Specific Binding Effects for Cucurbit[8]uril in 2,4,6‐Triphenylpyrylium–Cucurbit[8]uril Host–Guest Complexes: Observation of Room‐Temperature Phosphorescence and their Application in Electroluminescence

2,4,6‐Triphenylpyrylium (TP + ) forms host–guest complexes with cucurbiturils (CBs) in acidic aqueous solutions. 1 H NMR spectroscopic data indicates that complexation takes place by encapsulation of the phenyl ring at the four position within CB. Formation of the complex with CB[6] and CB[7] leads to minor shifts in the fluorescence wavelength maximum ( λ fl ) or quantum yield ( Φ fl ). In sharp contrast, for complexes with CB[8], the emission results in the simultaneous observation of fluorescence ( λ fl =480 nm, Φ fl =0.05) and room‐temperature phosphorescence ( λ ph =590 nm, Φ ph =0.15). The occurrence of room‐temperature phosphorescence can be used to detect the presence of CB[8] visually in solution. Molecular modeling and MM2 molecular mechanics calculations suggest that this effect arises from locking the conformational mobility of the 2‐ and 6‐phenyl rings as a result of CB[8] encapsulation. The remarkably high room‐temperature phosphorescence quantum yield of the TP + @CB[8] complex has been advantageously applied to develop an electroluminescent cell that contains this host–guest complex. In contrast, analogous cells prepared with TP + or TP + @CB[7] fail to exhibit electroluminescence.