ~6022, and ~6008, respectively. A computer simulation for molecular conformations of dendrimers 9 and 10 showed that 9 is asymmetrical and 10 is symmetrical along with the central linkers, and a diagram can show their differences [63]. As shown in Figure 6, dendrimer 10, with a 4-carbon linker in the dendritic core, allows the dendronic halves to be located at the anti-

Figure 4. Structural isomers of dendrons to form dendrimers (5) and (6).

2.4. LC Dendrimers with the Loss of C2 Symmetry by Changing the Central Cores

Partially changing the peripheral group to break C2-symmetry of molecules canlead dendrimers to contain isomers, and face-to-face π–π interactions between dendriticmolecules are thus modulated to form the LC phase with thermal treatment. This strategycan also be applied to central linkers of dendrimers. With the peripheral group fixedby the dialkylamino units, the central linker is modulated as C5H10, C4H8, C3H6, andC2H4 (Figure 5) [63]. Dendrimers 9 and 11, with the odd-number carbon linker in the

Nanomaterials 2021, 11, 2112 6 of 14

dendritic core, are LCs. Dendrimer 9 was observed to exhibit a columnar phase from~109 to ~168 ◦C on heating and from ~146 to ~87 ◦C on cooling. Dendrimer 11 showeda columnar phase between ~107 and ~144 ◦C on heating and between ~115 and ~81 ◦Con cooling. Dendrimers 10 and 12 with the even-number carbon linker in the dendriticcore are not liquid crystalline. The solid-phase-to-isotropic-phase (solid-to-iso) transitiontemperatures for dendrimers 10 and 12 are ~132 and ~147 ◦C, respectively, on heating, andthe isotropic-phase-to-solid-phase (iso-to-solid) transition temperatures for 10 and 12 are~100 and ~90 ◦C, respectively, on cooling.

Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 15

Figure 4. Structural isomers of dendrons to form dendrimers (5) and (6).

2.4. LC Dendrimers with the Loss of C2 Symmetry by Changing the Central Cores Partially changing the peripheral group to break C2-symmetry of molecules can lead

dendrimers to contain isomers, and face-to-face π–π interactions between dendritic mol-ecules are thus modulated to form the LC phase with thermal treatment. 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 C5H10, C4H8, C3H6, and C2H4 (Figure 5) [63]. Dendrimers 9 and 11, with the odd-number carbon linker in the dendritic core, are LCs. Dendrimer 9 was observed to exhibit a columnar phase from ~109 to ~168 °C on heating and from ~146 to ~87 °C on cooling. Dendrimer 11 showed a columnar phase be-tween ~107 and ~144 °C on heating and between ~115 and ~81 °C on cooling. Dendrimers 10 and 12 with the even-number carbon linker in the dendritic core are not liquid crystal-line. The solid-phase-to-isotropic-phase (solid-to-iso) transition temperatures for den-drimers 10 and 12 are ~132 and ~147 °C, respectively, on heating, and the isotropic-phase-to-solid-phase (iso-to-solid) transition temperatures for 10 and 12 are ~100 and ~90 °C, re-spectively, on cooling.

Figure 5. Structures of dendrimers (9–12).

Molecular weights for dendrimers 9–12 are similar: ~6050, ~6036, ~6022, and ~6008, respectively. A computer simulation for molecular conformations of dendrimers 9 and 10 showed that 9 is asymmetrical and 10 is symmetrical along with the central linkers, and a diagram can show their differences [63]. As shown in Figure 6, dendrimer 10, with a 4-carbon linker in the dendritic core, allows the dendronic halves to be located at the anti-

Figure 5. Structures of dendrimers (9–12).

Molecular weights for dendrimers 9–12 are similar: ~6050, ~6036, ~6022, and ~6008,respectively. A computer simulation for molecular conformations of dendrimers 9 and10 showed that 9 is asymmetrical and 10 is symmetrical along with the central linkers,and a diagram can show their differences [63]. As shown in Figure 6, dendrimer 10, witha 4-carbon linker in the dendritic core, allows the dendronic halves to be located at theanti-position of the C4H8 linker to reduce repulsions between Hs (at C2 and C3) and thedendronic halves. Therefore, the molecular conformation of 10 is almost symmetrical.Accordingly, dendrimer 11 with a 3-carbon linker leads both the dendronic halves toexist in the syn-position of the C3H6 linker to reduce corresponding repulsions; in thisconformation, two isomers of dendrimer 11 should exist since the left dendronic half maybe on the top or bottom of the right dendronic half (Figure 6). Because of the isomeric effect,the intermolecular face-to-face π–π interactions are reduced in the solid stacking, and thecolumnar phases of dendrimers 9 and 11 are thus formed on thermal treatment. To furtherunderstand the isomeric effect on influencing their stackings in LC state, their molecularvolumes (Vm) and cell volumes (Vcell) were calculated on the basis of powder-XRD data ofdendrimers 9 and 11 [63]. Since the calculated volumes are temperature-dependent, andthe XRD data for 9 and 11 were collected at different temperatures, the Vm/Vcell ratios,instead of Vm or Vcell, were compared to remove the temperature factor. The Vm/Vcellratios of 9 and 11 were calculated to be ~1.03 and ~0.95, respectively. For the non-mesogeniccompounds 10 and 12, crystalline XRD data were also used for calculation, assuming thatboth formed the columnar hexagonal crystalline phase; the Vm/Vcell ratios of 10 and 12were estimated to be ~0.92 and ~0.87, respectively.

Nanomaterials 2021, 11, 2112 7 of 14

Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 15

position of the C4H8 linker to reduce repulsions between Hs (at C2 and C3) and the den-dronic halves. Therefore, the molecular conformation of 10 is almost symmetrical. Accord-ingly, dendrimer 11 with a 3-carbon linker leads both the dendronic halves to exist in the syn-position of the C3H6 linker to reduce corresponding repulsions; in this conformation, two isomers of dendrimer 11 should exist since the left dendronic half may be on the top or bottom of the right dendronic half (Figure 6). Because of the isomeric effect, the inter-molecular face-to-face π–π interactions are reduced in the solid stacking, and the colum-nar phases of dendrimers 9 and 11 are thus formed on thermal treatment. To further un-derstand the isomeric effect on influencing their stackings in LC state, their molecular vol-umes (Vm) and cell volumes (Vcell) 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 Vm/Vcell ratios, in-stead of Vm or Vcell, were compared to remove the temperature factor. The Vm/Vcell ratios of 9 and 11 were calculated to be ~1.03 and ~0.95, respectively. For the non-mesogenic com-pounds 10 and 12, crystalline XRD data were also used for calculation, assuming that both formed the columnar hexagonal crystalline phase; the Vm/Vcell ratios of 10 and 12 were estimated to be ~0.92 and ~0.87, respectively.

