Jan
Balszuweit‡
a,
Meik
Blanke‡
a,
Marco
Saccone
ab,
Markus
Mezger
cd,
Constantin G.
Daniliuc
e,
Christoph
Wölper
f,
Michael
Giese
*a and
Jens
Voskuhl
*a
aFaculty of Chemistry (Organic Chemistry) and CENIDE, University of Duisburg Essen, Universitätsstraße 7, Essen 45141, Germany. E-mail: michael.giese@uni-due.de; jens.voskuhl@uni-due.de
bDipartimento di Ingegneria, Università degli Studi di Palermo, Viale delle Scienze 6, 90128 Palermo, Italy
cFaculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria
dMax Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
eOrganisch-Chemisches Institut, Westfälische Wilhelms Universität Münster, Corrensstraße 40, 48149 Münster, Germany
fFaculty of Chemistry (Inorganic Chemistry) and CENIDE, University of Duisburg-Essen, Universitätsstrasse 7, 45117 Essen, Germany
First published on 24th March 2021
A modular supramolecular approach towards hydrogen-bonded liquid crystalline assemblies based on naturally occurring polyphenols is reported. The combination of experimental observations, crystallographic studies and semi-empirical analyses of the assemblies provides insight into the structure–property relationships of these materials. Here a direct correlation of the number of donor OH-groups as well as their orientation with the mesomorphic behavior is reported. We discovered that the number and orientation of the OH-groups have a stronger influence on the mesomorphic behavior of the supramolecular assemblies than the connectivity (e.g. stilbenoid or chalconoid) of the hydrogen bond donors. Furthermore, the photo-switching behavior of selected complexes containing azopyridine ligands was investigated. This study will help future scientists to gain a deeper understanding of the underlying mechanisms and structure–property relationships of supramolecular assemblies with mesomorphic behavior, which is still one of the major challenges in current science.
Design, System, ApplicationWe investigated hydrogen-bonded assemblies (HBAs), consisting of naturally occurring polyphenols (HB donors) and alkylated stilbazoles (St) and azopyridines (Ap) (HB acceptors), concerning their liquid crystalline properties. To this end five different phenols, namely butein, isoliquiritigenin, resveratrol, piceatannol and oxyresveratrol, were assembled with St or Ap in all possible ratios yielding 36 HBAs, which were analyzed in detail using infrared spectroscopy, polarized optical microscopy, differential scanning calorimetry and small and wide angle X-ray scattering. We discovered mainly nematic phases for the complexes, whereas it is noteworthy that only the 1:3 complexes melted homogeneously leading to the assumption that this stoichiometry is favored for all complexes, which can be attributed to the substitution pattern of the phenols. Only oxyresveratrol reveals smectic behavior as a 1:3 complex and a nematic mesophase as a 1:4 complex. Additionally we discovered that the connectivity, chalconoid or stilbenoid, is less important for the mesomorphism than the substitution pattern of the phenols. This in-depth structure–property relationship of naturally occurring polyphenols will be appealing to a broader readership, since prediction of mesomorphic behavior is still challenging. We are sure that our study will help to gain a deeper understanding of the underlying principles in HBAs with liquid crystalline properties. |
Although a huge number of such systems have been reported, the complexity of the interplay of non-covalent interactions as well as their careful orchestration is still challenging and remains unpredictable over wide areas. Hence, stepwise variation of the structures remains the only way to gain deeper insights into the structure–property relationship of supramolecular liquid crystalline assemblies.
