Fluorophore-Appendant 5,5′-Bicalixarene Scaffolds for Host–Guest Sensing of Nitric Oxide

Abstract

Conjugated 5,5′-Bicalixarene scaffolds having fluorophores at the chain termini have been prepared and tested in the supramolecular detection of nitric oxide. Scaffolds bearing electron-rich fluorophores demonstrated a stronger turn-off response to the presence of NO than the fluorophore-free analogue in both organic and aqueous media, while no fluorescence quenching happened when the electron-deficient fluorophores were employed. Unprecedented ratiometric supramolecular sensing was observed when fluorophores of the opposite electronic demands were placed at the scaffold’s termini.

Electronic Tuning of Host-Guest Interactions within the Cavities of Fluorophore-Appended Calix[4]arenes

“…To verify this hypothesis, we developed synthetic protocols toward unsymmetrically substituted hosts 3a–d ( Scheme 1 , Scheme 2 and Scheme 3 ). Moreover, although we earlier reported the synthesis of the parent compound 3a (Φ = 0.17) [ 23 ], we have now modified the procedure to obtain the desired compound in only three steps ( Scheme 1 ).…”

“…To prepare compounds 3b and 3c , bearing, at the C ring, the electron-donating piperidine group and electron-withdrawing cyano group, respectively, the corresponding bromo-derivative 7 was prepared in three steps [ 23 ]. Reacting compound 7 with piperidine under the Buchwald–Hartwig amination conditions afforded the amino derivative 8 which was converted to the triflate 9 .…”

“…While studying the chemosensory properties of oxygen-depleted 5,5′-Bicalixarene scaffolds ( 1c ) bearing an alkyne function at the lower rim [ 21 ], we discovered that these compounds show strong NMR and fluorescence response upon the complexation of N-methyl pyridinium cation ( 2 ) [ 20 , 22 ]. Attachment of electron-donating fluorophores at the termini of the bicalixarene fragment expectedly increased the host–guest complexation properties of the scaffolds [ 23 ]. Yet, this observation alone does not provide compelling evidence for the π interactions with the oxygen-depleted part of the calixarene moiety.…”

Carbazole-fused calixarene cavities: single and mixed AIEgen systems for NO detection

“…20 Interestingly, while compound 1d or its 5,5'-bicalixarene congener (1b, Aryl = naphthalimide, Fig. 1) 20 did not show any fluorescence quenching upon passing NO through their DCE solutions, the fluorescence intensity of a carbazole-fused calixarene 2b decreased by ca. 37% from the original value under the same conditions.…”

“…To this end, we decided to explore whether the aggregation of a carbazole-containing calixarene host 1c in water-organic solvent mixtures will turn it into an AIE luminogen (AIEgen) and enhance its fluorescence response to NO gas compared with the moderate response obtained in organic solvents. 20 After the initial screening, we selected DMF-water system which showed strong AIE changes from blue emission in pure DMF solution to cyan fluorescent aggregates (λ em 450 nm) in a 60% water-DMF mixture. To our chagrin, the new aggregates did not show significant fluorescence changes after NO gas was passed through the solution for 5 minutes.…”