María J. Tapia

A comparative study of water soluble 5,10,15,20-tetrakis(2,6-dichloro-3-sulfophenyl)porphyrin and its metal complexes as efficient sensitizers for photodegradation of phenols

5,10,15,20-Tetrakis(2,6-dichloro-3-chlorosulfophenyl)porphyrin and its tin and zinc complexes were synthesized with high yields and fully characterized. The corresponding water-soluble 5,10,15,20-tetrakis(2,6-dichloro-3-sulfophenyl)porphyrins were obtained by hydrolysis with water. An extensive photophysical study of the new water soluble porphyrinic compounds was carried out including absorption and fluorescence spectra, fluorescence quantum yields, triplet absorption spectra, triplet lifetimes, triplet and singlet oxygen quantum yields. These sensitizers were successfully used in the photodegradation of 4-chlorophenol and 2,6-dimethylphenol. A comparison is made of their efficiencies, and some mechanistic considerations are highlighted.

Binding of polynucleotides to conjugated polyelectrolytes and its applications in sensing.

We provide a brief overview of the structural characteristics of the main groups of conjugated polyelectrolytes (CPEs) as well as the methods of synthesis and their behaviour in solution. Their tendency to form aggregates in solution, which is one of the key points to be taken into account for them to be used in polynucleotide sensing, is also considered and the various strategies adopted to avoid it will be discussed. These include the synthetic one (with the incorporation of charged and/or bulky substituents), the use of organic co-solvents and the addition of surfactants. The main physical chemical changes (optical, photophysical, electrical conductivity and viscosity) observed upon direct binding between polynucleotide and CPE, the kind of interactions involved and their applicability in sensing are considered as a function of the CPE structural rigidity. Moreover, more complex devices developed in CPE-polynucleotide sensing with the involvement of additional spectroscopic probes to induce Förster resonant energy transfer processes (FRET) or superquenching phenomena are reviewed. Finally, the main CPE applications in biosensing and the potential use of these systems in understanding DNA compaction and possible extension to the construction of supramolecular oligonucleotide structures are summarized.

Solubilization of Poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} in Water by Nonionic Amphiphiles

In the presence of the nonionic alkyloxyethylene surfactant n-dodecylpentaoxyethylene glycol ether (C12E5), the anionic conjugated polyelectrolyte (CPE) poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) dissolves in water, leading to a blue shift in fluorescence and dramatic increases in fluorescence quantum yields above the surfactant critical micelle concentration (cmc). No significant changes were seen with a poly(ethylene oxide) of similar size to the surfactant headgroup, confirming that specific surfactant-polyelectrolyte interactions are important. From UV-visible and fluorescence spectroscopy, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), and electrical conductivity, together with our published NMR and small-angle neutron scattering (SANS) results, we provide a coherent model for this behavior in terms of breakup of PBS-PFP clusters through polymer-surfactant association leading to cylindrical aggregates containing isolated polymer chains. This is supported by molecular dynamics simulations, which indicate stable polymer-surfactant structures and also provide indications of the tendency of C12E5 to break up polymer clusters to form these mixed polymer-surfactant aggregates. Radial electron density profiles of the cylindrical cross section obtained from SAXS results reveal the internal structure of such inhomogeneous species. DLS and cryo-TEM results show that at higher surfactant concentrations the micelles start to grow, possibly partially due to formation of long, threadlike species. Other alkyloxyethylene surfactants, together with poly(propylene glycol) and hydrophobically modified poly(ethylene glycol), also solubilize this polymer in water, and it is suggested that this results from a balance between electrostatic (or ion-dipole), hydrophilic, and hydrophobic interactions. There is a small, but significant, dependence of the emission maximum on the local environment.

Singlet-singlet energy transfer in self-assembled systems of the cationic poly{9,9-bis[6-N,N,N-trimethylammonium)hexyl]fluorene-co-1,4-phenylene} with oppositely charged porphyrins.

Electronic energy transfer has been studied between the cationic conjugated polyelectrolyte, poly{9,9-bis[6-N,N,N-trimethylammonium)hexyl]fluorene-co-1,4-phenylene} dibromide (HTMA-PFP), and three, oppositely charged meso-tetrakis-phenylporphyrinsulfonates in buffered (pH = 9.2), 4% (v/v) dimethyl sulfoxide-water (DMSO-water) solutions using steady-state and time-resolved fluorescence. Energy transfer was indicated by the decrease in intensity of the fluorescence band of the HTMA-PFP donor, by a corresponding increase in fluorescence of the porphyrin acceptors, by a band in the excitation spectrum of the porphyrin corresponding to the polymer absorption, and by the fact that the decay of the polymer emission observed at 423 nm was accompanied by the grow-in of porphyrin emission at 650 nm in time-resolved measurements. It is suggested that the energy transfer may involve upper excited states of the acceptor. The Förster equation and the experimental spectral overlap between donor fluorescence and acceptor absorption were used to calculate Förster radii for the three systems. Both steady-state and dynamic Stern-Volmer plots were nonlinear at high acceptor concentrations, and quenching rate constants calculated from the slopes of the initial linear region and the HTMA-PFP fluorescence lifetime were orders of magnitude greater than expected for a diffusion-controlled process, strongly supporting the idea that energy transfer occurs in self-assembled species formed by association (through ion pairing) of the polymer and porphyrins. There are indications that these aggregates involve more than one polymer chain. Picosecond time-resolved measurements on the HTMA-PFP fluorescence decay showed a short-lived component, attributed to the energy-transfer step, and two longer lived decays, which may be associated with exciton migration along the chain and the fluorescence decay of the polymer backbone, respectively. From considerations of the probable distance between donor and acceptor it is suggested that the Forster mechanism, assuming point dipoles, is inadequate for this system and that more detailed calculations, considering the actual sizes of the donor and acceptor, are necessary.

