100 Chemical Science Discovery of ‘bicontinuous cubic’ liquid crystalline mesophase from discotic molecules Liquid crystalline (LC) semiconductors have attracted increasing attention owing to their potential use in solution-processable and self-repairable organic electronics. Toward this goal, extensive studies have been performed on hexagonal columnar (Col h ) assembly of discotic LC molecules, since their π -stacked columnar arrays are expected to serve in electrical conduction. However, in a practical sense, the macroscopic alignment of such conducting columns with a desired configuration requires much effort to realize. From this point of view, discotic LCs with a bicontinuous cubic (Cub bi ) geometry might work, if their three-dimensionally interconnected networks serve as π -electronic channels. Here, we report that triphenylene (TP) derivatives (Fig. 1(a)) carrying paraffinic side chains with ionic liquid (IL) termini form an LC mesophase with a Cub bi geometry over a wide temperature range including room temperature. In 2005, we reported that 1 10 (see Fig. 1) forms an LC assembly with a Col h geometry [1]. The phase sequence upon cooling displayed, after the Col h mesophase (111–47°C), an optically isotropic phase, which however was not identified because of the lack of clear diffractions obtained in our conventional diffractometer for X-ray analysis (XRD). Recently, we have reinvestigated the phase diagram of 1 10 using a synchrotron radiation beam ( BL02B2 and BL44B2 ) and found that the optically isotropic phase is not amorphous but an LC mesophase with a Cub bi geometry (Fig. 1(b)) [2]. This unexpected observation prompted us to design 19 new TP derivatives with different paraffinic spacers and IL pendants (Fig. 1(a)). Interestingly, two of them with a long tetradecyl spacer between the TP core and IL pendants, i.e., 1 14 and 2 14 , exclusively form a Cub bi LC mesophase over a very wide temperature range (~200°C). The optically isotropic phase of 1 10 at 30°C displayed 14 distinct diffraction peaks in a small-angle region (2 θ = 1.8–5.6°, Fig. 2(a)). The reciprocal spacing ratios of these diffraction peaks (√6:√8:√14: √16:√20:√22:√24:√26:√30:√32:√38:√42:√46:√50) agreed well with those expected for the cubic symmetry with the space group Ia _ 3d . Upon heating to 87°C, these peaks disappeared completely, and instead, a set of new diffractions with d -spacings of 17.0, 14.7, and 11.2 Å appeared (Fig. 2(b)). As previously reported [1], these new peaks were indexed as the (110), (200), and (210) diffractions of a Col h mesophase (Fig. 1(b)). As summarized in Fig. 3, the phase transition behaviors of the other TP derivatives (Fig. 1(a)) on heating were likewise determined by synchrotron radiation XRD analysis. Ten TP derivatives formed a Cub bi mesophase, which in every case appeared in a lower temperature range than the Col h mesophase. In the comparison of 1 10 with 1 8 , 1 12 , and 1 14 with BF 4 – counterions, note that their phase behaviors are highly dependent on the length of the paraffinic side chains. Compound 1 8 with the shortest side chains among the family showed only a glass-to-isotropic transition (Fig. 3(a)). On the other hand, analogously to 1 10 , 1 12 with dodecyl chains exhibited a bicontinuous Ia _ 3d cubic phase, although, in a much wider temperature range (16 –160°C, on heating) than that observed for 1 10 . To our surprise, a further increase in the side-chain length from dodecyl ( 1 12 ) to tetradecyl ( 1 14 ) resulted in the exclusive formation of a Cub bi mesophase from 24 up to 221°C. To the best of our knowledge, there are no LC molecules that form a Cub bi mesophase in (a) (b) 1 n : X = H, Y= BF 4 , n = 8, 10, 12, 14 2 n : X = H, Y= PF 6 , n = 10, 12, 14 3 n : X = H, Y= Tf 2 N, n = 10, 12, 14 1 ' n : X = Me, Y= BF 4 , n = 8, 10, 12, 14 2 ' n : X = Me, Y= PF 6 , n = 10, 12, 14 3 ' n : X = Me, Y= Tf 2 N, n = 10, 12, 14 R(CH 2 ) n O R = X Me Y – R(CH 2 ) n O O(CH 2 ) n R O(CH 2 ) n R la 3 d Cub bi O(CH 2) n R O(CH 2) n R + N N – P 6 mm Col h Fig. 1. Schematic structures of (a) imidazolium ion-appended triphenylene derivatives and (b) bicontinuous cubic ( Ia _ 3d ) and hexagonal columnar ( P6mm ) liquid crystalline assemblies. 