Controlling the soft self-assembly of 1,3,4-oxadiazoles by carbosilane end-groups

https://doi.org/10.1016/j.molliq.2019.111362Get rights and content

Highlights

  • A new oxadiazole based building block for bent-core liquid crystals is introduced.

  • Ferroelectric switching in a silylated achiral 1,3,4-oxadiazole based LC.

  • Inversion one COO group changes ferroelectricity to para- and antiferroelectricity.

  • Silyl unit removal extinguishes polar order but results in dark-conglomerate phases.

  • First observation of a polar response in a Col phase formed by an oxadiazole LC.

Abstract

Herein we report the soft self-assembly of 1,3,4-oxadiazoles with carbosilane end groups in a series of polar and mirror-symmetry broken liquid crystalline (LC) phases. Most compounds are based on the new 4-[4-(5-hydroxyphenyl)-1,3,4-oxadiazol-2-yl)]benzoate building block and involve carbosilane end-groups. Compounds combining one silylated and one alkylated end form a sequence of lamellar, modulated lamellar and columnar phases with developing polar order and even leading to soft ferroelectrics. Silyl groups at both ends gives rise to a modulated lamellar phase with temperature induced transition to a hexagonal columnar phase, with the first observation of a polar switching in a columnar phase based on the oxadiazole heterocycle. Spontaneous mirror symmetry breaking (“dark conglomerate”) is observed after removal of the silyl group(s). The tilt correlation mode of the lamellar phases is controlled by combining a cyano-substituted with a silylated end. Overall, this work contributes to the understanding of the soft self-assembly of oxadiazole based and silylated bent-core mesogens, presenting new properties and potentiality for the oxadiazole materials. Also, it provides general guidelines for the targeted molecular design of functional soft matter with tailored morphologies and properties.

Introduction

Liquid crystals (LCs) represent a fascinating branch of soft condensed matter, characterized by the intrinsic combination of fluidity and long range order as required for numerous functions and applications [[1], [2], [3], [4]]. The fundamental understanding of the self-assembly of these fluid materials is of significant importance for the development of new materials and applications. Among the molecular building blocks, the 1,3,4-oxadiazole heterocycle received considerable attention since the first reports of their LC behaviour [[5], [6], [7], [8]], specially due their ease and versatility of synthesis, chemical stability and strong luminescence [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]]. Simultaneously, due their 134° bent structure, a significant interest arose as building block for bent-core mesogens [[26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45]] and in the search for the elusive thermotropic biaxial nematic phases (Nb) [[26], [27], [28], [29], [30], [31], [32], [33], [34],46]. Moreover, there are indications of polar order and spontaneous mirror symmetry breaking which have not yet been fully explored and understood [[47], [48], [49], [50], [51], [52], [53], [54], [55]]. However, despite their attractive features, these 1,3,4-oxadiazole derived bent-core compounds have significant drawbacks, like high melting points and having their LC ranges at relatively high temperatures, which need to be circumvented through molecular design [15,25,26,30,56,57].

Oligosiloxanes and related carbosilanes represent building blocks capable of affecting the LC self-assembly due to the formation of nano-segregated carbosilane/siloxane sublayers [[58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74]], receiving also contemporary interest for improving the performance of organic electronic materials [75,76], and as materials for nanolithographic applications [77]. However, there were no reports about silylated oxadiazole based bent-core mesogens, except few compounds with lateral siloxane groups [78,79]. Only in our recent work we reported first examples of bent-core mesogens with a silyl groups at one end of a 1,3,4-oxadiazole-4,4′-biphenol based bent-core unit [80].

Herein, we introduce a new bent building block involving a 1,3,4-oxadiazole unit, the 4-[4-(5-hydroxyphenyl)-1,3,4-oxadiazol-2-yl]benzoate moiety, and combine it with silyl groups at different ends. Thus, we explore the effects of position of silylation and the directions of the involved COO groups (compounds 2Si312 and 3Si312) on soft self-assembly in LC phases. The effect of insertion of two silyl end-groups (1Si3Si3) and replacement of the alkyl chain by a cyano group (1Si3CN) will be reported as well. The compounds explored in this work are displayed in Scheme 1. In 3.1 Inversion of the COO group in the alkyl terminated wing (, 3.2 Inversion of the COO group in the carbosilane terminated end ( it will be shown that inversion of the COO group direction leads to ferroelectric switching. That removal of the silyl group removes polar order and induces mirror symmetry breaking in dark conglomerate phases is described in Section 3.3, whereas Section 3.4 shows that the replacement of the alkyl chain by CN removes the synclinic tilt. The observations of a polar switching in a columnar phase formed by molecules with oxadiazole core is reported in Section 3.5 for a compound with two silyl groups.

Section snippets

Synthesis

Since all compounds have structural differences like ester group direction and alkyl chain type (length, presence or absence of carbosilane), every target compound had a different synthetic route. Compound 2Si312 was prepared by a multistep route, as outlined in Scheme 2. A similar procedure was employed for the non-silylated compound 2C6 with two hexyl chain, but using 4-hexyloxy benzoic acid for the final acylation step. On the other hand, due the inversion of both ester groups and

Investigation of liquid crystal self-assembly

The synthesized bent-shaped molecules were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM), small and wide angle X-ray scattering (SAXS, WAXS) and switching experiments under an triangular wave field. The used investigation methods are described in the ESI and the thermal data for each compound are summarized in Table 1.

