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 CB6] and CB7] leads to minor shifts in the fluorescence wavelength maximum (lambda(fl)) or quantum yield (Phi(fl)). In sharp contrast, for complexes with CB8], the emission results in the simultaneous observation of fluorescence (lambda(fl)=480 nm, Phi(fl)=0.05) and room-temperature phosphorescence (lambda(ph)=590 nm, Phi(ph)=0.15). The occurrence of room-temperature phosphorescence can be used to detect the presence of CB8] 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 CB8] encapsulation. The remarkably high room-temperature phosphorescence quantum yield of the TP(+)@CB8] 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(+)@CB7] fail to exhibit electroluminescence.