| Summary: | In order to explore how cucurbituril hosts accommodate an <i>N</i>-phenyl-pyridinium derivative guest, the complexation of the solvatochromic dye, 4-(4-(dimethylamino)styryl)-1-phenylpyridinium iodide (PhSt) with <i>,</i><i>′</i><i>,</i><i>δ</i><i>,</i><i>δ</i><i>′</i>-tetramethyl-cucurbit[6]uril (Me<sub>4</sub>CB6) and cucurbit[7]uril (CB7) was investigated by absorption spectroscopic, fluorescence and NMR experiments. In aqueous solutions, PhSt forms 1:1 complexes with both cucurbiturils, the complex with CB7 has a higher stability constant (<i>K</i><sub>a</sub> = 6.0 × 10<sup>6</sup> M<sup>−</sup><sup>1</sup>) than the complex with Me<sub>4</sub>CB6 (<i>K</i><sub>a</sub><i> </i>= 1.1 × 10<sup>6</sup> M<sup>−</sup><sup>1</sup>). As revealed by NMR experiments and confirmed by theoretical calculations, CB7 encapsulates the whole phenylpyridinium entity of the PhSt cation guest, whereas the cavity of Me<sub>4</sub>CB6 includes only the phenyl ring, the pyridinium ring is bound to the carbonyl rim of the host. The binding of PhSt to cucurbiturils is accompanied by a strong enhancement of the fluorescence quantum yield due to the blocking of the deactivation through a twisted intramolecular charge transfer (TICT) state. The TICT mechanism in PhSt was characterized by fluorescence experiments in polyethylene glycol (PEG) solvents of different viscosities. The PhSt-CB7 system was tested as a fluorescence indicator displacement (FID) assay, and it recognized trimethyl-lysine selectively over other lysine derivatives.
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