Aramid-film pH sensitive fluorescence enhancement based on benzimidazole intermolecular hydrogen bonds

Abstract

The aramid-film containing benzimidazole units was synthesized, and its pH sensitive fluorescence enhancement effect was investigated. After surface-treated by acidic ethanol solution (pH 1), the aramid film exhibits noticeable emissive enhancement as high as 33 times than the blank sample. The fluorescence lifetimes of aramid films increase significantly from 0.9 × 10−10 s to 8.2 × 10−10 s. The driving force is ascribed to the decrease of nonradiative transition probability, which comes from the strengthening molecular rigidity of phenyl benzimidazole chromophore within the polymer chains, induced by hydrogen bonds between N-H in benzimidazole group and C=O in amide group. The hydrogen-bond effect is thoroughly proved through examining the repeating molecular units of the aramid polymer chains by the methods of XRD, UV–vis, photoluminescence, molecular simulation, SEM and etc. This pH sensitive fluorescence change should have great potential in the sensor-device design of NH3, heavy metal ions or other systems due to the subsequent fluorescence quench effect, because of the excellent mechanical and processability of aramid polymers.