Michael G. Walter

Solar Water Splitting Cells

Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

Porphyrins and phthalocyanines in solar photovoltaic cells

This review summarizes recent advances in the use of porphyrins, phthalocyanines, and related compounds as components of solar cells, including organic molecular solar cells, polymer cells, and dye-sensitized solar cells. The recent report of a porphyrin dye that achieves 11% power conversion efficiency in a dye-sensitized solar cell indicates that these classes of compounds can be as efficient as the more commonly used ruthenium bipyridyl derivatives.

Photoelectrochemical Hydrogen Evolution Using Si Microwire Arrays

Arrays of B-doped p-Si microwires, diffusion-doped with P to form a radial n(+) emitter and subsequently coated with a 1.5-nm-thick discontinuous film of evaporated Pt, were used as photocathodes for H(2) evolution from water. These electrodes yielded thermodynamically based energy-conversion efficiencies >5% under 1 sun solar simulation, despite absorbing less than 50% of the above-band-gap incident photons. Analogous p-Si wire-array electrodes yielded efficiencies <0.2%, largely limited by the low photovoltage generated at the p-Si/H(2)O junction.

Electrical conductivity, ionic conductivity, optical absorption, and gas separation properties of ionically conductive polymer membranes embedded with Si microwire arrays

The optical absorption, ionic conductivity, electronic conductivity, and gas separation properties have been evaluated for flexible composite films of ionically conductive polymers that contain partially embedded arrays of ordered, crystalline, p-type Si microwires. The cation exchange ionomer Nafion, and a recently developed anion exchange ionomer, poly(arylene ether sulfone) that contains quaternary ammonium groups (QAPSF), produced composite microwire array/ionomer membrane films that were suitable for operation in acidic or alkaline media, respectively. The ionic conductivity of the Si wire array/Nafion composite films in 2.0 M H_(2)SO_4(aq) was 71 mS cm^(−1), and the conductivity of the Si wire array/QAPSF composite films in 2.0 M KOH(aq) was 6.4 mS cm^(−1). Both values were comparable to the conductivities observed for films of these ionomers that did not contain embedded Si wire arrays. Two Si wire array/Nafion membranes were electrically connected in series, using a conducting polymer, to produce a trilayer, multifunctional membrane that exhibited an ionic conductivity in 2.0 M H_(2)SO)4(aq) of 57 mS cm^(−1) and an ohmic electrical contact, with an areal resistance of ~0.30 Ω cm^2, between the two physically separate embedded Si wire arrays. All of the wire array/ionomer composite membranes showed low rates of hydrogen crossover. Optical measurements indicated very low absorption (<3%) in the ion-exchange polymers but high light absorption (up to 80%) by the wire arrays even at normal incidence, attesting to the suitability of such multifunctional membranes for application in solar fuels production.

Reaction of dichloromethane with pyridine derivatives under ambient conditions.

Pyridine derivatives and dichloromethane (DCM) are commonly used together in a variety of different applications. However, DCM slowly reacts with pyridine and a variety of other representative pyridine derivatives to form methylenebispyridinium dichloride compounds under ambient conditions. The proposed mechanism (two consecutive S(N)2 reactions) was studied by evaluating the kinetics of the reaction between 4-(dimethylamino)pyridine and DCM. The second-order rate constants for the first (k(1)) and second (k(2)) substitutions were found to be 2.56(+/-0.06) x 10(-8) and 4.29(+/-0.01) x 10(-4) M(-1) s(-1), respectively. Because the second substitution is so much faster than the first, the monosubstitution product could not be isolated or detected during the reaction; it was synthesized independently in order to observe its kinetics.

Syntheses and optoelectronic properties of amino/carboxyphenylporphyrins for potential use in dye-sensitized TiO2 solar cells

New mixed-substituent amino/carboxyphenylporphyrins for a dye-sensitized TiO2 solar cell were prepared using several synthetic routes. The reaction of 4-carbomethoxy- and 4-acetamidobenzaldehydes with pyrrole in propionic acid under aerobic conditions afforded mixtures of mixed amide/ester substituted tetraphenylporphyrins which were separated using centrifugal chromatography then deprotected to give the target compounds. Condensation of p-nitrophenyl- dipyrromethane with 4-carbomethoxybenzaldehyde in CH2Cl2 catalyzed by trifluoroacetic acid, followed by oxidation with dichlorodicyanoquinone gives trans-dicarbomethoxy/ dinitrophenylporphyrin, which when treated with SnCl2 and HCl affords the trans-diamino/dicarboxy derivative, trans-TA2C2PP. Commercially available tetrakis-5,10,15,20-(4-carboxyphenyl)porphyrin (TCPP) was converted to mixtures of mixed amino/carbomethoxyphenylporphyrins using hydroxylamine hydrochloride in polyphosphoric acid with methanol workup. Relative yields and product distributions from each route are discussed and the optoelectronic characteristics of the synthesized porphyrins were studied using UV-visible spectroscopy and cyclic voltammetry. Copyright

The effects of heavy atoms on the exciton diffusion properties in photoactive thin films of tetrakis(4-carbomethoxyphenyl)porphyrins

