Positive fluorosolvatochromism in N,N-dimethyl-4-amino-2,1,3-benzothiadiazole: A polarity-sensitive fluorescent probe

2,1,3-Benzothiadiazole (BTD) derivatives have emerged as versatile photofunctional platforms due to their strong electron-accepting character, synthetic accessibility, and tunable optical properties. Despite extensive studies on BTD-based fluorophores, the fluorosolvatochromic behavior of BTDs bearing N,N-dimethylamino substituents remains insufficiently explored. Here, we present a comprehensive investigation of N,N-dimethyl-4-amino-2,1,3-benzothiadiazole (BTDNMe2) in twenty-seven solvents using UV–vis absorption and fluorescence emission spectroscopy. BTDNMe2 exhibits weak solvatochromism in absorption but pronounced fluorosolvatochromism in emission, with a clear bathochromic shift of the fluorescence maxima as solvent polarity increases. The molar transition energy correlates positively with Reichardt’s normalized polarity scale ( E T N ), confirming the positive solvatochromic character of the probe. Multiparametric LSER analysis identified hydrogen-bond donor acidity and dipolarity as the dominant factors influencing solute–solvent interactions in the excited state, with the modified Kamlet–Abboud–Taft model providing the best statistical fit ( R 2 = 0.96 ). Dipole moment calculations using the Lippert–Mataga, Bakhshiev, and Kawski–Chamma–Viallet approaches revealed significantly higher values in the excited state compared to the ground state, consistent with intramolecular charge transfer (ICT). Theoretical calculations corroborated these findings, highlighting the rigidity of the molecular framework and the stabilization of the excited state by solvent polarity. Taken together, these results enhance the understanding of the fluorosolvatochromic properties of BTD derivatives and highlight the potential of BTDNMe2 as a straightforward, solvent-polarity-sensitive fluorescent probe.