CHIRALITY SYNCHRONISATION IN LIQUID CRYSTALLINE HELICAL NETWORKS OF π-CONJUGATED RODS

A robust design concept of liquid crystalline phases with cubic symmetry has been developed in which helical networks of π-conjugated rod-like molecules form two ranges of achiral double network phases, a long pitch and a short pitch, with a spontaneous mirror symmetry broken triple network phase between them.

COMPLEX SYSTEMS AND BIOMEDICAL SCIENCES

76 ESRF

High-spatial-resolution three-dimensional imaging of human spinal cord and column anatomy with postmortem X-ray phase- contrast micro-CT, G.E. Barbone (a,b), A. Bravin (c), A. Mittone (c), S. Grosu (b), J. Ricke (b), G. Cavaletti (d,e), V. Djonov (f) and P. Coan (a,b), Radiology (2020);

https://doi.org/10.1148/ radiol.2020201622. (a) Faculty of Physics, Ludwig-Maximilians- Universität München (Germany) (b) Department of Radiology, University Hospital, Ludwig-Maximilians-Universität München (Germany)

(c) ESRF (d) School of Medicine and Surgery, University of Milano-Bicocca (Italy) (e) Milan Center for Neuroscience, University of Milano-Bicocca (Italy) (f) Institute of Anatomy, University of Bern (Switzerland)

[1] J.D. Van Horn & A.W. Toga, Brain Imaging Behav. 8(2), 323-331 (2014). [2] O.J. Arthurs et al., Clin. Radiol. 70(8), 872-880 (2015). [3] M. Kampschulte et al., RöFo 188(2), 146-154 (2016). [4] F. Arfelli et al., Radiology 215(1), 286-293 (2000). [5] H. Hetterich et al., Radiology 271(3), 870-878 (2014). [6] A. Horng et al., Invest. Radiol. 49(9), 627-634 (2014). [7] A. Khimchenko et al., Adv. Sci. (Weinh) 5(6), 1700694 (2018). [8] G.E. Barbone et al., J. Neurosci. Methods 339, 108744 (2020). [9] M. Fratini et al., Sci. Rep. 5(1), 8514 (2015).

PRINCIPAL PUBLICATION AND AUTHORS

REFERENCES

This proof-of-concept study demonstrates that post-mortem X-ray phase-contrast micro-CT provides unique knowledge of vertebral bone, soft tissue, and vasculature in 3D at high spatial resolution within a single CT image. This makes

it attractive for multiscale tissue-preserving anatomic imaging work in human spinal cord and column specimens and provides an alternative or complementary method to traditional histologic or virtual 3D imaging techniques.

Liquid crystalline phases formed by self- assembled networks are receiving growing attention due to their capacity for charge transport in all three spatial directions and their potential use in photonics. However, a robust general design concept for these bicontinuous cubic (Cubbi) phases, especially for those involving π-conjugated rods, is still missing [1].

Such a concept has been developed for 5 , 5 - d i p h e n y l - 2 , 2 - b i t h i o p h e n e - b a s e d molecules, with one end containing three out-fanning flexible chains, and a range of substituents at the other end, the apex (Figure 62a). The cubic phases formed by these compounds are stable over broad temperature ranges, often down to ambient temperature, and tolerate a wide range of structurally diverse apex substituents, thus allowing tailoring of the electronic properties (Figure 62a). With an increasing number and size of apex substituents, a sequence of three different liquid crystalline (LC) network structures with cubic symmetry is observed. Two representative electron density maps, reconstructed from recorded high- resolution diffraction patterns at beamline BM28, are shown in Figures 62b and 62e.

Based on these, and complemented with optical, chiroptical and calorimetric investigations, the self-assembled structures of the Cubbi phases of the investigated compounds were uncovered. In the Cubbi phases, there are two or three continuous networks separated by a continuum of fluid pendant alkyl chains (Figures 62c,f). The π-conjugated rods are organised side-by-side in the networks and, on average, lie almost parallel to each other and perpendicular or slightly tilted to the network segments [2]. However, the steric repulsion between the end groups requires a slight twist between the molecular rods along the networks. Due to the interconnected network structure, the twist is synchronised throughout the entire network, leading to twisted networks with uniform helical sense as shown in Figures 62d and 62g [2-4]. In the double gyroid with Ia 3d space group, there are two networks in which the helical sense is opposite (Figures 62b-d), and therefore it is optically inactive. By contrast, the I23 phase is composed of three networks [4] (Figures 62e-g), where the chirality cannot cancel out, and therefore this Cubbi phase is optically active, having a unichiral structure with macroscopic chirality either as a conglomerate of left and right-handed domains