Functionality-oriented molecular gels: synthesis and properties of nitrobenzoxadiazole (NBD)-containing low-molecular mass gelators

Abstract

Two nitrobenzoxadiazole (NBD)-containing cholesteryl (Chol) derivatives were prepared by introducing D/L-phenylalanine into the linkers between the NBD and Chol units. The compounds were denoted as NLC and NDC, respectively. The gelation behaviors of them were tested in 34 liquids. It was found that the chirality of the linkers shows a great effect on the gelation ability and the gel properties of the two compounds. SEM studies demonstrated that the gelator in the gel of NDC/DMSO aggregated into uniform fibrous structures. FTIR, ¹H NMR and UV-Vis spectroscopy measurements revealed that intermolecular hydrogen bonding and π–π stacking are two main driving forces to promote the gel formation. Interestingly, the NDC/DMSO gel exhibits rapid and reproducible gel–sol phase transition and fluorescence quenching upon introduction of ammonia. Furthermore, both the gel and the fluorescence emission could be fully recovered upon evaporation of the ammonia gas introduced. Spectroscopy and model system studies revealed the association of ammonia with the nitro group of the NBD unit of the compound, which is recognized as the main reason for the chemical responses of the gel system. On the basis of the discovery, an ammonia sensing film had been fabricated and made into a device. Furthermore, a device-based and conceptual “ammonia leaking” monitoring instrument was developed. A preliminary test demonstrated that the performance of the system is exceptionally good, a typical and persuasive example to show the important real-life applications of molecular gels.