Recently Giese and co-workers established a modular approach towards hydrogen-bonded liquid crystalline materials with photo-responsive behavior by introducing alkylated azopyridine (Ap) or stilbazole (St) units as hydrogen acceptors and aromatic phenols such as phloroglucinol, resorcinol, hydroquinone and resveratrol (Res) as hydrogen donors.9 The latter, furthermore, responds to UV-light irradiation (300 nm) by undergoing photo-cyclisation to resveratrone, leading to a change in the mesophase.10 In this contribution, we expanded our scope to four additional, naturally occurring stilbenoids and chalconoids namely oxyresveratrol (Oxy), piceatannol (Pic), butein (But) and isoliquiritigenin (Iso) as hydrogen bond donors in combination with octyl-chain containing azopyridine (Ap8) and stilbazole (St8) as hydrogen bond acceptors (Fig. 1). Herein we report a systematic study on the structure–property relationship of hydrogen-bonded liquid crystals based on stilbenoids and chalconoids. Thereby, we focus on four aspects affecting the mesomorphic behavior: the role of the ratio of the hydrogen bond donor and acceptor, the influence of the hydrogen bond accepting moiety, the impact of the number and relative position of the OH-groups in the hydrogen bond donors and the role of the linking group of the hydrogen bond donating moiety.
Hydrogen-bonded mesogens were obtained by simple mixing of the acceptors and the donors, yielding a library of hydrogen-bonded assemblies, which were analyzed with respect to their mesomorphic behavior. Furthermore, computational analyses, small- and wide-angle X-ray scattering (SAXS, WAXS) and single X-ray diffraction analyses were performed to support the obtained findings for the mesomorphism.
Fig. 2 Summary of all hydrogen-bonded assemblies in the present study. Given are the transitions and transition temperatures as determined by POM with a heating and cooling rate of 10 °C min−1 (temperatures are given in °C). Transitions marked with * are monotropic and occur only on cooling. The scale bar represents 100 μm. The data of Res(St8)3 and Res(Ap8)3 have been reported before and are shown for comparative reasons.10 |
Fig. 3 Summary of the LC properties of the 1:3 complexes reported in the present study, measured on cooling with a rate of 10 °C min−1 under a POM. Clearing points are reported and complexes marked with * undergo a transition into a glassy state, preserving the texture of the liquid crystalline phase (temperatures are given in °C). The data of Res(St8)3 and Res(Ap8)3 have been reported before and are shown for comparative reasons.10 |
Comparing the Pic(Ap8)3 assembly with the corresponding Res(Ap8)3 assembly reveals a monotropic and very narrow temperature range of 2 °C of the nematic phase with simultaneous crystallization. Also in the case of the Pic(St8)3 assembly a drastic decrease in the temperature range of the mesophase was observed. In both cases the narrow temperature range of the mesophase is related to a significantly increased crystallization temperature. These findings were attributed to the fact that one vicinal OH group remains free and thus, does not bind to the hydrogen bond acceptor. However, in the densely packed crystalline phase, this free OH group interacts with neighbouring assemblies promoting the crystallization of the assemblies.13 This assumption is supported by single crystal X-ray diffraction analyses. For all donor:acceptor ratios (e.g. 1:3 and 1:4) only suitable crystals of the 1:3 complexes were obtained (see description of the crystal structures subchapter, vide infra). Interestingly, only Oxy(St8)3 shows a transition from the nematic (ΔTN = 23 °C) into a smectic phase (ΔTSmA = 48 °C). Upon cooling of the Oxy(St8)3 complex, first the “Schlieren” texture is observed, which undergoes a transition into a pseudo focal-conic texture (Fig. 2 and 3 and ESI† Fig. S9). The transition from the nematic to a smectic phase was confirmed by small angle X-ray scattering (SAXS) (Fig. 4 and S64 and S65†). Analysis of the radial averaged scattering patterns on the smectic mesophase reveals a periodicity of 4.3 nm and 0.45 nm for the long and short axis, respectively. The temperature range, in which the liquid-crystalline phase appears, is also significantly higher for Oxy (ΔT = 72 °C for St8) than for Pic (Fig. 2 and 3), which we attribute to the missing interference of intra- versus intermolecular hydrogen bonding for the Oxy-based assemblies. The 1:4 complexes of Oxy with Ap and St revealed only nematic behavior. To sum up, all three St8 complexes reveal mesomorphism over a broad temperature range revealing nematic (Res and Pic) or smectic (Oxy) phases.
