& Anion Recognition Differential Recognition of Anions with Selectivity towards F  by a Calix[6]arene–Thiourea Conjugate Investigated by Spectroscopy, Microscopy, and Computational Modeling by DFT Anita Nehra, Sateesh Bandaru, Deepthi S. Yarramala, and Chebrolu Pulla Rao* [a] Abstract: Anion recognition studies were performed with triazole-appended thiourea conjugates of calix[6]arene (i.e., compound 6 L) by absorption and 1 H NMR spectroscopy by using nineteen different anions. The composition of the spe- cies of recognition was derived from ESI mass spectrometry. The absorption spectra of compound 6 L showed a new band at l = 455 nm in the presence of F  due to a charge transfer from the anion to the thiourea moiety and the ab- sorbance increases almost linearly in the concentration range 5 to 200 mm. This is associated with a strong visual color change of the solution. Other anions, such as H 2 PO 4  and HSO 4  , exhibit a redshift of the l = 345 nm band and the spectral changes are associated with the formation of an isosbestic point at l = 343 nm. 1 H NMR studies further con- firm the binding of F  efficiently to the thiourea group among the halides by shifting the thiourea proton signals downfield followed by their disappearance after the addition of more than one equivalent of F  . The other anions also showed interactions with compound 6 L, however, their bind- ing strength follows the order F  > CO 3 2 > H 2 PO 4   CH 3 COO  > HSO 4  . The NMR spectral changes clearly re- vealed the anion-binding region of the arms in case of all these anions. The anion binding to compound 6 L indeed sta- bilizes a flattened-cone conformation as deduced based on the calix-aromatic proton signals and was further confirmed by VT 1 H NMR experiments. The stabilization of the flat- tened-cone conformation was further augmented by the in- teraction of the butyl moiety of the nBu 4 N + counterion. The structural features of the anion-bound species were demon- strated by DFT computations and the resultant structures carried the features that were predicted based on the 1 H NMR spectroscopic measurements. In addition, SEM images showed a marigold flower-type morphology for compound 6 L and this has been transformed into a chain- like structure of connected spherical particles in the pres- ence of F  . The anion-induced microstructural features are reflective of the binding strength, size, and shape of the anions. The binding strengths of the anions by compound 6 L were further compared with that of compound 4 L, a cal- ix[4]arene analogue of compound 6 L, in order to address the role of the number of arms built on the calixarene platform based on absorption spectroscopy, 1 H NMR spectroscopy, and DFT computations and it was found that compound 6 L is a better receptor for F  , which extends its interactions from all the three arms. Introduction Anion recognition is a subject of great interest in supramolec- ular and bioinorganic chemistry. [1–9] The anion–receptor interac- tions are mainly governed by weak interactions such as hydro- gen bonding and electrostatic interactions and thus, these differ distinctly from the cation–receptor interactions. [10, 11] Therefore, the design of anion receptors differ considerably from that of cation ones. In constructing the anion receptors, the thiourea unit has been widely utilized [12] because this unit acts as a good hydrogen donor for its interaction with anions and provides selective sensing. [13–21] Both N H groups of thio- urea can bind to anions by unidirectional hydrogen bonding. These features are evident from the crystal structures of anion- bound thiourea-based ligands. [22–27] In the literature N-benzami- dothioureas and thiourea-based receptors are being developed for anion recognition. [12] In this regard, herein we report the synthesis and characterization of the triazole-linked thiourea- appended calix[6]arene conjugate 6 L and its differential anion recognition by absorption and 1 H NMR spectroscopy and the stoichiometry of the species of recognition by ESI mass spec- trometry. Calix[6]arene is known to have more flexibility be- sides the fact that it can accommodate more arms and a bigger cavity size as compared to its calix[4]arene counter- part and hence would involve in a variety of applications. [28–34] The species of recognition of the conjugate with all the anions were computationally modeled and the corresponding com- plexation energies were established and compared with the spectral changes. Because the calixarenes are known to form [a] Dr. A. Nehra, Dr. S. Bandaru, D. S. Yarramala, Prof. Dr. C. P. Rao Department of Chemistry, Indian Institute of Technology Bombay Powai, Mumbai-400076 (India) Fax: (+ 91) 22-2572-3480 E-mail : cprao@iitb.ac.in Supporting information for this article (containing the characterization of the precursors and spectral data, absorption spectral traces, 1 H NMR titra- tion, ESI MS, and computational data) is available on the WWW under http ://dx.doi.org/10.1002/chem.201600844. Chem. Eur. J. 2016, 22, 8903 – 8914 # 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 8903 Full Paper DOI: 10.1002/chem.201600844