Figure 6. Isomers of dendrimers (10) and (11).

The ratios of Vm/Vcell for 9 and 11 are larger than those for 10 and 12, indicating that the stackings of 9 and 11 in the solid state are somewhat looser than those of 10 and 12. This further indicates that the isomeric effect on dendrimers can probably modulate the face-to-face π–π interactions between dendritic molecules.

As indicated previously, the repulsion between dendronic halves and the Hs (at C2 or C3) significantly influences the conformation of dendrimer 10. If the central linker of 10 was methylated on nitrogen, then the repulsion between Me and the Hs (at C2 or C3) should increase, although the repulsion should be smaller than that between the den-dronic halves and the corresponding Hs (at C2 or C3). However, this should convert sev-eral of the dendronic halves at the anti-position of C4H8 linker to the syn-position. Specifi-cally, dendrimer 13 should have three isomers in the solid state, as shown in Figure 7 [64], although most of the 13 exists as the form of isomer 13-I and, as expected, dendrimer 13 exhibited a columnar phase on thermal treatment (Figure 8). The mesogenic range is from ~111 to ~123 °C on heating and from ~115 to ~94 °C on cooling. Accordingly, dendrimer 14 has three isomers in the solid state but showed only a columnar phase between ~94 and ~72 °C on cooling without showing any mesogenic behavior on heating.

Figure 6. Isomers of dendrimers (10) and (11).

The ratios of Vm/Vcell for 9 and 11 are larger than those for 10 and 12, indicating thatthe stackings of 9 and 11 in the solid state are somewhat looser than those of 10 and 12.This further indicates that the isomeric effect on dendrimers can probably modulate theface-to-face π–π interactions between dendritic molecules.

As indicated previously, the repulsion between dendronic halves and the Hs (at C2 orC3) significantly influences the conformation of dendrimer 10. If the central linker of 10was methylated on nitrogen, then the repulsion between Me and the Hs (at C2 or C3)should increase, although the repulsion should be smaller than that between the dendronichalves and the corresponding Hs (at C2 or C3). However, this should convert several ofthe dendronic halves at the anti-position of C4H8 linker to the syn-position. Specifically,dendrimer 13 should have three isomers in the solid state, as shown in Figure 7 [64],although most of the 13 exists as the form of isomer 13-I and, as expected, dendrimer 13exhibited a columnar phase on thermal treatment (Figure 8). The mesogenic range is from~111 to ~123 ◦C on heating and from ~115 to ~94 ◦C on cooling. Accordingly, dendrimer 14has three isomers in the solid state but showed only a columnar phase between ~94 and~72 ◦C on cooling without showing any mesogenic behavior on heating.

The solid-to-iso transition temperatures of dendrimers 10 and 12 on heating are ~132and ~147 ◦C, respectively, which are higher than those of corresponding dendrimers 13and 14 (~118 and 123 ◦C, respectively). This indicates that methylation of nitrogen inthe central linker affects their molecular morphology and somewhat loosens their solidstacking. Based on the powder-XRD data of dendrimers 13 and 14, their ratios of Vm/Vcellwere also calculated. The Vm/Vcell ratio of 13 was ~0.92, similar to that of 10 (~0.92) andthe Vm/Vcell ratio of 14 was ~0.80, which is smaller to that of 12 (~0.87).

Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 15

Figure 7. Isomers of dendrimers (13-I), (13-II), and (13-III).

Figure 8. Structures of dendrimers (13) and (14).

The solid-to-iso transition temperatures of dendrimers 10 and 12 on heating are ~132 and ~147 °C, respectively, which are higher than those of corresponding dendrimers 13 and 14 (~118 and 123 °C, respectively). This indicates that methylation of nitrogen in the central linker affects their molecular morphology and somewhat loosens their solid stack-ing. Based on the powder-XRD data of dendrimers 13 and 14, their ratios of Vm/Vcell were also calculated. The Vm/Vcell ratio of 13 was ~0.92, similar to that of 10 (~0.92) and the Vm/Vcell ratio of 14 was ~0.80, which is smaller to that of 12 (~0.87).

Although methylation of nitrogens in the central linker of dendrimers converts non-mesogenic 10 and 12 to become mesogenic 13 and 14, respectively, the isomeric effect seems to not be significant in reducing intermolecular face-to-face π–π interactions, al-lowing dendrimers 13 and 14 to be looser in the solid stacking than dendrimers 10 and 12. Based on the optimal conformation of 13 [64], it was discovered that four intramolecular H-bond interactions arise between Na (a = 7, 8, 9, 10) and H (at Cb ; b = 1, 4, 5, 6), respec-tively, as shown in Figure 9. Accordingly, only two intramolecular H-bond interactions arise between Na (a = 5, 6) and H (at Cb; b = 1, 4,) in dendrimer 10. Therefore, dendrimer 13 that mostly exists in the form of 13-I should be more rigid and is less distorted than dendrimer 10, and therefore the void space inside the solid stacking of 10 should be larger than that inside the dendritic framework of 13-I. However, because of the isomeric effect, the face-to-face π–π interactions between dendritic molecules of 13 is reduced and there-fore its dendritic framework is fluffier. As a result, dendrimers 10 and 13 have almost the same Vm/Vcell ratio (~0.92) on the basis of balancing the two conflicting factors. When the central linker is shortened, congestion from dendritic halves of 12 is more vigorous and the distortion allows dendrimer 12 to become more porous. Therefore, the Vm/Vcell ratio of 12 (~0.87) is larger than that of 14 (~0.80).

Figure 7. Isomers of dendrimers (13-I), (13-II), and (13-III).

Nanomaterials 2021, 11, 2112 8 of 14

Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 15

Figure 7. Isomers of dendrimers (13-I), (13-II), and (13-III).

Figure 8. Structures of dendrimers (13) and (14).

