Tailoring dansyl probes: H vs CHO substitution for selective sub-nanomolar detection of picric acid

Fluorescent probes present an appealing alternative for the detection of environmental pollutants. Herein, we report the design and synthesis of -S-O-appended dansyl fluorophores 1 and 2, which exhibit enhanced photo-stability. Compound 1, bearing a phenyl functionality, shows excellent aggregation induced emission (AIE) behaviour along with positive solvatochromism, enabling the detection of picric acid (PA) at the nanomolar level through charge-transfer (CT) and π-π interactions. A small structural modification, from H to CHO at the C₄ position of the phenyl ring, yields compound 2, shows a remarkedly higher affinity toward PA, achieving sub-nanomolar detection. Compound 2 displays AIE and dual emission in moderately polar solvents. The enhanced sensitivity arises from ground-state complexation, which promotes π-π interactions and charge transfer interactions with PA. The interaction mechanisms were further investigated using Stern-Volmer (K_SV) analysis, TCSPC, and quantum-yield measurements. Finally, the sensing performance was validated through real-time PA detection in water, soil and in the vapour phase.