A coumarin-based fluorescent PET sensor utilizing supramolecular pKa shifts

• Published in: Tetrahedron letters 2011-10, Vol.52 (41), p.5249-5254
• Main Authors: Saleh, Na’il, Al-Soud, Yaseen A., Al-Kaabi, Leena, Ghosh, Indrajit, Nau, Werner M.
• Format: Article
• Language: English
• Subjects: Benzothiazole; Chemistry; Coumarin; Cross-disciplinary physics: materials science; rheology; Cucurbiturils; Exact sciences and technology; Fluorescence; Heterocyclic compounds; Heterocyclic compounds with n hetero atom and also o and/or s, se, te hetero atoms; Heterocyclic compounds with o, s, se, te hetero atom and condensed derivatives; Heterocyclic compounds with several n hetero atoms in the same ring, in separated rings or in fused rings; Host–guest systems; Materials science; Methods of nanofabrication; Organic chemistry; Photoinduced electron transfer (PET); Physics; Piperazine; pKa Shifts; Preparations and properties; Sensors; Supramolecular and biochemical assembly
• ISSN: 0040-4039; 1873-3581
• DOI: 10.1016/j.tetlet.2011.07.138

A new coumarin-based sensor (P-3) is synthesized. The photoinduced electron transfer in P-3 is hindered by protonation of its benzothiazole anchor. The CB7-induced protonation at pH 3.5, at which the uncomplexed P-3 would remain unprotonated, causes a fluorescence enhancement of ca. 45-fold. A new coumarin derivative containing benzothiazole and piperazine substituents was synthesized. Preferential inclusion of the benzothiazole group, over the coumarin and piperazine groups, inside the cavity of the molecular container cucurbit[7]uril (CB7) was evidenced by using optical and NMR techniques. The binding constant of the new complex with CB7 is higher in its protonated forms (e.g., K=2.8×106M−1) than in its neutral forms, which led to an increase in the pKa value associated with protonation of the aza nitrogen on the benzothiazole ring of ca. 2.5units. Such CB7-induced protonation disabled the photoinduced electron transfer (PET) in the included molecule, enhancing its coumarin fluorescence up to ca. 45-fold (pH 3.5, 410nm). The results are discussed in the context of designing sensitive analytical tools for reversible monitoring of optically inactive analytes by competitive displacement experiments.