Andras Marton

Johns Hopkins University, Baltimore, MD, United States

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Publications (10)36.46 Total impact

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    ABSTRACT: The oxidation of iodide to diiodide, I(2)˙(-), by the metal-to-ligand charge-transfer (MLCT) excited state of [Ru(deeb)(3)](2+), where deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, was quantified in acetonitrile and dichloromethane solution at room temperature. The redox and excited state properties of [Ru(deeb)(3)](2+) were similar in the two solvents; however, the mechanisms for excited state quenching by iodide were found to differ significantly. In acetonitrile, reaction of [Ru(deeb)(3)](2+*) and iodide was dynamic (lifetime quenching) with kinetics that followed the Stern-Volmer model (K(D) = 1.0 ± 0.01 × 10(5) M(-1), k(q) = 4.8 × 10(10) M(-1) s(-1)). Excited state reactivity was observed to be the result of reductive quenching that yielded the reduced ruthenium compound, [Ru(deeb(-))(deeb)(2)](+), and the iodine atom, I˙. In dichloromethane, excited state quenching was primarily static (photoluminescence amplitude quenching) and [Ru(deeb(-))(deeb)(2)](+) formed within 10 ns, consistent with the formation of ion pairs in the ground state that react rapidly upon visible light absorption. In both solvents the appearance of I(2)˙(-) could be time resolved. In acetonitrile, the rate constant for I(2)˙(-) growth, 2.2 ± 0.2 × 10(10) M(-1) s(-1), was found to be about a factor of two slower than the formation of [Ru(deeb(-))(deeb)(2)](+), indicating it was a secondary photoproduct. The delayed appearance of I(2)˙(-) was attributed to the reaction of iodine atoms with iodide. In dichloromethane, the growth of I(2)˙(-), 1.3 ± 0.4 × 10(10) M(-1) s(-1), was similar to that in acetonitrile, yet resulted from iodine atoms formed within the laser pulse. These results are discussed within the context of solar energy conversion by dye-sensitized solar cells and storage via chemical bond formation.
    Dalton Transactions 01/2011; 40(15):3830-8. · 3.81 Impact Factor
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    ABSTRACT: The coordination of halide ions to 5-(3,5-dicarboxyphenyl)-10,15,20-tri- p-tolylporphinatozinc(II) anchored to mesoporous nanocrystalline (anatase) TiO 2 thin films (TiO 2/ZnP) immersed in propylene carbonate was quantified. The addition of tetrabutylammonium halide salts to the external propylene carbonate electrolyte resulted in a red shift in the absorption spectrum with the maintenance of five isosbestic points. The absorption spectra were within experimental error the same for ZnP and ZnP-X (-) compared to TiO 2/ZnP and TiO 2/ZnP-X (-): A SoretZnP = 427 nm (epsilon = 574 000 M (-1) cm (-1)), A SoretZnP-Cl (-) = 435 nm (epsilon = 905 000 +/- 12 000 M (-1) cm (-1)), A SoretZnP-Br (-) = 436 nm (epsilon = 776 000 +/- 30 000 M (-1) cm (-1)), and A SoretZnP-I (-) = 437 nm (epsilon = 620 000 +/- 56 000 M (-1) cm (-1)). Titration studies with the halides revealed sharp isosbestic points consistent with formation of a 1:1 halide/porphyrin adduct. Equilibrium constants for ZnP were found to be 1670 M (-1) for Cl (-), 96 M (-1) for Br (-), and 5.5 M (-1) for I (-), and the corresponding values for TiO 2/ZnP were significantly smaller, 780 M (-1), 70 M (-1) and 3.4 M (-1). A quasi-reversible wave was observed by cyclic voltammetry of TiO 2/ZnP, E 1/2(ZnP (+/0)) = +790 mV vs Ag/AgCl, that was shifted 160 mV after addition of excess chloride, E 1/2(ZnP-Cl (0/-)) = +630 mV. In regenerative solar cells with quinone/hydroquinone redox mediators, TiO 2/ZnP and TiO 2/ZnP-X (-), where X is Cl, Br, or I, were found to convert light into electrical power. The photocurrent action spectrum demonstrated that energy conversion was initiated by light absorption of ZnP and/or the halide adduct.
