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Abstract
A fluorophore-anchor dye based on the 4-amino-1,8-naphthalimide chromophore was designed and characterised with respect to its ability of forming supramolecular host–guest complexes with cucurbit[6]uril (CB6) and cucurbit[7]uril (CB7) hosts. It was found that CB6 encapsulates the anchor of the dye with a high binding constant [K = (1.1–1.4) × 107 M− 1], which was independent of acidic or neutral pH conditions. The 1:1 binding was accompanied by fluorescence quenching (ca. 20%) and a bathochromic shift (Δλ = +22 nm) of the charge-transfer absorption band of the dye. This is indicative of the occurrence of hydrogen bonding between a carbonyl-lined host portal and the aromatic NH group of the guest dye. In comparison, CB7 encapsulated the dye much less efficiently (K = 4.8 × 104 M− 1) and showed a significant fluorescence enhancement (Φf = 0.15 versus 0.52 for free and CB7-complexed dye at pH 4, respectively). The occurrence of hydrogen bonding with the aromatic NH was much less evident for CB7 as judged by the only minor bathochromic shift (Δλ = +4 nm) of the charge-transfer absorption band of the dye. The dye–CB6 assembly has the potential for the detection of biogenic amines under physiological pH conditions and at low analyte concentrations.
Acknowledgements
The financial support by the Spanish Ministry of Economy and Competitiveness, Madrid (Grant CTQ2011-28390 for UP), the Portuguese Foundation for Science and Technology, Lisbon (doctoral fellowship SFRH/BD/81628/2011 for CPC, Grant REEQ/717/QUI/2005 for JPDS) and the POCTEP programme (I2TEP) is gratefully acknowledged. We thank Prof. W.M. Nau (Jacobs University Bremen, Germany) for a CB7 sample.