Structural studies on cationic poly{9,9-bis[6-(N,N,N-trimethylammonium)alkyl]fluorene-co-1,4-phenylene} iodides in aqueous solutions in the presence of the non-ionic surfactant pentaethyleneglycol monododecyl ether (C12E5)

Two cationic conjugated polyelectrolyte poly{9,9-bis[6-(N,N,N-trimethylammonium)alkyl]fluorene-co-1,4-phenylene} iodides in aqueous solution in the presence of the non-ionic surfactant pentaethyleneglycol monododecyl ether (C12E5) were studied using optical absorption and fluorescence, NMR, and small-angle neutron scattering (SANS) with a model of randomly arranged core?shell cylinders in a solvent. The polymers differed in both the size of the aromatic backbone and the length of the alkyl side chains. In agreement with studies on related conjugated polyelectrolytes, optical observations indicate that the surfactant breaks up clusters of the polymer and produces solutions of mixed polyelectrolyte?surfactant aggregates. The SANS data are in accord with the idea of dissolution and show that with C12E5 surfactant these polymers form long worm-like particles which contain rigid segments with the diameter of 5?nm. With the ternary system involving the polymer with a larger backbone, longer rigid segments were observed, for which a typical value of >60?nm was calculated. In contrast, for the smaller polymer the value is around 45?nm. This difference is rationalized on the basis of the difference in polymer size.

Photophysical properties of methyl β-carboline-3-carboxylate mediated by hydrogen-bonded complexes—a comparative study in different solvents

The hydrogen bonding interactions of methyl beta-carboline-3-carboxylate (BCCM) in both ground and first singlet excited electronic states have been studied in solvents with different properties in the presence of acetic acid, a hydrogen-bonding donor/acceptor. The methyl ester substituent reduces the pyridinic nitrogen basicity of this beta-carboline derivative. This fact has let us study the hydrogen bonding interactions in a higher range of acetic acid concentrations than for other beta-carboline derivatives previously studied. Steady and non-steady photophysical studies have been carried out in two non-polar solvents, benzene and p-dioxane; and in two polar solvents, acetonitrile and dichloromethane. Six different fluorescence emissions have been isolated corresponding to the uncomplexed BCCM, the protonated species and four different complexes between BCCM and acetic acid whose structures we have tried to elucidate.

Effect of metal ion hydration on the interaction between sodium carboxylates and aluminum(III) or chromium(III) ions in aqueous solution.

The interaction between sodium octanoate, decanoate, and dodecanoate and aluminum(III) and chromium(III) has been studied in water at natural pH values, starting well below the surfactant critical micelle concentration, using electrical conductivity, turbidity, and potentiometric measurements. With decanoate or dodecanoate, maximum interaction occurs at 3:1 stoichiometry, corresponding to charge neutralization. Although the solutions become turbid with both metal ions, indicating phase separation, differences are observed and attributed to the fact that aluminum(III) is relatively labile to substitution and rapidly replaces its water ligands, whereas chromium(III) is substitution inert. This shows up in well-defined floc formation with Al(3+), whereas Cr(3+) suspensions do not precipitate, probably because that replacement of coordinated water by carboxylate ligands is impeded. This can be overcome by increasing temperature, and differences in the thermal behavior with Al(3+) and Cr(3+) are suggested to be due to increased involvement of substitution reactions in the latter case. The effect of octanoate on the trivalent metal ions is less clear, and with Cr(3+) interaction only occurs when the carboxylate is in excess. Hydrophobic interactions between alkyl chains play a major role in driving phase separation. At high surfactant concentrations, the solid phases do not dissolve, in contrast to what is observed with the corresponding alkylsulfates. This has implications for use of these systems in metal separation through froth flotation. The concentration of metal ions in supernatant solution has been determined for sodium dodecanoate and sodium dodecylsulfate with Al(3+) and Cr(3+) over the whole surfactant concentration range by inductively coupled plasma-mass spectrometry (ICP-MS). From this, association constants have been determined and are found to be larger for the carboxylate than the alkylsulfate, in agreement with the greater Lewis basicity of the -CO(2)(-) group.

