The substitution of benzene with 1,3,5‐triazine leads to a new core for discotic liquid crystals: tris(aryloxadiazolyl)triazine (TOT, N1 – N15 , Scheme 1 , Figure 1 ), a fluorescent scaffold with excellent tendency towards columnar arrangement in the LC phase and broad mesophases. The synthesis as well as the thermal, optical, and luminescent properties of these compounds and some benzene analogues (TOBs C2 – C11 ) and the comparison of their properties are subject of this report.

1,3,5‐Tris‐(5‐phenyl‐1,3,4‐oxadiazol‐2‐yl)benzene (TOB), one of the first reported discotic oxadiazole compounds, 19 melts at 335 °C without any liquid‐crystalline phase, but a few molecules of this structure with flexible side chains attached are reported to form broad mesophases with the characteristic textures of a columnar arrangement, as observed by polarization optical microscopy. 20 , 21

Especially organic compounds are of great interest, they are required to replace rare, toxic, and expensive inorganic materials. 3 , 5 Organic optoelectronic devices have already been developed, like organic photovoltaics (OVPs), 6 organic light‐emitting diodes (OLEDs), 7 and organic field‐effect transistors (OFETs). 8 Although calamitic liquid crystals are the key compound in LCDs, 2 , 8 , 9 the technology of discotic liquid crystals (DLC) is only in its beginning. 10 , 11 Further interesting features of DLCs are self‐assembly, mostly to form columnar mesophases, self‐healing, and aggregation‐induced emission enhancement. 12 The columnar arrangement enables a one‐dimensional charge transfer (CT) 4 , 10 by overlapping of the aromatic π‐orbitals (π‐stacking). 13 Most DLCs consist of an electron‐rich aromatic core, for example triphenylene, (triaza)truxene, or thiophene, often with electron‐rich arms like alkoxybenzene or thiophene. 9 , 14 , 15 In contrast, electron‐deficient heterocycles like pyrimidines, triazines, or oxadiazoles are less common as components of the π‐conjugated core. 13 , 16 , 17 , 18

The development of new materials for electronic and opto‐electronic devices, for example nonlinear optical (NLO) absorbers, emitters, and semiconductors for the application in solar cells, light‐emitting diodes (LEDs), field‐effect transistors (FETs), and quantum computers is a highly active research field with an extraordinary commercial potential. 1 - 4

Results and Discussion

Synthesis The common route to 1,3,4‐oxadiazoles is the stepwise formation of diacylhydrazine followed by dehydration/cyclization with POCl 3 .21, 22, 23 This method has been successfully applied for the synthesis of star‐shaped molecules with a benzene center. An alternative way to construct oxadiazoles is opened by the Huisgen reaction of tetrazoles and acid chlorides in presence of pyridine bases.20, 24, 25 This method is particularly rewarding for the preparation of sensitive products.26 Hence, we followed this strategy to build discotic LCs with a tris‐1,3,4‐oxadiazolyl‐1,3,5‐triazine nucleus N1–N15 (Scheme 1). Initial results were disappointing because upon addition of tris(chlorocarbonyl)triazine 2 a to the tetrazole/collidine mixture, a brown precipitate was formed and tetrazoles were converted to nitriles (determined by mass spectrometry). Given that the acid chloride 2 a decomposed in the presence of bases and that no oxadiazole was formed, the reaction had to be