Sofia M. Fonseca

All-conjugated polyelectrolyte block copolymers

Novel, all-conjugated polyelectrolyte block copolymers of the rod-rod type can be generated in a “grafting from” scheme and exhibit a preferred tendency to self-assemble into layered aggregates both in solution and the solid state. Here, the rigid-rod structure of the individual, complex macromolecules favours the formation of low-curvature vesicular and lamellar aggregates. Our poly(9,9-dialkylfluorene)-b-poly[3-(6-ammoniumhexyl)thiophene] (PF2/6-b-P3TMAHT and PFO-b-P3TMAHT, where PF2/6 and PFO denote 2-(ethyl)hexyl and linear octyl alkyl pendant groups, respectively), and poly(9,9-dialkylfluorene)-b-poly[3-(6-pyridylhexyl)thiophene] (PF2/6-b-P3PyHT and PFO-b-P3PyHT) polyelectrolyte diblock copolymers allow for simple and reliable control of the occurring self-organisation process and the resulting nano-scaled architectures. They are, therefore, promising candidates for application as the active layer in electronic devices or as functional membranes (e.g. for sensor applications). Moreover, the electronic properties of the materials (especially the excitation energy transfer between both blocks) strongly depend on the aggregation state present. Aggregation can be further controlled via addition of oppositely charged surfactants resulting in the formation of ordered polyelectrolyte/surfactant complexes.

Selective Fluorescence Quenching in Cationic Fluorene-Thiophene Diblock Copolymers for Ratiometric Sensing of Anions

The cationic, all-conjugated AB diblock copolymer poly[9,9-bis(2-ethylhexyl)fluorene]-b-poly[3-(6-trimethylammoniumhexyl) thiophene] bromide (PF2/6-b-P3TMAHT) shows dual fluorescence from the poly(fluorene) (PF) and poly(thiophene) (PT) blocks. A comparison of fluorescence quenching of the cationic PT block fluorescence with unquenched PF block provides a sensitive ratiometric method for anion sensing. The application to analysis of halide ions, single- and double-stranded DNA is demonstrated. High selectivity is observed with halide ions, with the strongest quenching being seen with iodide. The quenching with DNA can be used for nucleic acid quantification at sub-μM concentrations.

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.

NMR, DFT and luminescence studies of the complexation of Zn(II) with 8-hydroxyquinoline-5-sulfonate.

Multinuclear ((1)H, (13)C and (27)Al) magnetic resonance spectroscopy (1D and 2D), DFT calculations and fluorescence have been used to study the complexation of 8-hydroxyquinoline-5-sulfonate (8-HQS) with Al(III). The study combines the high sensitivity of luminescence techniques, the selectivity of multinuclear NMR spectroscopy with the structural details accessible through DFT calculations, and aims to provide a detailed understanding of the complexation between the Al(3+) ion and 8-HQS. A full speciation study has been performed and over the concentration region studied, the Al(3+) ion forms complexes with 8-HQS in an aqueous solution in the pH range 2-6. At higher pH, the extensive hydrolysis of the metal limits complexation. Using Jobs method, three complexes were detected, with 1 : 1, 1 : 2 and 1 : 3 (metal : ligand) stoichiometries. These results are in agreement with those previously reported using potentiometric and electrochemical techniques. The geometries of the complexes are proposed based on the combination of NMR results with optimized DFT calculations. All the complexes in aqueous solutions at 25 °C are mononuclear species, and have an approximately octahedral geometry with the metal coordinated to one molecule of 8-HQS and four molecules of water (1 : 1 complex), two molecules of 8-HQS and two molecules of water mutually cis (1 : 2 complex), and to three molecules of 8-HQS in non-symmetrical arrangement (mer-isomer), for the 1 : 3 (metal : ligand) complex. On binding to Al(III), 8-HQS shows a more marked fluorescence than the weakly fluorescent free ligand. In addition, as previously noted, there are marked changes in the absorption spectra, which support the use of 8-HQS as a sensitive optical sensor to detect Al(3+) metal ions in surface waters and biological fluids. These complexes also show potential for applications in organic light emitting diodes (OLEDs).

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.

Photooxidation of cellulose acetate and cellobiose by the uranyl ion

The photooxidation of cellulose acetate by uranyl nitrate in acetone solutions has been investigated. Studies of the effect of the polymer on the uranyl luminescence showed an initial increase in intensity, followed by quenching. This is interpreted in terms of competition between complexation of uranyl ions by the polymer and dynamic quenching. In the quenching region, Stern-Volmer kinetics are followed. Upon photolysis of the solution, a decrease in viscosity was observed, consistent with chain scission. However, there was no sign of formation of reduced uranium species, suggesting that they are reoxidised by molecular oxygen. Model studies were carried out with cellobiose and it was confirmed that the luminescence quenching involves both dynamic and static processes. Photolysis of aqueous solutions of cellobiose and uranyl nitrate or perchlorate led to formation of uranium(v) and a decrease in pH. Upon interruption of photolysis, uranium(v) was seen to disproportionate. Yields of reduced uranium species were higher in degassed than aerated solutions, consistent with their oxidation by molecular oxygen in the latter case. Organic radicals were detected by electron paramagnetic resonance spectroscopy upon photolysis of cellulose acetate saturated with uranyl nitrate. The mechanism of photooxidation is suggested to involve hydrogen atom abstraction from the substrate by excited uranyl ions.

Fluorescence Study of Dehydroabietic Acid-Based Bipolar Arylamine-Quinoxalines

The absorption and fluorescence spectra, lifetimes and quantum yields of a series of triarylaminequinoxaline bipolar compounds, with and without the bulky dehydroabietic acid group, have been studied in toluene solution. This bulky group is introduced to improve solubility and thermal properties of these systems. It is shown that this does not affect their spectral or photophysical behavior. The compounds show relatively strong fluorescence, with the emission maximum strongly dependent upon the substituents present. Oxidation potentials have also been determined in acetonitrile solution, and again indicate that introduction of the resin acid moiety has no effect on these properties.

Direct observation of oxygen depletion and product formation during photocatalysis at a TiO2 surface using scanning electrochemical microscopy

Using scanning electrochemical microscopy (SECM) we have measured quantitatively the depletion of O2 during the photodegradation of 4-chlorophenol at supported TiO2 films for the first time and established the connection between Cl-formation and O2 depletion rates.

Luminescence from cerium(III) acetate complexes in aqueous solution: considerations on the nature of carboxylate binding to trivalent lanthanides

The luminescence of cerium(III) has been studied in aqueous solutions in the presence of acetate ion. In contrast to previous reports that cerium(III) carboxylate complexes do not luminesce, a weak emission is observed, which has a similar spectrum to the aquo cerium(III) species but a much shorter lifetime. Results from a variety of studies on complexation of various trivalent lanthanide ions with carboxylates, including NMR spectral and relaxation measurements, molecular mechanics calculations, deuterium isotope effects on Tb(III) luminescence lifetimes, together with literature data strongly suggest that the dominant mode of complexation between carboxylates and trivalent lanthanides involves a weak, predominantly bidentate, binding to the metal cation. We suggest that the observed emission arises from a 1 : 1 cerium acetate species involving such bidentate coordination.