Hydrogen-bonding interactions of norharmane in mixtures of acetic acid with benzene, p-dioxane and acetonitrile

Spectroscopic studies on the hydrogen-bonding interactions of norharmane with acetic acid (AcH), a proton acceptor/donor molecule, have been carried out in two non-polar solvents (benzene and p-dioxane) and in a polar solvent (acetonitrile). The steady and non-steady photophysical results show three different kinds of norharmane/AcH hydrogen-bonded complexes in the first excited state depending on the norharmane nitrogen implied in the complex formation: 1 ∶ 1 pyridinic nitrogen complex, 1 ∶ 1 pyrrolic nitrogen complex and 1 ∶ 2 pyridinic–pyrrolic nitrogen complex. In the three solvents and from the dependence of the reciprocal of norharmane lifetime on the acetic acid concentration, the quenching deactivation constants have been obtained. These constants are much higher than the diffusion constant indicating that the interaction between norharmane and AcH exists in the ground state. The 1 ∶ 1 stoichiometry of norharmane–AcH complexes in the ground state has been confirmed by the absorbance change at 354 nm. Benzene is the solvent in which complex formation is more favoured. Single, double and tri-exponential fluorescence decays were recorded, depending on the AcH concentration and the emission wavelength. Decay times of the neutral form and the 1 ∶ 1 complexes are quite close, ca. about 1 ns. The cation form has a lifetime of about 17 ns and the complex with two acetic molecules has a decay time of about 3 ns. With the formation of these complexes, it is possible to explain the fact that the emission around 500 nm disappears for high acetic acid concentrations.

On the flocculation and re-dissolution of trivalent lanthanide metal ions by sodium dodecyl sulfate in aqueous solutions

The interaction between aqueous solutions of trivalent lanthanide ions (M(3+): La(III) and Gd(III) and Tb (III)) at fixed (1mM) concentrations and various concentrations of sodium dodecyl sulfate (SDS), ranging from pre- to post-micellar, has been investigated by ICP-AES (La(III) and Gd(III)), luminescence spectra (Gd(III)) and lifetimes (Tb(III)) and (139)La NMR spectroscopy. It has been found that at concentration ratios, r=[SDS]/[M(3+)], around the charge neutralization value (ca. 3), dodecyl sulfate (DS(-)) anion interacts with the metal ions to form insoluble aggregates. The metal ion-DS(-) complexes remain flocculated for r values below 5-6 (Gd(III) and La(III), respectively), while at higher r values, re-dissolution takes place. The flocculated aggregates have been characterized by X-ray powder diffraction, and show a lamellar structure. Job plot method indicates that a complex with a 1:3 (M(3+):DS(-)) stoichiometry is formed. From ICP-AES analysis, a model based on a three-step mechanism has been developed and association constants calculated. For all systems the interaction between DS(-) and metal ions follows an associative process with K values ranging between K(1)=10 and K(3)=10(4). These data are discussed on the basis of the physical-chemical characteristics of the metal ions. Re-dissolution with increasing surfactant concentration is attributed to formation of mixed lanthanide/sodium dodecyl sulfate aggregates, with the relative lanthanide fraction in these species decreasing with increasing SDS concentration.

Interaction between Poly(9,9-bis(6′-N,N,N-trimethylammonium)hexyl)fluorene phenylene) Bromide and DNA as Seen by Spectroscopy, Viscosity, and Conductivity: Effect of Molecular Weights and DNA Secondary Structure

The interaction between three poly(9,9-bis(6-N,N,N-trimethylammonium)hexyl)fluorene phenylene) bromide (HTMA-PFP) samples of different molecular weights (Mn=14.5, 30.1 and 61.3 kg/mol) and both dsDNA and ssDNA secondary structures has been studied using UV-visible absorption and fluorescence spectroscopies (including steady-state, time-resolved, and anisotropy measurements for the latter), viscosity, and electrical conductivity in 4% (v/v) DMSO-water mixtures. At low nucleic acid concentrations, formation of a 1:1 complex in terms of HTMA-PFP repeat units and DNA bases occurs. This interaction results in quenching of polymer emission. For higher molar ratios of DNA to HTMA-PFP, corresponding to charge neutralization, a second process is observed that is attributed to aggregate formation. From the changes in the absorption spectra, the polymer aggregation constant and the aggregate absorption spectra were calculated by applying an iterative method. Polymer aggregation dramatically quenches HTMA-PFP fluorescence in the region of the electroneutrality point. Under these conditions, the ratio of the emission intensity at 412 nm (maximum) to that at 434 nm (I412/I434) reaches a minimum, the electrical conductivity decreases, and the viscosity of the solution remains constant, showing that the DNA concentration can be determined through various HTMA-PFP physicochemical properties. With respect to the photophysical parameters (emission quantum yield, shape and shift of emission spectra), no significant differences were observed between dsDNA and ssDNA or with conjugated polymer or DNA molecular weight. The two short-lived components in the fluorescence decays are attributed to the presence of aggregates. Aggregates are also suggested to be responsible for the decrease in the fluorescence anisotropy through interchain exciton migration.