Cucurbit[5]uril-metal complex-induced room-temperature phosphorescence of α-naphthol and β-naphthol

Similar to the larger members of the cucurbituril family, such as cucurbit[8]uril (Q[8]), the smallest member, cucurbit[5]uril (Q[5]), can also induce room-temperature phosphorescence (RTP) of alpha-naphthol (1) and beta-naphthol (2). The relationship between the RTP intensity of 1 and 2 and the concentration of Q[5] or Q[8] suggests that the mechanism underlying the Q[5] complex-induced RTP is different from that of the Q[8]-induced RTP for these luminophores. The crystal structures of 1-Q[5]-KI, 2-Q[5]-KI, 1-Q[5]-TlNO3, and 2-Q[5]-TlNO3 systems show that in each case Q[5] and the respective metal ions, K+ or Tl+, form infinite ...Q[5]-M+-Q[5]-M+... chains that surround the luminophores. Although these tube- or wall-like structures are likely destroyed in solution, the key interaction between the convex-shaped outer walls of Q[5] and the plane of the aromatic naphthols, via pi...pi stacking and C-H...pi interactions, is postulated to be essentially maintained leading to a microenvironment that holds the luminophore and the heavy atom perturber together; such a model is supported by the observed Q[5] complex-induced RTP of the above naphthols. With respect to this, a high Q[5]/luminophore concentration was employed in an endeavour to promote the formation of pi...pi stacking and C-H...pi interactions similar to those observed in the crystal structures of the 1- or 2-Q[5]-K+ and -Tl+ systems. In keeping with the proposed model, the RTP of each system is quenched when Q[5] is replaced by the alkyl-substituted Q[5] derivatives, decamethylQ[5] and pentacyclohexanoQ[5]. This is in agreement with the substituent groups on the surface of the metal-bond Q[5] obstructing the naphthol molecule from accessing the convex glycouril backbone of Q[5].