New entities for sensory chemistry based on calix[4]arene-carbazole conjugates: from synthesis to applications

Abstract

Two new calix[4]arene-carbazole conjugates (CALIX-CBZs) comprising 2- and 3-ethynyl-substituted carbazole derivatives attached to a central bis-calix[4]arene-containing phenylene ring have been designed for fluorescence-based detection of high explosive materials and explosive markers in vapour phase. The title compounds were prepared in good isolated yields and structurally fully characterised. CALIX-CBZs are highly fluorescent compounds that largely preserve their deep blue luminescence in solid state with no notorious emissions from electronic aggregated states. The excellent optical properties exhibited by casted films of both materials, including their photochemical stability, suggested their potential use as solid-state sensors. Remarkable high and fast responses were in fact retrieved upon contact with saturated vapours of 2,4,6-trinitrotoluene (TNT, a high explosive) and 2,4-dinitrotoluene (a common impurity in TNT batches, often used as its chemical marker), reaching near 80% of fluorescence quenching for the later on 30 s of exposure. Experiments performed with nitroaliphatic compounds (nitromethane (NM), a liquid explosive and 2,3-dimethyl-2,3-dinitrobutane, an explosive taggant) also revealed a high level of sensitivity (up to near 40% fluorescence quenching in only 10 s of exposure to NM). The quenching efficiencies were overall correlated with the extent and strength of CALIX-CBZs–analyte interactions, the vapour pressure of the analytes and the film thicknesses.

Keywords::

• calixarene
• carbazole
• fluorescence
• detection
• explosive
• nitroaromatic
• nitroaliphatic
• taggant

Acknowledgements

The authors thank Fundação para a Ciência e a Tecnologia/MCTES (Portugal) for partial financial support (PEst-OE/EQB/UI0702/2011). The authors gratefully acknowledge Prof. Manuel J. Matos (Instituto Superior de Engenharia de Lisboa and Instituto de Telecomunicações) assistance with profilometry measurements.

Notes

^1. Reported equilibrium vapour pressures (p) of analytes at 25°C are as follows: p_TNT = 8.79 × 10^− 6 mmHg (40), p_2,4-DNT = 1.44 × 10^− 4 mmHg (40), p_DMNB = 2.07 × 10^− 3 mmHg (41) and p_NM = 35.925 mmHg (42).

^2. A multitude of interactions should also come from the calixarene moieties (not involved in the aforementioned modelling studies due to computational limitations at this level of theory), giving the electron-rich surfaces of each of their sub-units.

^3. Molecular sizes (volumes) of TNT (194.37 Å³), 2,4-DNT (168.53 Å³), DMNB (181.30 Å³) and NM (59.91 Å³) were calculated from DFT/B3LYP/6-311G^* (35) using a 0.002 au isodensity surface.

^4. For an account of the literature references concerning DMNB and NM detection, see Ref. (32).