The solid-to-iso transition temperatures of dendrimers 10 and 12 on heating are ~132 and ~147 °C, respectively, which are higher than those of corresponding dendrimers 13 and 14 (~118 and 123 °C, respectively). This indicates that methylation of nitrogen in the central linker affects their molecular morphology and somewhat loosens their solid stack-ing. Based on the powder-XRD data of dendrimers 13 and 14, their ratios of Vm/Vcell were also calculated. The Vm/Vcell ratio of 13 was ~0.92, similar to that of 10 (~0.92) and the Vm/Vcell ratio of 14 was ~0.80, which is smaller to that of 12 (~0.87).

Although methylation of nitrogens in the central linker of dendrimers converts non-mesogenic 10 and 12 to become mesogenic 13 and 14, respectively, the isomeric effect seems to not be significant in reducing intermolecular face-to-face π–π interactions, al-lowing dendrimers 13 and 14 to be looser in the solid stacking than dendrimers 10 and 12. Based on the optimal conformation of 13 [64], it was discovered that four intramolecular H-bond interactions arise between Na (a = 7, 8, 9, 10) and H (at Cb ; b = 1, 4, 5, 6), respec-tively, as shown in Figure 9. Accordingly, only two intramolecular H-bond interactions arise between Na (a = 5, 6) and H (at Cb; b = 1, 4,) in dendrimer 10. Therefore, dendrimer 13 that mostly exists in the form of 13-I should be more rigid and is less distorted than dendrimer 10, and therefore the void space inside the solid stacking of 10 should be larger than that inside the dendritic framework of 13-I. However, because of the isomeric effect, the face-to-face π–π interactions between dendritic molecules of 13 is reduced and there-fore its dendritic framework is fluffier. As a result, dendrimers 10 and 13 have almost the same Vm/Vcell ratio (~0.92) on the basis of balancing the two conflicting factors. When the central linker is shortened, congestion from dendritic halves of 12 is more vigorous and the distortion allows dendrimer 12 to become more porous. Therefore, the Vm/Vcell ratio of 12 (~0.87) is larger than that of 14 (~0.80).

Figure 8. Structures of dendrimers (13) and (14).

Although methylation of nitrogens in the central linker of dendrimers converts non-mesogenic 10 and 12 to become mesogenic 13 and 14, respectively, the isomeric effect seemsto not be significant in reducing intermolecular face-to-face π–π interactions, allowingdendrimers 13 and 14 to be looser in the solid stacking than dendrimers 10 and 12. Basedon the optimal conformation of 13 [64], it was discovered that four intramolecular H-bondinteractions arise between Na (a = 7, 8, 9, 10) and H (at Cb; b = 1, 4, 5, 6), respectively, asshown in Figure 9. Accordingly, only two intramolecular H-bond interactions arise betweenNa (a = 5, 6) and H (at Cb; b = 1, 4,) in dendrimer 10. Therefore, dendrimer 13 that mostlyexists in the form of 13-I should be more rigid and is less distorted than dendrimer 10, andtherefore the void space inside the solid stacking of 10 should be larger than that insidethe dendritic framework of 13-I. However, because of the isomeric effect, the face-to-faceπ–π interactions between dendritic molecules of 13 is reduced and therefore its dendriticframework is fluffier. As a result, dendrimers 10 and 13 have almost the same Vm/Vcellratio (~0.92) on the basis of balancing the two conflicting factors. When the central linkeris shortened, congestion from dendritic halves of 12 is more vigorous and the distortionallows dendrimer 12 to become more porous. Therefore, the Vm/Vcell ratio of 12 (~0.87) islarger than that of 14 (~0.80).

Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 15

Figure 9. The numbering on carbons for indicating intramolecular H-bond interactions between H and N of dendrimers (10) and (13).

The central linkers in triazine-based dendrimers significantly influence their mor-phology and consequently affect the formation of the mesophase during the thermal pro-cess. 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 behav-ior. Dendrimers 15 and 16 were therefore further prepared for studying their LC proper-ties (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 tran-sition 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 co-lumnar 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 ex-plains 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

Figure 9. The numbering on carbons for indicating intramolecular H-bond interactions between H and N of dendrimers (10)and (13).

The central linkers in triazine-based dendrimers significantly influence their morphol-ogy 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 alsoundertaken to an odd-number carbon central linker. As dendrimer 11 has already beenobserved to exhibit a columnar phase on thermal treatment, a strong CN polar group wasincorporated in the central linker to investigate its influence on the mesogenic behavior.

Nanomaterials 2021, 11, 2112 9 of 14

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 onheating and between ~115 and ~82 ◦C on cooling. Dendrimer 16 showed a columnar phasefrom ~88 to ~144 ◦C on heating and from ~138 to ~68 ◦C on cooling. Compared with theDSC behaviors of 11, the solid-to-iso transition temperature of 15 (~118 ◦C) was lower thanthat of 11 (~144 ◦C) on heating, indicating that the alkylation on N in the central linkerincreased the steric congestion of triazine-based dendrimer and somewhat loosened theirsolid stacking as shown in the study of dendrimers 10, 12, 13 and 14. The solid-to-iso tran-sition temperature of 16 (~144 ◦C) was similar to that of 11 (~144 ◦C), and the mesogenicrange 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 temperaturebut also raised the isotropic-phase-to-columnar-mesophase (iso-to-col) transition temper-ature on cooling. In principle, the steric congestion from benzyl moiety in the centrallinker of dendrimers 15 should not differ from and 4-cyanobenzyl moiety in 16, but theirmesogenic behaviors on thermal treatment are significantly different. The strong polarityof the CN group strengthens intermolecular interactions between face-to-face dendriticmolecules. For further understanding, the Vm/Vcell ratios of 15 and 16 were estimated onthe basis of their powder-XRD data as ~0.80 and ~0.98, respectively. The Vm/Vcell ratioof 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 16all show a columnar phase in the mesogenic range, the face-to-face distances between twodendritic molecules (dslice) for 11, 15, and 16 can be obtained from the powder-XRD data in asimilar manner and are calculated to be 13.14, 13.28, and 13.68 Å, respectively. The Vm/Vcellratios and dslice distances show that the strong polarity of CN group significantly increasedthe face-to-face distance between dendritic molecules within columns. This reasonablyexplains that the col-to-solid transition temperature on cooling of 16 is lower than thoseof 11 and 15. However, as indicated previously, the solid-to-iso transition temperatureof 16 (~144 ◦C) is similar to that of 11 (~144 ◦C) on heating, which can also be ascribed tothe strong polarity of the CN group. During the heating process, the strong intermolecularpolar-to-polar interactions within columns led dendrimer 16 to have the same solid-to-isotransition temperature as 11, although the stacking of 16 was looser. Neither dendrimer 11nor 15 contains any functional group in the central linker, but the dslice distance of 15 withincolumns is 13.28 Å, only slightly larger than that of 11 (13.14 Å). The more void spacebetween dendritic molecules of 15 is occupied by the benzyl group, which results in thelower Vm/Vcell ratio of 15 (0.80) versus that of 11 (0.95). Since the dslice distances of 11 and15 do not differ from each other, the mesophase ranges of both dendrimer are also similarto each other.

Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 15

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.

Figure 10. Structures of dendrimers (15) and (16).

3. Formation of Triazine-Based LC Dendrimers by Adding Extra Components 3.1. Formation of LC Dendrimers by Intermolecular π–π Interaction

In addition to manipulating the morphology of dendritic molecules to form their LC phase, mixing extra components to non-liquid-crystalline (non-LC) triazine-based den-drimers can lead to the mesophase formation. Dendrimers 17 and 18 were reported to be non-LC on thermal treatment (Figure 11) [55]. Compound 17 became isotropic at ~66 °C on heating but no transition was observed on cooling. Compound 18 became isotropic at ~57 °C on heating and crystallized at ~32 °C on cooling. After mixing with 1:1 molar ratio of trinitrofluorenone (TNF), the mixture of 17-TNF showed a columnar phase between ~56 and ~78 °C on heating but still no transition was observed on cooling. The mixture of 18-TNF exhibited a columnar phase from ~51 to ~102 °C on heating and from ~97 °C to room temperature on cooling [55]. It is believed that TNF forms a complex mixture with 17 or 18 through the face-to-face π-π interactions, thereby adjusting the distance between dendritic molecules and showing LC phases on thermal treatment.

Figure 11. Structures of dendrimers (17) and (18).

Figure 10. Structures of dendrimers (15) and (16).

Nanomaterials 2021, 11, 2112 10 of 14

3. Formation of Triazine-Based LC Dendrimers by Adding Extra Components3.1. Formation of LC Dendrimers by Intermolecular π–π Interaction

In addition to manipulating the morphology of dendritic molecules to form theirLC phase, mixing extra components to non-liquid-crystalline (non-LC) triazine-baseddendrimers can lead to the mesophase formation. Dendrimers 17 and 18 were reported tobe non-LC on thermal treatment (Figure 11) [55]. Compound 17 became isotropic at ~66 ◦Con heating but no transition was observed on cooling. Compound 18 became isotropic at~57 ◦C on heating and crystallized at ~32 ◦C on cooling. After mixing with 1:1 molar ratioof trinitrofluorenone (TNF), the mixture of 17-TNF showed a columnar phase between~56 and ~78 ◦C on heating but still no transition was observed on cooling. The mixture of18-TNF exhibited a columnar phase from ~51 to ~102 ◦C on heating and from ~97 ◦C toroom temperature on cooling [55]. It is believed that TNF forms a complex mixture with 17or 18 through the face-to-face π-π interactions, thereby adjusting the distance betweendendritic molecules and showing LC phases on thermal treatment.

Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 15

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.

Figure 10. Structures of dendrimers (15) and (16).

3. Formation of Triazine-Based LC Dendrimers by Adding Extra Components 3.1. Formation of LC Dendrimers by Intermolecular π–π Interaction

In addition to manipulating the morphology of dendritic molecules to form their LC phase, mixing extra components to non-liquid-crystalline (non-LC) triazine-based den-drimers can lead to the mesophase formation. Dendrimers 17 and 18 were reported to be non-LC on thermal treatment (Figure 11) [55]. Compound 17 became isotropic at ~66 °C on heating but no transition was observed on cooling. Compound 18 became isotropic at ~57 °C on heating and crystallized at ~32 °C on cooling. After mixing with 1:1 molar ratio of trinitrofluorenone (TNF), the mixture of 17-TNF showed a columnar phase between ~56 and ~78 °C on heating but still no transition was observed on cooling. The mixture of 18-TNF exhibited a columnar phase from ~51 to ~102 °C on heating and from ~97 °C to room temperature on cooling [55]. It is believed that TNF forms a complex mixture with 17 or 18 through the face-to-face π-π interactions, thereby adjusting the distance between dendritic molecules and showing LC phases on thermal treatment.

Figure 11. Structures of dendrimers (17) and (18). Figure 11. Structures of dendrimers (17) and (18).

3.2. Formation of LC Dendrimers by Intermolecular H-Bond Interaction

Apart from the face-to-face π–π interactions, the LC dendritic mixtures can alsobe formed by the intermolecular H-bond interaction. Mixing 2,4-diamino-6-dodecyl-1,3,4- triazine with carboxylic acid-containing dendron in a 1:3 ratio generates H-bondeddendrimers 19 and 20, as shown in Figure 12 [66]. The acid-containing dendrons 21 and 22did not exhibit any mesophase on thermal treatment. Compound 21 showed only a glass-phase-to-isotropic-phase (glass-to-iso) transition temperature at ~52 ◦C on heating, andcompound 22 showed a glass-to-iso transition temperature at ~46 ◦C on heating. Insteadof a columnar phase, dendrimer 19 exhibited a nematic phase between ~4 and ~35 ◦C onheating, and dendrimer 20 was observed to exhibit a SmA phase from ~43 and ~70 ◦Con heating. Neither dendrimers nor dendrons were reported to show any mesophase oncooling. A carbazole unit was incorporated in dendrimers and dendrons for studying theirphotoconductivities, but the performances of dendrons 21 and 22 were slightly better thanthose of dendrimers 19 and 20, respectively.

Nanomaterials 2021, 11, 2112 11 of 14

Nanomaterials 2021, 11, x FOR PEER REVIEW 11 of 15

3.2. Formation of LC Dendrimers by Intermolecular H-Bond Interaction Apart from the face-to-face π–π interactions, the LC dendritic mixtures can also be

formed by the intermolecular H-bond interaction. Mixing 2,4-diamino-6-dodecyl-1,3,4- triazine with carboxylic acid-containing dendron in a 1:3 ratio generates H-bonded den-drimers 19 and 20, as shown in Figure 12 [66]. The acid-containing dendrons 21 and 22 did not exhibit any mesophase on thermal treatment. Compound 21 showed only a glass-phase-to-isotropic-phase (glass-to-iso) transition temperature at ~52 °C on heating, and compound 22 showed a glass-to-iso transition temperature at ~46 °C on heating. Instead of a columnar phase, dendrimer 19 exhibited a nematic phase between ~4 and ~35 °C on heating, and dendrimer 20 was observed to exhibit a SmA phase from ~43 and ~70 °C on heating. Neither dendrimers nor dendrons were reported to show any mesophase on cool-ing. A carbazole unit was incorporated in dendrimers and dendrons for studying their photoconductivities, but the performances of dendrons 21 and 22 were slightly better than those of dendrimers 19 and 20, respectively.

