José Virgílio Coelho Prata

Diffuse reflectance studies of β-phenylpropiophenone and benzophenone inclusion complexes with calix[4], [6] and [8]arenesDedicated to Professor Frank Wilkinson on the occasion of his retirement.

The formation of inclusion complexes of β-phenylpropiophenone and benzophenone with p-tert-butylcalix[4], [6] and [8]arenes (powdered solid samples) was studied with the use of diffuse reflectance techniques. Experimental evidence was obtained for inclusion in all cases. The benzophenone n → π* absorption band exhibits hypsochromic shifts with an increase of the calixarene ring aperture, providing evidence for an increase in cavity polarity. β-Phenylpropiophenone exhibits strong room temperature phosphorescence in contrast to its solution behaviour, and this is a clear evidence for the inclusion of this probe within the calixarene molecules. Calix[4]arene and calix[8]arene provide a more rigid environment to β-phenylpropiophenone whereas calix[6]arene appears to be more flexible. Transient absorption spectra of inclusion complexes with benzophenone (1 : 1 mol : mol, excited at 355 nm) show major triplet formation in the calix[4]arene case, while the ketyl radical of benzophenone is formed in the calix[6]arene case and is predominant in the calix[8]arene case. Phenoxyl radical transient absorption also becomes evident in the last two cases. For β-phenylpropiophenone (1 : 1 mol : mol) phenoxyl radicals are formed but no evidence was found for triplet or ketyl radical formation of this ketone in the calix[4]arene case. However, strong support for the ketyl radical of the aryl alkyl ketone was found for calix[6]arene and calix[8]arene inclusion complexes.

Novel laser-induced luminescence resulting from benzophenone/O-propylated p-tert-butylcalix[4]arene complexes. A diffuse reflectance study

Laser-induced room temperature luminescence of air-equilibrated benzophenone/O-propylated p-tert-butylcalix[4]arene solid powdered samples revealed the existence of a novel emission, in contrast with benzophenone/p-tert-butylcalix[4]arene complexes, where only benzophenone emits. This novel emission was identified as phosphorescence of 1-phenyl-1,2-propanedione, which is formed as the result of an hydrogen atom abstraction reaction of the triplet excited benzophenone from the propoxy substituents of the calixarene. Room temperature phosphorescence was obtained in air-equilibrated samples in all propylated hosts. The decay times of the benzophenone emission vary greatly with the degree of propylation, the shortest lifetimes being obtained in the tri- and tetrapropylated calixarenes. Triplet-triplet absorption of benzophenone was detected in all cases, and is the predominant absorption in the p-tert-butylcalix[4]arene case. where an endo-calix complex is formed. Benzophenone ketyl radical formation occurs with the O-propylated p-tert-butylcalix[4]arenes hosts, suggesting a different type of host/guest molecular arrangement. Diffuse reflectance laser flash photolysis and gas chromatography-mass spectrometry techniques provided complementary information, the former about transient species and the latter regarding the final products formed after light absorption. Product analysis and identification clearly show that the two main degradation photoproducts following laser excitation in the propylated substrates are 1-phenyl-1,2-propanedione and 2-hydroxybenzophenone, although several other minor photodegradation products were identified. A detailed mechanistic analysis is proposed. While the solution photochemistry of benzophenone is dominated by the hydrogen abstraction reaction from suitable hydrogen donors, in these solid powdered samples, the alpha-cleavage reaction also plays an important role. This finding occurs even with one single laser pulse which lasts only a few nanoseconds, and is apparently related to the fact that scattered radiation exists, due to multiple internal reflections possibly trapping light within non-absorbing microcrystals in the sample, and is detected until at least 20 micros after the laser pulse. This could explain how photoproducts thus formed could also be excited with only one laser pulse.

Fostering protein–calixarene interactions: from molecular recognition to sensing

Two 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 × 107 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.

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

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.

Studies Towards the Living Polymerisation of Phenylethynyl-calix[4]arene Compounds with Rh-based Ternary Catalytic Systems

The living polymerisation of mono and difunctional phenylethynylcalix[4]arene compounds 1 and 2 by Rh(I) ternary catalytic systems (TCS) was examined. Two TCS were tentatively prepared in situ, adapting known methodologies: (1) Rh(C = CPh)(norbornadiene)(PPh3) and (2) Rh(C(Ph)C = CPh2)(norbornadiene)(PPh3). Using the first TCS, the conjugated polymers poly 1 and poly 2 could be obtained in very good yields (77–86%), in short reaction times and freed from low-molecular-weight products, only when NEt3 was used as a co-catalyst. With the second TCS, excellent results were obtained. Indeed, this catalytic system proved to be quite efficient in the polymerisation of calix[4]arenes 1 and 2, affording the correspondent poly 1 and poly 2 essentially in almost quantitative yields (by GPC analysis), under appropriate conditions. The living nature of the polymerisation has been proved. For instance, in the case of calix[4]arene 1, the M n of the polymer obtained at high monomer conversion increased proportionally with the [1]:[Rh] molar ratio in the feed, keeping [1]o constant, thus showing that irreversible chain transfer or termination reactions did not occur to a major extent. Under the most favourable conditions {[1]:[Rh] = 50 and [2]:[Rh] = 50}, the polydispersities of poly 1 and poly 2 were kept in a narrow range (1.16–1.30).