    Inorganic Chemistry 09/2008; 47(17):7681-5. · 4.59 Impact Factor
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    ABSTRACT: The performance of five tetrapyrrole molecules as sensitizers in regenerative solar cells was evaluated. The tetrapyrroles form two sets. One set contains three meso-substituted porphyrins that differ only in the nature of their surface-binding tether:  isophthalic acid, ethynylisophthalic acid, or cyanoacrylic acid. The other set includes the ethynylisophthalic acid tether attached to porphyrin, chlorin, and bacteriochlorin macrocycles, which contain zero, one, and two saturated pyrrole rings, respectively. Incident photon-to-current efficiency was measured for each sensitizer loaded onto a mesoporous TiO2 semitransparent electrode in a solar cell. The porphyrin bearing the cyanoacrylic acid tether gives the largest peak and integrated (350−900 nm) photocurrent density of the five tetrapyrrole molecules. For this sensitizer, a quasi-monochromatic power conversion efficiency of 21% was obtained at the Soret maximum (450 nm), along with a fill factor of 0.69. To elucidate the molecular origins of the effects of tether and macrocycle reduction on photocurrent production, the measured redox potentials and optical absorption spectra were analyzed in terms of the characteristics (energies and electron-density distributions) of the frontier molecular orbitals obtained from density functional theory calculations. Additionally, first-principle simulations were performed for the production of photocurrent by hypothetical planar and actual mesoporous films of each sensitizer under AM 1.5 solar irradiation. Collectively, the findings give fundamental insights into the factors that govern the observed differences in photocurrent production characteristics for the five tetrapyrrole sensitizers. In addition, the results provide a framework for further tuning of the properties of these molecules and related sensitizers to enhance solar-cell performance.
    10/2007;
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    ABSTRACT: Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)](PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq)2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS)2], or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine, bq is 2,2'-biquinoline, and deeb and deebq are 4,4'-diethylester derivatives. These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface coverages of 8 (+/- 2) x 10(-8) mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were phi(417 nm) = 0.18 +/- 0.02, phi(532.5 nm) = 0.08 +/- 0.02, and phi(683 nm) = 0.05 +/- 0.01. Electron transfer to yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch-Williams-Watts model.
    Journal of the American Chemical Society 07/2006; 128(25):8234-45. · 10.68 Impact Factor
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    ABSTRACT: The excited states of [Ru(bpy)2(deeb)](PF6)2, where bpy is 2,2-bipyridine and deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine, were found to be efficiently quenched by triiodide (I3-) in acetonitrile and dichloromethane. In dichloromethane, I3- was found to quench the excited states by static and dynamic mechanisms; Stern-Volmer analysis of the time-resolved and steady-state photoluminescence data produced self-consistent estimates for the I3- + Ru(bpy)2(deeb)2+ <==> [Ru(II)(bpy)2(deeb)2+,(I3-)]+ equilibrium, K = 51,000 M(-1), and the bimolecular quenching rate constant, kq = 4.0 x 10(10) M(-1) s(-1). In acetonitrile, there was no evidence for ion pairing and a dynamic quenching rate constant of k(q) = 4.7 x 10(10) M(-1) s(-1) was calculated. Comparative studies with Ru(bpy)2(deeb)2+ anchored to mesoporous nanocrystalline TiO2 thin films also showed efficient excited-state dynamic quenching by I3- in both acetonitrile and dichloromethane, kq = 1.8 x 10(9) and 3.6 x 10(10) M(-1) s(-1), respectively. No reaction products for the excited-state quenching processes were observed by nanosecond transient absorption measurements from 350 to 800 nm under any experimental conditions. X-ray crystallographic, IR, and Raman data gave evidence for interactions between I3- and the bpy and deeb ligands in the solid state.
    Inorganic Chemistry 06/2006; 45(12):4728-34. · 4.59 Impact Factor
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    ABSTRACT: The metal-to-ligand charge-transfer (MLCT) excited states of Ru(bpy)(2)(deeb)(PF(6))(2), where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, in dichloromethane were found to be efficiently quenched by iodide at room temperature. The ionic strength dependence of the UV-visible absorption spectra gave evidence for ion pairing. Iodide was found to quench the excited states by static and dynamic mechanisms. Stern-Volmer and Benesi-Hildebrand analysis of the spectral data provided a self-consistent estimate of the iodide-Ru(bpy)(2)(deeb)(2+) adduct in dichloromethane, K = 59 700 M(-1). Transient absorption studies clearly demonstrated an electron-transfer quenching mechanism with transient formation of I(2)(*)(-) in high yield, phi = 0.25 for 355 or 532 nm excitation. For Ru(bpy)(2)(deeb)(PF(6))(2) in acetonitrile, similar behavior could be observed at higher iodide concentrations than that required in dichloromethane. The parent Ru(bpy)(3)(2+) compound also ion pairs with iodide in CH(2)Cl(2), and light excitation gave a higher I(2)(*)(-) yield, phi = 0.50. X-ray crystallographic, IR, and Raman data gave evidence for interactions between iodide and the coordinated deeb ligand in the solid state.
    Inorganic Chemistry 02/2006; 45(1):362-9. · 4.59 Impact Factor
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    ABSTRACT: The metal-to-ligand charge-transfer (MLCT) excited states of Ru(deeb)(bpy)(2)(PF(6))(2) [where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine] in acetonitrile or dichloromethane were found to be quenched by iodide at room temperature. The ionic strength dependence of the optical spectra gave evidence for ion pairing. Iodide is found to quench the photoluminescence (PL) intensity and influence the spectral distribution of the emitted light. A static component to the time-resolved PL quenching provided further evidence for ground-state adduct. Stern-Volmer analysis of the static component provided an estimate of the iodide-Ru(deeb)(bpy)(2)(2+) adduct equilibrium constant in dichloromethane, K(sv) = 40,000 M(-)(1). Transient absorption studies clearly demonstrate that an electron-transfer quenching mechanism is operative and that I(2)(-)(*) can be photoproduced in high yield, phi = 0.25. For Ru(bpy)(3)(PF(6))(2) in acetonitrile, similar behavior could be observed at iodide concentrations >100 times that required for dichloromethane.