The thermal stability of the products (Tdec) was determined by TGA measurements as the very beginning of weight loss under N2

Conclusions

1,3,4-Oxadiazole based LC compounds with carbosilane end groups form a wide variety of different LC phases, involving lamellar, columnar and sponge-like isotropic phases. Inversion of the COO group direction modifies the correlation length of polar order and the degree of tilt in the smectic phases. All compounds with only one silyl group self-assemble into a series of synclinic tilted smectic phases (SmCs) with a paraelectric range (SmCsPR) at high temperature, followed by a superparaelectric

Experimental section

The materials synthesis, the used analytical methods and the protocols of the characterization of the materials properties can be found in the ESI.

Declaration of Competing Interest

There are no conflicts to declare.

Acknowledgments

The authors thank the following institutions for financial support: DAAD, Germany; PRONEX, Brazil; CNPq, Brazil; FAPESC, Brazil and INCT-Catálise, Brazil (444061/2018-5) and the Deutsche Forschungsgemeinschaft, Germany (Grand TS 39/24-2). E. W. is also grateful to CNPq/DAAD for the support during the “PhD Sandwich” exchange program realized at the Martin-Luther-Universität Halle-Wittenberg in Germany. The authors would also like to thank the Brazilian Synchrotron Light Laboratory (LNLS) due to

References (113)

  • B.G. Kim et al.

    Tetrahedron Lett.

    (2001)
  • Y. Xiao et al.

    J. Mol. Liq.

    (2017)
  • V.S. Sharma et al.

    J. Mol. Liq.

    (2018)
  • J.W. Goodby et al.

    Handbook of Liquid Crystals

    (2014)
  • C. Tschierske

    Angew. Chem. Int. Ed.

    (2013)
  • T. Kato et al.

    Nat. Rev. Mater.

    (2017)
  • K. Dimitrowa et al.

    J. Prakt. Chem.

    (1980)
  • N.K. Chudgar et al.

    Mol. Cryst. Liq. Cryst.

    (1989)
  • D. Girdziunaite et al.

    Liq. Cryst.

    (1991)
  • A. Hetzheim et al.

    Liq. Cryst.

    (1999)
  • J. Han

    J. Mater. Chem. C

    (2013)
  • E. Westphal et al.

    J. Mater. Chem. C

    (2013)
  • H. Gallardo et al.

    Curr. Org. Synth.

    (2015)
  • M. Parra et al.

    Liq. Cryst.

    (2002)
  • A. Paun et al.

    J. Mater. Chem. C

    (2016)
  • M. Parra et al.

    Liq. Cryst.

    (2006)
  • M. Ferreira et al.

    New J. Chem.

    (2017)
  • M. Mitani et al.

    J. Mater. Chem. C

    (2017)
  • S.K. Pathak et al.

    J. Mater. Chem. C

    (2015)
  • J. Tang et al.

    RSC Adv.

    (2012)
  • S. Nath et al.

    Mol. Syst. Des. Eng.

    (2017)
  • C.K. Lai et al.

    Liq. Cryst.

    (2002)
  • C.F. He et al.

    Liq. Cryst.

    (2007)
  • S. Qu et al.

    Chem. Eur. J.

    (2011)
  • S. Varghese et al.

    Adv. Funct. Mater.

    (2009)
  • T.J. Dingemans et al.

    Liq. Cryst.

    (2000)
  • L.A. Madsen et al.

    Phys. Rev. Lett.

    (2004)
  • B.R. Acharya et al.

    Phys. Rev. Lett.

    (2004)
  • O. Francescangeli et al.

    Soft Matter

    (2010)
  • F. Speetjens et al.

    J. Mater. Chem.

    (2012)
  • Y.-K. Kim et al.

    Phys. Rev. E

    (2016)
  • C.D. Southern et al.

    Europhys. Lett.

    (2008)
  • M. Lehmann et al.

    Chem. Commun.

    (2008)
  • F. Vita et al.

    Chem. Mater.

    (2014)
  • T. Niori et al.

    J. Mater. Chem.

    (1996)
  • R.A. Reddy et al.

    J. Mater. Chem.

    (2006)
  • H. Takezoe et al.

    Jpn. J. Appl. Phys.

    (2006)
  • A. Eremin et al.

    Soft Matter

    (2013)
  • D.R. Link et al.

    Science

    (1997)
  • H. Takezoe et al.

    Bent-Shaped Liquid Crystals. Structures and Physical Properties

    (2017)
  • H. Wang et al.

    Liq. Cryst.

    (2012)
  • I.H. Chiang et al.

    ACS Appl. Mater. Interfaces

    (2014)
  • H.F. Gleeson et al.

    ChemPhysChem

    (2014)
  • S. Kaur et al.

    J. Mater. Chem. C

    (2013)
  • S. Srigengan et al.

    J. Mater. Chem. C

    (2018)
  • C. Tschierske et al.

    J. Mater. Chem.

    (2010)
  • H.K. Bisoyi et al.

    Liq. Cryst.

    (2019)
  • H. Wang et al.

    Liq. Cryst.

    (2018)
  • V. Görtz et al.

    Soft Matter

    (2009)
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