Evaluation of the supramolecular interaction of Congo red with cucurbiturils using mass spectrometry and spectroscopic methods

The ability of cucurbit[n]urils (CB[n]) to decolourise aqueous solutions of the azo dye Congo red (CR) was described more than a century ago alongside the first synthesis of CB[n]. No subsequent studies of the nature of the physical interactions have been reported despite the interest in using CB[n] as adsorbents for the removal of CR and related organic dyes from wastewaters. In the present work the supramolecular interaction between CB[n] (n = 7,8) and CR was studied by electrospray ionisation mass spectrometry (ESI-MS), 1H NMR, and solid-state characterisation of isolated complexes. Under positive ESI, the formation of host-guest complexes in the gas phase was not observed, suggesting that CR anions do not interact with the portals and the nonpolar inner cavity of the CB[n] molecules. Conversely, under negative ESI, 1:1 and higher order (1:2, 2:1, 3:1 and 2:2) CR:CB[7] and CR:CB[8] adducts were detected, which is attributed to interaction between CR and the outer surface hydrogens of CB[n]. Solid-state supramolecular adducts between CB[n] and CR were isolated from aqueous media under either ambient conditions, giving structures denoted as CR@CB[n](RT), or hydrothermal (100 °C) conditions, giving structures denoted as CR@CB[n](100). The adducts were characterised by elemental and thermogravimetric analyses (TGA), powder X-ray diffraction (PXRD), and spectroscopic methods (FT-IR, FT-Raman, 13C{1H} CP MAS NMR, UV/Vis and near-IR absorption, fluorescence excitation and emission). The interaction conditions and acid content of the starting CB[n] influenced the protonation state of CR molecules. CR@CB[7](RT) contained only unprotonated CR, while CR@CB[8](100) contained exclusively protonated CR (ammonium and azonium/quinoid structures). Other adducts contained mixtures of protonated/unprotonated forms.