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

authors	Costa, AL; Gomes, AC; Lopes, AD; Da Silva, JP; Pillinger, M; Goncalves, IS; de Melo, JSS
nationality	International
journal	NEW JOURNAL OF CHEMISTRY
keywords	ADSORPTION BEHAVIOR; WASTE-WATER; COMPLEXES; DYE; FLUORESCENT; CHEMISTRY; FERROCENE; DYNAMICS; REMOVAL
abstract	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), H-1 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 degrees 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, C-13{H-1} 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.
publisher	ROYAL SOC CHEMISTRY
issn	1144-0546
isbn	1369-9261
year published	2020
volume	44
issue	6
beginning page	2587
ending page	2596
digital object identifier (doi)	10.1039/c9nj05706d
web of science category	Chemistry, Multidisciplinary
subject category	Chemistry
unique article identifier	WOS:000514598200046