Triphenylene discotic liquid crystal trimers synthesized by Co2(CO)8-catalyzed terminal alkyne [2 + 2 + 2] cycloaddition

Summary The synthesis of star-shaped discotic liquid crystal trimers using Co2(CO)8-catalyzed terminal alkyne [2 + 2 + 2] cycloaddition reaction is reported. The trimers consist of three triphenylene discotic units linked to a central 1,2,4-trisubstituted benzene ring via flexible spacers. The trimers were synthesized in the yields up to 70% by mixing the monomers with 10 mol % of Co2(CO)8 as the catalyst in refluxing 1,4-dioxane. The liquid crystalline properties were investigated by using polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Trimer 4 with an ester connecting group and a longer spacer exhibited a rectangular columnar mesophase, while 5b and 5c possessing an ether linkage and a shorter spacer display a hexagonal columnar mesophase. The connecting functional group and the length of the flexible spacer between the central benzene ring and the triphenylene units have pivotal influence on the mesomorphism.

The reported DLC oligomers  are limited compared with the low-molar-mass DLCs and polymeric DLC materials, due to the construction methods for the oligomers. However, DLC oligomers usually possess wider mesophase ranges than the monomers as the crystallization was prevented, due to the size of molecules is enlarged and molecular symmetry is lowered. However DLC oligomers exhibit higher charged carrier mobility than DLC polymers, the most important parameter in determining the device performance, as the discotic unites can still self-assemble to higher order through the π-π interaction. In addition, DLC oligomers similar to polymers can be processed on flexible substrates by spin-casting, screen printing, doctor-blading, ink-jet printing and roll-to-roll processing, so that these cost-effective deposition methods can be used to manufacture electronic devices. Therefore, the exploration of efficient synthetic methods for the DLC oligomers and studying the properties of them are fundamental.
The construction methods of DLC oligomers can be divided into conventional organic reactions and transition metalcatalyzed synthetic methods. The use of transition metal-mediated reactions for constructing new organic functional materials is more efficient and therefore attractive. Our group has embarked upon a program to use new organic synthetic methods to prepare novel types of DLC materials and to study the relationship between their molecular structures and the mesomorphic properties [46][47][48][49][50]. We have reported that Cu(I)catalyzed alkyne-azide click reactions are emerging as an efficient method for the synthesis of discotic oligomers [46][47][48][49][50].
The transition metal-catalyzed [2 + 2 + 2] cycloaddition of three alkynes for the synthesis of polysubstituted benzene derivatives was studied due to its high efficiency and atomic efficiency [51]. The use of Co 2 (CO) 8 as a catalyst has been extensively applied to the synthesis of hexabenzocoronene DLCs through diarylethyne cyclotrimerization [51].
In this paper, we report four star-shaped DLC trimers with triphenylene discotic units by using a Co 2 (CO) 8 -catalyzed terminal alkynes [2 + 2 + 2] cycloaddition, and the trimers exhibit ordered rectangular (Col ro ) and hexagonal columnar mesophases (Col ho ). Furthermore, the structure-mesomorphic property relationship is discussed. The synthetic route is shown in Scheme 1 and Scheme 2.
The star-shaped DLC trimers with triphenylene discogens, 4 and 5a-c, were synthesized in yields of 36-71% by the selftrimerization of monomer 2 or 3a-c catalyzed by using 10 mol % of Co 2 (CO) 8 in refluxing 1,4-dioxane. Trimer 4, 5a and 5b were prepared in moderate yields, and the obvious lower synthetic yield of 5c might be caused by its shorter spacer and bigger steric hindrance. Considering the size of the trimers, we were satisfied with the preliminary synthetic yields, and did not further optimize the reaction conditions. Two isomers were obtained in the trimerization of mono-substituted alkynes, R-C≡CH: 1,2,4-and 1,3,5-trialkylbenzene. For the DLC trimers, the isomers could not be separated by thin-layer chromatography and column chromatography, and even high performance liquid chromatography (HPLC). However, the benzenes with three substituents, a 1,2,4-or 1,3,5-trisubstituted pattern, can be characterized by 1 H NMR spectroscopy [55,56]. According to this method, we find that the 1 H NMR peak of the 1,3,5-trisubstituted benzene isomer 4 appears at 6.83 ppm, for 5a at 6.81 ppm, and 7.03 ppm for 5b. There was no signal for the 1,3,5-trisubstituted isomer for 5c. The 1 H NMR peak area integration results showed that for 4, 5a and 5b, the 1,2,4-trisubstituted benzenes were present in more than 95% and 1,3,5-trisubstituted benzene isomers were less than 5%. For 5c, the symmetric isomer of the 1,3,5-trisubstituted benzene was not detected, and the yield of the 1,2,4-trisubstituted isomer was almost quantitative. Therefore, we came to the conclusion that this synthetic method and the following purification procedures supplied the 1,2,4-trisubstituted benzene-cored DLC oligomers.