NN

N

N

N

N

R = C12H25

R

O

O

OO

O

O

O

ONH

N

O

OO

O

ONH

N

O

Me HMe

HH H

O

OX

H

H

H

HHO

O

X

H

OO

X

H

H

X = Y =

O

O

OO

O

O

O

ONH

N

O

OO

O

ONH

N

O

Me HMe

HH HO

HO

O

HO

NN

N

N

N

N

RO

OY

H

H

H

HHO

OY

H

OO

Y

H

H

R = C12H25

Figure 12. Structures of dendrimers (19) and (20) and and dendrons (21) and (22).

4. Possible Applications As shown in the literature, the triazine-based LC dendrimers are thermally stable and

nearly transparent beyond 280 nm in the UV/Vis range [54]. Moreover, these dendrimers are electrochemically stable in solution and can survive in acidic media for a period of time [52]. These characteristics are important and are often required for opto- and electro-applications. In addition, LC dendrimers often have a high molecular weight, exhibiting columnar phases on thermal treatment and thus exhibiting good self-assembling ability in a long-range domain. In solvating guest materials, such as carbon nanotubes and nano-size catalysts, the LC dendrimers can lead the physical properties of guest molecules to be significantly displayed [28–34]. As shown in the introduction, dendrimers have been shown to be useful in the fields of catalysis [10–16] and light harvesting [17–22]. With the

Figure 12. Structures of dendrimers (19) and (20) and and dendrons (21) and (22).

4. Possible Applications

As shown in the literature, the triazine-based LC dendrimers are thermally stable andnearly transparent beyond 280 nm in the UV/Vis range [54]. Moreover, these dendrimersare electrochemically stable in solution and can survive in acidic media for a period oftime [52]. These characteristics are important and are often required for opto- and electro-applications. In addition, LC dendrimers often have a high molecular weight, exhibitingcolumnar phases on thermal treatment and thus exhibiting good self-assembling ability ina long-range domain. In solvating guest materials, such as carbon nanotubes and nano-size catalysts, the LC dendrimers can lead the physical properties of guest molecules tobe significantly displayed [28–34]. As shown in the introduction, dendrimers have beenshown to be useful in the fields of catalysis [10–16] and light harvesting [17–22]. With theassistance of LC dendrimers, the guest molecules should be more regularly arranged, thusexhibiting their physical characteristics significantly.

5. Conclusions and Prospect

Three approaches have been described to prepare triazine-based dendrimers withmesogenic phase on thermal treatment. The first is to properly incorporate mesogenic unitsonto triazine and piperazine framework in a straightforward manner by molecular design.

The second approach is to mix components to form LC dendritic mixtures by ad-justing intermolecular face-to-face π–π interactions, which is valuable for 1,3,5-triamino-s-triazine-based LC dendrimers. The mixing approach allows versatile preparation of1,3,5-triamino-s-triazine-based dendrimers, which were observed to form strong π–π inter-actions with tetrafluorbenzoquinone (TFB) in CH2Cl2, and the red shift of absorbance fromthe dendrimer-TFB complexes in the UV-Vis spectra can be up to ~180 nm [67].

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 acentral core with a C2 symmetry that can be efficiently prepared according to the litera-

Nanomaterials 2021, 11, 2112 12 of 14

tures [62–67]. Incorporating special functional groups at the periphery of dendrons cangive desired physical properties to the LC triazine-based dendritic mixtures. Althoughthe photoconductivity of the resulting dendrimers from the mixtures of 2,4-diamino-6-dodecyl-1,3,4-triazine and the corresponding dendrons is not better than those of dendronsalone, this property may be improved if columnar phases are formed on the thermal treat-ment of the dendritic mixtures since these columnar mesogens show better long-rangeself-assembling ability. Compound 23 may be likely to form columnar mesophases whentreated with proper acid-containing dendrons.

Nanomaterials 2021, 11, x FOR PEER REVIEW 12 of 15

assistance of LC dendrimers, the guest molecules should be more regularly arranged, thus exhibiting their physical characteristics significantly.

5. Conclusions and Prospect Three approaches have been described to prepare triazine-based dendrimers with

mesogenic phase on thermal treatment. The first is to properly incorporate mesogenic units onto triazine and piperazine framework in a straightforward manner by molecular design.

The second approach is to mix components to form LC dendritic mixtures by adjust-ing intermolecular face-to-face π–π interactions, which is valuable for 1,3,5-triamino-s-triazine-based LC dendrimers. The mixing approach allows versatile preparation of 1,3,5-triamino-s-triazine-based dendrimers, which were observed to form strong π–π interac-tions with tetrafluorbenzoquinone (TFB) in CH2Cl2, and the red shift of absorbance from the dendrimer-TFB complexes in the UV-Vis spectra can be up to ~180 nm [67].

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 C2 symmetry that can be efficiently prepared according to the litera-tures [62–67]. Incorporating special functional groups at the periphery of dendrons can give desired physical properties to the LC triazine-based dendritic mixtures. Although the photoconductivity of the resulting dendrimers from the mixtures of 2,4-diamino-6-do-decyl-1,3,4-triazine and the corresponding dendrons is not better than those of dendrons alone, this property may be improved if columnar phases are formed on the thermal treat-ment of the dendritic mixtures since these columnar mesogens show better long-range self-assembling ability. Compound 23 may be likely to form columnar mesophases when treated with proper acid-containing dendrons.

Figure 13. Structure of molecule (23).

Author Contributions: Conceptualization, methodology, writing—original draft preparation, L.-L.L.; writing—review and editing, L.-L.L., H.-F.H., and Y.-C.L.; software, formal analysis, investigation visualization, validation, data curation, Y.-C.L.; supervision, funding acquisition and project admin-istration, L.-L.L. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by the National Chi Nan University and the Ministry of Science and Technology, Taiwan (108-2113-M-260-008- and 109-2113-M-260 -001).

