Benzothiadiazole-Centered Donor-Acceptor-Donor Systems: Synthesis, Characterization, and PFAS-Induced Fluorochromism.

The development of π-conjugated donor-acceptor (D-A) systems has shown significant potential in optoelectronics, sensing, and biomedical applications. Benzothiadiazole (BTD), a key electron-accepting unit, has been widely utilized in designing luminescent materials due to its tunable electronic and photophysical properties. In this work, we present a new class of BTD-centered D-A-D organic fluorophores, where the central BTD core is symmetrically flanked by π-fused phenanthroimidazolyl or pyrenoimidazolyl donor/fluorogenic groups. This design promotes bidirectional electron delocalization, enhancing intramolecular charge transfer and enabling dual fluorescence emission. The synthesized fluorophores exhibit pronounced solvatofluorochromic effects, with emission properties highly sensitive to microenvironmental changes. We further demonstrate their application as effective molecular probes for per- and polyfluoroalkyl substances (PFAS), showing distinct fluorescence responses based on PFAS chain lengths. Notably, phenanthroimidazole-derived fluorophores exhibit significant blueshifts upon interaction with shorter-chain PFAS, while longer-chain PFAS induce quenching effects. These findings highlight the potential of our BTD-centered D-A-D fluorophores for developing advanced sensor arrays capable of rapidly discriminating PFAS in complex mixtures and further expand the toolbox for molecule-based luminescent sensing and molecular recognition.