Host-Enhanced Phenyl-Perfluorophenyl Polar−π Interactions

Phenyl-perfluorophenyl polar−π interactions have been revisited for the design and fabrication of functional supramolecular systems. The relatively weak associative interactions (ΔG ≈ −1.0 kcal/mol) have limited their use in aqueous self-assembly to date. Herein, we propose a strategy to strengthen phenyl-perfluorophenyl polar−π interactions by encapsulation within a synthetic host, thus increasing the binding affinity to ΔG= −15.5 kcal/mol upon formation of heteroternary complexes through social self-sorting. These heteroternary complexes were used as dynamic, yet strong, cross-linkers in the fabrication of supramolecular gels, which exhibited excellent viscoelasticity, stretchability, self-recovery, self-healing, and energy dissipation. This work unveils a general approach to exploit host-enhanced polar−π interactions in the design of robust aqueous supramolecular systems.

Continuous step-strain measurements were carried out at 1 rad s -1 in high-amplitude oscillatory (500 %) and low-amplitude oscillatory (5 %), respectively. All of the above data was collected at 293.15 K, analyzed by TRIOS software, TA Instruments, and then plotted by Origin 10.0. Time-temperature superposition (TTS) experiments were performed as the frequency sweep measurements at 1 % strain in the temperature range from 273.15 K to 353.15 K with 10 K intervals, and 303.15 K was used as the reference temperature. The obtained data was analyzed to calculate the activation energy of local chain motion.

SI-2 Molecular design and synthetic protocols
Chart S1 Molecular structures of all the guest molecules with the counter ion of bromide.
As shown in Chart S1, all of the guest molecules were rationally designed and easily prepared by salt formation reaction. [2] The representative synthetic route is shown in Figure S1 using 5FBVI as the example.

Figure S1
Representative synthetic route for the preparation of guest molecules.

Synthesis of 1-benzyl-3-vinylimidazolium bromide (BVI)
The same synthetic procedure used to prepare 5FBVI was followed using benzyl bromide as the starting material. The white solid of BVI was obtained, yielding ca. 94 %. 1

Synthesis of 1-(2,3,4,5,6-perfluoro)-benzyl-N,N,N-trimethylammonium bromide (5FBTMA)
The same synthetic procedure used to prepare 5FBVI was followed using trimethylamine as the starting material. The white solid of 5FBTMA was obtained, yielding ca. 90 %. 1   In a typical procedure, the amounts of acrylamide, supramolecular cross-linker of 5FBVI-BVI-CB[8] and photoinitiator I-2959 were predetermined and weighed out precisely in a glass vial. Then, the solid mixture was dissolved together in Milli-Q H 2 O with the precalculated volume under ultrasonication for 10 min. The obtained pre-gel solution was sealed and purged with nitrogen for 30 min. Next it was transferred into a laboratory-made glass mould with a certain thickness (1 or 2 mm), before being exposed to UV irradiation of 350 nm light with 4.8 mW/cm 2 for 6 hr. Finally, the hydrogel was removed from the glass mould and then cut into the required shape. Similarly, hydrogels could be directly prepared in a cell culture bottle or a small glass vial. The obtained hydrogels were cut using a dumbbell-shaped cutter or a razor blade into different sizes and shapes required for certain characterization. As shown in Figure S3, by adding 2×BVI-CB[8] into 2×5FBVI-CB[8], three proton peaks of the phenyl moiety on 2×BVI-CB[8] at 6.23, 6.49, 6.59 ppm gradually disappeared with the appearance of three new peaks at 6.39, 6.49, 6.59 ppm. This data is consistent with the 1 H NMR titrations in Figure 1a Figure S4, a new series of proton peaks attributed to the phenyl moiety of BVI gradually appears, which suggests the formation of a new heteroternary complex of 5FBVI-BVI-CB [8]. It is also observed that there are two series of vinyl groups that can be attributed to the vinyl groups of BVI and 5FBVI. This suggests the self-sorting phenomenon during the titration process.  Figure S5, three new fluorine peaks gradually appears with the addition of BVI into 5FBVI-CB[8], strongly suggesting the formation of a new heteroternary complex of 5FBVI-BVI-CB [8]. It should be mentioned that for clarity the intensity of 5FBVI in Figure 2a is one fourth of its original intensity in Figure S5, which is also labelled with 'x 0.25'. This is the extended version of Figure 2b. As shown in Figure S6, by adding BVI into 5FBVI-CB[8], the residual ion peak of BVI gradually appears at m/z of 185.1078 compared with the ion peak of 5FBVI at 275.0598 as the reference. Although electrospray is a soft ionization method, some of the host-guest complexes will disassociate, thus releasing plenty of free BVI and 5FBVI. The disassociated CB[8] will mostly be trapped in the MS apparatus. Thus, it is suggested to use the free guest solution or sodium chloride solution to wash out CB[8] in the MS apparatus after several experiments.   Figure S7, their enthalpy changes and entropy changes obtained from model fitting may not be precise for in-depth analysis. As the macroscopic binding constants (K 1 , K 2 ) were obtained, the interaction parameter (a) is therefore calculated by a = 4K 2 / K 1 based on consideration of statistical factors. [2,3]
As shown in Figure S7, CB[8]-mediated homoternary complexations with 5FBVI, 5FBMI, 5FBPy, and 5FBTMA all exhibit very strong negative cooperativity with a < 0.001 (Table S1). This data indicates that the perfluorophenyl group of the guest molecule plays the key role in its negatively cooperative binding with CB[8]. As for 5FBMTA (yellow data set), its second binding with CB[8] slightly increases. Compared with an imidazolium or pyridium group, the trimethyl group is more hydrophobic which could partially enter into the cavity and interfere with the inner electrostatic repulsion. [3]      cross-linker at 1 % strain and its fitting curve to obtain the activation energy of local chain motion (E a ).

SI-8 Photographs showing ionic conductivity of the 5FBVI-BVI-CB[8] gel
Figure S15 Photographs of a) an example of the obtained gel and b) a piece of gel twined around a plastic rod; c) the circuit diagram and d) photographs for demonstrating the ionic conductivity of the resultant gel in a series circuit with two LED lamps and 9 V battery.