Efficient fluorescence emission from protonated 1,3,4-oxadiazole derivatives with meta dimethylamino substitution

pH-sensing materials are vital in fields such as healthcare and environmental monitoring, and in industrial processes, due to their ability to detect and respond to changes in acidity and alkalinity. However, many proton-responsive luminescent materials suffer from fluorescence quenching after protonation, which limits their detection sensitivity. To address this challenge, we have designed and synthesized two donor-acceptor (D-A) type 1,3,4-oxadiazole derivatives (L-mDMAOXDBEN and B-mDMAOXDBEN), and their fluorescence and proton-responsive properties were investigated using a combination of experimental techniques and theoretical calculations. Our results show that altering the substitution position of the dimethylamino group on the terminal benzene ring from para to meta significantly enhances the intramolecular charge transfer (ICT) properties and leads to highly efficient fluorescence emission from the protonated molecules, with a remarkable quantum yield of 83.2%. Theoretical studies further reveal that this structural modification increases charge transfer over a slightly longer distance, enhancing the ICT properties. Additionally, the widened energy gap between the HOMO and the LUMO and the strengthened oscillator strength contribute to the observed blue-shift in the spectra and the high quantum efficiency. This research offers valuable insights into the relationship between the molecular structure and fluorescence behavior, paving the way for the development of more sensitive and effective pH-sensing materials.