Heating-Induced Switching to Hierarchical Liquid Crystallinity Combining Colloidal and Molecular Order in Zwitterionic Molecules

Hierarchical self-assemblies of soft matter involving triggerable or switchable structures at different length scales have been pursued toward multifunctional behaviors and complexity inspired by biological matter. They require several and balanced competing attractive and repulsive interactions, which provide a grand challenge in particular in the “bulk” state, i.e., in the absence of plasticizing solvents. Here, we disclose that zwitterionic bis-n-tetradecylphosphobetaine, as a model compound, shows a complex thermally switchable hierarchical self-assembly in the solvent-free state. It shows polymorphism and heating-induced reversible switching from low-temperature molecular-level assemblies to high-temperature hierarchical self-assemblies, unexpectedly combining colloidal and molecular self-assemblies, as inferred by synchrotron small-angle X-ray scattering (SAXS). The high-temperature phase sustains birefringent flow, indicating a new type of hierarchical thermotropic liquid crystallinity. The high-temperature colloidal-level SAXS reflections suggest indexation as a 2D oblique pattern and their well-defined layer separation in the perpendicular direction. We suggest that the colloidal self-assembled motifs are 2D nanoplatelets formed by the lateral packing of the molecules, where the molecular packing frustration between the tightly packed zwitterionic moieties and the coiled alkyl chains demanding more space limits the lateral platelet growth controlled by the alkyl stretching entropy. An indirect proof is provided by the addition of plasticizing ionic liquids, which relieve the ionic dense packings of zwitterions, thus allowing purely smectic liquid crystallinity without the colloidal level order. Thus, molecules with a simple chemical structure can lead to structural hierarchy and tunable complexity in the solvent-free state by balancing the competing long-range electrostatics and short-range nanosegregations.

A selected series of zwitterionic bis-n-alkylphosphobetaines CmH2m+1-O-(PO -)-O-C2H4-N + (CH3)2-CnH2n+1 (denoted as Cm-Cn) were synthesized.The synthesis was performed according to a published route where phosphobetaine zwitterionic amphiphiles with alkyl tail lengths from C4 to C22 have been obtained 1,2 .The yields corresponded with the previous synthesis protocols and were of ca.13-30 %.All synthesized compounds were characterized using 1 H & 13 C NMR, and FTIR.The molecules are hygroscopic and were previously shown to crystallize as hydrates. 1,2The amount of adsorbed water was determined by Karl Fischer titration as 6 wt% after storage in ambient conditions, and this adsorbed water can be removed during the typical drying procedure of heating the sample to 60 °C in vacuum overnight.

Supporting Section 2: Thermal stability of bis-n-alkylphosphobetaines (Cm-Cn)
Figure S3.The thermal stability of the compounds was studied with thermogravimetric analysis (TA Instruments TGA Q500) at 40-900 ºC under N2.The TGA-determined decomposition temperature, marking the decomposition of the n-alkyl chains starts at slightly above 200 ºC for all compounds.However, quaternary ammoniums are known to show melting accompanied with chemical degradation.This decomposition can occur in at least two mechanisms: reverse Menschutkin reaction or Hoffman elimination, 3 both of which conserve the alkyl tails, leading to non-volatile decomposition products which are not seen in the TGA thermogram.The thermostabilities were additionally studied by SMP30 melting point apparatus (Stuart) with visual inspection.The zwitterionic bis-n-alkylphosphobetaine molecules showed i) a transition to a visually gel-like state, followed by ii) a coloration process starting at ca. 180-190 ºC.

Supporting Section S3: Polarized optical microscopy studies of bis-nalkylphosphobetaines (Cm-Cn)
The series of bis-n-alkylphosphobetaine molecules were studied until the isotropization through crossing their respective clearing temperature (Tiso) to investigate their full thermal behavior.In the optical microscope studies, all synthesized compounds behaved in a similar manner with slightly shifted transition temperatures.
All samples showed a gradual increase in birefringence, a gradual softening of the crystals, and a short-lived smectic mesophase, where the typical oily streak and Maltese cross textures were observed right before the isotropization.The smectic textures were visible for longer time the longer the n-alkyl tails, owing to the increased amphiphilic nature.As expected, the clearing temperature was lower for the longer n-alkyl tail: Tiso was 195 °C for the C14-C14 (longest tail length) and 210 °C for C6-C6 (shortest tail length).Upon cooling from the isotropic state, the LC textures (bâtonnet, focal conic fan, and Maltese crosses) of bis-n-alkylphosphobetaine molecules were fully developed and unambiguously identified.
When heated to 160 °C (fixed as a safe temperature of investigation, vide supra), no characteristic LC textures could be observed, as seen by the POM microphotograph in the main text.The POM studies of both temperature ranges (i.e., below 160°C and till reaching the isotropic state) were therefore instrumental in understanding the complex thermal behavior of the bis-n-alkylphosphobetaine molecules.The structures of C14-C14 are listed for three selected temperatures: 25, 55, and 160 ºC, in the following tables.At room temperature, an orthorhombic model was fitted for Cr3D but with increased temperature, the fitting starts to deviate from ideality.We propose a gradual tilting of the crystallographic axis of the Cr3D structure, i.e., from orthorhombic to monoclinic, and ultimately to triclinic geometries.Due to the complexity of the triclinic structure, the lattice parameters (a, b, c, α, β, γ) are not explicitly stated.
In the tables, the following abbreviations are used: qexp experimentally observed peak position as a scattering vector q qtheor theoretical peak position as scattering vector q, as calculated using the lattice parameters a, b, c, α, β, γ