    Inorganic Chemistry 06/2005; 44(10):3383-5. · 4.59 Impact Factor
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    ABSTRACT: Dye-sensitized mesoporous nanocrystalline SnO2 electrodes and the pseudohalogen redox mediator (SeCN)2/SeCN- or (SCN)2/SCN- or the halogen redox mediator I3-/I- were implemented for regenerative solar cell studies. Adsorption isotherms of the sensitizers Ru(deeb)(bpy)2(PF6)2, Ru(deeb)2(dpp)(PF6)2, and Ru(deeb2(bpz)(PF6)2, where deeb is 4,4'-diethylester-2,2'-bipyridine, dpp is 2,3-dipyridyl pyrazine, and bpz is bipyrazine, binding to the SnO2 surface were well described by the Langmuir model from which the saturation coverage, Gamma0 = 1.7 x 10(-8) mol/cm2, and surface-adduct formation constant, Kad = 2 x 10(5) M(-1), were obtained. Following excited-state interfacial electron transfer, the oxidized sensitizers were reduced by donors present in the acetonitrile electrolyte as shown by transient absorption spectroscopy. With iodide as the donor, a rate constant k > 10(8) s(-1) was measured for sensitizer regeneration. In regenerative solar cells, it was found that the incident photon-to-current conversion efficiencies and open circuit voltages (Voc) were comparable for (SeCN)2/SeCN- and I3-/I- for all three sensitizers. The Voc varied linearly with the logarithm of the short circuit photocurrent densities (Jsc), with typical correlations of approximately 50-60 mV/decade. Capacitance measurements of the SnO2 electrode in the presence of I3-/I-, (SeCN)2/SeCN- or (SCN)2/SCN- are reported.
    The Journal of Physical Chemistry B 02/2005; 109(2):937-43. · 3.61 Impact Factor
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    ABSTRACT: The influence of electrolyte pH on the sensitization efficiency of porphyrin-derivatized TiO2 was studied by photoelectrochemical and transient absorption measurements. The porphyrins 5-(4-carboxyphenyl)-10,15,20-trimesitylporphinatozinc(II) (1), 5-(4-carboxyphenyl)-10,15,20-trimesitylporphine (2), 5-(4-carboxyphenyl)-10,15,20-trimesitylporphinatoplatinum(II) (3), and 5-(4-dihydroxyphosphorylphenyl)-10,15,20-trimesitylporphinatozinc(II) (4) were anchored to low-surface-area and nanocrystalline TiO2 films. The TiO2 conduction band edge potential (ECB) shifted 59 ± 2 mV/pH, from −0.43 V vs Ag/AgCl(aq) at pH 12 to +0.16 V vs Ag/AgCl(aq) at pH 2. Excited-state potentials (E1/2(P+/*)) of 1−4 ranged from −1.58 to −0.91 V vs Ag/AgCl(aq), well negative of ECB. Despite the thermodynamic favorability of electron injection, a 10-fold increase in sensitized photocurrent was measured for 1−4 upon acidification of the electrolyte from pH 10 to 4. Transient absorption data revealed that sensitization of nanocrystalline TiO2 by 1−4 depended on pH in an identical manner. A mechanism is proposed wherein protonation of a surface site is required for charge compensation of injected electrons. Thus, the magnitude of sensitized photocurrent is determined by surface protonation-deprotonation equilibria.
    Journal of Physical Chemistry B - J PHYS CHEM B. 06/2004; 108(31).
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    ABSTRACT: The pH dependence of sensitized photocurrent for porphyrin-derivatized planar TiO2 films in regenerative photoelectrochemical cells is reported. The porphyrin sensitizers 5-(4-carboxyphenyl)-10,15,20-trimesitylporphinatozinc(II) (1), 5-(4-carboxyphenyl)-10,15,20-trimesitylporphine (2), and 5-(4-carboxyphenyl)-10,15,20-trimesitylporphinatoplatinum(II) (3) were studied. The TiO2 conduction band edge potential (ECB) shifted from −0.43 V to +0.16 V vs Ag/AgCl(aq) from pH 12 to pH 2, a shift of 59 ± 2 mV per pH unit. Excited-state potentials (E1/2+/*) of 1−3 ranged from −1.57 to −0.91 V vs Ag/AgCl(aq), well negative of ECB. Nonetheless, for all three porphyrins a 10-fold increase in the magnitude of sensitized photocurrent was observed upon acidification of the electrolyte from pH 12 to pH 2. Photocurrent vs pH data did not depend on the sensitizer excited-state potential. A model is proposed wherein protonation of a surface state is required for charge compensation and photocurrent production. Therefore, the magnitude of sensitized photocurrent is determined by surface protonation/deprotonation equilibria.
    Journal of Physical Chemistry B - J PHYS CHEM B. 09/2003; 107(40).