Fig. 4 SAXS patterns of Oxy(St8)3 in a smectic (A), nematic (B) and isotropic (C) phase. The nematic phase (B) was oriented by a horizontal magnetic field B (arrow). |
Besides stilbenoids, the liquid crystalline properties of the chalconoid HBAs of But and Iso were investigated, showing the characteristic “Schlieren” texture of a nematic phase for all assemblies. But and Iso reveal the same substitution pattern of the hydroxyl-groups like Pic and Res respectively, but differ in the linkage between the two arene moieties featuring a carbonyl group (chalcone backbone). It is noticeable that Iso(St8)3 forms an enantiotropic mesophase with a broad temperature range upon cooling (ΔTN = 93 °C), whereas But(St8)3 shows monotropic phase behavior with the formation of a glassy state preserving the nematic mesophase (Fig. 3 and ESI† Fig. S21–S27). The additional free vicinal hydroxy-group of But seems to broaden the temperature range of the liquid crystalline state in the St-complex and thus prevents the crystallization on cooling. However, if the Ap complexes of the two compounds are considered, the Iso-complex has a much broader temperature range of the nematic phase as its substitution pattern is comparable with that of Res, featuring a single hydroxy-group para-oriented to the double bond bridge.
Fig. 5 POM images showing the photo-switchability of the nematic phase of A) Pic(Ap8)3 at 105 °C and B) Iso(Ap8)3 at 110 °C with a wavelength of 405 nm. |
This photo-switching is reversible and after removal of the UV light the “Schlieren” texture reappears within less than a second, which was reproduced for several cycles. A similar behavior was observed for Pic(Ap8)3 (Fig. 5) showing that this photo-isomerization is independent from the connectivity of the HB donors using either stilbenoids or chalconoids.
Fig. 6 A) X-ray structure of the 3:1 Pic(Ap8)3 assembly; B) space filling model of the packing in side view and C) in top view. Colored in red: azopyridine (Ap8), colored in blue: piceatannol (Pic). |
Pic was found to be disordered over two positions in the asymmetric unit. In the packing diagram a linear chain along the c-axis involving auxiliary hydrogen bond interactions between two adjacent piceatannol molecules (OH⋯O = 1.950 Å) and π⋯π interactions between pyridine–pyridine rings (3.348 Å; 3.352 Å) and phenyl–phenyl rings (3.375 Å, 3.388 Å) was formed (Fig. 6C). The 3D network is furthermore supported by numerous additional CH⋯π and π⋯π interactions. Replacing the azopyridine acceptor unit with St8 yielded the related solid state structure of Pic(St8)3. The three hydrogen bonds between the hydroxy-groups of the piceatannol and the pyridine nitrogen atom of stibazole-C8 are OH⋯Npy = 1.769, 1.811, and 1.831 Å (O⋯Npy = 2.691, 2.758 and 2.696 Å, Fig. 7A), respectively. These hydrogen bonds are slightly shorter compared to those found in the complex Pic(Ap8)3. In contrast to the linear chain found in the packing diagram of Pic(Ap8)3, which presents an alternating orientation of the Pic moiety, the solid state packing of Pic(St8)3 shows a parallel orientation of the Pic molecules, which are shifted with respect to each other (see Fig. 7C). Only OH⋯O (1.903 Å) and two types of CH⋯π interactions between pyridine–pyridine rings (2.898 Å) and pyridine–phenyl rings (2.826 Å) are involved in the formation of these chains (Fig. 7C). Additional CH⋯π and π⋯π interactions complete the 3D network. The parallel orientation of Pic(St8)3 is obviously beneficial for the formation of mesophases with a larger temperature range (see Fig. 2), whereas the linear orientation of Pic(Ap8)3 promotes faster crystallization.
