Bradley J. Brennan

Comparison of silatrane, phosphonic acid, and carboxylic acid functional groups for attachment of porphyrin sensitizers to TiO2 in photoelectrochemical cells

A tetra-arylporphyrin dye was functionalized with three different anchoring groups used to attach molecules to metal oxide surfaces. The physical, photophysical and electrochemical properties of the derivatized porphyrins were studied, and the dyes were then linked to mesoporous TiO2. The anchoring groups were β-vinyl groups bearing either a carboxylate, a phosphonate or a siloxy moiety. The siloxy linkages were made by treatment of the metal oxide with a silatrane derivative of the porphyrin. The surface binding and lability of the anchored molecules were studied, and dye performance was compared in a dye-sensitized solar cell (DSSC). Transient absorption spectroscopy was used to study charge recombination processes. At comparable surface concentration, the porphyrin showed comparable performance in the DSSC, regardless of the linker. However, the total surface coverage achievable with the carboxylate was about twice that obtainable with the other two linkers, and this led to higher current densities for the carboxylate DSSC. On the other hand, the carboxylate-linked dyes were readily leached from the metal oxide surface under alkaline conditions. The phosphonates were considerably less labile, and the siloxy-linked porphyrins were most resistant to leaching from the surface. The use of silatrane proved to be a practical and convenient way to introduce the siloxy linkages, which can confer greatly increased stability on dye-sensitized electrodes with photoelectrochemical performance comparable to that of the other linkers.

Oxidative coupling of porphyrins using copper(II) salts.

A method for radical coupling of porphyrins using copper(II) salts as one-electron oxidants was developed. A Zn(II)-porphyrin bearing an aminophenyl group yielded porphyrin oligomers, and two tri-arylporphyrins were oxidized to form doubly and triply linked dimers. Bromination of doubly linked dimers gave macrocycles with twisted skeletons.

Photoelectrochemical Cells Utilizing Tunable Corroles

Organic dyes with their wide range of molecular structures and spectroscopic features show great promise for solar energy applications. Corroles, structural analogues to porphyrins, are highly fluorescent molecules with tunable properties. We have synthesized a series of structurally similar corroles chelating gallium and phosphorus, along with a β-chlorinated phosphorus corrole, and determined their photophysical and electrochemical properties. The electrochemical potentials to oxidize the corroles range from 0.78 V vs NHE for the gallium corrole to 1.42 V for the β-octachlorinated phosphorus corrole. We are interested in developing photosensitizers for water oxidation on a metal oxide-based photoanode, so the corroles were modified to contain a meso-phenyl-COOH substituent for binding to metal oxide surfaces. The ability of these corrole dyes to act as photosensitizers was assessed by comparing the corroles in a model dye sensitized solar cell design. Transient absorption spectroscopy was utilized to analyze recombination dynamics and determine the kinetics of iodide oxidation. The most efficient photoelectrochemical cell was achieved for the phosphorus corrole P-2 with electrochemical properties and kinetics suitable for both photoinduced electron injection into TiO2 and oxidation of iodide. This structure-function study highlights the wide window for tuning corrole electrochemical potentials while still maintaining desirable photophysical properties, important variables when designing dyes for applications in photoelectrochemical water-oxidation cells.

Hole mobility in porphyrin- and porphyrin-fullerene electropolymers

Charge transport within films of several new types of electropolymerized porphyrin and porphyrin-fullerene dyad polymers was studied in order to obtain information on the suitability of these organic semiconductors for applications in solar energy conversion, sensor devices, etc. The films, prepared by electropolymerization on a conductive substrate, were immersed in acetonitrile and studied using chronocoulometric and cyclic voltammetric electrochemical methods. The charge diffusion coefficients were found to be dependent upon the electrolytic medium. Electrolyte anion size plays a significant role in determining the rate of migration of charge through the polymers, demonstrating that migration of positive charge is accompanied by migration of negative counterions. Bulkier anions markedly decrease the charge diffusion coefficient. This strong dependence suggests that anion mobility is the rate-limiting process for diffusional charge transport within the porphyrin polymer films and that the largest rates obtained are lower limits to the intrinsic cation mobility. With electrolytes containing the relatively small perchlorate anion, charge diffusion coefficients of the porphyrin polymers were similar to those reported for polyaniline under acidic conditions. The charge diffusion coefficient for a zinc porphyrin polymer was found to decrease 2 orders of magnitude in the presence of pyridine, suggesting that metal-containing porphyrins polymer films may have sensor applications. Cation (hole) mobilities previously reported in the literature for porphyrin-containing polymers with chemical structures quite different from those investigated here were much smaller than those found for the polymers in this study, but further investigation suggests that the differences are due to choice of electrode size and material.

Silatranes for binding inorganic complexes to metal oxide surfaces

A ruthenium complex containing silatrane functional groups has been synthesized and covalently bound to a conductive metal oxide film composed of nanoparticulate ITO (nanoITO). The silatrane-derived siloxane surface anchors were found to be stable in the examined range of pH 2 to 11 in aqueous phosphate buffer, and the ruthenium complex was found to have stable electrochemical features with repeated electrochemical cycling. The non-coordinating properties of the silatrane group to metals, which facilitates synthesis of silatrane-labeled coordination complexes, together with the facile surface-binding procedure, robustness of the surface linkages, and stability of the electrochemical properties suggest that incorporating silatrane motifs into ligands for inorganic complexes provides superior properties for attachment of catalysts to metal oxide surfaces under aqueous conditions.

