Nontraditional π Gelators Based on β-Iminoenolate and Their Difluoroboron Complexes: Effect of Halogens on Gelation and Their Fluorescent Sensory Properties Towards Acids

We have synthesized a series of new beta-iminoenolates and their corresponding difluoroboron complexes without any traditional gelation moieties, and some of them were able to gelatinize organic solvents. It was found that the presence of halogen atoms as substituents had a significant effect on gelation ability. In particular, bromo-containing compounds 4A and 4B exhibited excellent gelation abilities compared with other halogen-substituted gelators. By analyses of the single-crystal structure, the PXRD pattern of the xerogel, and electronic spectral changes during gelation, we deemed that pi-pi, C-H center dot center dot center dot Br, and C-H center dot center dot center dot Br interactions were the driving forces for the gelation of 4B. Interestingly, (Z)-1(4-bromophenyl)-2-(3-methylpyrazin-2-yl) ethen-1-ol (8A), prepared in this work, is the lowest-molecular-weight organogelator to have been reported. It should be noted that although beta-iminoenolates 3A-5A are nonemissive in solution, they emit strong yellow light in organogels, which suggests aggregation-induced emissive activity, whereas the difluoro-boron complexes 3B-5B show strong fluorescence in solutions, organogels, and xerogel-based films. Moreover, we found that the emission of 4B in a nanofiber-based film could be quenched significantly upon exposure to gaseous trifluoroacetic acid and that the decay time and detection limit were 0.5 s and 0.17 ppm, respectively. Thus, through this work we have provided a new strategy for the design of nontraditional pi gelators by introducing halogen atoms into pi-conjugated systems with moderate polarities.