Thermodynamics and kinetics of protonated merocyanine photoacids in water

Metastable-state photoacids (mPAHs) are chemical species whose photo-activated state is long-lived enough to allow for proton diffusion. Liao's photoacid (1) represents the archetype of mPAHs, and is being widely used on account of its unique capability to change the acidity of aqueous solutions reversibly. The behavior of 1 in water, however, still remains poorly understood. Herein, we provide in-depth insights on the thermodynamics and kinetics of 1 in water through a series of comparative 1H NMR and UV-Vis studies and relative modelling. Under dark conditions, we quantified a three-component equilibrium system where the dissociation (Ka) of the open protonated form (MCH) is followed by isomerization (Kc) of the open deprotonated form (MC) to the closed spiropyran form (SP) – i.e., in the absence of light, the ground state acidity can be expressed as KGSa = Ka(1 + Kc). On the other hand, under powerful and continuous light irradiation we were able to assess, for the first time experimentally, the dissociation constant (KMSa) of the protonated metastable state (cis-MCH). In addition, we found that thermal ring-opening of SP is always rate-determining regardless of pH, whereas hydrolysis is reminiscent of what is found for Schiff bases. The proposed methodology is general, and it was applied to two other compounds bearing a shorter (ethyl, 2) and a longer (butyl, 3) alkyl-1-sulfonate bridge. We found that the pKa remains constant, whereas both pKc and pKMSa linearly increase with the length of the alkyl bridge. Importantly, all results are consistent with a four-component model cycle, which describes perfectly the full dynamics of proton release/uptake of 1–3 in water. The superior hydrolytic stability and water solubility of compound 3, together with its relatively high pKGSa (low Kc), allowed us to achieve fully reversible jumps of 2.5 pH units over 18 consecutive cycles (6 hours).

Luminescent lanthanides bioprobes emitting in the visible and/or near-infrared ranges

Steve Comby

This Ph.D. work deals with the development of lanthanide-based bioprobes for biomedical analyses and imaging. For this purpose, two different approaches have been followed, one focusing on visible-emitting complexes, while the second addresses the sensitization of NIR-emitting LnIII ions. The homoditopic ligands H2LC2 and H2LC3 have been tailored to self-assemble with lanthanide ions (LnIII) into neutral triple-stranded bimetallic helicates of overall composition [Ln2(L)3] and also to provide a versatile platform for further derivatization. The grafting of polyoxyethylene groups onto the pyridine (H2LC2) or the benzimidazole (H2LC3) units ensures water solubility, while maintaining large thermodynamic stability and adequate antenna effect for the excitation of LnIII ions. The conditional stability constants are large at physiological pH, with logβ23 reaching values in the range 22-25 for helicates with H2LC2 and H2LC3. The ligands triplet states feature adequate energy to efficiently sensitize the EuIII luminescence (QEu L = 21 and 11 % for L = H2LC2 and H2LC3, respectively) in aerated water at pH 7.4. Conversely, energy back transfer occurring in [Tb2(LC3)3] precludes a good sensitization of the TbIII luminescence, and only the TbIII helicate with H2LC2 displays sizeable quantum yield (QTbL = 11 %). Furthermore, it has been demonstrated that ligand H2LC2 sensitizes the emission of other visible- and NIR-emitting LnIII ions, such as SmIII or YbIII, for which in cellulo luminescence is evidenced, a rare observation. The EuIII emission spectra of both helicates arise from a main species with pseudo-D3 symmetry and without coordinated water. Cytotoxicity experiments reveal no significant effect of the helicates on the viability of several cancerous cell lines and cell imaging properties of the EuIII helicates are demonstrated for the HeLa cell line by luminescence microscopy. Bright EuIII emission is seen after relatively short time (15-30 min) and low helicate incubation concentrations (10 µM). Similar images were obtained for [Sm2(LC2)3] and [Tb2(LC2)3] using slightly higher loading concentration. The helicates stain the cytoplasm and the permeation mechanism is likely to be endocytosis. The described luminescent helical stains are robust chemical species which remain mostly undissociated in the cell medium and which resist to the presence of other complexing agents, as demonstrated in the case of H2LC2. All these properties make these helicates potential bioprobes for interacting with biological material and for cell imaging applications. As near-infrared (NIR) light, in addition to be less energetic than visible light, displays better penetrability into the human body, the development of bioprobes emitting in the near-infrared range is of prime importance for deeper tissue imaging, especially for early diagnosis of tumors. We therefore investigate the ability of new polydentate podands to sensitize the NIR luminescence of NdIII and YbIII ions. These ligands have been designed in a way to take advantage of the chelating effect of bidentate (Tsox, TsoxMe) or tridentate 8-hydroxyquinolinate (8-HQ) subunits (T2soxMe). The interaction between the ligands and LnIII ions in aqueous solution leads to the formation of stable 1:1 chelates (pLn in the range of 12-16), all the chromophoric 8-HQ units being coordinated to the metal center, exploiting the entropic effect generated by the anchor. At physiological pH, one major species is present for all the ligands regardless of the LnIII ion used. The low energy triplet states of the ligands are particularly well-suited for an efficient sensitization of the NdIII- or YbIII-centered luminescence (QNdL = 0.02-0.04 % and QYbL = 0.13-0.37 %) in buffered aqueous solution. ErIII NIR luminescence is also detected in water for the chelates with Tsox and TsoxMe. Lifetime determinations point in all cases to the absence of coordinated water molecules in the inner coordination sphere of the LnIII ion. It has also been demonstrated that methylation of the amide functions of Tsox removes the quenching mechanism induced by the proximate N-H groups and increases both the lifetimes and quantum yields of the TsoxMe chelates, with the YbIII complex being one of the most NIR luminescent lanthanide-based edifices described so far. Ligand TsoxMe thus appears to be the best candidate of the series regarding to the development of NIR probes for bioanalyses. Furthermore, other applications, for instance in telecommunications, are still conceivable since the incorporation of the Yb-TsoxMe chelate into sol-gel silica films has proven to be successful, the chelate withstanding the sol-gel process, while providing a slight enhancement of photophysical properties compared to the same chelate in aqueous solution.

