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- Synthesis, structure, and optical properties of an alternating calix[4]arene-based meta-linked phenylene ethynylene copolymerPublication . D. Barata, Patrícia; Costa, Alexandra; Ferreira, Luis F. V.; Prata, José VirgílioNovel alternating copolymers comprising biscalix[4]arene-p-phenylene ethynylene and m-phenylene ethynylene units (CALIX-m-PPE) were synthesized using the Sonogashira-Hagihara cross-coupling polymerization. Good isolated yields (60-80%) were achieved for the polymers that show M-n ranging from 1.4 x 10(4) to 5.1 x 10(4) gmol(-1) (gel permeation chromatography analysis), depending on specific polymerization conditions. The structural analysis of CALIX-m-PPE was performed by H-1, C-13, C-13-H-1 heteronuclear single quantum correlation (HSQC), C-13-H-1 heteronuclear multiple bond correlation (HMBC), correlation spectroscopy (COSY), and nuclear overhauser effect spectroscopy (NOESY) in addition to Fourier transform-Infrared spectroscopy and microanalysis allowing its full characterization. Depending on the reaction setup, variable amounts (16-45%) of diyne units were found in polymers although their photophysical properties are essentially the same. It is demonstrated that CALIX-m-PPE does not form ground-or excited-state interchain interactions owing to the highly crowded environment of the main-chain imparted by both calix[4]arene side units which behave as insulators inhibiting main-chain pi-pi staking. It was also found that the luminescent properties of CALIX-m-PPE are markedly different from those of an all-p-linked phenylene ethynylene copolymer (CALIX-p-PPE) previously reported. The unexpected appearance of a low-energy emission band at 426 nm, in addition to the locally excited-state emission (365 nm), together with a quite low fluorescence quantum yield (Phi = 0.02) and a double-exponential decay dynamics led to the formulation of an intramolecular exciplex as the new emissive species.
- New entities for sensory chemistry based on calix[4]arene-carbazole conjugates: from synthesis to applicationsPublication . Barata, Patrícia; Prata, José VirgílioTwo 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 solidstate sensors. Remarkable high and fast responses were in fact retrieved upon contact with saturated vapours of 2,4,6trinitrotoluene (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 30s 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 10s 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.
- Highly selective and sensitive detection of toxic metals by fluorescent Bicyclic Calix[4]arene-based sensorsPublication . D. Barata, Patrícia; Fialho, Carina B.; Prata, José Virgílio; Costa, Alexandra I.Development of fast and portable chemosensors for trace detection of toxic metals, in particular those which are mostly present in the environment due to natural phenomenon and human activities (e.g. cadmium, mercury and lead), is a challenging area of current research.1 Calixarenes are one of the most widespread scaffolds in host-guest chemistry because of their rigid structures, which make them perfect candidates for complexation studies with ions and neutral molecules. Metal ions commonly bind at the lower rim of the calixarene moiety. Host-guest interaction can be enhanced by proper choice of additional binding sites containing nitrogen, oxygen, sulfur or a combination of them, and specifically designed calixarene architectures. Exploring the inherent capabilities of certain fluorescent calixarene-based compounds for establishing strong host:guest interactions, several sensing materials have been developed and tested by us towards the detection of neutral molecular species.2. We report in this communication the chemosensing ability of CALIX-OCP-CBZ and CALIX-OCP (Scheme 1) towards the detection of toxic metals, either by using the sensing element in fluid phase or solid state.
- Calix[4]arene-carbazole-containing polymers: synthesis and propertiesPublication . D. Barata, Patrícia; Costa, Alexandra; Prata, José VirgílioNew highly fluorescent calix[4]arene-containing phenylene-alt-ethynylene-3,6- and 2,7-carbazolylene polymers (CALIX-PPE-CBZs) have been synthesized for the first time and their photophysical properties evaluated. Both polymers were obtained in good isolated yields (70-84%), having M-w ranging from 7660-26,700 g mol(-1). It was found that the diethynyl substitution (3,6- or 2,7-) pattern on the carbazole monomers markedly influences the degree of polymerization. The amorphous yellow polymers are freely soluble in several nonprotic organic solvents and have excellent film forming abilities. TG/DSC analysis evidences similar thermal behaviors for both polymers despite their quite different molecular weight distributions and main-chain connectivities (T-g, in the range 83-95 degrees C and decomposition onsets around 270 degrees C). The different conjugation lengths attained by the two polymers dictates much of their photophysical properties. Thus, whereas the fully conjugated CALIX-PPE-2,7-CBZ has its emission maximum at 430 nm (E-g = 2.84 eV; Phi(F) = 0.62, CHCl3), the 3,6-linked counterpart (CALIX-PPE-3,6-CBZ) fluoresces at 403 nm with a significant lower quantum yield (E-g = 3.06 eV; Phi(F) = 0.31, CHCl3). The optical properties of both polymers are predominantly governed by the intrachain electronic properties of the conjugated backbones owing to the presence of calix[4]arenes along the polymer chain which disfavor significant interchain interactions, either in fluid- or solid-state.