The geometrical description of the crystal structures used above, in which we identified short intermolecular contacts and described them as contributors to the overall crystal packing, is common and easy to understand. However, this description gives only an incomplete picture of the factors that govern the crystal packing and it is of little help to quantify the intermolecular interactions in our structures. We thus used a methodology which is based on the intermolecular perturbation theory developed by Spackman,16 which is in turn based on the work of Gavezzotti,17 and implemented in the CrystalExplorer program.18 This methodology allows for direct and accurate quantification of the intermolecular interactions and recently provided us insight into the aggregation-induced-emission (AIE) behavior of a series of liquid crystalline aromatic thioethers,19 and explanation of subtle differences in the LC behavior of supramolecular liquid crystals based on natural products. Details on the methodology are given in the ESI.† In order to analyze the non-covalent interactions in the solid state a reference molecule in the crystal structure was defined (see Fig. 8 and 9 for details) and molecular pairs (structure determinants) with surrounding molecules were investigated with respect to their intermolecular interactions. In this respect, we selected the six most significant pairs (Fig. 8 and 9). In each structure determinant, the interaction energies are calculated and split into the individual contributions of electrostatics, polarization, dispersion and repulsion (ESI† Tables S2 and S3, for each individual contribution). Our structures are both disordered and we modelled them in the approximation that we are dealing with static disorder, so the disordered molecule can be in one conformation – or the other – in different cells. We present here the results for the most populated conformer of each of the two structures, but the conclusions do not change if the other two (less populated) conformers are considered. The main contributors to the crystal packing of this assembly are the two OH⋯Npy hydrogen bonds, which are formed by two Ap8 molecules and the “resorcinol” ring of the Pic molecule (44.9 and 44.2 kJ mol−1, Fig. 8, structure determinants 1 and 2).
The third OH⋯Npy hydrogen bond ranks fourth in the interaction hierarchy with a reduced interaction energy (36.6 kJ mol−1, Fig. 8, structure determinant 4) compared to the other two. We attributed this fact to the lower acidity of the hydroxy-protons in the “catechol” ring compared to those in the resorcinol ring of the Pic molecule.20 Furthermore, due to the steric repulsion of the two vicinal OH groups, the weakest OH⋯Npy hydrogen bond has a slightly distorted geometry with an O–H⋯N angle of 159°. This angle could be compared with the reference O–H⋯N angle of 176° present in the “ideal” phenol–pyridine complex.21 Interestingly, the remaining hydroxy-group, which does not interact with the pyridine rings of the Ap8 molecules, is involved in hydrogen bonding with another hydroxy-group of a neighbouring Pic molecule (Fig. 8, structure determinant 6).
The hydrogen bonds of structure determinants 1, 2, 4 and 6 are dominated by electrostatics (Fig. 8 and 9 and ESI† Tables S2 and S3), as expected. Important contributions to the crystal packing are also given by structure determinants 3 and 5, which feature CHalk⋯π interactions that are dispersive in nature (Fig. 8 and 9).
Similarly to the structure previously described, the OH⋯Npy hydrogen bonding encompassing structure determinants 1–3 dominates the overall interaction ranking. There are however important differences between the packing of Pic(St8)3 (Fig. 9) and the packing of Pic(Ap8)3 (Fig. 8), due to the fact that, in the present case, π⋯π edge-to-face and stacked interactions are much more important energetically (structure determinants 4–6). Although the packing of Pic(St8)3 (even that of any assembly) in the solid state cannot be directly compared with that in the LC state, the increased relevance of π⋯π and stacked interactions is important to promote the LC behavior, as the LC phase might be suppressed, when these interactions are hindered or disrupted.22
Footnotes |
† Electronic supplementary information (ESI) available: Additional DSC, POM and IR measurements as well as information on the Hirshfeld calculations. CCDC Pic(Ap8)3 (1945777) and Pic(St8)3 (1942435). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0me00171f |
‡ The two authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2021 |