Multi-stimuli responsive smart materials: cyanine amphiphile self-assembly for photochromic and pH-switching applications

Abstract

Smart hydrogels, known for their stimuli-responsive properties in drug delivery, tissue engineering, and sensors, are typically created using a top-down approach, which limits precise molecular control. In this study, we employ a bottom-up strategy to achieve greater molecular precision, enabling the development of innovative, multi-stimuli-responsive hydrogels. We designed and synthesized asymmetric cyanine amphiphiles incorporating diphenylimidazole as the donor and indole as the acceptor to create molecules with intramolecular charge transfer characteristics. These diphenylimidazole-indole amphiphiles, DPIIH and DPIIF, differ in that DPIIH lacks a fluorine atom at the indole terminal, while DPIIF includes this substitution. Additionally, diphenylimidazole reveals nonplanar conformations and twisted dihedral angles between the phenyl rings at the 4,5-position of imidazole, giving it aggregation-induced emission properties. In contrast to DPIIH, DPIIF can self-assemble into a hydrogel in water, probably due to the hydrogen bonding interactions between DPIIF and water molecules. Through detailed exploration of DPIIF, it was found to exhibit a reversible photochromic effect in polar solvents and can undergo reversible acid–base reactions. The photoisomerization and pH stimulus-response behaviors of DPIIF can be observed via colorimetric and fluorescence changes, making it suitable for applications such as invisible ink and pH sensors. In its hydrogel state, DPIIF reveals reversible photoswitching and pH-switching features, enabling reversible sol–gel transitions. These properties suggest potential applications in both cell culture (gel state) and cell recovery (solution state), offering versatile functionality in biomedical and research settings. Furthermore, the fluorescent properties of the hydrogel can be used to study and visualize cell–material interactions in detail, providing valuable insights for various biological studies.