performed in the absence of bases regardless of some side reactions due to HCl. Good yields of heterocyclic stars N1–N15 were obtained, accompanied by small amounts of 1,2,4‐triazoles as byproducts (determined by NMR, MS). The required triazine acid chloride 2 a was prepared in three steps by acid‐catalyzed trimerization of ethyl cyanoformate, saponification, and chlorination of the tripotassium salt 1 with POCl 3 to obtain pure 2 a in 71 % yield after distillation (method A). Similar results in the Huisgen reaction were obtained when 1 was converted to 2 a with thionyl chloride and the excess chlorinating agent was evaporated (method B).28 It appeared to be essential that 2 a was prepared just before the Huisgen reaction. The tetrazoles were synthesized in multistep reactions starting with protocatechuic nitrile 3 or methyl gallate 6 (Scheme 2).29, 30 3,4‐(Dialkoxyphenyl)‐tetrazoles 5 a,b are accessible through alkylation of 3 and 1,3‐dipolar cycloaddition of an azide.29 The 3,4,5‐analogues 9 a–c were synthesized from methyl gallate 6 through alkylation, saponification, chlorination/ammonolysis followed by dehydration/ azide transfer with triazidichlorosilane (see the Supporting Information for details).31 Scheme 2 Open in figure viewer PowerPoint Synthesis of tetrazoles. Biphenyl tetrazole 10 is accessible through Suzuki cross‐coupling reaction of iodophenyl tetrazole and di(decyloxy)phenyl boronic acid.32 5,6‐Di(decyloxy)naphth‐2‐yl‐tetrazole 11 was prepared by oxidation of 6‐bromo‐2‐naphthol to the o‐quinone, reduction, alkylation and Rosenmund‐von‐Braun cyanation followed by azide addition, similar to a literature procedure. With these tetrazoles in hand, 14 different tris(aryloxadiazolyl)triazines (TOT, N1–N15) and four tris(oxadiazolyl)benzenes (TOB, C2–C4, C9) were synthesized in yields ranging from 13–72 % (Figure 1, Table 1). Triazoles were formed as byproducts, their chromatographic behavior is generally very similar to the tris(oxadiazolyl) stars. Therefore, excessive chromatography was occasionally required—and responsible for reduced yields. Residual triazole in the yellow fluorescent TOT is easily distinguished by its blue fluorescence on solvent‐loaded TLC plates. The identity and purity of all materials is demonstrated by standard analytical methods, such as 1H and 13C NMR as well as TLC and HR‐MS. Table 1. Tris(aryl‐1,3,4‐oxadiazolyl)triazines and ‐benzenes, substitution pattern, chain length, yield (over two steps), and references. R Yield [%] R Yield [%] N1 R1=R3=H, R2=n‐propyloxy 47[a] N8 R1=R2=R3=n‐hexyloxy 31[b] N2 R1=H, R2=R3=n‐octyloxy 48[a] N9 R1=R2=R3=n‐octyloxy 34[a] C2 R1=H, R2=R3=n‐octyloxy 61 C9 R1=R2=R3=n‐octyloxy 13; 2821 N3 R1=H, R2=R3=n‐decyloxy 38[a] N10 R1=R2=R3=n‐decyloxy 45[a] C3 R1=H, R2=R3=n‐decyloxy 59[c][23] C10 R1=R2=R3=n‐decyloxy –[c][23] N4 R1=H, R2=R3=n‐dodecyloxy 47[a] C11 R1=R2=R3=n‐dodecyloxy –[c][27] C4 R1=H, R2=R3=n‐dodecyloxy 33; 8725 N12 R1=R2=R3=2‐ethylhexyloxy 55[b] N5 R1=H, R2=R3=n‐tetradecyloxy 70[b] N13 R1=R2=R3=3,7‐dimethyloctyloxy 72[b] N6 R1=H, R2=R3=4‐ethyloctyloxy 52[b] N14 R4=n‐decyloxy 46[b] N7 R1=H, R2=R3=3,7‐dimethyloctyloxy 29[b] N15 R5=n‐decyloxy 49[b] For the synthesis of the carbon analogues C2–C9, the reaction of trimesic acid trichloride 2 b in the presence of 2,4,6‐collidine successfully gave the pure TOBs in yields up to 61 % after chromatography.

