Simple substituents make big differences in photophysical performances of 2,1,3-benzothiadizole-conjugated spiropyrans

Developing multifunctional spiropyran dyes is of particular importance in diverse applications. In the present study, we synthesized two 2,1,3-benzothiadiazole-conjugated spiropyrans (BT-SP-NO₂ and BT-SP-NMe₂) with distinct substituents. These donor-acceptor-structured spiropyrans exhibited typical twisted intramolecular charge transfer features and strong emissions in low-polarity solvents with fluorescence quantum yields (QYs) of up to 90.7%. Like traditional spiropyrans, the electron-acceptor-substituted BT-SP-NO₂ exhibited excellent photochromic behavior under multiple alternating UV–Vis irradiation, while the electron-donor-substituted BT-SP-NMe₂ was an acidochromic dye. In addition, the substituent groups distinctly affected the packing modes of these spiropyrans in the solid state. BT-SP-NMe₂ showed a much stronger solid-state emission (QY of 59.0%) than BT-SP-NO₂. Moreover, these two dyes were utilized as biocompatible probes for the specific light-up imaging of lipid droplets.