Supporting Section 7: Electrochemical Impedance Spectroscopy (EIS)
Figure S14.Ionic conductivity as a function of temperature for mixtures C14-C14:BmimTFSI 1:1 and 1:0.33 mol:mol.Referencing was performed by measuring a similar protocol of EIS measurements of BmimTFSI (0:1) throughout the temperature regime.The referencing were performed in triplicates.The calibration of the set-up and determination of systemic impedance was performed for each temperature point separately (every 10 ºC, over the temperature range of 0 to 150 ºC).The ionic conductivity values of BmimTFSI were calculated according to the Vogel-Tamman-Fulcher equation that correspond relatively well with limitedly available experimental data. 4,5igure S15.The Nyquist plots of the two replicate measurements of C14-C14:BmimTFSI 1:1 (left panel) and 1:0.33 (right panel) mol:mol at selected temperature points.The equimolar sample shows diffusive phenomenon throughout the temperature scan area 0-150 °C whereas for 1:0.33 mol:mol, a depressed capacitive semicircle develops in the low temperature regime 0-50 °C.The Nyquist plots were analyzed using ZView software, using the data analysis models shown in Figure S16.

Figure S4 .
Figure S4.Clearing points for synthesized symmetric zwitterionic phosphobetaine compounds as a function of their alkyl tail lengths under rapid thermal scans.

Figure S5 .
Figure S5.Selected POM microphotographs for C14-C14 during the 1 st heating scan.The crystals show continuously increasing birefringence with heating.While the birefringence increases, the crystals also become gradually softer.The fluidity increases continuously, and by 180 °C the sample is spontaneously (in the absence of any mechanical solicitation like pressing or shearing) flowing though being viscous.It then starts to show the characteristic birefringent oily streak textures for a smectic liquid crystal at 195 °C that disappear on further stay on that temperature as the sample transitions into the isotropic state across its clarification temperature.The scale bars are 200 µm.

Figure S6 .Figure S7 .
Figure S6.Selected POM microphotographs for C14-C14 on cooling scan from the isotropic state.Typical birefringent LC textures are observed: bâtonnets that grow into focal-conic fans, Maltese crosses (clean and distorted), and finally paramorphotic mosaic textures, the latter ones being indicative of ordered smectic phases (e.g.SmBcryst or SmE) developing as a result of increasing intra-vs interlamellar correlations.The high viscosity of the liquid crystalline phases led to the freezing of the textures when decreasing temperature towards room temperature.The scale bars are 200 µm.

Figure S8 .
Figure S8.A close-up of the 1D SAXS pattern of C14-C14 at 160 ºC, showing the faint 2 nd and 3 rd order peaks of the Sm3 structure (blue) along with the other structures, namely Sm1 (yellow) and Cr2D (red).

Figure S9 .
Figure S9.WAXS 1D profiles for C14-C14 at selected temperatures during a cooling scan.The WAXS 1D profile recorded at 160 ºC indicates the loss of molecular crystallinity.The changes in peak pattern and intensity observed upon cooling are due to the slow crystallization of the alkyl chains.

Figure S11 .
Figure S11.The DSC thermograms of C14-C14 during the first three heating cycles suggest that after the 1 st heating cycle, the thermal transition between Cr3D and Coll phases are reversible.The 1 st heating scan is different than the subsequent ones which is typical for ionic amphiphiles.

Figure S16 .
Figure S16.Schematic pictures showing the applied data analysis methods to ensure a proper fitting of the EIS data while relying on physics-based realistic and relevant models.The ionic conductivities were calculated using the formula  =    , where K is the cell constant and Rbulk the bulk resistance extracted from the modelled EIS data.a) The straight line indicates a diffusion-controlled ion-conduction mechanism.b) Capacitive phenomena start to exist, although the major limiting factor is still diffusion-controlled. c) A highly depressed semicircle has started to develop, and no unambiguous inter-/extrapolation can be done, therefore an intermediate model is used.d) A more developed but still depressed semicircle reflects the prominent capacitive behavior of the sample.The equivalent circuit shown in the graph was employed to extract parameters for R1, R2, and CPE1.In this model, the R1 represents the resistance of the set-up, R2 the bulk resistance of the studied material, and the CPE1 the nonideal capacitive behavior of the sample.