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The data presented in this study are available on request from the corresponding author.

Conflicts of Interest: The authors declare no conflict of interest.

References 1. Tomalia, D.A.; Fréchet, J.M.J. Discovery of dendrimers and dendritic polymers: A brief historical perspective. J. Polym. Sci. A

Polym. Chem. 2002, 40, 2719–2728, https://doi.org/10.1002/pola.10301.

Figure 13. Structure of molecule (23).

Author Contributions: Conceptualization, methodology, writing—original draft preparation, L.-L.L.;writing—review and editing, L.-L.L., H.-F.H., and Y.-C.L.; software, formal analysis, investigationvisualization, validation, data curation, Y.-C.L.; supervision, funding acquisition and project adminis-tration, L.-L.L. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by the National Chi Nan University and the Ministry of Scienceand Technology, Taiwan (108-2113-M-260-008- and 109-2113-M-260 -001).

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The data presented in this study are available on request from thecorresponding author.

Conflicts of Interest: The authors declare no conflict of interest.

References1. Tomalia, D.A.; Fréchet, J.M.J. Discovery of dendrimers and dendritic polymers: A brief historical perspective. J. Polym. Sci. A

Polym. Chem. 2002, 40, 2719–2728. [CrossRef]2. Caminade, A.-M.; Turrin, C.-O.; Laurent, R.; Ouali, A.; Delavaux-Nicot, B. Dendrimers: Towards Catalytic, Material and Biomedical

Uses; Wiley: New York, NY, USA, 2011.3. Janaszewska, A.; Lazniewska, J.; Trzepinski, P.; Marcinkowska, M.; Klajnert-Maculewicz, B. Cytotoxicity of Dendrimers.

Biomolecules 2019, 9, 330. [CrossRef] [PubMed]4. Chauhan, A.S. Dendrimers for Drug Delivery. Molecules 2018, 23, 938. [CrossRef]5. Sherje, A.P.; Jadhav, M.; Dravyakar, B.R.; Kadam, D. Dendrimers: A versatile nanocarrier for drug delivery and targeting. Int. J.

Pharm. 2018, 548, 707–720. [CrossRef]6. Kaur, D.; Jain, K.; Mehra, N.K.; Kesharwani, P.; Jain, N.K. A review on comparative study of PPI and PAMAM dendrimers.

J. Nanopart. Res. 2016, 18, 146. [CrossRef]7. Kesharwani, P.; Jain, K.; Jain, N.K. Dendrimer as nanocarrier for drug delivery. Prog. Polym. Sci. 2014, 39, 268–307. [CrossRef]8. Madaan, K.; Kumar, S.; Poonia, N.; Lather, V.; Pandita, D. Dendrimers in drug delivery and targeting: Drug-dendrimer interactions

and toxicity issues. J. Pharm. Bioallied Sci. 2014, 6, 139–150. [PubMed]9. Mignani, S.; El Kazzouli, S.; Bousmina, M.; Majoral, J.-P. Expand classical drug administration ways by emerging routes using

dendrimer drug delivery systems: A concise overview. Adv. Drug Deliv. Rev. 2013, 65, 1316–1330. [CrossRef]10. Liu, Y.; Lopes, R.P.; Lüdtke, T.; Di Silvio, D.; Moya, S.; Hamon, J.-R.; Astruc, D. “Click” dendrimer-Pd nanoparticle assemblies as

enzyme mimics: Catalytic o-phenylenediamine oxidation and application in colorimetric H2O2 detection. Inorg. Chem. Front.2021, 8, 3301–3307. [CrossRef]

11. Yamamoto, K.; Imaoka, T.; Tanabe, M.; Kambe, T. New Horizon of Nanoparticle and Cluster Catalysis with Dendrimers.Chem. Rev. 2020, 120, 1397–1437. [CrossRef] [PubMed]

Nanomaterials 2021, 11, 2112 13 of 14

12. Tang, Y.-H.; Cangiotti, M.; Kao, C.-L.; Ottaviani, M.F. EPR Characterization of Copper(II) Complexes of PAMAM-Py Dendrimersfor Biocatalysis in the Absence and Presence of Reducing Agents and a Spin Trap. J. Phys. Chem. B 2017, 121, 10498–10507.[CrossRef]

13. Neumann, P.; Dib, H.; Caminade, A.-M.; Hey-Hawkins, E. Redox Control of a Dendritic Ferrocenyl-Based Homogeneous Catalyst.Angew. Chem. Int. Ed. 2015, 54, 311–314. [CrossRef] [PubMed]

14. Twyman, L.J.; King, A.S.H.; Martin, I.K. Catalysis inside dendrimers. Chem. Soc. Rev. 2002, 31, 69–82. [CrossRef]15. Astruc, D.; Chardac, F. Dendritic Catalysts and Dendrimers in Catalysis. Chem. Rev. 2001, 101, 2991–3024. [CrossRef]16. Crooks, R.M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L.K. Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characteriza-

tion, and Applications to Catalysis. Acc. Chem. Res. 2001, 34, 181–190. [CrossRef]17. Concellón, A.; Termine, R.; Golemme, A.; Romero, P.; Marcos, M.; Serrano, J.L. High hole mobility and light-harvesting in discotic

nematic dendrimers prepared via ‘click’ chemistry. J. Mater. Chem. C 2019, 7, 2911–2918. [CrossRef]18. Yim, D.; Sung, J.; Kim, S.; Oh, J.; Yoon, H.; Sung, Y.M.; Kim, D.; Jang, W.-D. Guest-Induced Modulation of the Energy Transfer

Process in Porphyrin-Based Artificial Light Harvesting Dendrimers. J. Am. Chem. Soc. 2017, 139, 993–1002. [CrossRef] [PubMed]19. Nantalaksakul, A.; Reddy, D.R.; Bardeen, C.J.; Thayumanavan, S. Light Harvesting Dendrimers. Photosyn. Res. 2006, 87, 133–150.

[CrossRef]20. Balzani, V.; Ceroni, P.; Maestri, M.; Vicinelli, V. Light-harvesting dendrimers. Curr. Opin. Chem. Biol. 2003, 7, 657–665. [CrossRef]

[PubMed]21. Adronov, A.; Fréchet, J.M.J. Light-harvesting dendrimers. Chem. Comm. 2000, 18, 1701–1710. [CrossRef]22. Gilat, S.L.; Adronov, A.; Fréchet, J.M.J. Light Harvesting and Energy Transfer in Novel Convergently Constructed Dendrimers.