Mesomorphism POM and DSC
Initially, we studied the mesomorphic properties of the monomers and trimers by using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The POM results of the monomers and trimers are summarized in Figure 1 and Figure 2, respectively. The DSC traces are shown in     [52] and 3b [57] were reported and the mesomorphism is comparable. Figure 3 and the phase transition data are summarized in Table 1.
For the mesomorphism of the functionalized triphenylene monomers, compound 2 [52] and 3b [57] have been reported, 3a and 3c are new. They all display typical optical textures with homeotropic alignment behavior of the hexagonal columnar (Col h ) mesophase ( Figure 1). The monomers display different phase-transition temperatures related to the connecting functional group and the length of the chain. Compound 2 displayed a Col phase at room temperature with a clearing point of 111 °C (for the heating run), and did not crystallize even when cooled to −50 °C. Monomer 3a exhibited a narrow columnar mesophase range between 41 °C to 60 °C for heating, and between 59 °C to 11 °C for cooling. Comparing the phase-transition temperatures of 2 and 3a, it was found that the ester connecting group has the effect of lowering the melting point and rising the clearing point. Compared with the symmetric discogen of 2,3,6,7,10,11-hexakis(pentyloxy)triphenylene (C5OTP) which possesses a Col phase between 69-122 °C [58], monomer 2 displays a lower melting point and 3a exhibits both lowered melting and clearing points. The ester connecting group of 2 increases its dipole-dipole interactions in the intermolecular columnar stacking and stabilizes the Col mesophase.
Both 3b and 3c having slightly lowered molecular symmetry compared to the parent compound C5OTP, exhibited a Col mesophase between 69-117 °C and 80-121 °C, respectively. Figure 2 shows the POM results for the trimers. Trimer 4 selfassembled into small-sized domains which were independent of the temperature and the cooling rate, and displayed a clearing point at 110 °C on heating, and on the cooling run the Col phase appeared at 100 °C. The crystallization did not occur for 4 even when it was cooled to −50 °C at the rate of 10 K/min.
Trimer 5a did not show mesomorphism as indicated both from the results of the POM and the DSC. The DSC curves of 5a showed only one phase transition peak at 15 °C on the first cooling run and at 25 °C on the second heating run. From the POM observations, we noted that the peak represents the crystal to isotropic liquid transition (Cr→Iso) and the reversed Iso→Cr transition.
Both trimer 5b and 5c exhibited a similar mesophase behavior to trimer 4: There is only one phase transition peak on the first cooling process and the second heating run. For 5b, the Col→Iso transition occurred at 106 °C on heating, and Iso→Col appeared at 104 °C on cooling. For 5c, the Col→Iso transition occurred at 125 °C on heating, and the reversed transition at 101 °C on cooling. No crystallization was observed for either 5b or 5c. Figure 4 depicts the X-ray diffraction patterns of 4, 5b and 5c at room temperature cooling from the isotropic liquid. Both 5b and 5c show a strong diffraction peak in the small-angle region (2θ = 3.8° or 4.5°), a broad halo peak of the alkyl chain at ca. 18°, and a core-core distance peak at 3.5-3.7 Å. Considering these XRD results together with their homeotropic alignment behavior displayed by the POM results shown in Figure 2C and Figure 2D, we assigned the mesophase of 5b and 5c as the Col ho phase. However, 4 exhibited a different XRD pattern from that of 5b and 5c: two strong diffraction peaks in the small-angle region (2θ = 3.7° and 4.7°) with a d value of 23.78 Å and 18.80 Å, respectively. We assigned the mesophase of 4 as the rectangular columnar phase (Col ro ), which was further confirmed by the temperature-dependent XRD results ( Figure 5), and the nonhomeotropic alignment behavior of the POM texture ( Figure 2A). The lattice parameters of the discotic Col mesophase of the trimers are summarized in Table 2. Trimer 4 displayed a smaller intracolumnar core-core distance of 3.57Å than that of 5b and 5c, due to the stronger π-π interactions between the triphenylene discogens with mono-ester group. The columnar parameter values of the trimers decreased with the spacer length shortened. So we deduced that the benzene cores were among the alkyl chains in the columnar stacking of the trimers. Further considering the relationship between the molecular structures of the trimers and the mesomorphism, we noted first the mesophase disappearance of 5a, which was synthesized by trimerization of liquid crystalline 3a. However, trimer 4 has displayed a stable Col ro mesophase while 5a did not. Both 4 and 5a possess the same length spacer, the ester-linker of 4 has a stronger polarity than the ether-linker of 5a. The isotropic point of 4 (111 °C) is higher than that of 5a (25 °C). More importantly, 4 is a room temperature liquid crystal and 5a has not shown a mesophase. Compared with 5a, both 5b and 5c have shorter spacers and have displayed a stable Col ho mesophase over a wide temperature range.

XRD results
Our study has demonstrated that for the star-shaped DLC trimers, both the length of spacer and the connecting linker group to the discotic units play a crucial role in the formation and stabilization of the discotic columnar mesophase.

Conclusion
The synthesis and mesomorphism of two new mono-functionalized triphenylene discotic monomers and four discotic trimers is reported. The trimers have been successfully synthesized for the first time by using a Co 2 (CO) 8

Supporting Information
Supporting Information File 1 Characterization instruments and methods. 1 H NMR spectra and 13 C NMR spectra for the monomers and trimers.