Eight new supramolecular crystals of 1,4-di(1H-imidazol-1-yl)benzene (DIB) have been prepared using water, 4,4′-biphenol, terephthalic acid, isophthalic acid, succinic acid, and 1,2,4,5-tetrafluoro-3,6-diiodobenzene. These materials were subsequently characterized by single crystal X-ray analysis. The results revealed that one-dimensional infinite chains were formed in these crystals through hydrogen bonding or halogen bonding interactions. The hydrogen atoms of the imidazole ring in DIB were found to be involved in three kinds of hydrogen-bonding ring motifs. These motifs could play an important role in the stabilization of the crystals and could therefore be beneficial to DIB in terms of its role as a good building block for supramolecular crystals.

A class of new compounds consisting of a tetraphenylethene (TPE) core and hydrophilic peripheral oligo(ethylene glycol) chains was synthesized. These newly synthesized compounds showed aggregation-induced emission (AIE) nature which originated from the TPE core, and their peripheral oligo(ethylene glycol) chains impart water-solubility for the whole molecules. These compounds exhibit a lower critical solution temperature behavior in water. The phase transition temperature of TPE derivatives increases with the increase in the length of oligo(ethylene glycol) chains. By taking advantage of the AIE properties and thermosensitivity, these compounds are able to sense temperature by showing different emission intensities. When being heated above its phase-transition temperature, these compounds start to aggregate, which results in the restricted intramolecular rotation of the TPE moiety. Consequently, the aqueous solutions of these compounds show a stronger fluorescence. The temperature-induced fluorescence change is highly reversible.

We have designed and synthesized two dithiooctanoic acid derivatives bearing N,N-disubstituted amide groups and used them to fabricate self-assembled monolayers (SAMs) on gold surface. These films showed a reversible changes in wettability, one of which was indicated by surface contact angle switching between 40° and 59° upon alternating treatment with ethanol and cyclohexane. NMR experimental results of a model molecule suggest that the solvent-responsive wettability of the SAMs could be related with the changes in the relative populations of two stereoisomers of amide. The solvent responsivity of SAMs fabricated from other two amides was also studied, and the results confirmed that N,N-disubstitution was essential for an amide-containing SAM to have stimuli responsivity. Thus, introduction of a functional group exhibiting controlled isomerization of conformation could be an effective strategy for designing new stimuli-responsive materials.

We investigated effect of cyclodextrins (CDs) on the cloud point of several thermosensitive polymers that are not ionizable. α-CD increased the cloud point of the poly(N-n-propylmethacrylamide) (PnPMAm) aqueous solution; by contrast, β-CD or γ-CD did not affect the cloud point of the PnPMAm solution. The cloud point of the PnPMAm solution increased gradually with an increase in the concentration of α-CD. Furthermore, we compared the effect of the CDs on the cloud points of four polymers with similar structures. As for poly(N-isopropylacrylamide) (PiPAAm), neither α-CD nor β-CD affected its cloud point. On the basis of the effect of the differently sized CDs on the cloud point of five polymers and the corresponding NOESY NMR data, we inferred that steric hindrance by the main chain of PiPAAm might be responsible for the bulky CD being unable to form a complex with the short isopropyl group.Keywords: cyclodextrin; inclusion complex; stimuli-responsive polymers;

We fabricated a layer-by-layer (LbL) film of temperature-responsive homopolymers at neutral pH and studied its temperature-dependent solubility. We first measured the cloud point of mixed solutions of temperature-responsive polymers. The significant decrease of cloud point suggested that the intermolecular interaction between two polymer chains of different kinds was stronger than that between two polymer chains of the same kind. Strong intermolecular interaction between two polymer chains of different kinds is a prerequisite for LbL assembly. On the basis of the decrease of cloud point of mixed solutions of temperature-responsive homopolymers, we selected poly(N-vinylcaprolactam) (PVCL) and poly(2-hydroxypropyl acrylate) (PHPA) for LbL assembly. LbL films of the two polymers were fabricated at neutral pH at a constant temperature. When the film was immersed in purified water at a temperature lower than the assembly temperature, it can be partially dissolved with a diffusion-limited dissolution process. The temperature-responsive solubility of the LbL film is closely connected to the phase behavior of mixed solutions of the two polymers. Additionally, as compared to multilayer films of neutral polymers and poly(carboxylic acid)s, the PVCL/PHPA multilayer film is relatively stable when it was immersed in buffer solutions near physiological pH at the assembly temperature. Such LbL films with temperature-responsive solubility might be used as a dissolvable film or a smart capsule.

