Pagoda[5]arene with Large and Rigid Cavity for the Formation of 1∶2 Host–Guest Complexes and Acid/Base-Responsive Crystalline Vapochromic Properties

Pagoda[5]arene (P5), which is composed of five 2,6-dimethoxylanthracene (2,6-DMA) subunits bridged by methylene groups at 1,5 positions, was conveniently synthesized in 43% by trifluoroacetic acid (TFA)-catalyzed one-pot condensation of commercially available 2,6-DMA and paraformaldehyde in 1,2-dichlorobenzene (o-DCB) at room temperature. P5 showed a highly symmetrical pentagonal structure with a large and deep electron-rich cavity, which made it form stable 1:2 host-guest complexes with various aryl-pyridinium, bipyridinium, and stilbazolium salts in solution. Interestingly, P5 could complex two or even three guests with its intrinsic cavity and the pseudo-cavity formed by the adjacent macrocycles in solid states. Moreover, the strong intermolecular charge-transfer (ICT) interactions between P5 and the guests were observed. Especially, the complexes formed by P5 and the protonated pyridinium or bipyridinium salts showed acid/base-responsive color changes in solution and remarkable crystalline vapochromic properties by two kinds of processes: from ICT to no ICT and from ICT to enhanced ICT interaction or vice versa. We believe that the easily available P5 with the specific structure and intriguing 1:2 host-guest complexation can find wide application in supramolecular assemblies and functional materials.