Design, Synthesis, and Pb2+-Selective Optical Sensing Properties of Novel π-Conjugated Dibenzosuberenone–Dihydropyridazine Fluorophores

Herein, five new π-conjugated small organic molecules (DD1-DD5) were synthesized starting from dibenzosuberone (1), applying a modular approach based on Suzuki coupling and Diels–Alder/retro-Diels–Alder (DA/rDA) reactions. The key intermediate, dibromodibenzosuberenone 4, was obtained in three steps, and its Suzuki couplings with aryl and heteroaryl boronic acids furnished compounds 10-12. A Sonogashira-type modification of compound 4 enabled the preparation of the diacetylene derivative 14, which, together with compounds 10-12, was reacted with tetrazines 20 and 22 via DA/rDA reactions to afford the target cycloadducts DD1-DD5. All compounds were characterized by NMR, FT-IR, and HRMS analyses. UV–vis and fluorescence studies demonstrated that all DD1-DD5 derivatives exhibited pronounced selectivity and high sensitivity toward Pb2+ ions, showing distinct absorbance changes, strong turn-on emission, and >100 nm mega-Stokes shifts. DD1 displayed the most significant Pb2+ response, while electron-withdrawing groups of DD4 (–NO2) and DD5 (–CF3) modulated their optical behavior. Additional but weaker selectivity toward Fe3+ was observed for DD4 and DD5. Job plot analysis indicated 1:1 binding for DD1-DD2 and 2:1 Pb2+–ligand stoichiometry for DD3-DD5. 1H NMR titrations supported Pb2+ complexation and revealed multi-dentate binding, particularly for DD3. Overall, the DD scaffold provides a tunable platform for selective Pb2+ sensing with complementary anion-responsive behavior.