A Modulated Helical Nanofilament Phase

Abstract

The modern study of the liquid crystal (LC) phases formed by bent-core molecules has led to many interesting and unexpected phenomena, including the first example of spontaneous reflection symmetry breaking and conglomerate formation in a bulk fluid phase. Indeed, exploration of the unique behavior of bent-core materials is currently one of the most productive frontier research areas in soft matter science. Perhaps the most complex of the known bent-core phases, the helical nanofilament (HNF) phase (also known as the B4 banana phase) has been under serious investigation since 1997, and continues to be a focus of interest in the bent-core materials constellation. Here, we report full characterization of the first example of a new phase in this family, HNF(mod), composed of simple alkoxybiphenylcarboxylate units and lacking the Schiff base groups found in previously known HNF mesogens. The classic HNF phase possesses a unique hierarchical nanostructure: An assembly of twisted layers stacked to form well-defined chiral nanorods (individual HNFs ca. 40 nm diameter), with a structure driven by intra-layer frustration leading to spontaneous saddle splay, and formation of layers with negative curvature. Solid state NMR data suggest that within individual HNF layers the structure is crystalline, though electron diffraction shows that no interlayer positional correlation exists. The HNFs in turn form a kind of hexatic LC phase, which freezes into a glassy state at ca. 110 8C.When the mesogens are achiral or racemic, an LC conglomerate of large heterochiral domains is easily seen in LC cells by polarized optical microscopy. The bulk HNF phase is porous, and when grown in the presence of other materials, can produce nanostructured composites. Potential applications of the HNF phase and composites include nonlinear optics, organic electronics, photovoltaics, and chiral separations. Interestingly, of the great many bent-core structures reported to date, with the exception of a single reported outlier all HNF mesogens include the hydrolytically unstable benzylideneaniline (Schiff base) moiety. This is problematic for some potential applications. Also, the rarity of the known HNF phases in the bent-core “structure space” motivates discovery of new HNF phases in order to better understand the molecular structural factors leading to their formation, and to allow future design of functional HNF materials. Here we report characterization of seven homologues of biphenyl-3,4’-diyl bis-(4’-alkoxybiphenyl-4-carboxylate), (diesters 1(n) (n= 9–15), Figure 1a). All of these exhibit a new HNF phase possessing in-layer modulation in addition to the characteristic negative curvature of the layers. Bent-core phases with in-layer modulation are known, including for example a tilted polar smectic (SmCP) phase possessing undulated smectic layers (also known as the B7 banana phase). Here, we designate the new modulated HNF phase HNF(mod), with the proposed hierarchical structure for individual HNFs illustrated in Figure 1. As for the classic HNF phase, we propose that the aromatic cores in the new phase are tilted, while the tails are extended almost normal to the local layer plane. However, for the HNF(mod), in-layer modulation with a spacing of about 40 , not seen in the HNF phase, is suggested by experimental data. We propose this secondary modulation results from a structural periodicity with wave vector parallel to the layers and normal to the polar axis (P), as indicated in Figure 1c. The precise nature of this periodic defect structure is not known, though the lack of two-fold symmetry for rotation about b in diesters 1(n) could reasonably produce such a periodic structural change by simple 1808 rotation about b, mediating in-layer “stripes” about eight molecules wide, as indicated (Figure 1c). Finally, as for the HNF phase, the twisted layers stack to form helical nanofilaments (Figure 1d). Evidence for this structural picture of the HNF(mod) phase formed by diesters 1(n) derives from: 1) polarized optical microscopy (POM); 2) differential scanning calorimetry (DSC); 3) X-ray diffraction (XRD) at small angle; and 4) transmission electron microscopy (TEM). The phase assignments, transition temperatures and enthalpies on cooling, and HNF(mod) layer spacing (d1) and in-layer modulation dimensions (d2) for diesters 1(n), are given in Table 1. A brief discussion of the key observations and interpretations leading to the model illustrated in Figure 1 follows. As indicated in Table 1, all of the homologues are very similar in their basic behavior. All show a transition from isotropic to a more conventional bent-core phase, tentatively assigned as either a B1 (columnar) phase, or a B2 (tilted smectic) phase. For the lower homologues, (n= 9–12) the high temperature phases are monotropic with respect to HNF(mod). [*] Dr. E. Tsai, Dr. J. M. Richardson, Dr. E. Korblova, Prof. Dr. D. M. Walba Department of Chemistry and Biochemistry University of Colorado, 215 UCB Boulder, CO 80309-0215 (USA) E-mail: walba@colorado.edu