Light-Tuning Surface Activity and Emulsion Stability of Macromolecular Surfactants Based on the Host-Guest Interaction

Supramolecular chemistry provides more possibilities to design responsive macromolecular surfactants. In this study, poly(N-isopropylacrylamide) (Pnipam) with (azobenzene) AZO as the end group (Pnipam-AZO) was synthesized (P1 and P2), and then an inclusion complex was formed by Pnipam-AZO and β-cyclodextrin-modified poly(ethylene oxide) (PEO-CD). The host-guest interaction was characterized by using UV-vis spectra and GPC. The surface activity of P2 and the PEO-CD inclusion complex (P2@CD-PEO) was studied. Compared with Pnipam-AZO, the surface tension of P2@CD-PEO was higher. The inclusion of the hydrophobic AZO by CD and the attachment of the hydrophilic PEO to P2 lead to the decreasing adsorption amount of Pnipam, which results in increasing surface tension of P2@CD-PEO. Under UV light, the photoisomerization of AZO leads to decoupling of P2 and PEO-CD. Accordingly, the surface tension decreased, and this process was recyclable. The aggregation behaviors of P2@CD-PEO were studied by dynamic laser light scattering. The size and structure of the aggregates formed by P2@CD-PEO were light-responsive. The decoupling of PEO-CD and Pnipam-AZO made the size of the aggregates increase under UV light due to the increasing hydrophobicity of Pnipam-AZO. The emulsification study showed that P2@CD-PEO was an effective emulsifier, which could form oil-in-water emulsion for the petroleum ether and water system. The SEM images of the freeze-dried emulsion showed that the emulsion was stabilized by P2@CD-PEO aggregates that adsorbed at the oil and water interface. Also, demulsification occurs under UV light, and the emulsification and demulsification processes are recyclable.

Chiral Induction of a Tetrakis(porphyrin) in Various Chiral Solvents

Non-covalent interactions offer an alternative way for developing stimulus-responsive materials such as sensors, machines, and drug-delivery systems. We recently reported that a urethane-equipped tetrakis(porphyrin) forms one-handed helical supramolecular polymers in solution in response to chirality of chiral solvents. Conformational changes in helical sense were detected using circular dichroism (CD) spectroscopy, which showed that the tetrakis(porphyrin) can possibly be used as a sensor for determining the enantiomeric excess of a chiral analyte. Hence, we studied the scope and limitations of the chiral-induction behavior of tetrakis(porphyrin) to deepen the understanding of tetrakis(porphyrin)-based chiral sensing systems. Herein, we report the chiral-induction behavior of tetrakis(porphyrin) in various chiral solvents, which was found to be CD-active in many chiral solvents. Notably, the tetrakis(porphyrin) was CD active in a cryptochiral molecular solvent, which is exciting because the chiralities of acyclic saturated hydrocarbons are hard to sense. Consequently, this study highlights the potential advantages of supramolecular chiral sensors capable of targeting a wide range of analytes, including molecules that are absorption-silent in the UV/vis region, ones devoid of anchoring functional groups, and acyclic, saturated hydrocarbons.

Host-Guest Complexation of Bisporphyrin Cleft and Electron-Deficient Aromatic Guests

The host-guest complexation of a bisporphyrin cleft with various electron-deficient guest molecules was studied in solution and in the solid-state. X-ray crystal structures of a bisporphyrin cleft with naphthalene dianhydride and 2,4,7-trinitrofluorenone reveal that these guest molecules were located within the bisporphyrin cleft and formed ideal π-π stacking interactions in a host-guest ratio of 1:1. Isothermal titration calorimetry determined the binding constants and thermodynamic parameters for the 1:1 host-guest complexations in 1,2-dichloroethane and toluene. Two types of enthalpy-entropy compensation effects were found: (1) The tightly stacked host-guest structures restrict guest movement within the cleft, which results in significant desolvation with large intrinsic entropies. (2) The loosely bound guests maintain their molecular freedom within the bisporphyrin cleft, which leads to less desolvation with small intrinsic entropies. Chiral guest encapsulation directed the clockwise and anticlockwise twisted conformations of the bisporphyrin units, which induced bisignate CDs.

Tyrosine-based photoluminescent diketopiperazine supramolecular aggregates

L-Tyrosine diketopiperazine (DKP) derivative 1 was synthesized, and the aggregation and photoluminescence behaviors were examined. A solution of 1 in tetrahydrofuran (THF) gradually became viscous at room temperature, and turned into the gel state 5 hours after preparation, as confirmed by dynamic viscoelasticity measurement. A solution of 1 in THF exhibited photoluminescence. Fibrous patterns were observed by transmission electron, atomic force and fluorescence microscopies. Dynamic light scattering, semiempirical molecular orbital and density functional theory calculations, as well as molecular dynamics simulations, indicated aggregate formation. This was attributed to intermolecular hydrogen bonding, mainly between the DKP moieties and partly between the urethane moieties, resulting in π-orbital overlap of the terminal phenyl groups leading to photoluminescence.