Thermal properties: DSC and POM Polarized optical microscopy revealed birefringend mesophases for 18 out of 20 star‐shaped molecules (including 2 mesogenes from literature).23, 27 The LC‐phases show textures typical for hexagonal‐columnar arrangements (Figure 2 a,b). A standard discotic liquid crystal exhibits usually two transitions in a DSC heating scan, melting and clearing. TOTs and TOBs are highly viscous liquid crystals in the high‐temperature range as evidenced by shearing experiments. Upon cooling, the structure and texture of the mesophase is maintained, even after complete congealing. Therefore, determination of the melting point is hard to observe by POM. In highly viscous mesophases (TOTs and TOBs), crystallization can be slow, resulting in partial or complete preservation of the mesophase structure. As a consequence, measured melting enthalpies in the cooling scan are lower than those of the first heating cycle. Thus, the values in Table 2 are for not completely relaxed materials. These phenomena have been observed for several TOTs and TOBs (N2–N8, N14, C2, C3). The partial recrystallization can give rise to a cold crystallization just below the melting point during the subsequent heating scan (N5, N7, N14). Glass transitions were detected for N12 and also transitions between crystal phases (N15, C3, C9) which occur only in the first heating curve or after prolonged storage. Figure 2 Open in figure viewer PowerPoint a) POM: fan texture of (N8) at 200 °C (0.5 K min−1) upon cooling. b) POM: focal conic fan texture of (N2) at 195 °C (10 K min−1) upon cooling. Table 2. Phase‐transition temperatures (°C) and corresponding enthalpies [kJ mol−1]. Compound Compound N1 Cr 276 I N8 Cr 55 [24.8][c] M 220 [1.2][c] I N2 Cr 97 [3.7][b] Col h 192 [3.4][b] I N9 Cr −10 [Tg][b] Col h 210 [4.5][b] I C2 Cr 106 [67.1] Col h 181 [4,8] I C9 Cr 1 [8.0] Cr 34 [20.9] Col h 190 [4.9] I N3 Cr 84 [3.9][b] M 203 [4.3][b] I N10 Cr 33 [21.9][a] M 179 [4.0][b] I C323 Cr 51 [92.7] Cr 83 [189.6] Cr 107 [26.7] M 176 [18.4] I C1023 Cr 46 [86.2] M 169 [15.5] I N4 Cr 87 [3.4][b] M [4.9][b] 202 I C1127 Cr approx. 20 M approx.145 I C425 Cr 103 [54.4] M 184 [3.9] I N12 I N5 Cr 89 [46.0][b] M 184 [3.9][b] I N13 Cr −19 [Tg][c] M 72 [0.9][b] I N6 Cr 56 [2.9][c] M 156 [2.3][c] I N14 Cr 121 [1.9][b] M 210 [3.0][b] I N7 Cr 95 [0.6][a] M 170 [2.1][b] I N15 Cr 93 [4.1][b] Cr 165 [26.9][b] I TOTs N6 and N8 are strongly inhibited in crystallization, a second measurement of the same DSC sample after two months did not exhibit any Cr→M transitions, only after two years a DSC experiment revealed a signal attributed to such a transition. Furthermore, some TOTs show exothermic transitions in their second heating cycles (e.g., N5, Figure 3) caused by thermally induced crystallization.33 This is followed by two endothermic transitions, melting (T=88.5 °C, ΔH=46.0 kJ mol−1) and clearing (T=184.3 °C, ΔH=3.9 kJ mol−1). Figure 3 Open in figure viewer PowerPoint DSC: second heating curve of (N5). The results of POM and DSC investigations on TOTs and TOBs are summarized in Figure 4 and Table 2. TOTs and TOBs show broad mesophases with phase widths of 69–220 K. Generally, the triazine‐based compounds have lower melting and higher clearing temperatures than their carbon analogues and consequently, the mesophase ranges are 20–64 K wider (Figure 4). The transition temperatures of 3,4‐dialkoxy TOTs N2–N5 are