A General Method for the Enantioselective Formation of Helical Nanofilaments

Abstract

Chirality has been an important subject in chemistry since Pasteur demonstrated the existence of molecules that were mirror images of each other. [1] Indeed, chirality has proven to be important in the fields of biology and pharmacy as well. [2] In the past, single enantiomers have been made by using asymmetric catalysis [3,4] and enantiomeric separation. [5] However, almost all methods rely on the use of chiral molecules. Recently, the development of asymmetric reactions that do not use chiral molecules is attracting lot of attention. In this respect, bent-core liquid crystals (BCLCs) [6][7][8] are very interesting because layer chirality occurs in nonchiral molecular systems as a result of the tilt of polar-ordered molecules from the normal of the smectic layer, [9] and is coupled with conformational molecular chirality. [10] Moreover spontaneous deracemization to give large chiral domains occurs particularly in the helical nanofilament (HNF) B4 phase because of negative splay elastic constants. [7][8][9][10][11][12] Each macroscopic chiral domain consisting of HNFs exhibits high optical activity. [11,[13][14][15] The formation of a helical structure in the B4 phase suggests that the chirality can be controlled, because in many other organic systems the handedness of the helical structure has been controlled by external chiral sources, such as circularly polarized light, [16,17] vortex flow, [18][19][20] and helicity memory induced by chiral molecules. [21] Previously, we used five different methods to control the chirality of the B4 phase and successfully obtained almost 100 % ee with some of them. These methods are: 1) the introduction of chiral analogues, [22] 2) the use of a chiral surface, [23] 3) irradiation of photoisomerizable molecules with circularly polarized ultraviolet (UV) light, [24] 4) the use of twisted nematic (TN) director orientation, [25] and 5) growth from a chiral phospholipid layer. [26] Methods 2 and 5 both involve the use of a chiral surface, however, with the former only 10 % ee was attained, whereas with the latter almost 100 % ee was attained. The difference originates from the absence (method 2) and presence (method 5) of a temperature gradient. Contrary to the use of chiral dopants in method 1 and chiral surfaces in methods 2 and 5, methods 3 and 4 do not require any chiral species and have the added advantage that they work on bulk samples. However, the phase sequences of the compounds used for methods 3 and 4 are restricted by necessary phase sequences, that is, a fluid smectic phase (method 3) and a nematic (N) phase (method 4) that occur at higher temperatures to the B4 phase. Namely, a gradual phase transition from fluid smectic phases to the B4 phase under circularly polarized UV irradiation triggers repeated photoisomerization, whereas UV irradiation in the B4 phase is not an effective method to control chirality. Only one example of method 4 is known [25]

Supramolecular Chirality in Self-Assembled Systems

“…The mixture of different molecules has the phase sequence N−Bx(B4/N), and homochiral helical nanofilaments can be obtained upon cooling ( Figure 76). 261 To understand the helical nanofilament phases assembled by banana-shaped (bent-core) achiral molecules, the corresponding hierarchical nanostructures have attracted much attention. It was found that the arrangement of different helical nanofilaments can be very important to the assembly process.…”

Chiral Superstructure Mesophases of Achiral Bent‐Shaped Molecules – Hierarchical Chirality Amplification and Physical Properties

“…d) The linear relationship of the CD intensity and the cell thickness in the range of 0.4–0.9 μm, as shown in the inset (reproduced from the original experimental data, since the cell gaps used in Figure c of ref. were erroneous). e) AFM (contact mode) image of HNFs made from a +60° TN cell at room temperature after removing the ZLI‐2293 component by n ‐hexane.…”

“…e) AFM (contact mode) image of HNFs made from a +60° TN cell at room temperature after removing the ZLI‐2293 component by n ‐hexane. Reproduced with permission . Copyright 2013, Wiley‐VCH.…”

Chirality Transfer and Modulation in LB Films Derived From the Diacetylene/Melamine Hydrogen‐Bonded Complex

“…The variation of the CD signal at all rotation angles was marginal compared with the intense CD signal corresponding to PDA chains (Fig. S6a), confirming the chirality origin of the CD signal, not the linear dichroism (LD) effect . Further, the CD signals of the peripheral PDA backbone seemed to be dependent on the helical packing of the melamine cores.…”

“…S6a), confirming the chirality origin of the CD signal, not the linear dichroism (LD) effect. 35 Further, the CD signals of the peripheral PDA backbone seemed to be dependent on the helical packing of the melamine cores. As shown in Figure S6b, the films with a positive CD signal at 245 nm and a negative CD signal at 270 nm for the melamine unit always exhibited the positive CD signal at about 560 nm and the negative CD signal at 660 nm for PDA backbone, whereas the films with a negative CD signal at 245 nm and a positive CD signal at 270 nm for the melamine unit always exhibited the negative CD signal at about 560 nm and the positive CD signal at 660 nm for PDA backbone.…”