A new method for the synthesis of β-cyano substituted porphyrins and their use as sensitizers in photoelectrochemical devices

β-Cyanoporphyrins have high positive potentials for oxidation and absorb light at longer wavelengths than most porphyrins, making them potential candidates for sensitizers in photoelectrosynthetic cells for water oxidation. In order to begin to evaluate this potential, two Zn(II) tetra-β-cyanoporphyrins have been synthesized and evaluated as sensitizers in dye sensitized solar cells using I−/I3− as the redox mediator. To prepare such specialized β-cyanoporphyrins, a new synthetic method has been developed. This approach involves reaction of Zn(CN)2 with β-brominated zinc porphyrins in the presence of tris-(dibenzylideneacetone)dipalladium. The tetra-cyanation reaction is complete under milder conditions as compared to those usually employed in previous methods and gives improved yields of up to ∼50%. The procedure allows for the cyanation of porphyrins with relatively sensitive functional groups. Examples of its application to a range of substituted tetra-arylporphyrins are reported, and the absorption and electrochemical properties of the compounds prepared are given. The results from using two of the molecules as sensitizers in dye sensitized solar cells are presented. It was found that the porphyrins produced no photocurrents in nanoparticulate TiO2-based cells, but both molecules produced photocurrents in SnO2-based cells, and are potential candidates for sensitizers in photoelectrosynthetic cells for water oxidation.

Synthesis and spectroscopic properties of a soluble semiconducting porphyrin polymer

A semiconducting porphyrin polymer that is solution processable and soluble in organic solvents has been synthesized, and its spectroscopic and electrochemical properties have been investigated. The polymer consists of diarylporphyrin units that are linked at meso-positions by aminophenyl groups, thus making the porphyrin rings an integral part of the polymer backbone. Hexyl chains on two of the aryl groups impart solubility. The porphyrin units interact only weakly in the ground electronic state. Excitation produces a local excited state that rapidly evolves into a state with charge-transfer character (CT) involving the amino nitrogen and the porphyrin macrocycle. Singlet excitation energy is transferred between porphyrin units in the chain with a time constant of ca. 210 ps. The final CT state has a lifetime of several nanoseconds, and the first oxidation of the polymer occurs at ca. 0.58 V vs. SCE. These properties make the polymer a suitable potential excited state electron donor to a variety of fullerenes or other acceptor species, suggesting that the polymer may find use in organic photovoltaics, sensors, and similar applications.

Selective oxidative synthesis of meso-beta fused porphyrin dimers

An efficient route to meso-β doubly connected fused porphyrin dimers was developed. Synthesis of the dimers incorporated two successive C–C bond-forming steps selectively coupling unsubstituted meso- and β-positions. Using Cu(BF4)2 as an oxidant in nitromethane solvent, the radical coupling of Cu(II)-porphyrins occurred in high yield and without side-products, allowing chromatography-free purification. Efficient demetalation of the product yielded free-base derivatives and the possibility to incorporate other metals into the macrocycles. The absorption and electrochemical properties vary with the inserted metal, showing broad UV-visible-NIR absorption and multiple one-electron oxidations/reductions in a relatively narrow electrochemical window.

Molecular titanium-hydroxamate complexes as models for TiO2 surface binding†

Hydroxamate binding modes and protonation states have yet to be conclusively determined. Molecular titanium(iv) phenylhydroxamate complexes were synthesized as structural and spectroscopic models, and compared to functionalized TiO2 nanoparticles. In a combined experimental-theoretical study, we find that the predominant binding form is monodeprotonated, with evidence for the chelate mode.

Preparation of Halogenated Fluorescent Diaminophenazine Building Blocks

A short, convenient, and scalable protocol for the one-pot synthesis of a series of fluorescent 7,8-dihalo-2,3-diaminophenazines is introduced. The synthetic route is based on the oxidative condensation of 4,5-dihalo-1,2-diaminobenzenes in aqueous conditions. The resulting diaminophenazines could be attractive intermediates for the preparation of polyfunctional phenazines and extended polyheteroacenes. We find that the undesired hydroxylation byproducts, typically obtained in aqueous conditions, are completely suppressed by addition of a stoichiometric amount of acetone during the oxidation step allowing for selective formation of 7,8-dihalo-2,2-dimethyl-2,3-dihydro-1H-imidazo[4,5-b]phenazine derivatives with good to excellent yields. Under reductive conditions, the imidazolidine ring can be hydrolyzed into the desired 7,8-dihalo-2,3-diaminophenazines. Furthermore, we report a selective route under highly reducing conditions to monohydrodeaminate the 2,3-di(methylamino) phenazine derivatives, which allows for further structural variations of these phenazine building blocks. All of these derivatives are luminescent, with measured fluorescence quantum-yields of up to 80% in ethanol for the more rigid structures, highlighting the potential of such materials to provide new fluorophores.