weakly affected by elongation of alkyl chains (octyl–tetradecyl); the maximal T c is reached with decyl and dodecyl chains. T c is more influenced by chain length than T m . The sensitivity of transition points to chain length is more pronounced in the 3,4,5‐trialkoxy series N8–N10. Compared with the 3,4‐dialkoxy derivatives, their LC phases are much wider (up to 220 K!) and appear at significantly lower temperatures. Figure 4 Open in figure viewer PowerPoint Overview of phase width of tris(aryloxadiazolyl)arene stars. Branching in the alkyl chains of the triazine stars (N6, N7, N12, N13) reduces transition temperatures and width of the LC phase. This effect is more pronounced in the 3,4,5‐trialkoxy series, even a complete loss of mesomorphism has been found (N12). This can be due to two reasons: diastereomeric mixtures and steric crowding, especially for the 3,4,5‐trialkoxy series. Extending the aromatic system of TOTs from peripheral phenyl (N3) to peripheral naphthalene N14 results in an increase of the melting point by 37 K and of the clearing point by 7 K. The even longer biphenyl derivative N15 reveals only a crystalline phase at much lower temperature, which may be attributed to the nonplanar structure of this building block and therefore reduced mobility in a column. The thermal behavior (Cr 25 (6) Col h 114 (4) iso) of a thiophene analogue to N9 and a higher homologue reveals a strong stabilization of the mesophase by the oxadiazole, probably due to CT interactions between electron‐deficient oxadiazoles and alkoxyphenyl rings.34 Similarly, 1,2,3‐triazoles stabilize the mesophase, phase widths are in the range of 160 K.17

X‐ray scattering Single crystals of p‐propyloxy‐TOT N1 were obtained through slow evaporation of toluene solution of N1.35 Compound N1 crystallizes in the triclinic space group P (a=8.62, b=13.59, c=15.79 Å; α=89.91, β=85.75, γ=76.78°) and contains two molecules per unit cell (Figure 5). Crystallographic data exhibit a nonplanar molecular structure. In contrast to the expected C 3 ‐symmetry (Figure 5), N1 adopts a Y‐shape because one phenyloxadiazole arm is flipped around the triazine–oxadiazole bond. These rings show a dihedral angle of 16° whereas all other biaryl units are essentially planar (Θ≤1°). The dihedral angles between the benzenes and oxadiazoles are about 10–14°. Furthermore, two propyloxy unit are disordered. Figure 5 Open in figure viewer PowerPoint Single‐crystal structure of N1 A) asymmetric molecular Y‐conformation, B) nearly perfect planarity, C) unit cell with alternating arranged molecules. The broken symmetry has also been observed for a tris‐1,2,3‐triazolyl‐1,3,5‐triazine,36 but a star similar to N1 with thiophene instead of oxadiazole34, 37, 38 shows nearly perfect C 3 ‐symmetry. This was explained by attractive S−N interactions. The high planarity, as expressed by dihedral angles between the heterocycles of 1–11°, was found to improve charge‐transport properties. Although these molecules are rotationally displaced with respect to adjacent molecules, the packing of Y‐shaped N1 corresponds to columns of alternating oriented molecules along a diagonal through the unit cell. Each two molecules have a center of inversion and the central triazines are off‐diagonal, bringing oxadiazoles and alkoxyphenyl in close proximity. These features correspond to the results of simulation of the mesophase structure (see below).