Angew. Chem. Int. Ed. 1999, 38, 1422–1427. [CrossRef]23. Wang, Y.; He, X.; Lu, Q. Polyamidoamine dendrimer functionalized cellulose nanocrystals for CO2 capture. Cellulose 2021, 28,

4241–4251. [CrossRef]24. El Kadib, A.; Katir, N.; Bousmina, M.; Majoral, J.P. Dendrimer–Silica hybrid mesoporous materials. New J. Chem. 2012, 36, 241–255.

[CrossRef]25. Bonaccorsi, L.; Lombardo, D.; Longo, A.; Proverbio, E.; Triolo, A. Dendrimer Template Directed Self-Assembly during Zeolite

Formation. Macromolecules 2009, 42, 1239–1243. [CrossRef]26. Yoo, S.; Lunn, J.D.; Gonzalez, S.; Ristich, J.A.; Simanek, E.E.; Shantz, D.F. Engineering Nanospaces: OMS/Dendrimer Hybrids

Possessing Controllable Chemistry and Porosity. Chem. Mater. 2006, 18, 2935–2942. [CrossRef]27. Kriesel, J.W.; Tilley, T.D. Dendrimers as Building Blocks for Nanostructured Materials: Micro- and Mesoporosity in Dendrimer-

Based Xerogels. Chem. Mater. 1999, 11, 1190–1193. [CrossRef]28. Martínez, N.P.; Inostroza-Rivera, R.; Durán, B.; Molero, L.; Bonardd, S.; Ramírez, O.; Isaacs, M.; Díaz Díaz, D.; Leiva, A.; Saldías,

C. Exploring the Effect of the Irradiation Time on Photosensitized Dendrimer-Based Nanoaggregates for Potential Applicationsin Light-Driven Water Photoreduction. Nanomaterials 2019, 9, 1316. [CrossRef] [PubMed]

29. Avila-Salas, F.; Pereira, A.; Rojas, M.A.; Saavedra-Torres, M.; Montecinos, R.; Bonardd, S.; Quezada, C.; Saldías, S.; Díaz Díaz,D.; Leiva, A.; et al. An experimental and theoretical comparative study of the entrapment and release of dexamethasone frommicellar and vesicular aggregates of PAMAM-PCL dendrimers. Eur. Polym. J. 2017, 93, 507–520. [CrossRef]

30. Axenov, K.V.; Laschat, S. Thermotropic Ionic Liquid Crystals. Materials 2011, 4, 206–259. [CrossRef] [PubMed]31. Rosen, B.M.; Wilson, C.J.; Wilson, D.A.; Peterca, M.; Imam, M.R.; Percec, V. Dendron-Mediated Self-Assembly, Disassembly, and

Self-Organization of Complex Systems. Chem. Rev. 2009, 109, 6275–6540. [CrossRef]32. Goodby, J.W.; Saez, I.M.; Cowling, S.J.; Görtz, V.; Draper, M.; Hall, A.W.; Sia, S.; Cosquer, G.; Lee, S.-E.; Raynes, E.P. Transmission

and Amplification of Information and Properties in Nanostructured Liquid Crystals. Angew. Chem. Int. Ed. 2008, 47, 2754–2787.[CrossRef]

33. Kim, H.-J.; Jung, E.-Y.; Jin, L.Y.; Lee, M. Solution Behavior of Dendrimer-Coated Rodlike Coordination Polymers. Macromolecules2008, 41, 6066–6072. [CrossRef]

34. Inoue, K. Functional dendrimers, hyperbranched and star polymers. Prog. Polym. Sci. 2000, 25, 453–571. [CrossRef]35. Hernández-Ainsa, S.; Barberá, J. Fluorinated liquid crystalline dendrimers. J. Fluor. Chem 2015, 177, 37–45. [CrossRef]36. Donnio, B.; Buathong, S.; Bury, I.; Guillon, D. Liquid crystalline dendrimers. Chem. Soc. Rev. 2007, 36, 1495–1513. [CrossRef]

[PubMed]37. Marcos, M.; Martín-Rapún, R.; Omenat, A.; Serrano, J.L. Highly congested liquid crystal structures: Dendrimers, dendrons,

dendronized and hyperbranched polymers. Chem. Soc. Rev. 2007, 36, 1889–1901. [CrossRef] [PubMed]38. Saez, I.M.; Goodby, J.W. Supermolecular liquid crystals. J. Mater. Chem. 2005, 15, 26–40. [CrossRef]39. Deschenaux, R.; Donnio, B.; Guillon, D. Liquid-crystalline fullerodendrimers. New J. Chem. 2007, 31, 1064–1073. [CrossRef]40. Gehringer, L.; Bourgogne, C.; Guillon, D.; Donnio, B. Liquid-Crystalline Octopus Dendrimers: Block Molecules with Unusual

Mesophase Morphologies. J. Am. Chem. Soc. 2004, 126, 3856–3867. [CrossRef]41. Gehringer, L.; Guillon, D.; Donnio, B. Liquid Crystalline Octopus: An Alternative Class of Mesomorphic Dendrimers.

Macromolecules 2003, 36, 5593–5601. [CrossRef]42. Meier, H.; Lehmann, M. Stilbenoid Dendrimers. Angew. Chem. Int. Ed. 1998, 37, 643–645. [CrossRef]43. Pesak, D.J.; Moore, J.S. Columnar Liquid Crystals from Shape-Persistent Dendritic Molecules. Angew. Chem. Int. Ed. Engl. 1997,

36, 1636–1639. [CrossRef]

Nanomaterials 2021, 11, 2112 14 of 14

44. Li, J.-F.; Crandall, K.A.; Chu, P.; Percec, V.; Petschek, R.G.; Rosenblatt, C. Dendrimeric Liquid Crystals: Isotropic−NematicPretransitional Behavior. Macromolecules 1996, 29, 7813–7819. [CrossRef]

45. Percec, V.; Chu, P.; Ungar, G.; Zhou, J. Rational Design of the First Nonspherical Dendrimer Which Displays Calamitic Nematicand Smectic Thermotropic Liquid Crystalline Phases. J. Am. Chem. Soc. 1995, 117, 11441–11454. [CrossRef]

46. Goossens, K.; Lava, K.; Bielawski, C.W.; Binnemans, K. Ionic Liquid Crystals: Versatile Materials. Chem. Rev. 2016, 116, 4643–4807.[CrossRef] [PubMed]

47. Donnio, B. Liquid–crystalline metallodendrimers. Inorg. Chim. Acta 2014, 409, 53–67. [CrossRef]48. Kato, T.; Mizoshita, N.; Kishimoto, K. Functional Liquid-Crystalline Assemblies: Self-Organized Soft Materials. Angew. Chem. Int. Ed.

