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- Metal ion recognition induced by calix[4]arene carbazole containing polymersPublication . D. Barata, Patrícia; Costa, Alexandra; Fialho, Carina B.; Prata, José VirgílioSensing and recognition of ions and neutral molecules via synthetic receptors are of current interest in supramolecular chemistry because of their significant importance in several areas, such as chemistry, biology and environment. Compared with small molecules, polymers-based sensors displayed several importante advantages like signal amplification. In this way, the incorporation of molecular receptors such as calixarenes with conjugated polymer backbones is expected to enhance the signaling events related to a host–guest interaction. The preorganized binding sites, easy derivatization and flexible three-dimensional steric structures make calixarenes ideal construction platforms for molecular design to generate fluorescente receptors. The use of calixarenes as supramolecular scaffolds for this type of architectures has been explored and the sensing abilities of resultant polymers toward metal and molecular ions established. Based on the high sensitivity shown by the non-polymeric analogue CALIX-OCP-CBZ (notshown), to toxic metal cations, we decide two extend the sensing study to polymer materials. Herein, we report the preliminar results of the chemosensing ability of a new bicyclic calix[4]arene-carbazole-polymer (CALIX-OCP-PPE-CBZ) towards the detection of toxic metals in fluid phase.
- Fostering protein-calixarene interactions: from molecular recognition to sensingPublication . Prata, José Virgílio; D. Barata, PatríciaTwo isomeric bis-calixarene-carbazole conjugates (CCC-1 and CCC-2) endowed with carboxylic acid functions at their lower rims have been found to display a high sensing ability (KSV up to 6 x 10(7) M-1) and selectivity toward cytochrome c, a multi-functional protein, in an aqueous-based medium. After targeting basic amino acid residues on the protein surface residing near the prosthetic heme group through electrostatic and hydrophobic interactions, a rapid photoinduced electron transfer ensues between the integrated transduction element (aryleneethynylene chromophore) of CCCs and the iron-oxidized heme of cytochrome c, enabling direct detection of the protein at nanomolar levels. Our results show that CCCs are capable of efficiently discriminating heme proteins (cytochrome c vs. myoglobin) and non-heme proteins (lysozyme) in an aqueous medium. Studies performed in two solvent systems (organic and aqueous) strongly suggest that in an organic medium a Forster-type resonance energy transfer is responsible for the observed reduction in CCCs emission upon contact with heme proteins while in an aqueous medium a specific photoinduced electron transfer mechanism prevails.
- 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.
- Cooperative effects in the detection of a nitroaliphatic liquid explosive and na explosive taggant in the vapor phase by calix[4]arene-based carbazole-containing conjugated polymersPublication . D. Barata, Patrícia; Prata, José VirgílioTwo fluorescent molecular receptor based conjugated polymers were used in the detection of a nitroaliphatic liquid explosive (nitromethane) and an explosive taggant (2,3-dimethyl-2,3-dinitrobutane) in the vapor phase. Results have shown that thin films of both polymers display remarkably high sensitivity and selectivity toward these analytes. Very fast, reproducible, and reversible responses were found. The unique behavior of these supramolecular host systems is ascribed to cooperativity effects developed between the calix[4] arene hosts and the phenylene ethynylene-carbazolylene main chains. The calix[4]-arene hosts create a plethora of host-guest binding sites along the polymer backbone, either in their bowl-shaped cavities or between the outer walls of the cavity, to direct guests to the area of the transduction centers (main chain) at which favorable photoinduced electron transfer to the guest molecules occurs and leads to the observed fluorescence quenching. The high tridimensional porous nature of the polymers imparted by the bis-calixarene moieties concomitantly allows fast diffusion of guest molecules into the polymer thin films.
- 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.
- Inherently chiral calix[4]arenes with planar chirality: two new entries to the familyPublication . Prata, José Virgílio; D. Barata, Patrícia; Pescitelli, GennaroThe synthesis of two new inherently chiral calix[4]arenes (ICCs, 1 and 2), endowed with electron-rich concave surfaces, has been achieved through the desymmetrization of a lower rim distal-bridged oxacyclophane (OCP) macrocycle. The new highly emissive ICCs were resolved by chiral HPLC, and the enantiomeric nature of the isolated antipodes proved by electronic circular dichroism (CD). Using time-dependent density functional calculations of CD spectra, their absolute configurations were established. NMR studies with (S)-Pirkle's alcohol unequivocally showed that the host-guest interactions occur in the chiral pocket comprehending the calix-OCP exo cavities and the carbazole moieties.
- 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.