Chiroptical and emissive properties of a calix[4]arene-containing chiral poly (P-phenylene ethynylene) with enantioselective recognition ability

Supramolecular chirality was achieved in solutions and thin films of a calixarene-containing chiral aryleneethynylene copolymer. The observed chiroptical activity, which is primarily allied with the formation of aggregates of high molecular weight polymer chains, is the result of a combination of intrachain and interchain effects. The former arises by the adoption of an induced helix-sense by the polymer main-chain while the latter comes from the exciton coupling of aromatic backbone transitions. The co-existence of bulky bis-calix[4]arene units and chiral side-chains on the polymer skeleton prevents efficient π-stacking of neighbouring chains, keeping the chiral assembly highly emissive. In contrast, for a model polymer lacking calixarene moieties, the chiroptical activity is dominated by strong interchain exciton couplings as a result of more favourable packing of polymer chains, leading to a marked decrease of photoluminescence in the aggregate state. The enantiomeric recognition abilities of both polymers towards (R)- and (S)-α-methylbenzylamine were examined. It was found that a significant enantiodiscrimination is exhibited by the calixarene-based polymer in the aggregate state.

Intramolecular addition of acyldiazenecarboxylates onto double bonds in the synthesis of heterocycles

Appropriate aryl-substituted unsymmetrical azodicarbonyl compounds, generated from bishydrazides by oxidation, undergo intramolecular cyclisations to furnish a variety of useful heterocycles such as N-substituted oxindoles, carbostyrils, benzazepinones, benzazocinones, benzimidazolones, benzoxazinones and pyrazolones in varying degrees of efficiency. Methods are described to remove the N-acyl groups from the heteroaromatic compounds. Under mildly acidic conditions where equal opportunities are available for an ipso or a normal cyclisation it is the former process that occurs preferentially. Evidence is presented in favour of a C-to-C migration in the ipso product for the formation of a methoxy-substituted carbostyril derivative. One of the spiro substances is shown to participate in dienone–phenol rearrangement to provide the corresponding quinolone–phenol in high yield.

How an environmental issue could turn into useful high-valued products: the olive mill wastewater case

Carbon-based nanomaterials have been directly synthesized from olive mill wastewaters (OMWWs) for the first time, using expedite and simple environmental-friendly procedures. The OMWWs collected from a mill operating by a two-phase centrifugation system, after being heated (150-300 °C) solely or in the presence of additives for 2-12 h, furnished nanostructured materials in high yields. Under an optimised set of reaction conditions here described, the resultant as-synthetized aqueous dispersions of carbon nanoparticles exhibit outstanding fluorescence emission properties, which encompass an astonishing quantum yield (ΦF > 0.4). The as-prepared carbon nanomaterials show excitation-dependent emissions covering the whole visible spectrum, with a predominant high glow in the blue-green region, and a remarkable photostability. The relevant features attained by the nanomaterials here reported, allied to their easy synthesis and carbon source affordability, render them with unique capabilities to be used in several current and emerging technological applications, namely in bioimaging and nanomedicine, sensorial analysis, (photo)catalysis and optoelectronics. The as-synthesized nanoparticles show a remarkable high sensitivity and selectivity towards haemoglobin.

Inherently chiral calix[4]arenes with planar chirality: two new entries to the family

Abstract The 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.

Comparative Study of the Copolymerization Kinetics of Mono and Divinylbenzyl p-tert-Butylcalix[4]arene Derivatives and Styrene

A study of the copolymerization kinetics of 25,27-bis-(4-vinyl-benzyloxy)-26,28-dihydroxy-p-tert-butylcalix[4]arene (1) and 25,26,27-tripropoxy-28-(4-vinyl-benzyloxy)-p-tert-butylcalix[4]arene (2) with styrene (St) was undertaken. The radical copolymerizations were carried out in THF in the presence of benzoyl peroxide at 75°C for a certain period. Six molar feed ratios, ranging from 1:1 to 1:20 (1 or 2 to St), were used to calculate the reactivity parameters. The copolymer composition was determined by FT-IR spectroscopy using a Beers law plot obtained from the corresponding homopolymers. The reactivity ratio calculations were performed with the linearization methods of Fineman-Ross (F-R) and Kelen-Tüdös (K-T), assuming the validity of the so-called terminal model. In the copolymerization of the monoene 2, similar reactivity ratios were found for the comonomers (ca. 1.2; K-T). On the other hand, the reactivity ratios calculated for the copolymerization of 1 with St yielded r St = 0.67 and r calix = 3.0 (K-T method). The higher reactivity of monomer 1 as compared to styrene is discussed in connection with our previously postulated cyclocopolymerization route.