2D‐Scattering on oriented fibers Two‐dimensional wide‐angle X‐ray scattering (WAXS) experiments on macroscopically aligned extruded fibers were carried out to determine the intra‐ and intercolumnar organization of the TOTs and TOBs in their liquid‐crystal phases. Filaments were obtained by extrusion of N2 and N9 in their LC‐phase (175 °C, 10 min annealing) as well as of C2 and C9 (140 °C, 5 min annealing). Table 3 and Figure 6 summarize the results. The patterns in Figure 6A, B reveal characteristic features of well‐aligned columnar LC samples: i) reflections centered at the equator attributed to a hexagonal 2D lattice of columns, ii) a halo corresponding to the liquid‐like chains with an average distance between 4.0–4.6 Å (see Table 3) and iii) a rather broad signal at wider angles reminiscent of the average aromatic distances (π–π stacking). The corresponding integration of the patterns along the equator and the meridian (Figure 6 C, D) uncover general trends. First, the reflections with larger and mixed indices are obviously more intense for the carbon derivatives than for the nitrogen derivatives. For N2 the 11 and 20 reflections appear only at lower temperature. This points to a higher two‐dimensional order of columns for the derivatives C2 and C9 when compared with N2 and N9. The cell parameters of the hexagonal unit cells decrease with increasing temperature (see also Table 3), whereas simultaneously the distances along the columns increase. However, this effect is surprisingly small because a changes only by 0.7 Å, when heating N2 from 25 to 176 °C. These observations are the same for the nitrogen and the carbon series. The distinct difference in both series is the intensity of the π–π signal corresponding to distances between 3.2–3.5 Å, which is much more intense for the compounds C2 and C9 and very small or almost absent for N2 and N9. The correlation length calculated by the Scherrer formula39 amounts to 6–7 repeating units for C2, C9 and only about 4 repeating units for N2 and N9. To gain insight in the packing of these star‐shaped, shape persistent mesogens, the density was measured at 23.5 °C and extrapolated to the temperatures of the LC phases (Table 4, for details see the Supporting Information). Table 3. Unit cell parameters, miller indices and corresponding distances. Compound hkl (d in Å) N2 176 °C 100 (26.7) 200 (13.3) 210 (10.1) halo (4.6) π–π (3.5) a hex =30.8 Å N2 25°C 100 (27.3) 200 (13.7) 210 (10.3) halo (4.2) π–π (3.2) a hex =31.5 Å C2 125 °C 100 (27.3) 200 (13.6) halo (4.0) π–π (3.5) a hex =31.5 Å N9 100°C 100 (27.1) 200 (13.6) 210 (10.2) halo (4.5) π–π (3.5) a hex =31.2 Å N9 25 °C 100 (27.3) 200 (13.7) 210 (10.3) halo (4.3) π–π (3.3) a hex =31.5 Å C9 155 °C 100 (26.6) 110 (15.3) 200 (13.4) 210 (10.1) halo (4.3) π–π (3.5) a hex =30.7 Å C9 125 °C 100 (26.7) 110 (15.4) 200 (13.3) 210 (10.1) halo (4.1) π–π (3.4) a hex =30.8 Å C9 60 °C 100 (27.1) 110 (15.6) 200 (13.5) 210 (10.2) halo (4.0) π–π (3.4) a hex =31.3 Å Figure 6 Open in figure viewer PowerPoint Diffraction pattern of N2 at 176 °C (A) and C9 at 125 °C (B). (C) and (D) show the integrated intensity of the equator and the meridian at various temperatures. Table 4. Experimental, extrapolated densities, and number of molecules in a columnar repeating unit h. ρ exp [a] [g cm−3] V mol [b] [Å3] V Ar [c] [Å3] V IAr [d] (crystal) [Å3] ρ[e] [g cm−3] (T [°C]) Z[f] h[g] [Å] N2 1.065 2000 698 609 1.02 (100) 3 7.50 N9 1.012 2736 783 625 0.96 (100) 2 6.84 C2 1.029 2065 763 616 0.97 (125) 3 7.68 C9 1.022 2704 751 632 0.95 (125) 2 7.08 From the density data, the molecular volume was calculated and subsequently the aromatic volumes are available when the known volumes of the aliphatic chains are subtracted.40 Here it is evident that all aromatic volumes are larger than the volumes occupied in a crystal, which were calculated by an increment method.41 The volumes are 15–25 % higher in the mesophases. This is certainly a consequence of the less dense packing in the soft‐crystalline and liquid‐crystalline matter but the large values points also to the fact that the intrinsic free space between the shape‐persistent branches cannot be completely filled. There is clearly