2006, 45, 38–68. [CrossRef]49. Schnabel, W.J.; Rätz, R.; Kober, E. The Synthesis of Substituted Melams. J. Org. Chem. 1962, 27, 2514–2519. [CrossRef]50. Thurston, J.T.; Dudley, J.R.; Kaiser, D.W.; Hechenbleikner, I.; Schaefer, F.C.; Holm-Hansen, D. Cyanuric Chloride Derivatives. I.

Aminochloro-s-triazines. J. Am. Chem. Soc. 1951, 73, 2981–2983. [CrossRef]51. Takagi, K.; Hattori, T.; Kunisada, H.; Yuki, Y. Triazine dendrimers by divergent and convergent methods. J. Polym. Sci. A

Polym. Chem. 2000, 38, 4385–4395. [CrossRef]52. Lai, L.-L.; Wang, L.-Y.; Lee, C.-H.; Lin, Y.-C.; Cheng, K.-L. Nanomaterials of Triazine-Based Dendrons: Convergent Synthesis and

Their Physical Studies. Org. Lett. 2006, 8, 1541–1544. [CrossRef] [PubMed]53. Steffensen, M.B.; Hollink, E.; Kuschel, F.; Bauer, M.; Simanek, E.E. Dendrimers based on [1,3,5]-triazines. J. Polym. Sci. A

Polym. Chem. 2006, 44, 3411–3433. [CrossRef]54. Lai, L.-L.; Lee, C.-H.; Wang, L.-Y.; Cheng, K.-L.; Hsu, H.-F. Star-Shaped Mesogens of Triazine-Based Dendrons and Dendrimers as

Unconventional Columnar Liquid Crystals. J. Org. Chem. 2008, 73, 485–490. [CrossRef] [PubMed]55. Kotha, S.; Kashinath, D.; Kumar, S. Synthesis of liquid crystalline materials based on 1,3,5-triphenylbenzene and 2,4,6-triphenyl-

1,3,5-s-triazine. Tetrahedron Lett. 2008, 49, 5419–5423. [CrossRef]56. Thiebaut, O.; Bock, H.; Grelet, E. Face-on Oriented Bilayer of Two Discotic Columnar Liquid Crystals for Organic Donor−Acceptor

Heterojunction. J. Am. Chem. Soc. 2010, 132, 6886–6887. [CrossRef]57. Shearman, G.C.; Yahioglu, G.; Kirstein, J.; Milgrom, L.R.; Seddon, J.M. Synthesis and phase behaviour of β-octaalkyl porphyrins.

J. Mater. Chem. 2009, 19, 598–604. [CrossRef]58. Yang, C.-W.; Hsia, T.-H.; Chen, C.-C.; Lai, C.-K.; Liu, R.-S. Synthesis and Columnar Mesophase of Fluorescent Liquid Crystals

Bearing a C2-Symmetric Chiral Core. Org. Lett. 2008, 10, 4069–4072. [CrossRef]59. Li, Z.; Zhi, L.; Lucas, N.T.; Wang, Z. Triangle-shaped polycyclic aromatics based on tribenzocoronene: Facile synthesis and

physical properties. Tetrahedron 2009, 65, 3417–3424. [CrossRef]60. Lai, L.-L.; Hsu, S.-J.; Hsu, H.-C.; Wang, S.-W.; Cheng, K.-L.; Chen, C.-J.; Wang, T.-H.; Hsu, H.-F. Formation of Columnar Liquid

Crystals on the Basis of Unconventional Triazine-Based Dendrimers by the C3-Symmetric Approach. Chem. Eur. J. 2012, 18,6542–6547. [CrossRef]

61. Alfa Aesar. Research Chemicals, Metals and Materials; Alfa Aesar: Ward Hill, MA, USA, 2011; p. 124.62. Lai, L.L.; Wang, S.W.; Cheng, K.L.; Lee, J.J.; Wang, T.H.; Hsu, H.F. Induction of the columnar phase of unconventional dendrimers

by breaking the C2 symmetry of molecules. Chem. Eur. J. 2012, 18, 15361–15367. [CrossRef] [PubMed]63. Lai, L.-L.; Hsieh, J.-W.; Cheng, K.-L.; Liu, S.-H.; Lee, J.-J.; Hsu, H.-F. A Small Change in Central Linker Has a Profound Effect

in Inducing Columnar Phases of Triazine-Based Unconventional Dendrimers. Chem. Eur. J. 2014, 20, 5160–5166. [CrossRef][PubMed]

64. Tsai, M.-J.; Hsieh, J.-W.; Lai, L.-L.; Cheng, K.-L.; Liu, S.-H.; Lee, J.-J.; Hsu, H.-F. Converting Nonliquid Crystals into LiquidCrystals by N-Methylation in the Central Linker of Triazine-Based Dendrimers. J. Org. Chem. 2016, 81, 5007–5013. [CrossRef]

65. Lee, C.-H.; Huang, C.-C.; Li, C.-Y.; Lai, L.-L.; Lee, J.-J.; Hsu, H.-F. Both increasing the Iso-to-Col transition and lowering thesolidifying temperatures of a triazine-based dendrimer by introducing CN polar groups in the dendritic core. J. Mater. Chem. C2019, 7, 14232–14238. [CrossRef]

66. Castelar, S.; Barberá, J.; Marcos, M.; Romero, P.; Serrano, J.-L.; Golemme, A.; Termine, R. Supramolecular dendrimers based on theself-assembly of carbazole-derived dendrons and triazine rings: Liquid crystal, photophysical and electrochemical properties.J. Mater. Chem. C 2013, 1, 7321–7332. [CrossRef]

67. Lai, L.-L.; Hsu, H.-C.; Hsu, S.-J.; Cheng, K.-L. A Convenient Synthesis of Triazine-Based Dendrons and Dendrimers viaa Convergent Approach and a Study of Their Interactions with Tetrafluorobenzoquinone. Synthesis 2010, 2010, 3576–3582.[CrossRef]