We investigated the influence of addition of cyclodextrin (CD) on the cloud point of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) aqueous solution. Both β-CD and randomly methylated β-cyclodextrin (RM-β-CD) raised the cloud point of PDMAEMA solution; by contrast, the addition of α-CD or γ-CD did not affect the cloud point of PDMAEMA solution. The inclusion complexation between side group of PDMAEMA and RM-β-CD was confirmed by NMR experiments. The cloud point of PDMAEMA solution was raised gradually with the increase of concentration of RM-β-CD. On the basis of inclusion complexation equilibrium, we deduced an equation, which clearly described the relationship between cloud point and concentration of RM-β-CD. This analysis will provide an important theoretic guide for other related research. In comparison with copolymerization with another monomer, such a supramolecular approach avoids a series of synthesis and facilitates fine-tuning of cloud point.

We studied the charge-transfer (CT) interaction between poly(9-vinylcarbazole) (PVK) and 3,5-dinitrobenzamido (DBA) group or 3-nitrobenzamido (NBA) group. The complexation equilibrium constant of PVK and N-butyl-3,5-dinitrobenzamide was found to be larger than that of PVK and N-butyl-3-nitrobenzamide using UV−vis spectroscopy. The desorption process of PVK from a DBA-modified substrate was studied with single molecule force spectroscopy. A lot of force−extension curves with a plateau were observed, which indicated the existence of trainlike conformation. The average desorption force of a PVK chain from the DBA-modified substrate was 28 pN. For comparison, the desorption process of PVK from an NBA-modified substrate was also studied, and an average desorption force of 18 pN was obtained. The apparent interaction energy of a CT complex was calculated on the basis of the desorption force and the surface density of electron acceptors. The obtained apparent interaction energies were 14 and 8 kJ·mol−1 for the carbazolyl−DBA complex and carbazolyl−NBA complex, respectively. On the basis of the energy of the frontier molecular orbital of carbazole, 3,5-dinitrobenzamide, and 3-nitrobenzamide, the difference in strength between the interactions was interpreted as the influence of the substituent group on the energy of the frontier molecular orbital.

Layer-by-layer assembly technique was applied to fabricate multilayer films of poly(N-acryloyl-N'-propylpiperazine) (PAcrNPP) and poly(acrylic acid). The film buildup was followed by Quartz Crystal Microbalance, and the effect of temperature and pH on the layer-by-layer assembly was investigated. Below the lower critical solution temperature of PAcrNPP, the adsorption amount increased with increase of temperature. As the pH of dipping solutions was systematically varied from 2.0 to 7.5, adsorption amount of polymers initially increased and then decreased. The relationship between adsorption amount and degree of ionization of polymers was discussed. When an eight-bilayer film was alternately exposed to HCl aqueous solution (pH = 3.5) and neutral water (pH = 6.5), the film showed pH-response, which could be the result of polymer rearrangement.

Poly(allylamine hydrochloride) (PAH), which is frequently used in fabricating polyelectrolyte multilayer films, was studied by single molecule force spectroscopy (SMFS). Plenty of force-extension curves with a long plateau were obtained in water, indicating that train-like structure was predominant when PAH was adsorbed on the substrate. It was found that the peak-type force-extension curves of PAH in water were not able to be fitted by the modified freely-jointed chain model. Additionally, there was a flat region in the derivative of force-extension curves. Thus, it was inferred that PAH chain in water was in a special conformation and underwent a “conformational transition” under the stretching of an external force. This phenomenon did not appear in the SMFS experiment in 1 mol/L urea solution, which indicated that urea was able to break the special conformation.

A class of new compounds consisting of a tetraphenylethene (TPE) core and hydrophilic peripheral oligo(ethylene glycol) chains was synthesized. These newly synthesized compounds showed aggregation-induced emission (AIE) nature which originated from the TPE core, and their peripheral oligo(ethylene glycol) chains impart water-solubility for the whole molecules. These compounds exhibit a lower critical solution temperature behavior in water. The phase transition temperature of TPE derivatives increases with the increase in the length of oligo(ethylene glycol) chains. By taking advantage of the AIE properties and thermosensitivity, these compounds are able to sense temperature by showing different emission intensities. When being heated above its phase-transition temperature, these compounds start to aggregate, which results in the restricted intramolecular rotation of the TPE moiety. Consequently, the aqueous solutions of these compounds show a stronger fluorescence. The temperature-induced fluorescence change is highly reversible.