# Fluorescent Dihomooxacalix[4]arenes for the Detection of Nitroaromatic Compounds in Solution and in the Vapour Phase: Structural and Supramolecular Insights

## TL;DR

Researchers developed fluorescent calixarene compounds that can detect nitroaromatic compounds in both liquid and vapor forms, with structural insights into their sensing mechanism.

## Contribution

The study introduces new fluorescent calixarene derivatives with structural adaptability for efficient and reversible detection of nitroaromatic compounds.

## Key findings

- The calixarene derivatives show selectivity for 2,4,6-trinitrophenol (TNP) with a specific quenching order.
- Both calixarenes enable fast and reversible detection of nitrobenzene (NB) and nitrotoluene (NT) vapors.
- Structural flexibility and solvent effects influence the sensing performance of the calixarene framework.

## Abstract

Two fluorescent ureido-dihomooxacalix[4]arene derivatives containing naphthyl residues at the lower rim (1 and 2) were studied for the detection of nitroaromatic compounds (NACs) in solution and in vapour phases. Their affinity in solution was determined by UV-Vis absorption, fluorescence and NMR spectroscopy. For NAC vapour sensing, calixarenes were dispersed in a polytetrafluoroethylene (PTFE) matrix. Four new solvated crystals of dihomooxacalix[4]arene 2 were obtained and the solvent’s influence on its structural characteristics was investigated. The solvent-dependent structural variations observed in the crystal structures highlight the intrinsic flexibility of the calixarene framework. Such conformational adaptability, evident in the disruption and reorganization of hydrogen bonding and π–π interactions, is directly relevant to nitroaromatic sensing, where a rapid and reversible host response is crucial for effective detection. Theoretical calculations were also performed to provide further insights on the binding process. The corrected Stern–Volmer constants (KSV) obtained showed that both receptors present selectivity for TNP and follow the same quenching order (TNP > NT > NB > DNT > TNT > DNB). Factors other than electron density distribution should dominate the quenching extent and therefore the values of the SV constants, which will be greatly overestimated if no correction to the inner filter effect is applied. Detection of NB and NT and vapours by both calixarenes produced a complete, very fast (2 to 5 s), and reversible quenching, indicating the potential use of this porous PTFE–calixarene matrix for the sensing of volatile NACs.