Perylene bisimide hydrogels and lyotropic liquid crystals with temperature-responsive color change

Tuning of the temperature response of perylene bisimide hydrogels exhibiting LCST behavior has been achieved by social self-assembly in water.


Materials and Methods
NMR spectroscopy. 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AV 400 spectrometer using CDCl 3 as solvent. The chemical shifts are reported in ppm and refer to the proton signal of the solvent as internal standard. Multiplicities for proton signals are abbreviated as s, d, and m for singlet, doublet, and multiplet, respectively.
UV/Vis spectroscopy. UV/Vis absorption spectra were recorded using a Perkin Elmer Lambda 40P spectrophotometer. The spectra were measured in quartz glass cuvettes using either spectroscopic grade tetrahydrofuran (Uvasol ® ) or deionized water prepared by water purification system PURELAB classic (ELGA, France). Temperature control was accomplished by Perkin Elmer PTP-1+1 Peltier system. Extinction coefficients were calculated from Lambert-Beer's law.
Fluorescence spectroscopy. Fluorescence spectra were recorded on a PTI QM-4/2003 fluorescence spectrometer and are corrected against photomultiplier and lamp intensity.
Fluorescence quantum yield (Φ) of PBI 1 was calculated from the integrated intensity under the emission band (I) using the following equation: Where OD is the optical density of the solution at the excitation wavelength and n is the refractive index. The optical density of the solution for the calculation of quantum yields was less than 0.05 at the excitation wavelength. N,N'-Bis(2,6-diisopropylphenyl)-1,6,7,12tetraphenoxyperylene-3,4:9,10-tetracarboxylic acid bisimide (Φ r = 0.96) in chloroform was used as reference. S1 Fluorescence spectra of hydrogel samples were recorded with an ECLIPSE LV100 POL polarizing microscope (Nikon, Japan) using scope mode, ten-fold magnification and two seconds exposure time. An Intenslight C-HGFI mercury vapor lamp (Nikon, Japan) served as external light source. The spectra are averaged from ten measurements. Excitation wavelength region was 380-420 nm. The hydrogel sample was carefully placed as a film between two glass plates.

S3
Cryogenic scanning electron microscopy. Cryo-SEM measurements were performed using a Zeiss Ultra Plus Field Emission SEM operating at 2-5 kV with an aperture size set of 30 m to avoid excessive charging and radiation damage of imaged areas. Sample preparation consisted of placing a small drop of the PBI hydrogel onto copper stub sample holder. Prior to examination, the specimen was plunged into liquid nitrogen slush at -210 °C. The sample was then transferred under vacuum using the loading transfer rod into the high vacuum cryo-preparation chamber (Quorum PP2000T) at -180 °C, fractured and then transferred into a SEM sample chamber maintained at about -150 °C.
Polarizing optical microscopy (POM) and optical microscopy (OM). The materials were examined under an Olympus Bx41 (Nikon, Japan) polarizing microscope equipped with a THMS 600 heating stage (Linkam, Great Britain).
Thermotropic and lyotropic liquid-crystalline samples were prepared by fiber extrusion using a mini-extruder. The measurements were carried out in Mark capillaries (Hilgenberg) positioned perpendicular to the incident X-ray beam.
The hydrogel samples for the XRD measurements were prepared as follows: The viscous PBI hydrogels (80 wt% water content) were placed inside a glass Mark-tube (Ø=1.5 mm). The tube, open at both sides, was placed vertically in a vial to allow the sample to reach the middle part of the tube by gravity (≈6 h). Then, the tube was sealed by melting both sides. This tube was inserted into another Mark-tube (Ø=2 mm) which was also sealed by melting. This sample preparation prevented the evaporation of the water during the measurements that were performed under vacuum.
It should be noted that a certain degree of alignment is observed in the 2D X-ray diffraction patterns of PBI hydrogels (80 wt% water content) above the LCST (Figure 4e and Figure S10b).
The reflections corresponding to the columnar hexagonal lattices appear along the equator, which indicates that the columns are oriented nearly parallel to the glass tube. This anisotropic organization is attributed to the partial orientation of the gel fibers during the sample preparation or to directing effects of the glass tube during the LCST phase transition. This alignment was not studied in detail.

S4
Preparation of the hydrogels and lyotropic liquid crystals. Hydrogels and lyotropic liquid crystal samples were prepared by weighing the PBI compounds in flasks and subsequent addition of the appropriate amount of water (20 wt% for lyotropic liquid crystals and 80 wt% for the hydrogels).
The flasks were closed and stored at r.t. until the mixtures became homogeneous (ca. 24 h). In the case of the lyotropic samples, the mixtures were treated with a spatula to ensure the homogenous distribution of water in the PBI material.
The hydrogels based on mixtures of PBI 1 and PBI 2 were prepared as follows: PBI 1 and PBI 2 were mixed in the desired molar ratio by dissolving in dichloromethane. Subsequent removal of the solvent resulted in an amorphous mixed solid that was dried under vacuum. An appropriate amount of pure water (80 wt%) was added to the PBIs mixture in a flask that was closed and stored overnight at r.t..