David G. Cobbold

Bio: David G. Cobbold is an academic researcher from University College London. The author has contributed to research in topics: Resonance & Hexamethylphosphoramide. The author has an hindex of 2, co-authored 2 publications receiving 144 citations.

Characterization of sulfur radical anions in solutions of alkali polysulfides in dimethylformamide and hexamethylphosphoramide and in the solid state in ultramarine blue, green, and red

Abstract: The characterization of the Sjradical anion as the blue species present in hexamethylphosphoramide and dimethylformamide solutions of alkali polysulfides and in ultramarine blue has been extended by the measurement of the depolarization ratios for the two a l bands of the ion, by the observation of two distinct resonance Raman progressions for the ion in solution, by the calculation of the S-S-S bond angle and the force constants for the ion, and by the estimation of the S-S bond length in the ion. The vibrational spectra of the red species formed by adding sulfur to blue solutions of Na2S4 in dimethylformamide are reported; the red species is thought to be the Sc radical anion. Both the Sc and S3radical anions have been identified as the species responsible for the color of ultramarine green. Oxidation of the latter to ultramarine blue is associated with an increase in the S3-/S< ratio. The force constants, harmonic frequencies, and anharmonicity constants of both the S c and S3radical anions are compared with those of other isoelectronic or effectively isoelectronic species. The resonance Raman spectrum of ultramarine red reveals the presence of the S3and S r radical anions together with a third, predominant species which may be the neutral Sq molecule.

Resonance Raman Spectroscopy, and Its Application to Inorganic Chemistry. New Analytical Methods (27)

Abstract: Resonance Raman spectra are obtained when the wave number of the exciting radiation is close to, or coincident with, that of an electronic transition of the scattering species. Such spectra are usually characterized by a very large enhancement of the intensities of particular Raman bands, sometimes with the appearance of intense overtone and combination tone progressions. The technique provides detailed information about excited electronic states because it is only the vibrational modes associated with the chromophore that are resonance-Raman active. Additionally, the high sensitivity is such that compounds at concentrations as low as 10−6 mol/L may be detected, enabling resonance Raman spectroscopy to be used as an analytical tool and for the study of chromophores in molecules of biological interest.

Understanding the Role of the Sulfide Redox Couple (S2–/Sn2–) in Quantum Dot-Sensitized Solar Cells

Abstract: The presence of sulfide/polysulfide redox couple is crucial in achieving stability of metal chalcogenide (e.g., CdS and CdSe)-based quantum dot-sensitized solar cells (QDSC). However, the interfacial charge transfer processes play a pivotal role in dictating the net photoconversion efficiency. We present here kinetics of hole transfer, characterization of the intermediates involved in the hole oxidation of sulfide ion, and the back electron transfer between sulfide radical and electrons injected into TiO2 nanoparticles. The kinetic rate constant (107–109 s–1) for the hole transfer obtained from the emission lifetime measurements suggests slow hole scavenging from CdSe by S2– is one of the limiting factors in attaining high overall efficiency. The presence of the oxidized couple, by addition of S or Se to the electrolyte, increases the photocurrent, but it also enhances the rate of back electron transfer.