The singlet exciton diffusion properties of solution-cast thin films prepared from mixed substituent monoiodophenyl or monobromophenyl derivatives of 5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrin (TCM4PP) were investigated for use in solar energy conversion applications. The photoluminescent singlet decay lifetime PL(t) of pristine porphyrin films and films lightly doped (0.1–0.6% wt) with [6,6]-phenyl-C61-butryic acid methyl ester (PCBM) were used to obtain relative quenching efficiencies (Q). The exciton diffusion coefficient (D) and exciton diffusion length (LD) for each derivative were obtained by modeling the quenching efficiency and PL lifetime decay data using a 3D exciton Monte Carlo diffusion simulation. Although the three TCMPP derivatives showed nearly identical absorbances and electrochemical properties, the monobromophenyl or monoiodophenyl substituted porphyrins exhibited significantly lower steady-state emission intensities and fluorescent lifetimes in solution. Amorphous thin films of the halogenated derivatives also exhibited a decrease in the PL decay lifetimes, relative quenching efficiencies, and reduced singlet exciton diffusion lengths. The singlet exciton diffusion length for TCM4PP was calculated to be 15 nm and decreased by 71% to 4.4 nm for TCM3IPP with the addition of a single iodo substituent. Photocurrent–voltage measurements of the derivatives in a PCBM bulk heterojunction device suggest that lowered exciton diffusion and enhanced singlet to triplet exciton conversion, due to the heavy atom effect, decreases photoconversion efficiency.

Enhancing exciton diffusion in porphyrin thin films using peripheral carboalkoxy groups to influence molecular assembly

The effects of molecular arrangement on the singlet exciton diffusion properties of free-base carboalkoxyphenylporphyrins containing a variety of alkyl substituents were investigated in solution-cast thin films. Exciton diffusivity was calculated using relative quenching efficiencies (Q) obtained by measuring the singlet photoluminescent decay lifetime PL(t) of pristine porphyrin films and films doped with 0.06–0.2% volume fraction of [6,6]-phenyl-C61-butryic acid methyl ester (PCBM). The quenching efficiency and PL lifetime decay data was used in a 3D exciton diffusion Monte Carlo simulation model to extract the exciton diffusion coefficient (D) and diffusion length (LD). Five carboalkoxyphenyl porphyrins (TCAPPs) were synthesized and analyzed in solution-cast thin films, tetra(4-carbomethoxyphenyl)porphyrin (TCM4PP), tetra(4-carbobutoxyphenyl) porphyrin (TCB4PP), tetra(4-carbohexoxyphenyl)porphyrin (TCH4PP), tetra(4-carbo-2-ethylhexoxyphenyl) porphyrin (TCEH4PP), and tetra(4-carbooctoxyphenyl)porphyrin (TCO4PP). Longer alkyl chain derivatives yielded increased PL decay lifetimes and extended the exciton diffusion lengths (LD) for octyl (TCO4PP) and hexyl (TCH4PP) porphyrin derivatives. We observed an increase in the LD from 15 nm for TCM4PP to 25 nm for TCH4PP, while a branched alkyl chain derivative (TCEH4PP) showed the lowest LD of 14 nm. UV-Vis and XRD data indicate that molecular organization is strongly dependent upon the peripheral carboalkoxy chain, and that nematic molecular organization resulted in an increase in the exciton diffusion length. Our findings are an important step toward a deeper understanding of the exciton diffusivity and molecular packing relationship of simple free-base porphyrins.

Synthesis and Characterization of Electropolymerized Porphyrin Nanofibers

ABSTRACT Electrochemical oxidation of tetrakis-5,10,15,20-(4-aminophenyl)porphyrin (TAPP) on fluorine-doped tin oxide electrodes leads to a conductive polymeric film (poly-TAPP) with a nanostructured fibrous morphology. The nanofiber structure and growth rate are enhanced by the addition of pyridine (5 - 15%) to the dichloromethane electrochemical solution. Electropolymerization in the absence of pyridine leads to a more highly bundled poly-TAPP structure with a spectrum indicative of protonated porphyrin units. Testing of poly-TAPP electrodes in an electrochemical iodide cell and in a 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C 61 (PCBM) integrated solid state photovoltaic cell indicates modest photoactivity. INTRODUCTION Porphyrins and porphyrin-containing materials continue to attract a great deal of attention for organic photonic devices because of their versatile molecular framework, varied optoelectronic properties, and strongly light-absorbing chromophores. 1 The electropolymerization of porphyrins offers the advantage of controlling the film deposition and conductivity through varying porphyrin monomer, the potential window, and the electrolytic conditions.

Linking design and properties of purine-based donor–acceptor chromophores as optoelectronic materials

Creating new building blocks for donor–acceptor conjugated systems is an important task for continued development of materials for organic electronics. Purines were introduced into small-molecule π-conjugated systems via Stille cross-coupling using stannylated derivatives of benzodithiophene, thiophene, or dithienylbenzothiadiazole to generate a series of “purine–π–purine” chromophores having high thermal stability, long excited-state lifetimes, and high quantum yields. Photophysical and electrochemical property characterization indicate that depending on the choice of a conjugated bridging unit, purines behave as either an electron-donating or an electron-accepting unit in these small-molecule donor–acceptor chromophores. Specifically, while purine chromophores do not exhibit charge transfer character when linked to a thiophene unit, purinyl units act as a weak acceptor when coupled with benzodithiophene and as a weak donor when coupled with dithienylbenzothiadiazole. In addition to fundamental insights into the molecular design of purine-based chromophores and their charge-transfer character, the results and synthetic tailorability of purines suggest that they may be compelling building blocks in conjugated materials for optical and electronic devices and sensors.