101 such an extremely wide temperature range (~200°C) from room temperature. When the counteranions were changed from BF 4 – to larger ones such as PF 6 – ( 2 ), an essentially identical side-chain dependence of the phase transition emerged, where 2 14 exclusively formed a Cub bi LC mesophase over a very wide temperature range (21–177°C, upon heating). Nonetheless, 2 10 with decyl chains, in contrast to its BF 4 – analogue ( 1 10 ), did form no LC assembly. When much larger counteranions such as bis(trifluoromethanesulfonyl)imide (Tf 2 N – ) were employed, no LC assembly was obtained from 3 12 with dodecyl side chains, and even 3 14 with the longest side chains among the family formed not only Cub bi mesophase but also gave Col h mesophase. Nevertheless, as an overall tendency, TP with larger counteranions (BF 4 – → PF 6 – → Tf 2 N – ) requires longer paraffinic side chains (decyl → dodecyl → tetradecyl) to form the Cub bi mesophase. Having this trend in mind, we also investigated the phase behaviors of 1’– 3’ with bulkier imidazolium ion pendants than 1– 3 . As shown in Fig. 3(b), only four TP derivatives showed, though not exclusively, the Cub bi LC mesophase. With respect to the effects of paraffinic side chains and counteranions, the trends observed for 1’– 3’ appear to be similar to those for 1 – 3 . Possibly owing to the larger IL pendants in 1’– 3’ , the formation of the Cub bi LC assembly required much longer paraffinic side chains than that in the case of 1– 3 . Typically, 1’ 10 and 2’ 12 , in contrast to 1 10 and 2 12 , showed no Cub bi mesophase, only a Col h mesophase, between the glassy and melt states. Overall, both Col h and Cub bi LC mesophases are most likely composed of π -stacked TP columns. We consider that (1) the π -stacking of the discotic core and (2) the interionic interaction of IL pendants are two major parameters for determining the phase diagram. When the paraffinic chains are sufficiently long, LC mesophases result, where the core and IL pendants can undergo π -stacking and interionic interactions, respectively. Meanwhile, when the paraffinic chains are rather short, LC mesophases hardly emerge unless the IL pendants are sufficiently small, since the columnar π -stacking of the discotic core is sterically disturbed. Semiconducting Cub bi LC materials, unveiled in the present work, are quite attractive for many applications. 7 6 5 4 3 2 20 15 10 5 0 (a) (b) Iso Col h Cub bi 2 θ θ (degree) Intensity (arb. units) Intensity (arb. units) × 3 × 3 × 3 × 3 30°C 67°C 87°C 117°C 2 θ θ (degree) 211 321 400 332 422 220 431 521 541 631 543 440 532, 611 420 (a) (b) –50 0 50 100 150 200 250 (°C) (°C) 1 8 1 10 2 10 2 12 2 14 3 10 3 12 3 14 1 12 1 14 –50 0 50 100 150 200 250 1’ 8 1’ 10 2 ’ 10 2 ’ 12 2 ’ 14 3 ’ 10 3 ’ 12 3 ’ 14 1’ 12 1’ 14 G Cub bi Col h Iso Fig. 2. (a) XRD pattern of 1 10 at 30°C. The numbers indicate the Miller indices. (b) Temperature-dependent XRD patterns of 1 10 on heating in 2 θ ranges of 0–20°. The wavelength of the X-ray was set at 1.08 Å. Fig. 3. Phase transition behaviors of imidazolium ion- appended triphenylene derivatives upon heating. G: glass, Cub bi : bicontinuous cubic, Col h : hexagonal columnar, and Iso: isotropic liquid. Compound 1’ 14 showed a Pn _ 3m cubic-to- Ia _ 3d cubic transition at 129°C. Yohei Yamamoto Graduate School of Pure and Applied Sciences, University of Tsukuba E-mail: yamamoto@ims.tsukuba.ac.jp References [1] J. Motoyanagi et al. : Chem. Commun. (2005) 101. [2] M.A. Alam, J. Motoyanagi, Y. Yamamoto, T. Fukushima, J. Kim, K. Kato, M. Takata, A. Saeki, S. Seki, S. Tagawa, T. Aida: J. Am. Chem. Soc. 131 (2009) 17722.
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