Biomedical subjects

David F Watson

Publications and source records attributed to David F Watson.

4 recordsLinked to original sources

PubMed2006

Theoretical solar-to-electrical energy-conversion efficiencies of perylene-porphyrin light-harvesting arrays.

The efficiencies of organic solar cells that incorporate light-harvesting arrays of organic pigments were calculated under 1 sun of air mass 1.5 solar irradiation. In one set of calculations, photocurrent efficiencies were evaluated for porphyrin, phthalocyanine, chlorin, bacteriochlorin, and porphyrin-bis(perylene) pigment arrays of different length and packing densities under the assumption that each solar photon absorbed quantitatively yielded one electron in the external circuit. In another more realistic set of calculations, solar conversion efficiencies were evaluated for arrays comprising porphyrins or porphyrin-(perylene)2 units taking into account competitive excited-state relaxation pathways. A system of coupled differential equations for all reactions in the arrays was solved on the basis of previously published rate constants for (1) energy transfer between the perylene and porphyrin pigments, (2) excited-state relaxation of the perylene and porphyrin pigments, and (3) excited-state electron injection into the semiconductor. This formal analysis enables determination of the optimal number of pigments in an array for solar-to-electrical energy conversion. The optimal number of pigments depends on the molar absorption coefficient and the density at which the arrays can be packed on an electrode surface. Taken together, the ability to employ fundamental photophysical, kinetic, and structural parameters of modular molecular architectures in assessments of the efficiency of solar-to-electrical energy conversion should facilitate the design of molecular-based solar cells.

Electrons↗

PubMed2005

Electron injection at dye-sensitized semiconductor electrodes.

Electron injection at dye-sensitized semiconductors is reviewed. Particular emphasis is placed on theoretical and photoelectrochemical studies of dye-sensitized planar and single-crystal electrodes. The accepted mechanism of electron injection, which was derived from these classical studies, is introduced. Selected photoelectrochemical studies of dye-sensitized nanocrystalline semiconductors are reviewed; emphasis is given to factors that influence the efficiencies of electron injection and charge recombination. The development of quasi-solid-state nanocrystalline dye-sensitized solar cells is also discussed. Recent time-resolved spectroscopic studies of electron injection and charge recombination are reviewed. These studies have led to a better understanding of electron injection mechanisms, and have revealed the limitations of the classical models.

Coloring Agents↗

PubMed2002

Excited-state electronic coupling and photoinduced multiple electron transfer in two related ligand-bridged hexanuclear mixed-valence compounds.

The synthesis, characterization, electrochemical, photophysical, and photochemical properties of two hexanuclear mixed-valence compounds are reported. Each supramolecular species consists of two cyano-bridged [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(3)L-NC-Fe(II)(CN)(5)] triads that are linked to each other through a Pt(IV)-L-Pt(IV) bridge, where L = 4,4'-dipyridyl (bpy) or 3,3'-dimethyl-4,4'-dipyridyl (dmb). The major difference between the two compounds is the electronic nature of the bridging ligand between the two Pt atoms. Both species exhibit a broad Fe(II) --> Pt(IV) intervalent (IT) absorption band at 421 nm with an oscillator strength that is approximately four times that for [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(5)] and twice that for [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(4)-NC-Fe(II)(CN)(5)].(4-) When L = bpy, the resonance Raman spectrum obtained by irradiating the IT band at 488 nm exhibits several dipyridyl ring modes at 1604, 1291, and 1234 cm(-1) which are not present in the spectrum when L = dmb. In addition, femtosecond pump-probe spectroscopy performed at 400 nm yields a transient bleach of the IT absorption band with a single exponential decay of 3.5 ps for L = bpy, compared with only 1.8 ps for L = dmb and 2.1 ps for [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(4)-NC-Fe(II)(CN)(5)].(4-) Last, prolonged irradiation of the complexes at 488 nm leads to the formation of 4 equiv of ferricyanide with a quantum efficiency of 0.0014 for L = bpy and 0.0011 for L = dmb. The transient absorption, resonance Raman, and photochemical data suggest that the degree of excited electronic coupling in these compounds is tunable by changing the electronic nature of the Pt-L-Pt bridging ligand.

Journal Article↗

PubMed2002

Photochemical image generation in a cyanogel system synthesized from tetrachloropalladate(II) and the trimetallic mixed-valence complex [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(4)-NC-Fe(II)(CN)(5)](4-): consideration of photochemical and dark mechanistic pathways of Prussian blue formation.

The cyanogel system involving PdCl(4)(2-) and the mixed-valence complex [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(4)-NC-Fe(II)(CN)(5)](4-) is reported. The system has been characterized by UV-vis absorption, diffuse reflectance infrared, and resonance Raman spectroscopies. Gelation occurs through coordination of Pd(II) to the nitrogen atom of terminal cyanide ligands in the mixed-valence complex. Irradiation into the Fe(II) --> Pt(IV) intervalent electron transfer (IT) band of [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(4)-NC-Fe(II)(CN)(5)](4-) results in the formation of a variety of Prussian-blue-like species within the rigid cyanogel matrix. Photochemical and dark mechanisms involving coupled cyanide loss and Fe(II) oxidation are proposed for the formation of Prussian-blue-like species. The optical contrast between irradiated and nonirradiated regions of the gel enables photochemical image generation with at least 12 microm resolution. This capability is demonstrated through the production of a series of diffraction gratings in cyanogel samples.

Journal Article↗