EPFL2008

Electrocatalysis at liquid/liquid interfaces

Raheleh Partovi Nia

This thesis is devoted to the study of oxygen reduction reaction catalysed by porphyrins at the interface between two immiscible electrolyte solutions (ITIES). Electrochemical and spectrophotometric techniques are introduced to these interfaces in order to gather more information about the transfer mechanism. Furthermore, the reduction of oxygen and the oxidation of decamethylferrocene (DMFc) in 1,2-DCE and production of hydrogen peroxide (H2O2) in the aqueous phase, on the basis of the two-phase reaction controlled by a common ion are investigated. Mass spectrometric measurements were carried out for the 1,2-DCE phase before and after two-phase reaction with an aqueous phase containing acid to indicate the stability of DMFc and the DMFc+ over the course of the two-phase reaction. Density function theory (DFT) computations have been performed based on developed a reaction pathway. Catalytic effect of 5,10,15,20-tetraphenylporphyrinatocobalt(II) [Co(tpp)], 2,3,7,8,12,13,17,18-Octaethyl-porphyrin cobalt(II) (CoOEP) and two free-base porphyrins 5,10,15,20-meso-tetraphenylporphyrin (H2TPP) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin (H2OEP) have been investigated as a catalyst for a two electron reduction of O2 in presence of an electron donor at various pH values at the polarized water|1,2-DCE interface. Using voltammetry, it is possible to drive this catalytic reduction at the interface as a function of the applied potential difference, where aqueous protons and organic electron donors combine to reduce O2. The signal observed corresponds to a proton-coupled electron transfer (PCET) reaction, as no current and no reaction can be observed in the absence of either catalyst, acid or O2. [Co(tpp)] and CoOEP catalysis work like conventional cobalt porphyrins, activating O2 via coordination by the formation of a superoxide structure. The advantages of the present system is that, by controlling the interfacial potential difference, the proton transfer from water to 1,2-DCE can be accurately controlled. Accordingly, the driving force for proton-coupled electron transfer reactions is also effectively harnessed. Assisted proton transfer (APT) reactions were studied across the water|1,2-DCE interface facilitated by two free-base porphyrins such as H2TPP and H2OEP. At a water|1,2-DCE interface, the interfacial formation of di-acid H4TPP2+ and H4OEP2+ are observed by ion-transfer voltammetry and UV-Visible spectroscopy, due to the double protonation of H2TPP and H2OEP at the tertiary nitrogens in the ring. Additionally, "Ionic Partition Diagram" of neutral and ionisable H2TPP compounds is plotted to illustrate the various contributions of H2TPP.

EPFL2010

Amorphous Molybdenum Sulfide Films as Hydrogen Evolution Catalysts in Water

Daniel Andreas Merki

Molecular hydrogen is a promising candidate to replace fossil fuels as the energy carrier. Hydrogen does not exist in its molecular form on earth and must therefore be generated, starting from hydrogen-rich compounds. Water would be a renewable resource for hydrogen. It can be split into oxygen and hydrogen. Water splitting, however, needs electrical energy. If this energy is produced from renewable resources, such as solar or wind power, water splitting would be a sustainable and green process. One key step in water splitting is the reduction of protons to form hydrogen. To achieve a high efficiency in this reaction, catalysts are required. This dissertation is devoted to the development of hydrogen evolution catalysts that are made of earth-abundant and inexpensive materials. First, transition metal complexes of tetrathiotungstate and tetrathiomolybdate were studied as potential homogeneous catalysts for hydrogen evolution in organic or aqueous solutions (chapter 1). The redox activity of these complexes was investigated by cyclic voltammetry in the absence and presence of an acid in organic solutions. The first reduction peak of (Pr4N)2[Ni(MoS4)2] became more intense and was shifted to less negative potentials upon addition of an acid, e.g. p-cyanoanilinium tetrafluoroborate. This was considered as a sign for catalytic proton reduction. Electrolysis of a solution containing (Pr4N)2[Ni(MoS4)2] and an acid produced hydrogen. However, the complex turned out to be a precursor of the real catalytically active species, which was deposited on the working electrode’s surface during the electrochemical experiment. Consecutive cyclic voltammetric scans were found to be a good method to deposit the catalytically active film in a controllable manner from an aqueous solution of (Pr4N)2[Ni(MoS4)2]. Furthermore, a catalytically active film could be deposited from a solution of MoS42-, i.e. in the absence of Ni2+. Chapter 2 describes the investigation and characterization of films made using MoS42- as the precursor. The films consist of amorphous molybdenum sulfide. They are efficient hydrogen evolution catalysts in water. The films were characterized by XPS and electron microscopy. Whereas the pre-catalysts could be MoS3 or MoS2, the active form of the catalysts was identified as amorphous MoSx, with x close to 2. Significant geometric current densities were achieved at low overpotentials (e.g., ca. 15 mA/cm2 at η = 200 mV) using these catalysts. The catalysis was compatible with a wide range of pH values. The current efficiency for hydrogen production was quantitative. A Tafel slope of 39 mV/dec was observed, suggesting a rate-determining Heyrovsky step. The turnover frequency per active site was estimated.

EPFL2012