much less free space in the aromatic part for the derivative N2 with six chains, which has the highest density. The integer number of molecules filling the columnar repeating h unit (twice the π–π distance) was calculated to be three molecules for the six‐chains derivatives and two molecules for the columnar stratum of the nine‐chain mesogens. For all systems, the number of aliphatic chains is identical, the a‐parameter is also almost identical and, therefore, the value h must increase for N2 and C2 owing to the larger number of aromatic units in the core area of the column. However, this increase amounts only to 0.6–0.7 Å and assuming coplanar stacked mesogens N2 and C2 would be only 2.5–2.6 Å apart, whereas the intramolecular distances for N9 and C9 are in agreement with π‐stacks (3.4–3.5 Å). Given that the distances between the six‐chain derivatives are smaller than van der Waals distances, modelling of the LC phases has been performed to unravel packing details. Thus, the columnar phases of N2, N9, and C9 were constructed with the program Materials Studio (Forcite, COMPASS II) on the bases of XRS and density data. Figure 7 highlights the results for compound N2. Various starting setups with non‐C 3 ‐symmetric and C 3 ‐symmetric conformers were geometry optimized to obtain large attractive noncovalent interaction energies after several minimization and annealing procedures. In general, the van der Waals energy was large and negative (−1670 to −1768 kcal mol−1, values are given for a set of four unit cells with 24 molecules), whereas the electrostatic energy was always positive but decreased slightly when compared with the single molecule (850–898 kcal mol−1 in the LC phases versus 973 kcal mol−1 for the single molecule, see the Supporting Information). Figure 7 Open in figure viewer PowerPoint Packing of N2 in the columnar assembly. Molecules are displaced from the center of the column and occupy spaces from different columnar slices to fill efficiently the space. The best results were obtained for the Y‐shaped conformer similar to the one found in the single crystal. It was observed that the core position deviates from the center of the column (Figure 7 C) and that the aromatic units contribute to the space filling of different columnar slices (Figure 7 D). The chains densely occupy the peripheral space. As a consequence of this dense packing, the intracolumnar order is reduced and almost no π‐stacking is visible in the experimental pattern. However, note that C2, with the same number of mesogens per columnar stratum, must pack in as similar way but nevertheless, exhibit a π‐stacking signal. Consequently, the electrostatic repulsion discussed in detail below must also be important for the LC self‐assembly. When comparing N9 and C9, it was evident that the columnar stratum of height h consists of two molecules with 18 chains and the intracolumnar spacing was calculated to be 3.4 (N9) and 3.5 Å (C9). This can be also confirmed by modelling (see the Supporting Information). Although C9 shows a clear π‐stacking of the mesogens, N9 lacks this signal. The missing π‐stacking for N9 may be explained by the increase in the electrostatic interactions for N9 compared with single molecules, whereas the electrostatic interactions do not change for C9 (see the Supporting Information).42 Thus, mesogens N9 avoid evidently the cofacial stacking.

Electrical conductivity of N2, C2 and N9 Conductivity was studied using the Time‐of‐Flight method to get information about their charge‐transfer ability (see the Supporting Information). For the 3,4‐substituted derivative N2, charge carrier mobilities of μ=10−3 cm2 V−1 s−1 in the crystalline (25 °C) and mesophase (120 °C) are significantly higher than those of its carbon analogue C2 and the 3,4,5‐substituted N9, which are in the range of μ=10−5 cm2 V−1 s−1 at 120 °C. The latter can be rationalized by the much denser packing of the aromatic scaffold in the center of the columns when considering N2 and N9. The lower charge‐carrier mobility of C2 is not yet well understood, because the aromatic units pack as compact as N2, and moreover, show a clear signal for a higher intracolumnar order in contrast to N2.